Name: resonance fluorescence of an ingaas quantum dot in a planar cavity using orthogonal excitation and detection
Uploaded: 2017-10-13T15:00:00
Description: Watch this Scientific Journal Video about Resonance Fluorescence of an InGaAs Quantum Dot in a Planar Cavity Using Orthogonal Excitation and Detection at JoVE.com

The authors would like to acknowledge Glenn S. Solomon for providing the sample. This work was supported by the National Science Foundation (DMR-1452840).

Caution: Please be aware of the possible dangers of laser scattering during the alignment. Wear proper safety goggles for protection. To facilitate the alignment process, an infrared viewer (IR-viewer) is necessary. An IR-sensitive fluorescent card is also helpful but not necessary.
1. Sample Preparation
<ol>
	<li>Use a diamond scribe to make a miniscule scratch on the edge of the top surface of the sample at the desired location of the cleave. Use two pairs of flat-ended tweezers to hold th...

<ol>
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The critical steps in the protocol are: the mode-matching and alignment of the excitation beam to the waveguide mode; and proper alignment and focusing of the collection optics. The most difficult parts of these steps are the initial alignment; optimizing the coupling of an already aligned setup is relatively straightforward. Overlapping the collection and excitation areas is a step that is simple with the ability to image the sample on the camera, but is very difficult without this capability. In order to have high-qual...

Resonant excitation of a single quantum emitter combined with fluorescence detection was a long-term experimental challenge mainly due to the inability to spectrally discriminate the weak fluorescence from the strong excitation scattering. This difficulty, however, has been successfully overcome in the past decade by two different approaches: dark-field confocal excitation based on polarization discrimination1,2,3,4,5, and orthogonal excitation-detection based on spatial mode discrimination6,7,8,9,10,11,12,13,14. Both approaches demonstrate a strong capability to significantly suppress laser scattering and thus are widely adopted in various experiments, for example, observation of spin-photon entanglement5,15,16, demonstration of dressed states2,7,12,17,18,19,20,21,22,23,24,25,26, and coherent manipulation of confined spins3,27,28,29,30. Neither approach can be universally applied to every situation; each is limited to some specific conditions. The dark-field technique utilizes the polarization degree of freedom of photons to suppress the excitation laser scattering. This technique has several advantages. For example, there is no requirement for a well-defined waveguide mode, which enables confocal-only implementation. The confocal implementation allows for circularly polarized excitation and possibly tighter focus of the excitation beam at the quantum emitter, resulting in higher excitation intensity. However, this polarization-selective method restricts the detection polarization to be orthogonal to the excitation polarization, and thus prevents a complete characterization of the polarization properties of the fluorescence. In comparison, spatial mode discrimination preserves the complete freedom of detection polarization by utilizing the orthogonality between the propagation modes of excitation and detection beams to suppress the laser scattering4. The constraints of this technique are the necessity of a waveguide structure in the sample to provide an excitation mode orthogonal to the detection mode, and the restriction of the excitation polarization to be perpendicular to the propagation direction of the beam.
Here, we demonstrate a protocol for constructing a free-space-based orthogonal excitation-detection setup for resonance fluorescence experiments. Compared to the pioneering work on spatial mode discrimination where an optical fiber was used to couple light into the cavity6, this protocol provides a solution in free space, and does not require kinetic components to mount either the sample or the fiber in cryostat. Fine control of the directions of the excitation beam and the detection path are manipulated by optics external to the cryostat, while aspheric singlet lenses act as focusing objectives inside the cold region of the cryostat. We provide representative images of the key alignment steps in the process of achieving resonant excitation and detection of fluorescence from a single quantum dot.
The sample used for this demonstration is grown by molecular beam epitaxy (MBE). The InGaAs quantum dots (QDs) are embedded in a GaAs spacer that is bounded by two distributed Bragg reflectors (DBRs), as shown in the zoom-in view of the sample in Figure 1. The GaAs spacer between the DBRs acts as a waveguide, where the excitation beam is confined by total internal reflection. The DBRs also act as high-reflectivity mirrors for wavevectors that are nearly normal to the sample plane. This forms a Fabry-Perot mode to which the QDs couple when emitting fluorescence. The Fabry-Perot mode must be resonant with the emission wavelength &#955; of the QDs, which requires the GaAs spacer to be an integer multiple of&#160;&#955;/n, where n is the index of refraction of GaAs. For this demonstration, the thickness of the GaAs spacer is chosen to be 4&#955;/n, which is approximately 1 &#181;m, so as to be near the diffraction limited spot size of the incident excitation beam. A narrower spacer would result in a lower coupling efficiency of the excitation beam into the waveguide mode.
The experimental setup is shown in Figure 1. To maximize the coupling efficiency, an aspheric single-lens objective Eobj with numerical aperture NA=0.5 and focal length of 8 mm is chosen to focus the excitation beam onto the cleaved face of the sample. The function of the Keplerian telescope (composed of lens pair E1 and E2) in the excitation path is two-fold: (1) to fill the aperture of the excitation objective Eobj so the excitation beam is tightly focused for better mode-matching to the waveguide (in this realization the collimated beam diameter is 2.5 mm), and (2) to provide three degrees of freedom to maneuver the focal point of the excitation beam at the cleaved face of the sample. Lens E1 is mounted on an X-Y translational mount that provides the two degrees of freedom to shift the excitation spot freely in the plane of the cleaved sample face. Lens E2 is mounted on a non-rotating zoom housing which provides the freedom to choose the depth of the focal point in the sample. These three degrees of freedom allow us to optimize the resonant excitation of a single QD without requiring movement of the sample itself.
In the fluorescence collection path, a similar lens configuration (Lobj, L1, and L2) is used to allow detection of fluorescence from different parts of the sample. The light from the sample is focused by one of two tube lenses onto either an IR-sensitive camera (Lcam) or the entrance slit of the spectrometer (Lspec). Motion of L1 along the z-axis adjusts the focus of the image, and lateral translation of L2 causes the image to scan across the plane of the sample. The focal lengths of L1 and L2 are equal so their magnification is unity. This is done to maximize the range L2 can be translated before vignetting occurs.
To facilitate alignment and location of a QD, a home-built illuminator based on Kohler illumination is incorporated into the setup, as shown in Figure 1. The purpose of Kohler illumination is to provide uniform illumination to the sample and ensure that an image of the illumination light source is not visible in the sample image. The lens configurations of both the illuminator and the collection path are carefully designed to separate the conjugate image planes of the sample and the light source. Every lens in the collection path is separated from its neighbors by the sum of their focal lengths. This ensures that wherever the sample image is in focus &#8211; such as at the sensor of the camera &#8211; the light source image is completely defocused. Similarly, where the light source image is in focus &#8211; such as at the back focal plane of the objective &#8211; the sample image is completely defocused. The light source is a commercial light emitting diode (LED) emitting at 940 nm. The aperture diaphragm enables the adjustment of the illumination intensity, and the field diaphragm determines the field of view to be illuminated. The keys to realizing uniform illumination are to set the distance between lens K4 and L2 to be the sum of the focal lengths of the two lenses, and to ensure that the aperture of Lobj is not overfilled by the illumination. In this protocol, the illumination is also used to optimize the distance between&#160;Lobj and the sample.
The objective&#160;Lobj and either tube lens provides a magnification of 20x on the camera or the spectrometer. The lens pair L3 and L4 between&#160;Lobj and&#160;Lspec forms another Keplerian telescope that provides an extra 4x magnification to the image on the charge-coupled device (CCD) of the spectrometer. The addition of lenses L3 and L4 results in a total magnification of 80x, which is necessary to spatially distinguish fluorescence from nearby QDs. L3 and L4 are mounted on flipping mounts to facilitate switching of the magnification because 20x magnification provides a larger field of view on the sample.
To overlap the field of view of the collection path with the path of the excitation beam through the waveguide, the emission from the continuum of the quantum dot wetting layer is helpful. One can determine the emission wavelength of the wetting layer by measuring the emission spectrum of the sample under above band-gap excitation. For our sample, wetting layer emission occurs at approximately 880 nm at 4.2 K. By coupling a cw laser beam at 880 nm into the waveguide of the sample, one can observe a streak pattern formed by the PL from the wetting layer, which is shown in the accompanying video. The streak reveals the propagation path of the excitation light that has been coupled into the waveguide. The presence of this streak combined with the ability to image the surface of the sample makes alignment straightforward.

<table>
	<tbody>
		<tr>
			<td>Tunable external cavity diode laser</td>
			<td>Toptica Photonics</td>
			<td>DL-Pro</td>
			<td></td>
		</tr>
		<tr>
			<td>Closed-cycle cryostat</td>
			<td>Montana Instruments</td>
			<td>Cryostation</td>
			<td></td>
		</tr>
		<tr>
			<td>Spectrometer, 750 mm focal length</td>
			<td>Princeton Instruments</td>
			<td>SpectraPro 2750</td>
			<td></td>
		</tr>
		<tr>
			<td>Thermoelectrically cooled charge-coupled device</td>
			<td>Princeton Instruments</td>
			<td>Pixis 100BR-eXcelon</td>
			<td></td>
		</tr>
		<tr>
			<td>HeNe laser</td>
			<td>JDSU</td>
			<td>1125P</td>
			<td></td>
		</tr>
		<tr>
			<td>Infrared sensitive camera</td>
			<td>Sony</td>
			<td>NEX-5TL</td>
			<td>IR blocking filter removed</td>
		</tr>
		<tr>
			<td>Power meter and detector</td>
			<td>Newport</td>
			<td>1918-C, 918D-IR-OD3</td>
			<td></td>
		</tr>
		<tr>
			<td>Adjustable aspheric fiber collimator</td>
			<td>Thorlabs</td>
			<td>CFC-8X-A</td>
			<td></td>
		</tr>
		<tr>
			<td>Air-Spaced Doublet Collimator</td>
			<td>Thorlabs</td>
			<td>F810APC-842</td>
			<td></td>
		</tr>
		<tr>
			<td>Protected Silver Mirrors x 5</td>
			<td>Thorlabs</td>
			<td>PF10-03-P01</td>
			<td></td>
		</tr>
		<tr>
			<td>Flip mounts x 2</td>
			<td>Thorlabs</td>
			<td>FM90</td>
			<td></td>
		</tr>
		<tr>
			<td>Aspheric condenser lens, f = 20 mm; K1</td>
			<td>Thorlabs</td>
			<td>ACL2520-B</td>
			<td></td>
		</tr>
		<tr>
			<td>Best form spherical lens, f = 50 mm; E2, L1, L2, K2</td>
			<td>Thorlabs</td>
			<td>LBF254-050-B</td>
			<td></td>
		</tr>
		<tr>
			<td>Best form spherical lens, f = 100 mm; E1, L4, K3, K4</td>
			<td>Thorlabs</td>
			<td>LBF254-100-B</td>
			<td></td>
		</tr>
		<tr>
			<td>Best form spherical lens, f = 200 mm; Lspec, Lcam</td>
			<td>Thorlabs</td>
			<td>LBF254-200-B</td>
			<td></td>
		</tr>
		<tr>
			<td>Plano-convex lens, f = 400 mm; L3</td>
			<td>Thorlabs</td>
			<td>LA1172-B</td>
			<td></td>
		</tr>
		<tr>
			<td>Molded glass aspheric lens, f = 8 mm; Eobj</td>
			<td>Thorlabs</td>
			<td>C240TME-B</td>
			<td></td>
		</tr>
		<tr>
			<td>Precision asphere, f = 10 mm; Lobj</td>
			<td>Thorlabs</td>
			<td>AL1210-B</td>
			<td></td>
		</tr>
		<tr>
			<td>Longpass Filters, 800 nm, x2</td>
			<td>Thorlabs</td>
			<td>FEL0800</td>
			<td></td>
		</tr>
		<tr>
			<td>Non-polarizing beam splitter cube (NPBS)</td>
			<td>Thorlabs</td>
			<td>BS029</td>
			<td></td>
		</tr>
		<tr>
			<td>Pellicle beam splitter</td>
			<td>Thorlabs</td>
			<td>BP108</td>
			<td></td>
		</tr>
		<tr>
			<td>Polarizer</td>
			<td>Thorlabs</td>
			<td>LPNIRE100-B</td>
			<td></td>
		</tr>
		<tr>
			<td>Light emitting diode, 940 nm</td>
			<td>Thorlabs</td>
			<td>M940D2</td>
			<td></td>
		</tr>
	</tbody>
</table>

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.

Figure 1 shows one particular realization of the necessary equipment to accomplish resonant excitation of a single quantum dot. Other realizations are possible, but the critical components are: an excitation path to couple to the waveguide; a collection path to guide fluorescence to detectors; a confocal excitation path to excite along the collection path; and an illumination path to enable imaging of the sample surface.
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Watch this Scientific Journal Video about Resonance Fluorescence of an InGaAs Quantum Dot in a Planar Cavity Using Orthogonal Excitation and Detection at JoVE.com

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

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). ...

The overall goal of this protocol is to achieve resonant excitation of a quantum dot and simultaneous fluorescence detection using orthogonal excitation and detection modes. The realization of resonant excitation with fluorescence detection is essential for studying light matter interactions in low dimensional nano structures and it has been used to illustrate quantum optical effects such as dress states. The main advantage of this technique is that it uses orthogonality between the excitation and detection modes to minimize the collection of laser scattering which preserves the fluorescence polarization.This technique has implications for quantum dot-based sources of highly indistinguishable single photons and for coherent control and measurement of electron spins in quantum dots. Set up the elements of the experiment on an optical bench. A cryostat to hold the sample is at the center of the experiment.The optical elements direct and focus the output of three light sources. This schematic provides an overview of the setup. Again, the sample held in a cryostat is at the center.One light source is an external cavity diode laser used for resonant excitation. Another is a helium neon laser used for above bend excitation. There is also an LED for illumination.The resonant excitation laser beam passes through a polarizer and enters through one window of the cryostat. The lenses E1 and E2 form a Capillarian telescope. E1 is on an XY stage to allow shifting the excitation spot laterally.E2 is in a zoom housing to allow varying the depth of focus. Light from the illumination source and the helium neon laser enters the other window of the cryostat. Within the sample, the light interacts with the indium gallium arsenide quantum dots embedded between two distributed Bragg reflectors.The cleaved face of the sample is exposed to the resonant excitation laser. Photoluminescence from the quantum dot travels back along the optical path of the helium neon laser. It passes through a second Capillarian telescope with a design similar to the first but with unity magnification.A camera is in position to capture the light from the sample in order to facilitate the alignment. A spectrometer is in position to collect the photoluminescence from the sample for intensity and spectral analysis. The position of a flip mirror M1 determines whether the light goes to the camera or the spectrometer.The first step is to prepare a sample for the experiment. Experiment samples are cleaved from a larger sample grown using molecular beam epitaxy. In addition to the sample, have ready a copper sample plate, thermally conductive paint, a diamond scribe, and flat-ended tweezers.Approach the sample with the diamond scribe. Use it to make a small scratch on the edge of the sample's top surface. Then use the tweezers to hold the sample on each side of the scratch and apply an outward rotating torque.Move the cleaved sample to the copper sample plate and attach it with the thermally conductive paint. Next, take the copper sample plate to the cryostat to mount it. Ensure that the sample is oriented correctly for the experiment.The next step is to install an aspheric objective lens in the cryostat. This lens is secured in a translational mount with three degrees of freedom. It belongs in the resonant excitation laser path and focuses light to the cleaved end of the sample.At the beginning, the lens should be more than one focal length from the sample. With the lens in the cryostat, turn on the excitation laser and center the lens on it. The lens height should be the same as that of the center of the sample.Next, set up a white paper screen in the laser path behind the sample and observe the screen with an infrared viewer. There should be a bright spot due to the light transmitted through the sample. Slowly translate the lens toward the sample.Observe the screen through the viewer and stop when a clear silhouette image of the sample appears. Adjust the lens position to center the silhouette at the center of the bright spot. Continue slowly translating the lens toward the sample.The silhouette image on the screen will become magnified and may shift horizontally. Keep adjusting the lens to center the image and magnify it until interference fringes are visible. Slowly translate the lens further toward the sample.At each depth position, monitor the fringes by sliding the objective left and right. Move the lens to a position that maximizes the spacing between the two groups of fringes and secure it. Then adjust the lateral lens position to minimize fringes on the paper screen.Next, adjust the Capillarian telescope in the excitation path. The two lenses should be centered on the resonant excitation laser beam. Referring to the schematic, position lens E2 so that it and the aspheric lens are separated by the sum of their focal lengths.Also, set the separation between E1 and E2 to be the sum of their focal lengths. Use the infrared viewer to observe the diffraction pattern on the paper screen while adjusting the height of the lens E1.The goal is to again observe the interference fringes. Slide E2 forward and backward while monitoring the fringes by sliding E1 left and right.Place E2 at the position where the fringe groups are farthest apart. Finally, adjust lens E1 laterally to have the fringes disappear or be minimized. Remove the screen and continue placing and aligning the remaining optical elements.Cool the sample to 4.2 Kelvin and prepare for spectrometry. The apparatus should be configured as in this schematic. Excite the sample with the helium neon laser and direct the photoluminescence from the sample to the spectrometer.From the resulting measured emission spectrum, identify the emission peak between 860 and 900 nanometers. This corresponds to emission from the wetting layer. For the next measurement, change the configuration to use the camera.Use a long-pass filter in front of the camera to block the helium neon light. Also, turn on the illumination light. Shift the lens L2 laterally while observing the camera image.The sample image will be panned by the lens motion. Stop after locating the cleaved edge of the sample. Now turn on the resonant excitation laser with the wavelength set to be resonant with the wetting layer.On the camera, identify a bright scattering spot at the cleaved edge of the sample. Next, adjust the horizontal position of E1.The aim is to observe a streak pattern of photoluminescence on the camera and to maximize its intensity. Then move E1 vertically to overlap the streak with the photoluminescence spot caused by the helium neon excitation.Monitor and record the intensity of the photoluminescence. Systematically adjust lenses E1 and E2 to maximize the intensity. Change the configuration back to use the spectrometer.Set it up to monitor the first order diffraction at the center of the emission wavelength of the quantum dot ensemble. Next, move to the excitation laser. Tune its frequency across the energy range of the quantum dot ensemble.Observe the emitted photoluminescence with the CCD camera attached to the spectrometer. A resonantly excited quantum dot will appear as a disc surrounded by rings known as an area pattern. Select a bright quantum dot to work with.Maximize the dot's photoluminescence intensity by fine tuning the wavelength of the resonant excitation laser. Jointly adjust E1 laterally and E2 axially to achieve maximum intensity. This series of images is from a neutral quantum dot at different detunings as indicated in gigahertz at the top of each image.For comparison, this series is from a charged quantum dot at different detunings also indicated at the top of the images. The charged state cannot be determined solely using the spectrum. Using the neutral quantum dot images and integrating the intensity within a radius of four pixels of the image center yields the spectrum.This spectrum is the result of a similar procedure with the charged quantum dot images and a six pixel radius. The orange dots represent the normalized resonant photoluminescence excitation intensity. The blue squares are data from the corresponding set of images.These images are from eight different quantum dots with different resonant wavelengths that monotonically increase from left to right. The pattern of a central area disc with surrounding rings is the typical image of a quantum dot. The radii of the rings and disc vary because of the interdependence of the wavelengths and emission angles of the cavity modes.Generally, individuals new to this method will struggle with laser light into the wave guide of the sample because the alignment is very sensitive and the cost element must be done at room temperature. Once the experimental components are prepared, the alignment can be done in a couple of hours if it is performed properly. While attempting this procedure, it's important to monitor the interference fringes to correctly position the excitation lasers and high quality imaging of the sample surface is critical to allow alignment of the excitation and detection paths.Following this procedure, the detection path can be modified to include further elements such as a polarization analyzer or a tunable Fabry-Perot interferometer. This technique paved the way for researchers in the field of solid-state quantum optics to explore resonant phenomena in zero dimensional structures such as quantum dots. After watching this video, you should have a good understanding of how to achieve resonant excitation of a quantum dot and the simultaneous fluorescence detection using orthogonal excitation and detection modes.

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Research

JoVE Journal

Engineering

Gradient Echo Quantum Memory in Warm Atomic Vapor

Gradient echo memory (GEM) is a protocol for storing optical quantum states of light in atomic ensembles. The primary motivation for such a technology is that quantum key distribution (QKD), which uses Heisenberg uncertainty to guarantee security of cryptographic keys, is limited in transmission distance. The development of a quantum repeater is a possible path to extend QKD range, but a repeater will need a quantum memory. In our experiments we use a gas of rubidium 87 vapor that is contained in a warm gas cell. This makes the scheme particularly simple. It is also a highly versatile scheme that enables in-memory refinement of the stored state, such as frequency shifting and bandwidth manipulation. The basis of the GEM protocol is to absorb the light into an ensemble of atoms that has been prepared in a magnetic field gradient. The reversal of this gradient leads to rephasing of the atomic polarization and thus recall of the stored optical state. We will outline how we prepare the atoms and this gradient and also describe some of the pitfalls that need to be avoided, in particular four-wave mixing, which can give rise to optical gain.

The gradient echo memory is a protocol for storing optical quantum states of light in atomic ensembles. Quantum memory is a key element of a quantum repeater, which can extend the range of quantum key distribution. We outline the operation of the scheme when implemented in a 3-level atomic ensemble.

Gradient  echo memory (GEM) is a protocol for storing optical quantum states of light in  atomic ensembles. The primary motivation for such a technology ...

Analysis of Contact Interfaces for Single GaN Nanowire Devices

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO2 interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices. This procedure determines the degree of adhesion of the contact films to the substrate and NWs and allows for the characterization of the morphology and composition of the contact interface with the substrate and nanowires. This technique is also useful for assessing the amount of residual contamination that remains from the NW suspension and from photolithographic processes on the NW-SiO2 surface prior to metal deposition. The detailed steps of this procedure are presented for the removal of annealed Ni/Au contacts to Mg-doped GaN NWs on a SiO2 substrate.

A technique was developed that removes Ni/Au contact metal films from their substrate to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN nanowire devices.

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit  a strong degradation after annealing due to the  occurrence of void formation at the ...

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.

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 ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

Detection and Recovery of Palladium, Gold and Cobalt Metals from the Urban Mine Using Novel Sensors/Adsorbents Designated with Nanoscale Wagon-wheel-shaped Pores

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in industrialized countries. Here, the development of optical mesosensors/adsorbents (MSAs) for efficient recognition and selective recovery of Pd(II), Au(III), and Co(II) from urban mine was achieved. A simple, general method for preparing MSAs based on using high-order mesoporous monolithic scaffolds was described. Hierarchical cubic Ia3d wagon-wheel-shaped MSAs were fabricated by anchoring chelating agents (colorants) into three-dimensional pores and micrometric particle surfaces of the mesoporous monolithic scaffolds. Findings show, for the first time, evidence of controlled optical recognition of Pd(II), Au(III), and Co(II) ions and a highly selective system for recovery of Pd(II) ions (up to ~95%) in ores and industrial wastes. Furthermore, the controlled assessment processes described herein involve evaluation of intrinsic properties (e.g., visual signal change, long-term stability, adsorption efficiency, extraordinary sensitivity, selectivity, and reusability); thus, expensive, sophisticated instruments are not required. Results show evidence that MSAs will attract worldwide attention as a promising technological means of recovering and recycling palladium, gold and cobalt metals.

Because of the importance and extensive use of palladium, gold and cobalt metals in high-technology equipment, their recovery and recycling constitute an important industrial challenge. The metal recovery system described herein is a simple, low-cost means for the effective detection, removal, and recovery of these metals from the urban mine.

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in ...

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we demonstrate an optical spectroscopy method that is capable of resolving spectral features beyond that of the spin fine structure and homogeneous linewidth of single quantum dots (QDs) using a standard, easy-to-use spectrometer setup. This method incorporates both laser and photoluminescence spectroscopy, combining the advantage of laser line-width limited resolution with multi-channel photoluminescence detection. Such a scheme allows for considerable improvement of resolution over that of a common single-stage spectrometer. The method uses phonons to assist in the measurement of the photoluminescence of a single quantum dot after resonant excitation of its ground state transition. The phonon&#39;s energy difference allows one to separate and filter out the laser light exciting the quantum dot. An advantageous feature of this method is its straight forward integration into standard spectroscopy setups, which are accessible to most researchers.

The manuscript describes a method of phonon-assisted quasi-resonant fluorescence spectroscopy that incorporates both laser-limited resolution and photoluminescence (PL) spectroscopy. This method utilizes optical phonons to provide linewidth-limited resolution spectra of atom-like semiconductor structures in the energy domain. The method is also easily realized with a single spectrometer optical spectroscopy setup.

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we ...

Frequency Mixing Magnetic Detection Scanner for Imaging Magnetic Particles in Planar Samples

The setup of a planar Frequency Mixing Magnetic Detection (p-FMMD) scanner for performing Magnetic Particles Imaging (MPI) of flat samples is presented. It consists of two magnetic measurement heads on both sides of the sample mounted on the legs of a u-shaped support. The sample is locally exposed to a magnetic excitation field consisting of two distinct frequencies, a stronger component at about 77 kHz and a weaker field at 61 Hz. The nonlinear magnetization characteristics of superparamagnetic particles give rise to the generation of intermodulation products. A selected sum-frequency component of the high and low frequency magnetic field incident on the magnetically nonlinear particles is recorded by a demodulation electronics. In contrast to a conventional MPI scanner, p-FMMD does not require the application of a strong magnetic field to the whole sample because mixing of the two frequencies occurs locally. Thus, the lateral dimensions of the sample are just limited by the scanning range and the supports. However, the sample height determines the spatial resolution. In the current setup it is limited to 2 mm. As examples, we present two 20 mm &#215; 25 mm p-FMMD images acquired from samples with 1 &#181;m diameter maghemite particles in silanol matrix and with 50 nm magnetite particles in aminosilane matrix. The results show that the novel MPI scanner can be applied for analysis of thin biological samples and for medical diagnostic purposes.

A scanner for imaging magnetic particles in planar samples was developed using the planar frequency mixing magnetic detection technique. The magnetic intermodulation product response from the nonlinear nonhysteretic magnetization of the particles is recorded upon a two-frequency excitation. It can be used to take 2D images of thin biological samples.

The setup of a planar Frequency Mixing Magnetic Detection (p-FMMD) scanner for performing Magnetic Particles Imaging (MPI) of flat samples is presented. ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Silicon photonic chips have the potential to realize complex integrated quantum systems. Presented here is a method for preparing and testing a silicon photonic chip for quantum measurements.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit ...

Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation displays and solid-state lighting technologies. The brightness and efficiency of blue QDs-based light-emitting diodes (LEDs) remain inferior to their red and green counterparts, due to the inherently unfavorable energy levels of different colors of light. To solve these problems, a device structure should be designed to balance the injection holes and electrons into the emissive QD layer. Herein, through a simple autoxidation strategy, pure blue QD-LEDs which are highly bright and efficient are demonstrated, with a structure of ITO/PEDOT:PSS/Poly-TPD/QDs/Al:Al2O3. The autoxidized Al:Al2O3 cathode can effectively balance the injected charges and enhance radiative recombination without introducing an additional electron transport layer (ETL). As a result, high color-saturated blue QD-LEDs are achieved with a maximum luminance over 13,000 cd m-2, and a maximum current efficiency of 1.15 cd A-1. The easily controlled autoxidation procedure paves the way for achieving high-performance blue QD-LEDs.

A protocol is presented for fabricating high-performance, pure blue ZnCdS/ZnS-based quantum dots light-emitting diodes by employing an autoxidized aluminum cathode.

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation ...