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 border="0" cellpadding="0" cellspacing="0">
	<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&#160;</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&#160;</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 CO2 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>CO2 laser spectrum analyzer</td>
			<td>Optical Engineering&#160;</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&#160;</td>
			<td></td>
			<td>Primarily used for aligning the CO2 reference lasers. Currently sold by Macken Instruments Inc.</td>
		</tr>
		<tr>
			<td>Resistors</td>
			<td>Ohmite&#160;</td>
			<td>L225J100K</td>
			<td>100 kW, 225 W. Between 4 to 6 resistors are used in each ballast system. Each CO2 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 CO2 laser</td>
		</tr>
		<tr>
			<td>Amplifier&#160;</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.&#160; 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 CO2 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>CO2 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

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Spatial Separation of Molecular Conformers and Clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

We present a technique that allows the spatial separation of different conformers or clusters present in a molecular beam. An electrostatic deflector is used to separate species by their mass-to-dipole moment ratio, leading to the production of gas-phase ensembles of a single conformer or cluster stoichiometry.

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold ...

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 ...

Automation of Mode Locking in a Nonlinear Polarization Rotation Fiber Laser through Output Polarization Measurements

When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a new and inexpensive procedure to force mode locking in a pre-adjusted nonlinear polarization rotation fiber laser. This procedure is based on the detection of a sudden change in the output polarization state when mode locking occurs. This change is used to command the alignment of the intra-cavity polarization controller in order to find mode-locking conditions. More specifically, the value of the first Stokes parameter varies when the angle of the polarization controller is swept and, moreover, it undergoes an abrupt variation when the laser enters the mode-locked state. Monitoring this abrupt variation provides a practical easy-to-detect signal that can be used to command the alignment of the polarization controller and drive the laser towards mode locking. This monitoring is achieved by feeding a small portion of the signal to a polarization analyzer measuring the first Stokes parameter. A sudden change in the read out of this parameter from the analyzer will occur when the laser enters the mode-locked state. At this moment, the required angle of the polarization controller is kept fixed. The alignment is completed. This procedure provides an alternate way to existing automating procedures that use equipment such as an optical spectrum analyzer, an RF spectrum analyzer, a photodiode connected to an electronic pulse-counter or a nonlinear detecting scheme based on two-photon absorption or second harmonic generation. It is suitable for lasers mode locked by nonlinear polarization rotation. It is relatively easy to implement, it requires inexpensive means, especially at a wavelength of 1550 nm, and it lowers the production and operation costs incurred in comparison to the above-mentioned techniques.

A protocol to detect and automate mode locking in a pre-adjusted nonlinear polarization rotation fiber laser is presented. The detection of a sudden change in the output polarization state when mode locking occurs is used to command the alignment of an intra-cavity polarization controller in order to find mode-locking conditions.

When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a ...

Design and Use of a Full Flow Sampling System (FFS) for the Quantification of Methane Emissions

The use of natural gas continues to grow with increased discovery and production of unconventional shale resources. At the same time, the natural gas industry faces continued scrutiny for methane emissions from across the supply chain, due to methane&#39;s relatively high global warming potential (25-84x that of carbon dioxide, according to the Energy Information Administration). Currently, a variety of techniques of varied uncertainties exists to measure or estimate methane emissions from components or facilities. Currently, only one commercial system is available for quantification of component level emissions and recent reports have highlighted its weaknesses.
In order to improve accuracy and increase measurement flexibility, we have designed, developed, and implemented a novel full flow sampling system (FFS) for quantification of methane emissions and greenhouse gases based on transportation emissions measurement principles. The FFS is a modular system that consists of an explosive-proof blower(s), mass airflow sensor(s) (MAF), thermocouple, sample probe, constant volume sampling pump, laser based greenhouse gas sensor, data acquisition device, and analysis software. Dependent upon the blower and hose configuration employed, the current FFS is able to achieve a flow rate ranging from 40 to 1,500 standard cubic feet per minute (SCFM). Utilization of laser-based sensors mitigates interference from higher hydrocarbons (C2+). Co-measurement of water vapor allows for humidity correction. The system is portable, with multiple configurations for a variety of applications ranging from being carried by a person to being mounted in a hand drawn cart, on-road vehicle bed, or from the bed of utility terrain vehicles (UTVs). The FFS is able to quantify methane emission rates with a relative uncertainty of &#177; 4.4%. The FFS has proven, real world operation for the quantification of methane emissions occurring in conventional and remote facilities.

Design and Use of a Full Flow Sampling System FFS for the Quantification of Methane Emissions

We have designed, developed, and implemented a novel full flow sampling system (FFS) for quantification of methane emissions and greenhouse gases from across the natural gas supply chain.

The use of natural gas continues to grow with increased discovery and production of unconventional shale resources. At the same time, the natural gas ...

Optical Trap Loading of Dielectric Microparticles In Air

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly scattered dielectric microparticles adhered to a glass substrate are momentarily detached using ultrasonic vibrations generated by a piezoelectric transducer (PZT). Then, the optical beam focused on a selected particle lifts it up to the optical trap while the vibrationally excited microparticles fall back to the substrate. A particle may be trapped at the nominal focus of the trapping beam or at a position above the focus (referred to here as the levitation position) where gravity provides the restoring force. After the measurement, the trapped particle can be placed at a desired position on the substrate in a controlled manner. 
In this protocol, an experimental procedure for selective optical trap loading in air is outlined. First, the experimental setup is briefly introduced. Second, the design and fabrication of a PZT holder and a sample enclosure are illustrated in detail. The optical trap loading of a selected microparticle is then demonstrated with step-by-step instructions including sample preparation, launching into the trap, and use of electrostatic force to excite particle motion in the trap and measure charge. Finally, we present recorded particle trajectories of Brownian and ballistic motions of a trapped microparticle in air. These trajectories can be used to measure stiffness or to verify optical alignment through time domain and frequency domain analysis. Selective trap loading enables optical tweezers to track a particle and its changes over repeated trap loadings in a reversible manner, thereby enabling studies of particle-surface interaction.

A protocol for launching and stably trapping selected dielectric microparticles in air is presented.

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly ...

Generation of Size-controlled Poly (ethylene Glycol) Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. This paper proposes a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation. The polydimethylsiloxane (PDMS) chip was fabricated using the multi-layer soft lithography method. Hexadecane containing surfactant was used as the continuous phase, and PEGDA with the ultraviolet (UV) photo-initiator was the dispersed phase. Surfactants allowed the local surface tension to drop and formed a more cusped tip which promoted breaking into tiny micro-droplets. As the pressure of dispersed phase was constant, the size of droplets became smaller with increasing continuous phase pressure before dispersed phase flow was broken off. As a result, droplets with size variation from 1 &#181;m to 80 &#181;m in diameter could be selectively achieved by changing the pressure ratio in two inlet channels, and the average coefficient of variation was estimated to be below 7%. Furthermore, droplets could turn into micro-beads by UV exposure for photo-polymerization. Conjugating biomolecules on such micro-beads surface have many potential applications in the fields of biology and chemistry.

Generation of Size-controlled Poly ethylene Glycol Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Here, we present a protocol to illustrate the fabrication processes and verifying experiments of a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation.

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. ...

Quantitative Analysis of Vacuum Induction Melting by Laser-induced Breakdown Spectroscopy

Vacuum induction melting is a popular method for refining high purity metal and alloys. Traditionally, standard process control in metallurgy involves several steps, include drawing samples, cooling, cutting, transport to the laboratory, and analysis. The whole analysis process requires more than 30 minutes, which hinders on-line process control. Laser-induced breakdown spectroscopy is an excellent on-line analysis method that can satisfy the requirements of vacuum induction melting because it is fast and noncontact and does not require sample preparation. The experimental facility uses a lamp-pumped Q-switched laser to ablate melted liquid steel with an output energy of 80 mJ, a frequency of 5 Hz, a FWHM pulse width of 20 ns, and a working wavelength of 1,064 nm. A multi-channel linear charge coupled device (CCD) spectrometer is used to measure the emission spectrum in real time, with a spectral range from 190 to 600 nm and a resolution of 0.06 nm at a wavelength of 200 nm. The protocol includes several steps: standard alloy sample preparation and an ingredient test, smelting of standard samples and determination of the laser breakdown spectrum, and construction of the elements concentration quantitative analysis curve of each element. To realize the concentration analysis of unknown samples, the spectrum of a sample also needs to be measured and disposed with the same process. The composition of all main elements in the melted alloy can be quantitatively analyzed with an internal standard method. The calibration curve shows that the limit of detection of most metal elements ranges from 20-250 ppm. The concentration of elements, such as Ti, Mo, Nb, V, and Cu, can be lower than 100 ppm, and the concentrations of Cr, Al, Co, Fe, Mn, C, and Si range from 100-200 ppm. The R2 of some calibration curves can exceed 0.94.

During vacuum induction melting, laser-induced breakdown spectroscopy is used to perform real-time quantitative analysis of the main-ingredient elements of a molten alloy.

Vacuum induction melting is a popular method for refining high purity metal and alloys. Traditionally, standard process control in metallurgy involves ...

Method Development for Contactless Resonant Cavity Dielectric Spectroscopic Studies of Cellulosic Paper

The current analytical techniques for characterizing printing and graphic arts substrates are largely ex situ and destructive. This limits the amount of data that can be obtained from an individual sample and renders it difficult to produce statistically relevant data for unique and rare materials. Resonant cavity dielectric spectroscopy is a non-destructive, contactless technique which can simultaneously interrogate both sides of a sheeted material and provide measurements which are suitable for statistical interpretations. This offers analysts the ability to quickly discriminate between sheeted materials based on composition and storage history. In this methodology article, we demonstrate how contactless resonant cavity dielectric spectroscopy may be used to differentiate between paper analytes of varying fiber species compositions, to determine the relative age of the paper, and to detect and quantify the amount of post-consumer waste (PCW) recycled fiber content in manufactured office paper.

A protocol for the non-destructive analysis of the fiber content and relative age of paper.

The current analytical techniques for characterizing printing and graphic arts substrates are largely ex situ and destructive. This limits the amount of ...

Measuring Sub-23 Nanometer Real Driving Particle Number Emissions Using the Portable DownToTen Sampling System

The current particle size threshold of the European Particle Number (PN) emission standards is 23 nm. This threshold could change because future combustion engine vehicle technology may emit large amounts of sub-23 nm particles. The Horizon 2020 funded project DownToTen (DTT) developed a sampling and measurement method to characterize particle emissions in this currently unregulated size range. A PN measurement system was developed based on an extensive review of the literature and laboratory experiments testing a variety of PN measurement and sampling approaches. The measurement system developed is characterized by high particle penetration and versatility, which enables the assessment of primary particles, delayed primary particles, and secondary aerosols, starting from a few nanometers in diameter. This paper provides instruction on how to install and operate this Portable Emission Measurement System (PEMS) for Real Drive Emissions (RDE) measurements and assess particle number emissions below the current legislative limit of 23 nm.

Presented here is the DownToTen (DTT) portable emission measurement system to assess real driving automotive emissions of sub-23 nm particles.