This work was supported in part by the Washington Space Grant Consortium under Award NNX10AK64H.

1. Planning of Experiments
<ol>
	<li>Conduct a survey of the literature to assess prior work performed using the laser medium of interest, which for this experiment is CH2F2. Identify all known laser emissions along with all information about the lines such as their wavelength and frequency. Several surveys of known laser emissions are available13,31&#8211;37.</li>
	<li>Compile all spectroscopic investigations of the molecule used as the laser medium with a focus on prior Fourier transform34 and optoacoustic studies38,39.</li>
</ol>
2. Generating Far-Infrared Laser Emissions
<ol>
	<li>Safety Overview.
	<ol>
		<li>Develop a standard operating procedure for the lab that includes proper eye protection when working with the CO2 and far-infrared laser systems.</li>
	</ol>
	</li>
	<li>Alignment and Calibration.
	<ol>
		<li>Calibrate each CO2 laser using a grating-based spectrum analyzer designed for the CO2 laser according to the manufacturer&#8217;s protocol.</li>
		<li>Align the end mirrors and the coupling mirror in the far-infrared laser cavity using a He-Ne laser so that their radiation is focused onto the MIM diode detector.</li>
		<li>Direct the radiation from the CO2 pump laser into the far-infrared laser cavity through a sodium chloride window at an angle of approximately 72o with respect to the cavity axis.</li>
		<li>Direct the radiation from the two CO2 reference lasers to either their respective low-pressure fluorescence reference cell or co-linearly onto the MIM diode detector using beam splitters and additional mirrors.</li>
	</ol>
	</li>
	<li>Detection of far-infrared laser radiation.
	<ol>
		<li>Polish the Nickel base every several days using a standard metal polish.</li>
		<li>Crimp a 25 &#956;m tungsten wire into a copper post and bend the wire into the configuration shown in Figure 3.</li>
		<li>Adjust the length of the wire so that it is between 10 to 20 wavelengths of the radiation being measured.</li>
		<li>Electrochemically etch the tip of the wire in a saturated sodium hydroxide (NaOH) solution by applying a voltage (approximately 3.5 to 5 VAC) to the solution.</li>
		<li>Re-etch the tip with a low voltage (less than 1 VAC). This roughens the tip of the wire and improves the diode&#8217;s performance.</li>
		<li>Rinse the wire with distilled water.</li>
		<li>Insert the copper post into the MIM diode&#8217;s housing once the wire is dry.</li>
		<li>Place the wire in contact with the Nickel base using a fine screw and level system. Contacts yielding a resistance across the diode between 100 and 500 &#937;&#160;are typically used when detecting and measuring far-infrared laser radiation.</li>
	</ol>
	</li>
	<li>Generation of far-infrared laser radiation.
	<ol>
		<li>Set the CO2 pump laser on a specific laser emission, e.g., 9P36.</li>
		<li>Rotate the micrometer dial on the CO2 pump laser back and forth to achieve maximum intensity on the beam stop.</li>
		<li>Adjust the tilt of the CO2 pump laser&#8217;s grating to achieve maximum intensity on the beam stop.</li>
		<li>Repeat steps 2.4.2 and 2.4.3 until the output power for the CO2 pump laser appears optimized on the beam stop.</li>
		<li>Remove the beam stop from the path of the CO2 pump laser.</li>
		<li>Turn on and align the optical chopper into the beam path of the CO2 pump laser.</li>
		<li>Open the valve on the CH2F2 cylinder to introduce the far-infrared laser medium into the far-infrared laser cavity.</li>
		<li>Adjust the metering valve on the inlet line until a pressure of approximately 10 Pa is achieved. 
		Note: Only the approximate pressure is necessary since it is used as a way of systematically scanning the far-infrared laser cavity.</li>
		<li>Set the position of the output coupler such that its outermost tip is approximately 1 cm from the middle of the laser cavity as indicated by a calibrated scale on the outside of the laser cavity. 
		Note: Only the approximate location is necessary since it is used as a way of systematically scanning the far-infrared laser cavity.</li>
		<li>Adjust the position of the moveable far-infrared laser mirror in approximately 0.25 mm increments by rotating the calibrated micrometer dial back and forth. Simultaneously tune the frequency of the CO2 pump laser through its gain curve by changing the voltage applied across the CO2 pump laser&#8217;s piezoelectric transducer (PZT).</li>
		<li>If no signal is observed on the oscilloscope display, repeat step 2.4.10 with the output coupler moved to its next position where the tip is approximately 1.5 cm from the middle of the laser cavity as indicated by a calibrated scale on the outside of the laser cavity.</li>
		<li>If no signal is observed on the oscilloscope display, repeat step 2.4.10 with the output coupler moved to its next position where the tip is approximately 2 cm from the middle of the laser cavity as indicated by a calibrated scale on the outside of the laser cavity.</li>
		<li>If no signal is observed on the oscilloscope display, repeat steps 2.4.9 through 2.4.12 with a far-infrared laser pressure of approximately 19 Pa as adjusted with the metering valve on the inlet line.</li>
		<li>If no signal is observed on the oscilloscope display, repeat steps 2.4.9 through 2.4.12 with a far-infrared laser pressure of approximately 27 Pa as adjusted with the metering valve on the inlet line.</li>
		<li>If no signal is observed on the oscilloscope display, insert the beam stop into the path of the CO2 pump laser and close the valve on the CH2F2 cylinder until the far-infrared laser pressure is approximately 0 Pa.</li>
		<li>Set the CO2 pump laser to the next laser emission, e.g., 9P34, and optimize the output power using steps 2.4.2 through 2.4.4.</li>
		<li>Repeat steps 2.4.5 through 2.4.16 until all emissions generated by the CO2 pump laser are used. When searching for far-infrared laser lines, place a focus on CO2 pump laser emissions whose frequencies overlap with any absorption regions identified in step 1.2.</li>
	</ol>
	</li>
	<li>Characterizing far-infrared laser emissions.
	<ol>
		<li>Simultaneously adjust the pressure of the far-infrared laser medium, the voltage applied to the CO2 pump laser&#8217;s PZT, and the position of the output coupler until the far-infrared laser emission&#8217;s output power is maximized (determined by a maximum peak-to-peak signal from the MIM diode detector as observed on the oscilloscope display, similar to Figure 4).</li>
		<li>Turn the micrometer dial clockwise until the far-infrared laser emission is observed on the oscilloscope display. Record the position of the micrometer dial.</li>
		<li>Turn the micrometer dial clockwise for an additional 20 modes corresponding to the same far-infrared laser emission. Record the position of the micrometer dial.</li>
		<li>Subtract the position of the micrometer dial in steps 2.5.2 and 2.5.3. Divide this difference by 10 to obtain the wavelength of the far-infrared laser emission.</li>
		<li>Repeat steps 2.5.2 through 2.5.4 a total of five times and average the wavelength of the far-infrared laser emission. Average laser wavelengths measured by traversing at least 20 adjacent longitudinal modes have a one-sigma uncertainty of &#177; 0.5 &#956;m.</li>
		<li>Measure the polarization of the far-infrared laser radiation, relative to the CO2 pump radiation, using either a gold wire-grid polarizer (394 lines/cm) or a Brewster polarizer.</li>
	</ol>
	</li>
</ol>
3. Determining Far-Infrared Laser Frequencies
<ol>
	<li>Identifying the CO2 reference laser emissions.
	<ol>
		<li>Calculate the frequency of the far-infrared laser emission based on its measured wavelength.</li>
		<li>Identify sets of CO2 reference laser lines whose frequency difference is within several GHz of the calculated frequency for the far-infrared laser emission40. A typical list used for such measurements is shown in Table 1.</li>
	</ol>
	</li>
	<li>Searching for the heterodyne beat signal.
	<ol>
		<li>Identify the first set of CO2 reference laser lines and set each CO2 reference laser on their respective laser emission.</li>
		<li>Optimize the output power for each CO2 reference laser using steps 2.4.2 through 2.4.4 and the monitor power meter.
		<ol>
			<li>Adjust an iris, either internal or external to each reference laser, so that the power from each CO2 reference laser is approximately 100 mW as measured by the monitor power meter shown in Figure 2.</li>
		</ol>
		</li>
		<li>Block the radiation from the CO2 pump laser using a beam stop while unblocking the radiation from the CO2 reference lasers.</li>
		<li>Turn on and align the optical chopper into the co-linear beam path of the CO2 reference lasers.</li>
		<li>Optimize for maximum peak-to-peak voltage each CO2 reference laser emission on the MIM diode detector using several mirrors, beam splitters, and a 2.54 cm focal length ZnSe plano-convex lens while observing the output on the oscilloscope, similar to Figure 5.</li>
		<li>Block the radiation from the CO2 reference lasers using a beam stop while unblocking the radiation from the CO2 pump laser.</li>
		<li>Re-optimize the CO2 pump laser and the far-infrared laser, as necessary, so that the far-infrared laser emission has a maximum peak-to-peak voltage as observed on the oscilloscope.</li>
		<li>Disconnect the MIM diode detector&#8217;s output from the oscilloscope and connect it to an amplifier whose output is observed on a spectrum analyzer.</li>
		<li>Unblock the radiation from the CO2 reference lasers.</li>
		<li>Remove the optical choppers modulating the CO2 pump and reference lasers.</li>
		<li>Set the spectrum analyzer on a 40 MHz span and search for the beat signal in 1.5 GHz increments by manually scanning this frequency range using the spectrum analyzer&#8217;s adjustment knob.</li>
		<li>If no beat signal is observed, disconnect the MIM diode&#8217;s output from the amplifier and connect it to the oscilloscope.</li>
		<li>Block the radiation from the CO2 reference lasers and reinsert the optical chopper into the path of the CO2 pump laser.</li>
		<li>Repeat steps 3.2.2 through 3.2.13 as necessary until the spectrum analyzer has been used to search for the beat signal between 0 and 12 GHz.</li>
		<li>If no beat signal is observed, repeat steps 3.2.2 through 3.2.14 with another set of CO2 reference laser lines until either the beat signal is observed or all possible sets of CO2 reference laser lines are exhausted.</li>
	</ol>
	</li>
	<li>Stabilizing the CO2 reference frequencies.
	<ol>
		<li>Apply a voltage between 0 and 900 V to the first CO2 reference laser&#8217;s PZT so that the signal from its respective fluorescence reference cell is at the center of the Lamb dip, illustrated in Figure 6 and as viewed on an oscilloscope as in Figure 7.</li>
		<li>Activate the feedback voltage applied to the first CO2 reference laser&#8217;s PZT using a custom built lock-in/servo amplifier so that it remains locked to the center of the Lamb dip.</li>
		<li>Repeat steps 3.3.1 and 3.3.2 for the second CO2 reference laser.</li>
		<li>Visually monitor the output of the pre-amp on an oscilloscope, as in Figure 7, to ensure the reference lasers remains locked.</li>
	</ol>
	</li>
	<li>Measurement of the beat frequency.
	<ol>
		<li>Center the beat signal on the spectrum analyzer display and adjust its amplitude to maximize its size on the display.</li>
		<li>Set the spectrum analyzer to view two simultaneous traces of the beat signal, as in Figure 8, by selecting the Clear Write feature for both Trace 1 and Trace 2. One trace will display the instantaneous signal while the other will record the maximum signal (using a Max Hold feature on the spectrum analyzer for the second trace).</li>
		<li>Rotate the micrometer dial on the far-infrared laser cavity back and forth across the gain curve for a given cavity mode.</li>
		<li>Use the View feature on the spectrum analyzer to freeze the second (Max Hold) trace once a symmetric pattern is obtained.</li>
		<li>Slightly rotate the micrometer dial clockwise to decrease the length of the far-infrared laser cavity. Simultaneously observe the subsequent small shift in the beat frequency on the spectrum analyzer due to this slight increase in the frequency of the far-infrared laser.</li>
		<li>Place markers at the full width at half maximum points of the symmetric pattern (Max Hold trace) using the Marker function with the Delta feature on the spectrum analyzer.</li>
		<li>Measure the center frequency of the beat signal using the Span Pair feature on the spectrum analyzer.</li>
		<li>Repeat steps 3.4.1 through 3.4.7.</li>
		<li>Disengage the lock in/servo amplifier for each CO2 reference laser to unlock each laser from its center frequency and re-optimize each CO2 reference laser.</li>
		<li>Re-lock the reference lasers using steps 3.3.1 through 3.3.4.</li>
		<li>Repeat steps 3.4.1 through 3.4.10 for a total of 6 measurements. Once complete, unlock each CO2 reference laser from its center frequency.</li>
		<li>Calculate the revised frequency of the far-infrared laser emission using these beat frequencies to obtain an accurate prediction for the second set of CO2 reference laser lines.</li>
		<li>Identify a different set of CO2 reference laser lines whose frequency difference is within several GHz of the calculated frequency for the far-infrared laser emission.</li>
		<li>Optimize the next set of CO2 reference laser lines on the MIM diode detector and obtain the beat signal using steps 3.2.2 through 3.2.15 as necessary.</li>
		<li>Lock the new set of CO2 reference laser lines using steps 3.3.1 through 3.3.4.</li>
		<li>Repeat steps 3.4.1 through 3.4.10 for a total of 6 measurements. Once complete, unlock each CO2 reference laser from its center frequency.</li>
		<li>Insert beam stops into the paths of the CO2 pump and reference lasers.</li>
	</ol>
	</li>
	<li>Calculation of the far-infrared laser frequency.
	<ol>
		<li>Calculate the unknown far-infrared laser frequency, &#957;FIR, using the measured beat frequency through the relation 
		&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;FIR = |&#957;CO2(I)&#8211;&#957;CO2(II)| &#177; |&#957;beat|&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;Eq. 1 
		where |&#957;CO2(I)&#8211;&#957;CO2(II)| is the magnitude of the difference frequency synthesized by the two CO2 reference lasers and |&#957;beat| is the magnitude of the beat frequency. The &#177; sign in Eq. 1 is determined experimentally from step 3.4.5.</li>
		<li>Obtain an average frequency and calculate the uncertainty.</li>
	</ol>
	</li>
</ol>

Certain commercial equipment is identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the authors, nor does it imply that the equipment identified is necessarily the best available for the purpose.

There are several critical steps within the protocol that require some additional discussion. When measuring the far-infrared laser wavelength, as outlined in step 2.5.3, it is important to ensure the same mode of the far-infrared laser emission is being used. Multiple modes of a far-infrared laser wavelength (i.e., TEM00, TEM01, etc.) can be generated within the laser cavity and thus it is important to identify the appropriate adjacent cavity modes being used to measure the wavelength...

The measurement of far-infrared laser frequencies was first performed by Hocker and co-workers in 1967. They measured the frequencies for the 311 and 337 &#956;m emissions from the direct-discharge hydrogen cyanide laser by mixing them with high order harmonics of a microwave signal in a silicon diode1. To measure higher frequencies, a chain of lasers and harmonic mixing devices were used to generate the laser harmonics2. Eventually two stabilized carbon dioxide (CO2) lasers were chosen to synthesize the necessary difference frequencies3,4. Today, far-infrared laser frequencies up to 4 THz can be measured with this technique using only the first harmonic of the difference frequency generated by two stabilized CO2 reference lasers. Higher frequency laser emissions can also be measured using the second harmonic, such as the 9 THz laser emissions from the methanol isotopologues CHD2OH and CH318OH.5,6 Over the years, the accurate measurement of laser frequencies has impacted a number of scientific experiments7,8 and permitted the adoption of a new definition of the meter by the General Conference of Weights and Measures in Paris in 1983.9&#8211;11
Heterodyne techniques, such as those described, have been immensely beneficial in the measurement of far-infrared laser frequencies generated by optically pumped molecular lasers. Since the discovery of the optically pumped molecular laser by Chang and Bridges12, thousands of optically pumped far-infrared laser emissions have been generated with a variety of laser media. For example, difluoromethane (CH2F2) and its isotopologues generate over 250 laser emissions when optically pumped by a CO2 laser. Their wavelengths range from approximately 95.6 to 1714.1 &#956;m.13&#8211;15 Nearly 75% of these laser emissions have had their frequencies measured while several have been spectroscopically assigned16&#8211;18.
These lasers, and their accurately measured frequencies, have played a crucial role in the advancement of high-resolution spectroscopy. They provide important information for infrared spectral studies of the laser gases. Often these laser frequencies are used to verify the analysis of infrared and far-infrared spectra because they provide connections among the excited vibrational state levels that are often directly inaccessible from absorption spectra19. They also serve as the primary radiation source for studies investigating transient, short-lived free radicals with the laser magnetic resonance technique20. With this extremely sensitive technique, rotational and ro-vibrational Zeeman spectra in paramagnetic atoms, molecules, and molecular ions can be recorded and analyzed along with the ability to investigate the reaction rates used to create these free radicals.
In this work, an optically pumped molecular laser, shown in Figure 1, has been used to generate far-infrared laser radiation from difluoromethane. This system consists of a continuous wave (cw) CO2 pump laser and a far-infrared laser cavity. A mirror internal to the far-infrared laser cavity redirects the CO2 laser radiation down the polished copper tube, undergoing twenty six reflections before terminating at the end of the cavity, scattering any remaining pump radiation. Therefore the far-infrared laser medium is excited using a transverse pumping geometry. To generate laser action, several variables are adjusted, some simultaneously, and all are subsequently optimized once laser radiation is observed.
In this experiment, far-infrared laser radiation is monitored by a metal-insulator-metal (MIM) point contact diode detector. The MIM diode detector has been used for laser frequency measurements since 1969.21&#8211;23 In laser frequency measurements, the MIM diode detector is a harmonic mixer between two or more radiation sources incident on the diode. The MIM diode detector consists of a sharpened Tungsten wire contacting an optically polished Nickel base24. The Nickel base has a naturally occurring thin oxide layer that is the insulating layer.
Once a laser emission was detected, its wavelength, polarization, strength, and optimized operating pressure were recorded while its frequency was measured using the three-laser heterodyne technique25&#8211;27 following the method originally described in Ref. 4. Figure 2 shows the optically pumped molecular laser with two additional cw CO2 reference lasers having independent frequency stabilization systems that utilize the Lamb dip in the 4.3 &#956;m fluorescence signal from an external, low pressure reference cell28. This manuscript outlines the process used to search for far-infrared laser emissions as well as the method for estimating their wavelength and in accurately determining their frequency. Specifics regarding the three-laser heterodyne technique as well as the various components and operating parameters of the system can be found in Supplemental Table A along with references 4, 25&#8211;27, 29, and 30.

<table><tbody><tr><td>Vacuum pump</td><td>Leybold</td><td>Trivac D4A</td><td>HE-175 oil; Quantity = 3</td></tr><tr><td>Vacuum pump</td><td>Leybold</td><td>Trivac D8B or D16B</td><td>Fomblin Fluid; Quantity = 1 of each</td></tr><tr><td>Vacuum pump</td><td>Leybold</td><td>Trivac D25B</td><td>HE-175 oil; Quantity = 1</td></tr><tr><td>Optical chopper with controller</td><td>Stanford Research Systems</td><td>SR540</td><td></td></tr><tr><td>Lock-in amplifier</td><td>Stanford Research Systems</td><td>SR830</td><td></td></tr><tr><td>Spectrum analyzer</td><td>Agilent</td><td>E4407B</td><td>ESA-E Series, 9 kHz to 26.5 GHz Spectrum Analyzer</td></tr><tr><td>Amplifier&amp;nbsp;</td><td>Miteq</td><td>AFS-44</td><td>Provides amplification of signals between 2 and 18 GHz. The amplifier is powered by a Hewlett Packard triple output DC power supply, model E3630A.</td></tr><tr><td>Amplifier&amp;nbsp;</td><td>Avantek</td><td>AWL-1200B</td><td>Provides amplification of signals less than 1.2 GHz.</td></tr><tr><td>Power supply</td><td>Hewlett Packard</td><td>E3630A</td><td>Low voltage DC power supply for amplifier.</td></tr><tr><td>Power supply</td><td>Glassman</td><td>KL Series</td><td>High voltage power supply for the CO&lt;sub&gt;2&lt;/sub&gt; lasers; Quantity = 2; negative polarity</td></tr><tr><td>Power supply</td><td>Fluke</td><td>412B</td><td>High voltage power supply used with the NIST Asymmetric HV Amp</td></tr><tr><td>Detector</td><td>Judson Infrared Inc</td><td>J10D</td><td>For fluorescence cell; Quantity = 2</td></tr><tr><td>CO&lt;sub&gt;2&lt;/sub&gt; laser spectrum analyzer</td><td>Optical Engineering&amp;nbsp;</td><td>16-A</td><td>Currently sold by Macken Instruments Inc.</td></tr><tr><td>Thermal imaging plates with UV light</td><td>Optical Engineering&amp;nbsp;</td><td></td><td>Primarily used for aligning the CO&lt;sub&gt;2&lt;/sub&gt; reference lasers. Currently sold by Macken Instruments Inc.</td></tr><tr><td>Resistors</td><td>Ohmite&amp;nbsp;</td><td>L225J100K</td><td>100 kW, 225 W. Between 4 to 6 resistors are used in each ballast system. Each CO&lt;sub&gt;2&lt;/sub&gt; laser has its own ballast system. Fans are used to cool the resistors.</td></tr><tr><td>HV relay, SPDT</td><td>CII Technologies</td><td>H-17</td><td>Quantity = 3; one for each CO&lt;sub&gt;2&lt;/sub&gt; laser</td></tr><tr><td>Amplifier&amp;nbsp;</td><td>Princeton Applied Research</td><td>PAR 113</td><td>Used with fluorescence cell; Quantity = 2</td></tr><tr><td>Oscilloscope</td><td>Tektronix</td><td>2235A</td><td>Similar models are also used; Quantity = 2</td></tr><tr><td>Oscilloscope/Differential amplifier</td><td>Tektronix</td><td>7903 oscilloscope with 7A22 differential amplifier</td><td></td></tr><tr><td>Power meter with sensor</td><td>Coherent</td><td>200</td><td>For use below 10 W.&amp;nbsp; This is the power meter shown in Figure 2.</td></tr><tr><td>Power meter with sensor</td><td>Scientech, Inc</td><td>Vector S310</td><td>For use below 30 W</td></tr><tr><td>Multimeter</td><td>Fluke</td><td>73III</td><td>Similar models are also used; Quantity = 3</td></tr><tr><td>Data acquisition</td><td>National Instruments</td><td>NI cDAQ 9174 chassis with NI 9223 input module</td><td>Uses LabVIEW software</td></tr><tr><td>Simichrome polish</td><td>Happich GmbH</td><td></td><td>Polish for the Nickel base used in the MIM diode detector. Although the Nickel base can be used immediately after polishing, a 12 hour lead time is typically recommended.</td></tr><tr><td>Pressure gauge</td><td>Wallace and Tiernan</td><td>61C-1D-0050</td><td>Series 300; for CO&lt;sub&gt;2&lt;/sub&gt; laser; Quantity = 3</td></tr><tr><td>Pressure gauge with controller</td><td>Granville Phillips</td><td>Series 375</td><td>For far-infrared laser</td></tr><tr><td>Zirconium Oxide felt</td><td>Zircar Zirconia</td><td>ZYF felt</td><td>Used as a beam stop</td></tr><tr><td>Zirconium Oxide board</td><td>Zircar Zirconia</td><td>ZYZ-3 board</td><td>Used as a beam stop; Quantity = 4</td></tr><tr><td>Teflon sheet</td><td>Scientific Commodities, Inc</td><td>BB96312-1248</td><td>1/32 inch thick; used for the far-infrared laser output window</td></tr><tr><td>Polypropylene</td><td>C-Line sheet protectors</td><td>61003</td><td>used for the far-infrared laser output window</td></tr><tr><td>Vacuum grease</td><td>Apiezon</td><td></td><td></td></tr><tr><td>Power supply</td><td>Kepco</td><td>NTC 2000</td><td>PZT power supply</td></tr><tr><td>PZT tube</td><td>Morgan Advanced Materials</td><td></td><td>1 inch length, 1 inch outer diameter, 0.062 inch thickness, reverse polarity (positive voltage on outside); Quantity = 3</td></tr><tr><td>ZnSe (AR coated)</td><td>II-VI Inc</td><td></td><td>CO&lt;sub&gt;2&lt;/sub&gt; laser window (Quantity = 3), lens, and beam splitter (Quantity 3)</td></tr><tr><td>NaCl window</td><td>Edmond Optics</td><td></td><td>Quantity = 1</td></tr><tr><td>CaF window</td><td>Edmond Optics</td><td></td><td>Quantity = 2</td></tr><tr><td>Laser mirrors and gratings</td><td>Hyperfine, Inc</td><td></td><td>Gold-coated; includes positioning mirrors</td></tr><tr><td>Glass laser tubes and reference cells</td><td>Allen Scientific Glass</td><td></td><td></td></tr><tr><td>MIM diode detector</td><td>Custom Microwave, Inc</td><td></td><td></td></tr><tr><td>Other</td><td></td><td></td><td>Other materials include magnetic bases, base plates, base clamps, XYZ translation stage, etc.</td></tr></tbody></table>

characterizing far-infrared laser emissions and the measurement of their frequencies

The generation and subsequent measurement of far-infrared radiation has found numerous applications in high-resolution spectroscopy, radio astronomy, and Terahertz imaging. For about 45 years, the generation of coherent, far-infrared radiation has been accomplished using the optically pumped molecular laser. Once far-infrared laser radiation is detected, the frequencies of these laser emissions are measured using a three-laser heterodyne technique. With this technique, the unknown frequency from the optically pumped molecular laser is mixed with the difference frequency between two stabilized, infrared reference frequencies. These reference frequencies are generated by independent carbon dioxide lasers, each stabilized using the fluorescence signal from an external, low pressure reference cell. The resulting beat between the known and unknown laser frequencies is monitored by a metal-insulator-metal point contact diode detector whose output is observed on a spectrum analyzer. The beat frequency between these laser emissions is subsequently measured and combined with the known reference frequencies to extrapolate the unknown far-infrared laser frequency. The resulting one-sigma fractional uncertainty for laser frequencies measured with this technique is &#177; 5 parts in 107. Accurately determining the frequency of far-infrared laser emissions is critical as they are often used as a reference for other measurements, as in the high-resolution spectroscopic investigations of free radicals using laser magnetic resonance. As part of this investigation, difluoromethane, CH2F2, was used as the far-infrared laser medium. In all, eight far-infrared laser frequencies were measured for the first time with frequencies ranging from 0.359 to 1.273 THz. Three of these laser emissions were discovered during this investigation and are reported with their optimal operating pressure, polarization with respect to the CO2 pump laser, and strength.

As mentioned, the frequency reported for a far-infrared laser emission is an average of at least twelve measurements performed with at least two different sets of CO2 reference laser lines. Table 2 outlines the data recorded for the 235.5 &#956;m laser emission when using the 9P04 CO2 pump laser. For this far-infrared laser emission, fourteen individual measurements of the beat frequency were recorded. The first set of measurements were recorded while using the 9R1...

Watch this Scientific Journal Video about Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies at JoVE.com

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

We describe the generation of far-infrared radiation using an optically pumped molecular laser along with the measurement of their frequencies with heterodyne techniques. The experimental system and techniques are demonstrated using difluoromethane (CH2F2) as the laser medium whose results include three new laser emissions and eight measured laser frequencies.

The generation and subsequent measurement of far-infrared radiation has found numerous applications in high-resolution spectroscopy, radio astronomy, and ...

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12.0K Views.  Central Washington University. We describe the generation of far-infrared radiation using an optically pumped molecular laser along with the measurement of their frequencies with heterodyne techniques. The experimental system and techniques are demonstrated using difluoromethane (CH2F2) as the laser medium whose results include three new laser emissions and eight measured laser frequencies.

Video: Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

Implementation of a Reference Interferometer for Nanodetection

A thermally and mechanically stabilized fiber interferometer suited for examining ultra-high quality factor microcavities is fashioned. After assessing its free spectral range (FSR), the module is put in parallel with a fiber taper-microcavity system and then calibrated through isolating and eliminating random shifts in the laser frequency (i.e. laser jitter noise). To realize the taper-microcavity junction and to maximize the optical power that is transferred to the resonator, a single-mode optical fiber waveguide is pulled. Solutions containing polystyrene nanobeads are then prepared and flown to the microcavity in order to demonstrate the system&#8217;s ability to sense binding to the surface of the microcavity. Data is post-processed via adaptive curve fitting, which allows for high-resolution measurements of the quality factor as well as the plotting of time-dependent parameters, such as resonant wavelength and split frequency shifts. By carefully inspecting steps in the time-domain response and shifting in the frequency-domain response, this instrument can quantify discrete binding events.

A reference interferometer technique, which is designed to remove undesirable laser jitter noise for nanodetection, is utilized for probing an ultra-high quality factor microcavity. Instructions for assembly, setup, and data acquisition are provided, alongside the measurement process for specifying the cavity quality factor.

A thermally and mechanically stabilized fiber interferometer suited for examining ultra-high quality factor microcavities is fashioned. After assessing ...

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 ...

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 ...

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 ...

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb explosion imaging setup in which a hitherto-impractical camera angle is realized. In our imaging technique, diatomic molecules are irradiated with a circularly polarized strong laser pulse. The ejected atomic ions are accelerated perpendicularly to the laser propagation. The ions lying in the laser polarization plane are selected through the use of a mechanical slit and imaged with a high-throughput, 2-dimensional detector installed parallel to the polarization plane. Because a circularly polarized (isotropic) Coulomb exploding pulse is used, the observed angular distribution of the ejected ions directly corresponds to the squared rotational wave function at the time of the pulse irradiation. To create a real-time movie of molecular rotation, the present imaging technique is combined with a femtosecond pump-probe optical setup in which the pump pulses create unidirectionally rotating molecular ensembles. Due to the high image throughput of our detection system, the pump-probe experimental condition can be easily optimized by monitoring a real-time snapshot. As a result, the quality of the observed movie is sufficiently high for visualizing the detailed wave nature of motion. We also note that the present technique can be implemented in existing standard ion imaging setups, offering a new camera angle or viewpoint for the molecular systems without the need for extensive modification.

We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the wavelength of the strong-field ionizing pump pulse from the commonly used 800 nm into the near-infrared (1200-1600 nm), the contribution of adiabatic electron tunneling to the ionization process increases relative to multiphoton absorption. Adiabatic ionization results in predominant population of the ground electronic state of the ion upon electron removal, which effectively prepares a coherent vibrational state (&#34;wave packet&#34;) amenable to subsequent excitation. In our experiments, the coherent vibrational dynamics are probed with a weak-field 800 nm pulse and the time-dependent yields of dissociation products measured in a time-of-flight mass spectrometer. We present the measurements on the molecule dimethyl methylphosphonate (DMMP) to illustrate how using 1500 nm pulses for excitation enhances the amplitude of coherent oscillations in ion yields by a factor of 10 as compared to 800 nm pulses. This protocol may be implemented in existing pump-probe setups through the incorporation of an optical parametric amplifier (OPA) for wavelength conversion.

We present a protocol for probing ultrafast vibrational coherences in polyatomic radical cations that result in molecular dissociation.

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the ...

Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing

We present a protocol for performing gas spectroscopy using infrared degenerated four-wave mixing (IR-DFWM), for the quantitative detection of gas species in the ppm-to-single-percent range. The main purpose of the method is the spatially resolved detection of low-concentration species, which have no transitions in the visible or near-IR spectral range that could be used for detection. IR-DFWM is a nonintrusive method, which is a great advantage in combustion research, as inserting a probe into a flame can change it drastically. The IR-DFWM is combined with upconversion detection. This detection scheme uses sum-frequency generation to move the IR-DFWM signal from the mid-IR to the near-IR region, to take advantage of the superior noise characteristics of silicon-based detectors. This process also rejects most of the thermal background radiation. The focus of the protocol presented here is on the proper alignment of the IR-DFWM optics and on how to align an intracavity upconversion detection system.

Here, we present a protocol to perform sensitive, spatially resolved gas spectroscopy in the mid-infrared region, using degenerate four-wave mixing combined with upconversion detection.

We present a protocol for performing gas spectroscopy using infrared degenerated four-wave mixing (IR-DFWM), for the quantitative detection of gas species ...

Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional &#960;-conjugate Systems

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near infrared (near-IR) transitions, because it can overcome the low sensitivity of spontaneous Raman spectrometers in the near-IR region. Here, we describe technical details of a femtosecond time-resolved near-IR multiplex stimulated Raman spectrometer that we have recently developed. A description of signal generation and optimization, measurement, data acquisition, and calibration and correction of recorded data is provided as well. We present an application of our spectrometer to analyze the excited-state dynamics of &#946;-carotene in toluene solution. A C=C stretch band of &#946;-carotene in the second lowest excited singlet (S2) state and the lowest excited singlet (S1) state is clearly observed in the recorded time-resolved stimulated Raman spectra. The femtosecond time-resolved near-IR stimulated Raman spectrometer is applicable to the structural dynamics of &#960;-conjugate systems from simple molecules to complex materials.

Details of signal generation and optimization, measurement, data acquisition, and data handling for a femtosecond time-resolved near-IR stimulated Raman spectrometer are described. A near infrared stimulated Raman study on the excited-state dynamics of &#946;-carotene in toluene is shown as a representative application.

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near ...

High-definition Fourier Transform Infrared (FT-IR) Spectroscopic Imaging of Human Tissue Sections towards Improving Pathology

High-definition Fourier Transform Infrared (FT-IR) spectroscopic imaging is an emerging approach to obtain detailed images that have associated biochemical information. FT-IR imaging of tissue is based on the principle that different regions of the mid-infrared are absorbed by different chemical bonds (e.g., C=O, C-H, N-H) within cells or tissue that can then be related to the presence and composition of biomolecules (e.g., lipids, DNA, glycogen, protein, collagen). In an FT-IR image, every pixel within the image comprises an entire Infrared (IR) spectrum that can give information on the biochemical status of the cells that can then be exploited for cell-type or disease-type classification. In this paper, we show: how to obtain IR images from human tissues using an FT-IR system, how to modify existing instrumentation to allow for high-definition imaging capabilities, and how to visualize FT-IR images. We then present some applications of FT-IR for pathology using the liver and kidney as examples. FT-IR imaging holds exciting applications in providing a novel route to obtain biochemical information from cells and tissue in an entirely label-free non-perturbing route towards giving new insight into biomolecular changes as part of disease processes. Additionally, this biochemical information can potentially allow for objective and automated analysis of certain aspects of disease diagnosis.

High-definition Fourier Transform Infrared FT-IR Spectroscopic Imaging of Human Tissue Sections towards Improving Pathology

Fourier Transform Infrared (FT-IR) spectroscopic imaging is a fast and label-free approach to obtain biochemical data sets of cells and tissues. Here, we demonstrate how to obtain high-definition FT-IR images of tissue sections towards improving disease diagnosis.

High-definition Fourier Transform Infrared (FT-IR) spectroscopic imaging is an emerging approach to obtain detailed images that have associated ...

Medicine

Exploring Multiphase Polymeric Systems by Nanoscale Infrared Spectroscopy

Advances in Nanoscale Infrared Spectroscopy to Explore Multiphase Polymeric Systems

Multiphase polymeric systems encompass local domains with dimensions that can vary from a few tens of nanometers to several micrometers. Their composition is commonly assessed using infrared spectroscopy, which provides an average fingerprint of the various materials contained in the volume probed. However, this approach does not offer any details on the arrangement of the phases in the material. Interfacial regions between two polymeric phases, often in the nanoscale range, are also challenging to access. Photothermal nanoscale infrared spectroscopy monitors the local response of materials excited by infrared light with the sensitive probe of an atomic force microscope (AFM). While the technique is suitable for interrogating small features, such as individual proteins on pristine gold surfaces, the characterization of three-dimensional multicomponent materials is more elusive. This is due to a relatively large volume of material undergoing photothermal expansion, defined by the laser focalization onto the sample and by the thermal properties of the polymeric constituents, compared to the nanoscale region probed by the AFM tip. Using a polystyrene (PS) bead and a polyvinyl alcohol (PVA) film, we evaluate the spatial footprint of photothermal nanoscale infrared spectroscopy for surface analysis as a function of the position of PS in the PVA film. The effect of the feature position on the nanoscale infrared images is investigated, and spectra are acquired. Some perspectives on the future advances in the field of photothermal nanoscale infrared spectroscopy are provided, considering the characterization of complex systems with embedded polymeric structures.

Author Spotlight: Advances in Nanoscale Infrared Spectroscopy to Explore Multiphase Polymeric Systems  

This protocol describes the application of atomic force microscopy and nanoscale infrared spectroscopy to evaluate the performance of photothermal nanoscale infrared spectroscopy in the characterization of three-dimensional multi-polymeric samples.

Multiphase polymeric systems encompass local domains with dimensions that can vary from a few tens of nanometers to several micrometers. Their composition ...

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

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