Name: construction and characterization of external cavity diode lasers for atomic physics
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Description: Watch this Scientific Journal Video about Construction and Characterization of External Cavity Diode Lasers for Atomic Physics at JoVE.com

1. Component Selection
<ol>
	<li>Select a diode at the appropriate wavelength for the atom of interest. It is critical that the selected diode be single mode (sm and has sufficient power for the application. An anti-reflection coated diode is ideal. These diodes will not lase without the addition of an external cavity and they are designed explicitly for ECDL operation. They have significantly better performance, particularly for applications where scanning the wavelength of the laser is important. The laser diode used here is listed in List of Materials). 
	As in MacAdam&#160;et al.3, the ECDL must be designed to tightly fit the diode and a collimating lens. Mechanical stability and thermal contact are critical for good operation of the laser. For ease of construction, and minimal machining, success has been had using a diode laser mount with integrated lens tube (List of Materials).</li>
	<li>Select a lens to collimate the diode. It is important that the numerical aperture be comparable or larger than the numerical aperture of the diode otherwise there will be substantial losses. Most diodes have a high numerical aperture (&#62;0.5) and require aspheric lenses, otherwise aberrations will result in very low feedback efficiency. Make sure the lens is anti-reflection coated at the operating wavelength, choose a lens with a long focal length to increase the beam size on the grating and a design wavelength near the operating wavelength to reduce aberration. Refer to List of Materials for the lens used in the demonstrated system.</li>
	<li>Select the appropriate external grating for the laser diode&#39;s frequency range and the grating tuning arm central angle. The wavelength of light diffracted into the first order, Littrow configuration, is given by &#955; = 2dsin(&#952;), where d is the grating line spacing, &#952; is the grating angle of incidence and &#955; the wavelength21 (Figure 1B). There are two main types of diffraction grating, holographic and ruled, and both may be blazed or not. Depending on the type of grating the diffracted power may vary substantially. Aim for a holographic grating with a diffraction efficiency of between 20-30%. Refer to List of Materials for the grating used in the demonstrated system.</li>
	<li>Use the simplest design manageable - complexity often means instability. There are a vast number of ECDL designs but the simplest is the Littrow3,5,7,22. Read the papers and decide whether a large mode hop free range (the frequency range over which the diode can continuously tune without suddenly jumping to a different frequency), a very narrow linewidth or reduced pointing variation is of greatest importance for the application. Obtain as much information as possible before beginning ECDL design. Often the grating ECDL is more than adequate for applications in atomic physics.</li>
	<li>It is important to realize that the performance of an ECDL is most strongly rooted in the electronics that drive the diode current and stabilize the temperature of the laser. Without a good set of electronics the mechanical design will under-perform. Included is a comparison of different current and temperature controllers in Table 1. The lower the current noise, the better the laser will perform23.</li>
</ol>
2. Assembly
<ol>
	<li>For the purposes of this paper the starting point for the ECDL assembly will be a complete ECDL mechanical system mounted on a thermoelectric cooler (TEC) without the frequency selecting components (i.e. grating and laser diode).</li>
	<li>Begin by placing the laser diode in its respective mounting hole and secure it using its mounting ring. Be careful not to over-torque the mounting ring. It should be snug but not tight.</li>
	<li>Before connecting the laser diode to the current supply, check the diode specification sheet for the anode, cathode and ground pin assignments. This varies from diode to diode and putting the current through the diode backwards will destroy it.
	<ol>
		<li>Laser diodes are low voltage devices, typically 5-10 V&#160;maximum, and care must be taken to ensure no static is discharged to them. It is good practice to wear a grounding strap when handling diodes and install a protection circuit (e.g. Figure 2) across the laser diode pins to prevent high voltages. The diode can and ground pins should be permanently grounded and the use of thin wires can aid in reducing the coupling of mechanical vibrations.</li>
	</ol>
	</li>
	<li>Set the maximum and minimum temperatures and the maximum diode and TEC current limits on the diode controller according to the values in the diode specification sheet. If the minimum operating temperature is below dew point for the lab then use a minimum temperature of ~ 2 &#176;C above dew point. This will avoid condensation.</li>
	<li>The diode specification sheet usually has a wavelength vs. temperature figure at a given diode current. Use this figure as a reference to initially set the diode temperature (and current) to match the wavelength of interest. If a temperature vs wavelength graph is unavailable adjust the set temperature to room temperature.</li>
	<li>Turn the temperature controller on and allow the temperature to stabilize.</li>
	<li>Turn ON the diode and turn the current up so that the output beam may be clearly observed with a viewing card. Use an IR card to view the beam.</li>
	<li>Insert the aspheric collimating lens and collimate the laser diode by adjusting the separation between the diode and the lens. In order to ensure good collimation make sure the beam has a clear path, ideally &#62;3 m, and adjust the lens position until the beam diameter just after the ECDL and at the end of the beam path are the same, being sure to check that the beam is not focusing at any point along the path.</li>
	<li>Check the polarization from the diode laser is in the desired plane for the diffraction grating (S or P). In most cases the polarization of the diode is along the short axis of the elliptical beam shape but it is good practice to check the polarization axis using a polarizing beam splitter.
	<ol>
		<li>If the beam axis is not in the desired plane, loosen the diode mounting ring and rotate the diode until the proper orientation is achieved. Some ECDL designs allow this to be done with the laser on and connected to the current source and others do not. If the current supply wires must be removed to rotate the diode, turn off the current supply at the control box and remove the wires. The ECDL temperature control can remain on during this process. Remember to always wear a grounding strap when handling the diode.</li>
		<li>If it was necessary to reposition the diode repeat the previous step to recollimate the diode.</li>
	</ol>
	</li>
	<li>The diffraction plane of the grating is usually labelled by the manufacturer with an arrow perpendicular to the grating lines and in the direction of the blazed reflection. Double check this by observing the reflection from a broad-band light source, such as a light bulb, as a function of angle.
	<ol>
		<li>If the grating is held with the arrow pointing back towards the observer and a broad band light source over head, the reflected light will change in color as a function of grating angle.</li>
		<li>Mount the grating so that the arrow points back towards the diode and thus adjusting the grating angle varies the wavelength reflected back into the diode (Figures 1A and&#160;1B).</li>
	</ol>
	</li>
	<li>Once the grating orientation has been confirmed glue the grating on the ECDL tuning arm using fast setting glue such as Loctite.</li>
</ol>
3. Feedback Alignment
<ol>
	<li>Place a viewing card aligned to the ECDL output beam. This will be used to monitor the laser power as adjustments are made to the pointing of the diffracted beam. A power meter could also be used but is slower in its response.</li>
	<li>Adjust the set current on the diode control box to just below the threshold current for reflective front facet diodes and 1/3 the maximum current for AR coated diode gain chips. Reflective front facet diodes will have a threshold current on their specification or data sheets while AR coated gain chips do not.</li>
	<li>Adjust the angle of the grating arm both horizontally and vertically, to steer the diffracted beam back into the diode, effectively making an external feedback cavity. When the beam is directed into the laser diode there will be a significant increase in the output power, observable as a marked increase or bright flash on a viewing card or a dramatic increase of power when measured using a power meter or photodiode.
	<ol>
		<li>A viewing card is not a very quantitative measure of power so it may be necessary to incrementally lower the laser diode current and readjust the feedback beam until the above behavior can be seen at the lowest possible current.</li>
		<li>Adjusting the collimation lens focus or axial position to optimize focusing at the diode facet can further lower the threshold and increase output power after which it will be necessary to reoptimize the grating angle horizontally and vertically.</li>
	</ol>
	</li>
</ol>
4. Initial Frequency Selection
<ol>
	<li>For the initial frequency alignment of the laser an absolute measurement of wavelength with a precision of &#60;1 nm and ideally &#60;0.1 nm is ideal. This coarse frequency measurement will make it much easier to tune the laser frequency onto an atomic transition in a later step. There are many options including using a wavemeter, an optical spectrum analyzer, spectrometer, or a monochromator with a camera. Make sure a calibrated accurate device is used or check its calibration for example, using a HeNe laser. Alternatively, the coarse frequency adjustment can usually be accomplished by walking the grating angle and current while the laser is scanning until an absorption or fluorescence signal from a vapor reference cell can be seen.
	<ol>
		<li>Generally a secondary beam picked off from the main beam, using a glass wedge prism or &#955;/2 waveplate and polarizing beam splitter, will be used as an input for the wavemeter. This optics setup is seen in Figure 1D. Refer to List of Materials for materials used in this demonstration.</li>
	</ol>
	</li>
	<li>Adjust the ECDL until the desired output wavelength is obtained. The diode driving current, temperature, grating angle and external cavity length will all affect the laser frequency24 (Figure 3).
	<ol>
		<li>Begin by adjusting the grating angle, either by hand or using the piezo. Secondly, adjust the diode current.</li>
		<li>If the desired frequency is to the blue of the grating sweep range, the diode temperature should be decreased and vice versa if the desired wavelength is to the red.</li>
	</ol>
	</li>
</ol>
5. Fine Frequency Adjustments and Frequency Locking
<ol>
	<li>Set up saturated absorption spectroscopy on the ECDL output using the configuration in Figure 1F 3,14,17. The use of an optical isolator immediately after the laser is essential (Figure 1C). It is important to avoid back reflection into the laser, which can cause instability. Saturated absorption spectroscopy using a reference cell, containing the atom of interest is a simple way to lock a laser to a narrow atomic transition25.
	<ol>
		<li>Ensure the reference cell is on an angle to avoid back reflections and that the mirror retro reflects the beam back through the vapor cell with maximum overlap. The double pass transmitted power may be monitored using the photo diode as the ECDL wavelength is scanned.</li>
	</ol>
	</li>
	<li>Most diode controllers will have a built in scan function that will scan the wavelength by adjusting the grating piezo voltage and hence the grating angle and external cavity length or by modulating the diode current. The width, scan offset and laser temperature and current should be adjusted until an absorption signal can be viewed on a scope connected to the photo detector. When the laser is scanning over the atomic transition it should be possible to see the laser beam path in the vapor cell fluoresce or flash with the naked eye or through an IR viewer.</li>
	<li>The power per unit area in the reference beam for saturated absorption spectroscopy must be at or above the saturation intensity for the atomic transition. Use the &#955;/2 wave plate before the polarizing beam splitter to increase the power until a clear absorption signal can be seen. Calculations of saturation intensities can be found in Foot16.</li>
	<li>With the laser scanning over the 780 nm Rb atomic transition, a wide Doppler broadened absorption signal should be seen, ~5 GHz width, with several sharp transitions ~10 MHz burned in Foot16 (Figure 4). Minimizing the power used for generating the saturated absorption signal is necessary to reduce power broadening and produce a sharper feature to lock to.</li>
	<li>In order to lock the ECDL frequency, an error signal is needed. By placing coils around the reference cell as in Figure 510 and oscillating the magnetic field, the Zeeman levels and thus the frequencies of the transitions are modulated. In this case the current passing through the Zeeman coils is modulated at around 250 kHz with a magnitude of ~1 G.</li>
	<li>Mix the absorption signal from the saturated absorption photo detector with the modulation signal from the function generator. When the output from the mixer is viewed on a scope it should be an error signal similar to Figure 4. The magnitude of the error signal will depend on the relative phase between the two mixed signals. Rotate the &#955;/4 beam splitter before the vapor cell to adjust the phase.</li>
	<li>Progressively reduce the scan range and adjust the offsets to center the scan over the transition of interest with no other transitions present.</li>
	<li>A proportional-integral-derivative (PID) circuit (for example see MacAdam&#160;et al.3) may then be used to lock the ECDL wavelength using the error signal. The PID gain should be reduced below the point at which ringing is observed by looking for the presence of modulation in the error signal (e.g. using a spectrum analyzer or Fourier transform of the error signal trace).</li>
</ol>
6. Linewidth Measurement
<ol>
	<li>In order to attain an accurate linewidth measurement it is necessary to have either a known narrow linewidth source (another laser with linewidth significantly less than the ECDL), two of the same ECDLs or a delay line long compared with the coherence length of the ECDL. Here two ECDLs will be interfered to measure linewidth. Alternatively, it may be easier to lock to a resonance produced by an atomic transition or a Fabry-Perot cavity and fit to the noise above the bandwidth of the locking loop.</li>
	<li>Lock the two lasers to different hyperfine transitions, ideally around 100 MHz offset. This will minimize the impact of electronic noise.</li>
	<li>Mode, power and polarization match the two beams and interfere them together using a 50/50, nonpolarizing beam splitter. Align the resulting beam on a photo detector. The signal output on the photo detector should be a sine wave with a frequency of the two laser&#39;s frequency offset. It may be necessary to attenuate or defocus the resulting beam so as not to damage or saturate the photodiode.
	<ol>
		<li>The overlap of the two beating beams will determine the fringe contrast as viewed on a scope during the linewidth measurement. If the fringe contrast is poor, spend additional time improving the mode matching and overlap of the beams on the beam splitter and detector. A good method is to overlap the two beams using two iris&#39;, or pin holes, separated by a relatively large distance, ~1 m.</li>
	</ol>
	</li>
	<li>It will be difficult to resolve the frequency fluctuations on a scope. For the best measurement use a spectrum analyzer, which will give a Voigt profile centered on the beat frequency with a linewidth &#916;f, equal to the convolved laser linewidth (Figure 6). To a good approximation the trace can be fit to a Gaussian and the linewidth obtained from the fit. The measured noise or linewidth will depend on the acquisition or integration time, which may be set by adjusting the resolution bandwidth on the spectrum analyzer. For this reason it is important to quote the integration time when quoting the measured linewidth.</li>
</ol>

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The authors have nothing to disclose. Specific product and company citations are for the purpose of clarification only and are not an endorsement by the authors.

This publication has shown how to move from a disassembled ECDL through the alignment and frequency locking to produce a measurement of the laser linewidth. The mechanical design and the design of the electronics such as PID servos, diode drivers and temperature controllers is too specialized to be discussed here but have been comprehensively discussed in referenced publications1,3,5.
Although diode ECDL&#39;s have become a staple in atomic physics labs, the species and transitions that these devises can reach is limited. Much progress has been made in broadening the wavelength range from diode based lasers however currently many gaps remain especially in the UV. Power limitations of ECDL systems continue to restrict their applications. Bare single mode diodes range in power from &#181;Watts to 100&#39;s of mWatts. Additionally, tapered amplifiers can be added to an ECDL system to increase the single mode total laser power up to the Watt level. If single mode powers much greater than a Watt or other wavelengths are required alternative laser architectures are required. These include fiber lasers26, solid state lasers27 such as TiSaph lasers or they may rely on nonlinear frequency conversion processes27 such as Raman lasers, four wave mixing, sum frequency generation, or an optical parametric oscillator.
This publication focuses on a locking mechanism that is dependent on an atomic vapor cell. For many applications in atomic physics a simple glass vapor cell, as discussed here, may not be available, such is the case for species like Yb. Many other techniques for obtaining a reference sample with a variety of species have been demonstrated such as, hot atomic beams, discharge lamps, buffer gas cells, iodine cells, and sputtering cells.
This laser system design is inherently limited to linewidths of &#8776;30 kHz 28 and typically closer to 100 kHz. If the application requires a narrower linewidth other stabilization techniques or alternative laser designs26 are required.
Whenever working with optical systems, cleanliness is of the utmost importance. It is good practice when first being introduced to and handling optics that gloves be worn to prevent accidentally touching the optical surface. If an optic is scratched it should not be used in a laser system. In most cases optics with finger prints or dust can be cleaned with acetone or compressed air respectively. Any imperfection in an optical surface can and will introduce loss and potentially noise into the system. Optics mounts should be fixed to the optics bench at all times and should be firmly bolted down once in place.
When aligning optics such as waveplates and polarizing beam splitters, ensure the light is incident near perpendicular to the optical surface while avoiding reflections back into the laser. As the incident angle deviates from 90&#176; the behavior of these optical elements becomes further and further from ideal. To minimize aberration and maximize numerical aperture beams should always travel through the center of lenses and be normal to the lens. In contrast, a vapor cell should be placed at a slight angle to the incident beam to avoid etalon effects. For this reason many vapor cells are manufactured with nonparallel end facets.
The lasers used here are class 3B. Even stray reflections have the potential for eye damage. Work with lasers of this type should only be undertaken by trained personnel familiar with the hazards of lasers. Laser safety goggles should be worn at all times. Never look directly down the path of any laser for optical alignment and take particular care to avoid generating hazardous specular reflections off optical components. Always positively terminate beam lines using a beam dump.

Measuring and manipulating the quantum state of atoms is at the heart of atomic physics and requires the ability to address specific transitions between atomic electronic states. For example consider rubidium, a typical and much used alkali atom. Here, the wavelength of light coupling the ground and first excited electronic state is ~780 nm (384 THz) and the excited state lifetime due to spontaneous emission is ~26 nsec giving an absorption linewidth of 6 MHz 4. Thus, a light source with frequency stability of at least one part in 108 is required to reliably address this transition.
Before the development of ECDLs, dye lasers and Titanium Sapphire lasers were typically used for atomic physics. These are large, expensive, complex systems that offer optical gain over a large bandwidth and thus can be tuned to overlap an atomic transition. The potential to replace these gain media with a cheap, simple diode laser engineered with a bandgap matching the desired wavelength was recognized in the early 1980s1,2. Simple, easy to build designs which achieve 100 kHz linewidths were well understood and common place by the early 1990s3,5,6. Many different configurations and designs have been demonstrated each with advantages and disadvantages. Probably the most common configurations are the Littrow3,5,7,8 and Littman 9 configurations. This discussion focuses on the simplest, the Littrow configuration shown in Figure 1A.
A number of tuning mechanisms are simultaneously used to achieve a high precision in the laser frequency. Firstly, a diode is required with a bandgap producing sufficient gain at the desired wavelength at an achievable operating temperature. The typical laser diode will have gain over several nanometers (THz). Secondly, a reflective diffraction grating is angle tuned to provide optical feedback into the diode at the desired wavelength. Depending on the grating, the diode, the focusing lens used and their alignment, the grating will select a frequency range of typically 50-100 GHz. The laser will oscillate at a wavelength resonant with the external laser cavity (between the diode rear facet and the grating). Tuning this cavity length across a wavelength allows the laser to be tuned across a free spectral range (c/(2L)) around the grating gain peak where c, is the speed of light and L, is the cavity length, typically 1-5 cm (FSR 3-15 GHz). When two cavity modes are a similar wavelength from the peak grating feedback wavelength the laser may run multimode. As the oscillating cavity mode is tuned further from the gain peak than its neighboring mode the laser will mode hop limiting the tuning range. The behavior of the cavity modes with respect to the grating mode can be seen in Figure 3. The mode hop free tuning range is a key performance metric for an ECDL. By simultaneously tuning the grating angle and the cavity length it is possible to continuously tune across many free spectral ranges without mode hops, making locating and locking to spectral features much easier8. Electronic tuning of the optical path length of the cavity for locking may be achieved by a combination of tuning the grating angle/position using a piezo actuator (Figure 1A) (scanning bandwidth ~1 kHz) and tuning the diode current which primarily modulates the refractive index of the diode (scanning bandwidth &#8805;100 kHz). Using laser diodes rather than anti-reflection (AR) coated gain chips for the gain medium adds the additional complication of adding the laser diode internal cavity response which may have a typical free spectral range of 100-200 GHz. In this case the cavity must be temperature tuned to match the response from the grating. Using a laser diode rather than an AR coated gain chip will dramatically reduce the mode hop free tuning range unless there is a means to synchronously tune the diode current or temperature. Finally, to achieve a linewidth better than 100 kHz careful attention must be paid to eliminate other noise sources. This requires careful mechanical design of the mounts to minimize acoustic vibration, mK level temperature stabilization, rms current stability of the diode at the &#8804;30 nA level and careful tuning of the gain of all locking loops10. Selecting the proper electronics for the application is just as important as the laser and optics design. A list of diode controllers and specifications can be found in Table 1.
Once stable lasing has been achieved, the next requirement is to lock the laser frequency to a reference such as an atomic transition, an optical cavity or another laser. This removes the effects of slow drifts such as small temperature fluctuations, essentially eliminating noise for frequencies within the bandwidth of the locking loop. There are a myriad of locking techniques that have been developed for obtaining an error signal, each suited for a particular reference system. An error signal for phase locking two lasers can be obtained by mixing the two lasers on a beam splitter. Pound-Drever hall11 or tilt-locking12 can be used to lock to a cavity. To lock to an atomic absorption line DAVLL13 or saturated absorption spectroscopy3,14 in combination with current modulation10, Zeeman modulation10, or tilt-locking15 may be used.
The locking of an ECDL to a rubidium transition using Zeeman modulation of saturated absorption in a vapor cell will be described here. If a low intensity beam passes through a rubidium vapor cell at room temperature and the frequency is tuned in the vicinity of the 780 nm atomic transition a number of Doppler broadened absorption features ~500 MHz wide will be observed rather than the 6 MHz wide natural linewidth (calculations for natural and Doppler linewidths can be found in Foot16). If, however, this beam is retro reflected, the second pass will have less absorption on resonance as atoms with a zero longitudinal velocity have already been partially excited by the first pass17. Other frequencies will be absorbed by different velocity populations on each pass and therefore absorption will not be saturated. In this way an apparent transmission feature overlaid on the Doppler broadened absorption at transitions with a width about the natural linewidth can be obtained. This provides a sharp absolute frequency reference to lock to. The frequency of the atomic transition may be modulated using the Zeeman effect by dithering the magnitude of a magnetic field in the reference cell. A suitable homogeneous magnetic field may be produced using a solenoid setup as shown in Figure 5. Electronically mixing the modulated waveform with the saturated absorption transmission generates an error signal which can be used to adjust the diode current and integrated to adjust the piezo voltage. Thus, the laser may be locked to the transition without needing to modulate the laser frequency.
The linewidth of an ECDL is generally measured by interfering two frequency locked lasers of the same type on a beam splitter18. The beat frequency between the lasers is then measured using a fast photodiode and an RF spectrum analyzer. The noise spectrum beyond the locking loop bandwidth is then fitted to a Voigt (convolution of a Gaussian and Lorentzian) profile. The noise from the different lasers add in quadrature. In the case of two equivalent lasers this gives a fitted linewidth of &#8730;(2) times the single laser linewidth. If a laser is available with a known linewidth significantly smaller than that expected from the ECDL and it is within the tuning range of the ECDL, then that could be used instead. Another method commonly used for measuring linewidth is the delayed self homodyne technique19,20 where part of the beam is sent along an optical delay line such as a fiber and then mixed on a beam splitter with the laser. This technique relies on the delay being longer than the coherence length of the laser under measurement. This works well for noisy lasers but for a 100 kHz linewidth laser the coherence length is around 3 km, which begins to become impractical. Alternatively, an atomic transition in a saturated absorption cell or a Fabry-Perot cavity can be used to provide a frequency reference for laser linewidth measurement. In this system the laser frequency will need to sit at a linear portion of ether a saturated absorption or Fabry-Perot resonance rather than allowed to scan in frequency. By measuring the signal noise on a photo diode and knowing the resonance linewidth, the frequency noise can be found. The lower limit of the linewidth measurement is then limited by the slope of the transmission resonance.
The presence of higher order lasing modes may be checked for by looking at intensity noise at the frequency of the free spectral range using an RF spectrum analyzer or by using a scanning Fabry-Perot or an optical spectrum analyzer with a resolution better than the free spectral range of the ECDL. The coarse tuning range may be measured by measuring the power as a function of wavelength (using a wavemeter, monochromator, or optical spectrum analyzer) while tuning the laser across its limits using the grating. The mode hop free tuning range is generally measured using a scanning Fabry-Perot cavity where a mode hop can be detected as a discontinuous jump in frequency.

<table border="0" cellpadding="0" cellspacing="0" width="870">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="20">
			<td height="94" rowspan="3" style="height:94px;width:253px;">Laser Diode 
			(Rubidium, 780nm)</td>
			<td rowspan="3" style="width:181px;">Roithner</td>
			<td rowspan="3" style="width:195px;">ADL-78901TX</td>
			<td rowspan="3" style="width:241px;">Various wavelengths, powers, case sizes and AR coatings are available (Thor Labs, Eagle Yard Photonics, Rothnier)</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="53">
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:253px;">Diffraction Grating 
			(Rubidium, 780nm)</td>
			<td style="width:181px;">Newport</td>
			<td><a href="http://search.newport.com/?q=*&amp;x2=sku&amp;q2=05HG1800-500-1">05HG1800-500-1&#160;</a></td>
			<td style="width:241px;">Holographic or rullered 
			&#160;(Optional blazing) 
			(Thor Labs, Newport)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:253px;">Viewing Card</td>
			<td style="width:181px;">Thor Labs</td>
			<td>VRC5</td>
			<td style="width:241px;">Infared viewing card</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:253px;">Diode&#160; Lens</td>
			<td style="width:181px;">Thor Labs</td>
			<td>C330TME-B</td>
			<td>Coated for 780 nm</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:253px;">Glass Wedge</td>
			<td style="width:181px;">Thor Labs</td>
			<td>PS814</td>
			<td>10 &#176; wedge</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:253px;">1/2 Waveplate</td>
			<td style="width:181px;">Thor Labs</td>
			<td>WPH10M-780</td>
			<td style="width:241px;">780 nm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:253px;">1/4 Waveplate</td>
			<td style="width:181px;">Thor Labs</td>
			<td>WPQ10M-780</td>
			<td style="width:241px;">780 nm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:253px;">Rotation mounts</td>
			<td style="width:181px;">Thor Labs</td>
			<td style="width:195px;">RSP1C</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:253px;">PBS</td>
			<td style="width:181px;">Thor Labs</td>
			<td style="width:195px;">PBS252</td>
			<td style="width:241px;">780 nm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:253px;">Isolator</td>
			<td style="width:181px;">Thor Labs</td>
			<td style="width:195px;">IO-5-780-HP</td>
			<td style="width:241px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:253px;">Vapor Cell</td>
			<td style="width:181px;">Thor Labs</td>
			<td><a href="http://www.thorlabs.com/thorProduct.cfm?partNumber=GC25075-RB">GC25075-RB</a></td>
			<td>Rubidium&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:253px;">Photo Detector</td>
			<td style="width:181px;">Moglabs</td>
			<td>PDD-001-400-1100-&#955;</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:253px;">Scope</td>
			<td style="width:181px;">Tektronix</td>
			<td style="width:195px;">TDS1001B</td>
			<td></td>
		</tr>
		<tr height="78">
			<td height="78" style="height:78px;width:253px;">Wavemeter</td>
			<td style="width:181px;">Yokogawa</td>
			<td style="width:195px;">AQ-6515A</td>
			<td style="width:241px;">We use an optical spectrum analyzer but a cheaper wavemeter would also be sufficient</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:253px;">Electronic spectrum analyzer</td>
			<td style="width:181px;">Agilent</td>
			<td style="width:195px;">E4411B</td>
			<td style="width:241px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:253px;">&#160;IR Viewer</td>
			<td style="width:181px;">FJW Optical Systems Inc</td>
			<td>84499A-5</td>
			<td style="width:241px;">Infared viewer</td>
		</tr>
	</tbody>
</table>

construction and characterization of external cavity diode lasers for atomic physics

Since their development in the late 1980s, cheap, reliable external cavity diode lasers (ECDLs) have replaced complex and expensive traditional dye and Titanium Sapphire lasers as the workhorse laser of atomic physics labs1,2. Their versatility and prolific use throughout atomic physics in applications such as absorption spectroscopy and laser cooling1,2 makes it imperative for incoming students to gain a firm practical understanding of these lasers. This publication builds upon the seminal work by Wieman3, updating components, and providing a video tutorial. The setup, frequency locking and performance characterization of an ECDL will be described. Discussion of component selection and proper mounting of both diodes and gratings, the factors affecting mode selection within the cavity, proper alignment for optimal external feedback, optics setup for coarse and fine frequency sensitive measurements, a brief overview of laser locking techniques, and laser linewidth measurements are included.

There are 5 main steps involved in aligning, frequency locking and characterizing the linewidth of the ECDL. These are: obtaining feedback from the grating and using this to set the coarse ECDL frequency measured on a wavemeter, observing laser absorption in the reference cell, viewing the atomic transition with a resolution around the natural linewidth in a saturated absorption spectroscopy setup, obtaining an error signal around the desired transition and locking to it, and finally observing the beat note of two lasers and measuring the laser linewidth. Step one is successfully completed, fairly trivially when the wavelength as read on the wavemeter corresponds to the atomic transition of interest. When attempting to achieve absorption in the reference cell, florescence can be seen along the beam path in the cell with an IR viewer when the transition is hit. If the ECDL is scanning the cell will flash. A saturated absorption signal may be difficult to spot when first aligning because the transmission lines may be very small compared to the Doppler absorption peak. When peaks, similar to those shown in Figure 4, can be seen, the saturated absorption system is working properly. By adjusting the phase and scan parameters an error signal similar to that shown in Figure 4 should be obtained. In order to measure the ECDL linewidth it is necessary to obtain a beat signal between two beams. As the beams become more and more overlapped a sine wave will begin to appear, as seen on a scope from a photo detector. Keep aligning until the contrast between the nodes and anti-nodes is largest. When the beat signal is then passed through an electronic spectrum analyzer a signal similar to Figure 6 should be seen. The laser linewidth can be measured from this signal. The complete optics setup can be seen in Figure 1.
<img alt="Figure 1" fo:content-width="5in" fo:src="/files/ftp_upload/51184/51184fig1highres.jpg" src="/files/ftp_upload/51184/51184fig1.jpg" /> 
Figure 1. Complete optics setup. This is an example of a complete optics setup for the ECDL system discussed. A: This shows the Littrow configuration of an ECDL. A percentage, typically 20-30%, of the beam incident on the grating is diffracted back into the diode. The diffraction angle and reflection angle are equal. The grating is mounted to a tuning stage which uses a piezo to control the grating angle. B: The output beam from the laser diode is incident on the grating at angle &#952; with the 0th order reflecting off and the 1st order diffraction being sent back along the incident beam path. The wavelength of diffracted light is given by &#955; = 2dsin(&#952;) in Littrow configuration. C: Position, and orientation of the optical isolator to reduce unwanted feedback to the laser diode. D: The output beam from the laser box passes through a &#955;/2 waveplate and PBS and is aligned to the wavemeter. The power in the reflected and transmitted beams can be adjusted by rotating the waveplate. E: Beam line used for experiment. This line will contain the majority of the laser&#39;s power. F: Pass a reference beam at or above saturation intensity through a PBS, &#955;/4 waveplate, reference gas cell, and retro reflect it back onto the PBS. It is important that the two beams are well overlapped to get proper saturation spectroscopy. The waveplate will ensure the polarization of light on the retro reflected beam will be rotated 90&#176; from the incident beam allowing it to exit the opposite port of the beam splitter. <a href="https://www.jove.com/files/ftp_upload/51184/51184fig1highres.jpg" target="_blank">Click here to view larger image.</a>
<img alt="Figure 2" fo:content-width="2in" src="/files/ftp_upload/51184/51184fig2highres.jpg" width="200" /> 
Figure 2.&#160;Laser diode protection circuit. Example protection circuit for the laser diode current. R1 and C1 form a basic RC circuit and will filter out high frequency noise. D1 and D2 are Schottky and Zener diodes respectively. The Schottky diode, which has a fast response time, is in place to protect against reverse voltages, and the Zener diode, which has a slower response time, is designed to allow current to pass if above the laser diodes maximum operating voltage, thereby avoiding damage to the laser diode. Typical values for the components will be R1 = 1 &#937;, C1 = 1 mF, D1 = 30 V, D2 = 6 V. The values chosen for R1 and C1 will limit the current modulation bandwidth of the diode. This may be less than ideal if a error signal is being produced via current modulation instead of the Zeeman modulation discussed.
<img alt="Figure 3" fo:content-width="5in" fo:src="/files/ftp_upload/51184/51184fig3highres.jpg" src="/files/ftp_upload/51184/51184fig3.jpg" /> 
Figure 3.&#160;Competing modes in an ECDL. Green: Line width of grating diffraction order &#8776;50 GHz depending on the grating. Red solid: The Internal cavity mode of a laser diode with a line width &#8776;10 MHz and free spectral range &#8776;80 GHz. Red dash: The internal cavity of an anti-reflection coated diode. These diodes will have a line width in the nm range. Blue: External cavity modes with a line width of &#8776;500 kHz and a free spectral range of &#8776;5 GHz. From a 3 cm long external cavity. Adjusting the grating angle will shift the center of the green curve and simultaneously change the external cavity length in turn shifting the blue curve as well. Adjustment of the diode current and temperature will shift the red curves.
<img alt="Figure 4" fo:content-width="5in" fo:src="/files/ftp_upload/51184/51184fig4highres.jpg" src="/files/ftp_upload/51184/51184fig4.jpg" /> 
 
Figure 4.&#160;Saturated absorption spectroscopy and corresponding error signal. For Rubidium 87. Lower Curve: Saturated absorption peaks on the much broader Doppler absorption peak formed from Doppler free spectroscopy. Upper Curve: Error signal for the corresponding saturated absorption system. The labels above the error signal correspond to the atomic transition (F&#8594;F&#39;).
<img alt="Figure 5" fo:content-width="5in" fo:src="/files/ftp_upload/51184/51184fig5highres.jpg" src="/files/ftp_upload/51184/51184fig5.jpg" /> 
 
Figure 5.&#160;Zeeman Coil. Coil wrapped around a rubidium vapor cell used in Zeeman modulation.
<img alt="Figure 6" fo:content-width="5in" fo:src="/files/ftp_upload/51184/51184fig6highres.jpg" src="/files/ftp_upload/51184/51184fig6.jpg" /> 
Figure 6.&#160;Laser linewidth. Signal acquired from a spectrum analyzer of the beat note formed by two similar lasers. From the figure, the beat has a frequency of 206.24 MHz and a linewidth of 0.3 MHz with an integration time of 20 msec.
<table border="0" cellpadding="0" cellspacing="0" width="635">
	<colgroup>
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="24">
			<td height="24" style="height:24px;width:237px;">Current Controls</td>
			<td style="width:123px;">Range</td>
			<td style="width:275px;">Noise</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Thor Labs:</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">LDC200CV</td>
			<td>0-20 mA</td>
			<td>&#60;1 &#181;A (10 Hz -10 MHz)&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">LDC201CU</td>
			<td>0-100 mA</td>
			<td>&#60;0.2 &#181;A (10 Hz -10 MHz)&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">LDC202C</td>
			<td>0-200 mA</td>
			<td>&#60;1.5 &#181;A (10 Hz -1 MHz)&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">LDC205C</td>
			<td>0-500 mA</td>
			<td>&#60;3 &#181;A (10 Hz -1 MHz)&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Moglabs:</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">DLC-202</td>
			<td>0-200 mA</td>
			<td>&#60;300 pA/&#8730;Hz</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">DLC-252</td>
			<td>0-250 mA</td>
			<td>&#60;300 pA/&#8730;Hz</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">DLC-502</td>
			<td>0-500 mA</td>
			<td>&#60; 300 pA/&#8730;Hz</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Stanford Research Systems:</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">LDC500</td>
			<td>0-100 mA</td>
			<td>&#60; 0.9 &#181;A RMS (10 Hz -1 MHz)&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">LDC501</td>
			<td>0-500 mA</td>
			<td>&#60; 4.5 &#181;A RMS (10 Hz -1 MHz)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Toptica:</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">DCC 110/100</td>
			<td>0-100 mA</td>
			<td>200 nA RMS (5 Hz-1 MHz)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">DCC 110/500</td>
			<td>0-500 mA</td>
			<td>1 &#181;A RMS (5 Hz-1 MHz)</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;">Temperature Controllers</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Thor Labs:</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">TED200C</td>
			<td>-45 to 145 &#176;C</td>
			<td>&#177;2 mK</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Moglabs:</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">DLC-202</td>
			<td>-40 to 50 &#176;C</td>
			<td>&#177;5 mK</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">DLC-252</td>
			<td>-40 to 50 &#176;C</td>
			<td>&#177;5 mK</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">DLC-502</td>
			<td>-40 to 50 &#176;C</td>
			<td>&#177;5 mK</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Stanford Research Systems:</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">LDC500</td>
			<td>-55 to 150 &#176;C</td>
			<td>&#177;2 mK</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">LDC501</td>
			<td>-55 to 150 &#176;C</td>
			<td>&#177;2 mK</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Toptica:</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">DTC 110</td>
			<td>0-50 &#176;C</td>
			<td>&#177;2 mK</td>
		</tr>
	</tbody>
</table>
Table 1.&#160;Diode Current and Temperature Controllers. Various companies&#39; diode current and temperature controllers with their ranges and noise levels.

Watch this Scientific Journal Video about Construction and Characterization of External Cavity Diode Lasers for Atomic Physics at JoVE.com

Construction and Characterization of External Cavity Diode Lasers for Atomic Physics

This is an instructional paper to guide the construction and diagnostics of external cavity diode lasers (ECDLs), including component selection and optical alignment, as well as the basics of frequency reference spectroscopy and laser linewidth measurements for applications in the field of atomic physics.

Since their development in the late 1980s, cheap, reliable external cavity diode lasers (ECDLs) have replaced complex and expensive traditional dye and ...

construction-characterization-external-cavity-diode-lasers-for-atomic

Research

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Engineering

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

Construction of a High Resolution Microscope with Conventional and Holographic Optical Trapping Capabilities

High resolution microscope systems with optical traps allow for precise manipulation of various refractive objects, such as dielectric beads 1 or cellular organelles 2,3, as well as for high spatial and temporal resolution readout of their position relative to the center of the trap. The system described herein has one such "traditional" trap operating at 980 nm. It additionally provides a second optical trapping system that uses a commercially available holographic package to simultaneously create and manipulate complex trapping patterns in the field of view of the microscope 4,5 at a wavelength of 1,064 nm. The combination of the two systems allows for the manipulation of multiple refractive objects at the same time while simultaneously conducting high speed and high resolution measurements of motion and force production at nanometer and piconewton scale.

The system described herein employs a traditional optical trap as well as an independent holographic optical trapping line, capable of creating and manipulating multiple traps. This allows for the creation of complex geometric arrangements of refractive particles while also permitting simultaneous high-speed, high-resolution measurements of the activity of biological enzymes.

High resolution microscope systems with optical traps allow for precise manipulation of various refractive objects, such as dielectric beads 1 or cellular ...

Sputter Growth and Characterization of Metamagnetic B2-ordered FeRh Epilayers

Chemically ordered alloys are useful in a variety of magnetic nanotechnologies. They are most conveniently prepared at an industrial scale using sputtering techniques. Here we describe a method for preparing epitaxial thin films of B2-ordered FeRh by sputter deposition onto single crystal MgO substrates. Deposition at a slow rate onto a heated substrate allows time for the adatoms to both settle into a lattice with a well-defined epitaxial relationship with the substrate and also to find their proper places in the Fe and Rh sublattices of the B2 structure. The structure is conveniently characterized with X-ray reflectometry and diffraction and can be visualised directly using transmission electron micrograph cross-sections. B2-ordered FeRh exhibits an unusual metamagnetic phase transition: the ground state is antiferromagnetic but the alloy transforms into a ferromagnet on heating with a typical transition temperature of about 380 K. This is accompanied by a 1% volume expansion of the unit cell: isotropic in bulk, but laterally clamped in an epilayer. The presence of the antiferromagnetic ground state and the associated first order phase transition is very sensitive to the correct equiatomic stoichiometry and proper B2 ordering, and so is a convenient means to demonstrate the quality of the layers that can be deposited with this approach. We also give some examples of the various techniques by which the change in phase can be detected.

A method to prepare epitaxial layers of ordered alloys by sputtering is described. The B2-ordered FeRh compound is used as an example, as it displays a metamagnetic transition that depends sensitively on the degree of chemical order and the exact composition of the alloy.

Chemically ordered alloys are useful in a variety of magnetic nanotechnologies. They are most conveniently prepared at an industrial scale using ...

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement is used. Here we operate a Nd:YAG laser at a frequency of 10 Hz at the fundamental wavelength of 1,064 nm. The 14 nsec pulses with anenergy of 190 mJ/pulse are focused to a 50 &#181;m spot size to generate a plasma from optical breakdown or laser ablation in air. The microplasma is imaged onto the entrance slit of a 0.6 m spectrometer, and spectra are recorded using an 1,800 grooves/mm grating an intensified linear diode array and optical multichannel analyzer (OMA) or an ICCD. Of interest are Stark-broadened atomic lines of the hydrogen Balmer series to infer electron density. We also elaborate on temperature measurements from diatomic emission spectra of aluminum monoxide (AlO), carbon (C2), cyanogen (CN), and titanium monoxide (TiO).
The experimental procedures include wavelength and sensitivity calibrations. Analysis of the recorded molecular spectra is accomplished by the fitting of data with tabulated line strengths. Furthermore, Monte-Carlo type simulations are performed to estimate the error margins. Time-resolved measurements are essential for the transient plasma commonly encountered in LIBS.

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Time-resolved atomic and diatomic molecular species are measured using LIBS. The spectra are collected at various time delays following the generation of optical breakdown plasma with Nd:YAG laser radiation&#160;and are analyzed to infer electron density and temperature.

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement ...

Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator

Most modern radiation therapy devices allow the use of very small fields, either through beamlets in Intensity-Modulated Radiation Therapy (IMRT) or via stereotactic radiotherapy where positioning accuracy allows delivering very high doses per fraction in a small volume of the patient. Dosimetric measurements on medical accelerators are conventionally realized using air-filled ionization chambers. However, in small beams these are subject to nonnegligible perturbation effects. This study focuses on liquid ionization chambers, which offer advantages in terms of spatial resolution and low fluence perturbation. Ion recombination effects are investigated for the microLion detector (PTW) used with the Cyberknife system (Accuray). The method consists of performing a series of water tank measurements at different source-surface distances, and applying corrections to the liquid detector readings based on simultaneous gaseous detector measurements. This approach facilitates isolating the recombination effects arising from the high density of the liquid sensitive medium and obtaining correction factors to apply to the detector readings. The main difficulty resides in achieving a sufficient level of accuracy in the setup to be able to detect small changes in the chamber response.

A growing number of radiation therapy devices offer the advantage of delivering the dose through very small beams to the tumor, allowing for increased conformity and higher doses per fraction. Many different detectors can be used for the dosimetry of these small fields. In the present study, the effect of ion recombination is investigated for a liquid ionization chamber using a stereotactic radiation therapy system.

Most modern radiation therapy devices allow the use of very small fields, either through beamlets in Intensity-Modulated Radiation Therapy (IMRT) or via ...

Writing and Low-Temperature Characterization of Oxide Nanostructures

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions. Oxide interfaces exhibit a wide range of properties that can be controlled include conduction, piezoelectric behavior, ferromagnetism, superconductivity and nonlinear optical properties. Recently, methods for controlling these properties at extreme nanoscale dimensions have been discovered and developed. Here are described explicit step-by-step procedures for creating LaAlO3/SrTiO3 nanostructures using a reversible conductive atomic force microscopy technique. The processing steps for creating electrical contacts to the LaAlO3/SrTiO3 interface are first described. Conductive nanostructures are created by applying voltages to a conductive atomic force microscope tip and locally switching the LaAlO3/SrTiO3 interface to a conductive state. A versatile nanolithography toolkit has been developed expressly for the purpose of controlling the atomic force microscope (AFM) tip path and voltage. Then, these nanostructures are placed in a cryostat and transport measurements are performed. The procedures described here should be useful to others wishing to conduct research in oxide nanoelectronics.

Oxide nanostructures provide new opportunities for science and technology. The interfacial conductivity between LaAlO3 and SrTiO3 can be controlled with near-atomic precision using a conductive atomic force microscopy technique. The protocol for creating and measuring conductive nanostructures at LaAlO3/SrTiO3 interfaces is demonstrated.

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions.  Oxide ...

Convergent Polishing: A Simple, Rapid, Full Aperture Polishing Process of High Quality Optical Flats &amp; Spheres

Convergent Polishing is a novel polishing system and method for finishing flat and spherical glass optics in which a workpiece, independent of its initial shape (i.e., surface figure), will converge to final surface figure with excellent surface quality under a fixed, unchanging set of polishing parameters in a single polishing iteration. In contrast, conventional full aperture polishing methods require multiple, often long, iterative cycles involving polishing, metrology and process changes to achieve the desired surface figure. The Convergent Polishing process is based on the concept of workpiece-lap height mismatch resulting in pressure differential that decreases with removal and results in the workpiece converging to the shape of the lap. The successful implementation of the Convergent Polishing process is a result of the combination of a number of technologies to remove all sources of non-uniform spatial material removal (except for workpiece-lap mismatch) for surface figure convergence and to reduce the number of rogue particles in the system for low scratch densities and low roughness. The Convergent Polishing process has been demonstrated for the fabrication of both flats and spheres of various shapes, sizes, and aspect ratios on various glass materials. The practical impact is that high quality optical components can be fabricated more rapidly, more repeatedly, with less metrology, and with less labor, resulting in lower unit costs. In this study, the Convergent Polishing protocol is specifically described for fabricating 26.5 cm square fused silica flats from a fine ground surface to a polished ~&#955;/2 surface figure after polishing 4 hr per surface on a 81 cm diameter polisher.

A novel optical polishing process, called &#8220;Convergent Polishing&#8221;, which enables faster, lower cost polishing, is described. Unlike conventional polishing processes, Convergent Polishing allows a glass workpiece to be polished in a single iteration and with high surface quality to its final surface figure without requiring changes to polishing parameters.

Convergent Polishing is a novel polishing system and method for finishing flat and spherical glass optics in which a workpiece, independent of its initial ...

Electrochemical Etching and Characterization of Sharp Field Emission Points for Electron Impact Ionization

A new variation of the drop-off method for fabricating field emission points by electrochemically etching tungsten rods in a NaOH solution is described. The results of studies in which the etching current and the molarity of the NaOH solution used in the etching process were varied are presented. The investigation of the geometry of the tips, by imaging them with a scanning electron microscope, and by operating them in field emission mode is also described. The field emission tips produced are intended to be used as an electron beam source for ion production via electron impact ionization of background gas or vapor in Penning trap mass spectrometry applications.

A method for electrochemically etching field emission tips is presented. Etching parameters are characterized and the operation of the tips in field emission mode is investigated.

A new variation of the drop-off method for fabricating field emission points by electrochemically etching tungsten rods in a NaOH solution is described. ...

Resonance Fluorescence of an InGaAs Quantum Dot in a Planar Cavity Using Orthogonal Excitation and Detection

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). Resonant excitation without fluorescence detection &#8211; for example, a differential transmission measurement &#8211; can determine some properties of the emitting system, but does not allow applications or measurements based on the emitted photons. For example, the measurement of photon correlations, observation of the Mollow triplet, and realization of single photon sources all require collection of the fluorescence. Incoherent excitation with fluorescence detection &#8211; for example, above band-gap excitation &#8211; can be used to create single photon sources, but the disturbance of the environment due to the excitation reduces the indistinguishability of the photons. Single photon sources based on QDs will have to be resonantly excited to have high photon indistinguishability, and simultaneous collection of the photons will be necessary to make use of them. We demonstrate a method to resonantly excite a single QD embedded in a planar cavity by coupling the excitation beam into this cavity from the cleaved face of the sample while collecting the fluorescence along the sample&#39;s surface normal direction. By carefully matching the excitation beam to the waveguide mode of the cavity, the excitation light can couple into the cavity and interact with the QD. The scattered photons can couple to the Fabry-Perot mode of the cavity and escape in the surface normal direction. This method allows complete freedom in the detection polarization, but the excitation polarization is restricted by the propagation direction of the excitation beam. The fluorescence from the wetting layer provides a guide to align the collection path with respect to the excitation beam. The orthogonality of the excitation and detection modes enables resonant excitation of a single QD with negligible laser scattering background.

Resonant excitation of a single self-assembled quantum dot can be achieved using an excitation mode orthogonal to the fluorescence collection mode. We demonstrate a method using the waveguide and Fabry-Perot modes of a planar microcavity surrounding the quantum dots. The method allows complete freedom in the detection polarization.

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). ...

Production and Characterization of Vacuum Deposited Organic Light Emitting Diodes

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex donor-acceptor emitters is presented. With a step-by-step procedure, readers will be able to repeat and produce OLED devices based on simple organic emitters. A patterning procedure allowing the creation of personalized indium tin oxide (ITO) shape is shown. This is followed by the evaporation of all layers, encapsulation and characterization of each individual device. The end goal is to present a procedure that will give the opportunity to repeat the information presented in cited publication but also using different compounds and structures in order to prepare efficient OLEDs.

A protocol for the production of simple structured organic light-emitting diodes (OLEDs) is presented.

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex ...