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

Caution: Please be aware of the possible dangers of laser scattering during the alignment. Wear proper safety goggles for protection. To facilitate the alignment process, an infrared viewer (IR-viewer) is necessary. An IR-sensitive fluorescent card is also helpful but not necessary.
1. Sample Preparation
<ol>
	<li>Use a diamond scribe to make a miniscule scratch on the edge of the top surface of the sample at the desired location of the cleave. Use two pairs of flat-ended tweezers to hold the sample on both sides of the scratch. Apply an outward-rotating torque with the tweezers and the sample will cleave. 
	NOTE: A long scratch is unnecessary to promote cleaving, and it will likely cut through the waveguide layer making light coupling impossible. The cleaved face is delicate enough that any touch on its surface might damage the waveguide face.</li>
	<li>Attach the cleaved sample piece onto a copper sample plate using thermally conductive silver paint or silver epoxy. 
	NOTE: The cleaved face should be flush with the edge of the mounting plate so that the excitation laser will hit the sample face without interference.</li>
	<li>Mount the copper plate in the cryostat so that both the cleaved face and the sample surface are optically accessible through the cryostat windows.</li>
</ol>
2. Alignment of Resonant Excitation Path
NOTE: To maximize the coupling efficiency into the waveguide, the profile of the incident excitation beam has to be matched with that of an imaginary backwards propagating beam exiting the waveguide.
<ol>
	<li>Coarse alignment of excitation laser beam to the cleaved face of the sample.
	<ol>
		<li>Use the degrees of freedom of optical fiber coupler FC0 and mirror M0 to direct the excitation beam onto the cleaved face of the sample before the excitation lenses are installed.</li>
		<li>Level the excitation beam horizontally both with respect to the optical table and with respect to the plane of the sample.</li>
	</ol>
	</li>
	<li>Installation of excitation objective&#160;Eobj 
	<ol>
		<li>Put the aspheric lens Eobj in a translational mount with three degrees of translational freedom. Center Eobj on the laser and set the height of&#160;Eobj to be the same as the center of the sample.</li>
		<li>Set up a white paper screen behind the sample along the excitation path. Use an IR-viewer to observe a bright spot on the paper due to the laser light passing by the sample.</li>
		<li>Slide&#160;Eobj towards the sample slowly until a clear silhouette image of the sample can be seen on the paper. Adjust the height and lateral position of&#160;Eobj to center the silhouette at the middle of the bright spot.</li>
		<li>Keep sliding Eobj towards the sample slowly, and the silhouette image on the screen experiences a magnification. Meanwhile, adjust the lateral position of Eobj (left/right) to compensate the horizontal shifting of the silhouette image. 
		NOTE: During the slow movement of Eobj to the sample, diffraction fringes will start to appear at some point. This provides a new reference to put the focused spot at the surface layer of the sample.</li>
		<li>Keep sliding&#160; Eobj towards the sample slowly. At each location of Eobj, shift Eobj left/right increase the fringe spacing until there is only one fringe visible to look for fringes on the screen. 
		Note: There will be two groups of fringes, one to the left and one to the right of the sample surface.</li>
		<li>Locate Slide Eobj Eobj at a position that minimizes the number of fringes visible.</li>
	</ol>
	</li>
	<li>Alignment of telescope lenses E1 and E2
	<ol>
		<li>Insert lenses E1 and E2 into the excitation path centered on the laser beam. Position E2 separated from&#160;Eobj by the sum of their focal lengths. Set the separation between E1 and E2 to be the sum of their focal lengths, e.g., f1 + f2 = 150 mm.&#160;</li>
		<li>Observe the silhouette and diffraction pattern on the paper with an IR-viewer. Adjust the height of E1 to center the silhouette at the center of the bright laser illumination spot.</li>
		<li>Slide E2 towards or away from E1 while adjusting the lateral position of E1. Secure E1 and E2 at positions that both fringe groups either disappear or show a minimum number of fringes in the view.&#160;</li>
		<li>Insert a vertically oriented polarizer POL before E1, and center it on the excitation beam. &#160; 
		Note: Some polarizers have a slight wedge angle, in which case the excitation beam will experience an angular deviation. Use E1 and E2 to compensate this deviation.</li>
	</ol>
	</li>
</ol>
3. Alignment of Photoluminescence Collection Path
NOTE: The performance of the imaging system built in the collection path is mostly determined by the precision of the positioning of Lobj because of its short focal length (fobj = 10mm, NA=0.55). Two general steps are involved in the alignment of Lobj: coarse alignment by using a HeNe laser, and fine tweaking by using the illuminator and the bulk exciton emission of GaAs. These alignment steps are performed with the sample at room temperature.
<ol>
	<li>Alignment of the above band-gap (HeNe) excitation path and installation of the camera:
	<ol>
		<li>Couple an above band-gap laser beam (HeNe) into a single mode fiber.</li>
		<li>Direct the output beam from the fiber collimator FC1 onto the sample via a mirror M3.</li>
		<li>Tilt FC1 horizontally to center the laser spot on the sample about 1 mm away from the cleaved edge. Tilt M3 horizontally to shift the back reflection of the laser beam to be just above or below the incident beam. Repeat this process several times till meeting both criteria.</li>
		<li>Tilt FC1 and M3 vertically to level the HeNe beam with respect to the optical table and keep it directed at the sample.</li>
		<li>Use the IR viewer to locate both the resonant laser and the HeNe laser spots on the sample. Check that the center of the HeNe laser spot is at the same height as the resonant laser spot. If not, use FC1 and M3 to match the beam heights while keeping the HeNe beam level with the table.</li>
		<li>Insert the non-polarizing beam splitter cube (90:10), NPBS, into the HeNe path. Center the cube in the incident HeNe beam.</li>
		<li>Locate two beams at the beam splitter output to the collection path, one from reflection from the sample, and one from internal reflection within the cube.</li>
		<li>Rotate the cube by a small angle (~5 degrees) such that the two beams can be easily separated at the exit. The light reflected off the sample surface can be used as a crude guide to align the camera. 
		NOTE: The direction of the internally reflected beam will not change when the cube is rotated about the vertical axis.</li>
		<li>Level the cube with respect to the optical table by ensuring the HeNe beam corresponding to the internal reflection inside the cube is at the same height as the incoming beam.</li>
		<li>Put an IR-sensitive camera in the path of the back reflected HeNe beam. Use a tube lens Lcam with a focal length of 200 mm to focus the sample image onto the camera. 
		NOTE: A home-built tube system is used to house the lens Lcam, as shown in Figure 1, which prevents stray room light from being detected by the camera.</li>
		<li>Set up an 800 nm long-pass filter, F1, in front of Lcam to filter out the HeNe light, which allows the observation of PL from the sample with the camera.</li>
	</ol>
	</li>
	<li>Installation and optimization of the position of lens Lobj
	<ol>
		<li>Put the aspheric lens&#160;Lobj in a translational mount with three degrees of translational freedom. Center&#160;Lobj on the HeNe laser and set the separation from the sample to be the focal length, fobj = 10 mm.</li>
		<li>Set up lens pair L1 and L2 (f1 = f2 = 50 mm) by using an X-Y translational mount where one side is fixed and the other side is movable in the lateral plane controlled by micrometers. 
		NOTE: Lens L2 goes into the movable side of the mount. L1 is held by a lens tube and attached to the fixed side of the mount. The tube system provides the freedom to adjust the distance between the two lenses by screwing in/out the lens tube holding L1 along the optical axis.</li>
		<li>Set the distance between the two lenses to be 100 mm. Set L2 at the center of the mount by adjusting the micrometers.</li>
		<li>Insert the L1 and L2 lens combo into the HeNe path between NPBS and the cryostat. Set the distance between L1 and Lobj to be fobj + f1. Center L1 and L2 on the incident HeNe light.</li>
		<li>Insert the illuminator and pellicle into the collection path as shown in Figure 1. Set the distance between lens K4 and L2 to be the sum of their focal lengths.</li>
		<li>Center the illumination beam on L2 by adjusting the angle of the illuminator.</li>
		<li>Adjust the angle of the pellicle to center the back reflected illumination light visible in the camera image to the PL spot caused by HeNe excitation. 
		NOTE: For the purpose of alignment, one can close the field diaphragm to locate the center of the illuminated area.</li>
		<li>Using only the illuminator light, find a surface defect or dust on the sample by looking at the camera. Search other parts of the sample as needed by moving L2 laterally.</li>
		<li>Slightly tap Lobj in/out along the optical axis to make the edge of the defect or dust sharpest.</li>
		<li>Shift L2 back to the center of the mount.</li>
		<li>View the HeNe-excited PL spot on the camera, and move Lobj horizontally so that the PL spot is 1-2 mm from the cleaved edge of the sample. 
		NOTE: For a distance less than 1 mm, the laser scattering from the cleaved edge of the sample would be collected by objective Lobj. While for a distance too far from the cleaved face, the excitation beam can experience attenuation before reaching the QD, which reduces the maximum excitation power available.</li>
		<li>Shift L2 horizontally till the cleaved edge of the sample is shown on the camera under illumination.</li>
		<li>Slowly shift Lobj vertically to look for a bright laser spot on the cleaved edge of the sample, which is caused by the scattering of the resonant excitation beam at the cleaved face of the sample.</li>
		<li>Level the PL spot caused by HeNe excitation to the bright laser spot on the cleaved edge of the sample.</li>
	</ol>
	</li>
	<li>Realignment of the HeNe excitation path with respect to the new location of Lobj. 
	NOTE: To maximize scannable area and minimize vignetting, it is necessary to re-center the excitation optics and the excitation beam with respect to the location of Lobj.
	<ol>
		<li>Remove L1 and L2. Center the excitation beam on&#160;Lobj while ensuring the beam is in the surface normal direction of the sample.</li>
		<li>Center L2 on the mount. Center both L1 and L2 on the incident excitation beam. Set the distance between L1 and&#160;Lobj to be the sum of the two focal lengths, i.e., f1 + fobj.</li>
		<li>Reposition Lcam such that it is centered on reflected HeNe beam. Reposition the camera such that the HeNe excited PL (use a long-pass filter) is centered on the image.</li>
		<li>Adjust the angle of the illuminator and the pellicle to center the illumination light on L2 and on the PL spot caused by HeNe excitation.</li>
	</ol>
	</li>
	<li>Alignment of mirrors M1 and M2. 
	NOTE: A laser directed backwards through the spectrometer will facilitate alignment.
	<ol>
		<li>Monitor the HeNe-excited PL from the sample at the camera. Center an iris (Iris A) on the PL between the pellicle and M1.</li>
		<li>Center lens Lspec on the reverse beam and place it one focal length fspec away from the entrance slit of the spectrometer.</li>
		<li>Send the reverse beam from the spectrometer to the sample by reflecting off of two mirrors, M1 and M2.</li>
		<li>Set up another iris (Iris B) between M2 and Lspec and center it on the reverse beam.</li>
		<li>Steer M2 to center the reverse beam on Iris A. Steer M1 to center the PL on Iris B. Repeat this process several times until both criteria are satisfied.</li>
		<li>Locate the center of the entrance slit (30 &#956;m width) of the spectrometer on the CCD by monitoring the zero-order diffraction of the room light.</li>
		<li>Open the entrance slit of the spectrometer. By using an 800 nm long pass filter, the PL from the sample under HeNe excitation can be observed on the CCD.</li>
		<li>Steer M1 to center this spot at the entrance slit of the spectrometer and at the middle height of the CCD, and steer M2 to center the reverse beam on Iris A. Repeat this process several times until both criteria are met.</li>
		<li>Alignment of lens pair L3 and L4: Position L3 in the PL collection path at a location that is f2 + f3 away from lens L2. Place L4 into the collection path separated from L3 by the sum of their focal lengths, f3 + f4. Adjust the lateral position of L4 to center the PL spot on the CCD.</li>
	</ol>
	</li>
</ol>
4. Overlap of the PL Collection Path with respect to the Resonant Excitation Path
<ol>
	<li>Cool down the sample to 4.2 K. With the above-band excitation, use the spectrometer to pinpoint the emission wavelength of the wetting layer (typically around 880 nm).</li>
	<li>Set up an 800 nm long-pass filter F1 in front of Lcam to block the HeNe light. With the help of the illuminating light, shift L2 horizontally to locate the cleaved edge of sample on the camera.</li>
	<li>Set the side excitation wavelength to be resonant with the wetting layer. Locate a bright scattering spot at the cleaved edge of the sample on the camera.</li>
	<li>Observe a &#34;streak pattern&#34; of photoluminescence on the camera by adjusting the lateral position of E1. Maximize the intensity of the streak by shifting E1 laterally. 
	NOTE: The &#34;streak&#34; is the wetting layer emission, which implies that the excitation beam is coupled into the waveguide of the sample.</li>
	<li>Adjust E1 vertically to move the streak to overlap with the PL spot caused by the HeNe excitation.</li>
	<li>Record the intensity of the wetting layer PL. Adjust E2 in one direction, then re-optimize the position of E1; again record the intensity of the PL and compare to the earlier value.</li>
	<li>If the intensity has increased, repeat the adjustment of E2 in the same direction. If the intensity has decreased, then reverse the adjustment of E2. Repeat this procedure to find the optimum positions for both E1 and E2.</li>
</ol>
5. Resonant Excitation of a Single Quantum Dot
NOTE: There are two possible approaches to realize resonant excitation of a single QD: (1) tune the excitation frequency of the laser to match a specific QD resonance; or (2) scan the laser frequency across the resonance energies of the QD ensemble until resonance fluorescence from a single QD is observed.
<ol>
	<li>Method (1) - Targeted excitation:
	<ol>
		<li>Set the spectrometer to monitor the first-order diffraction at the center of the emission wavelength of the QD ensemble under above band-gap excitation. Open the entrance slit of the spectrometer.</li>
		<li>Adjust the power of the above-band excitation until a glowing background appears due to excitation of the continuum tail of the wetting layer states. Close the entrance slit to 30 &#956;m.</li>
		<li>Shift L2 laterally to find a suitable QD - for example, the brightest one in view. Record the wavelength of the QD&#160;&#955;QD as measured by the spectrometer.</li>
		<li>Tune the wavelength of the resonant excitation laser to be the same value as &#955;QD. 
		NOTE: Often, the spectrometer can pick up the weak signal of the scattering of the resonant excitation laser from the optics. If not, direct a split-off of the excitation beam into the spectrometer.</li>
		<li>Maximize the QD&#39;s PL intensity at on the CCD by fine tuning the frequency of the excitation laser. 
		NOTE: For some QDs, a small amount of HeNe light is needed to allow the QD to be resonantly excited10,31,32. The required HeNe laser power is usually so low - a few hundred nanowatts - that no fluorescence caused solely by this HeNe beam can be detected by the CCD.</li>
		<li>Maximize the PL intensity of the QD by adjusting the height and the lateral position of lens E1 and the axial position of lens E2. Jointly optimize the positions of lenses E1 and E2 to maximize the intensity of the resonance fluorescence from the QD.</li>
	</ol>
	</li>
	<li>Method (2) - Spectral searching:
	<ol>
		<li>Set the spectrometer to monitor the first-order diffraction at the center of the emission wavelength of the QD ensemble. Open the entrance slit of the spectrometer.</li>
		<li>Tune the frequency of the excitation laser across the energy range of the QD ensemble. A resonantly excited QD will appear on the CCD as a dot surrounded by couple of Airy rings. Pick a QD that is bright.</li>
		<li>Maximize its PL intensity by fine tuning the wavelength of the excitation laser.</li>
		<li>Maximize the PL intensity of the dot by adjusting the height and the lateral position of E1and the axial position of lens E2. Jointly optimize the positions of lenses E1 and E2 to maximize the intensity of the resonance fluorescence from the QD.</li>
	</ol>
	</li>
</ol>

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M., Ro&#223;bach, R., Jetter, M., Michler, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cascaded+single-photon+emission+from+the+Mollow+triplet+sidebands+of+a+quantum+dot.">Cascaded single-photon emission from the Mollow triplet sidebands of a quantum dot.</a> Nat. Photonics. 6 (4), 238-242 (2012).</li><li>Ulrich, S. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dephasing+of+Triplet-Sideband+Optical+Emission+of+a+Resonantly+Driven+InAs/GaAs+Quantum+Dot+inside+a+Microcavity.">Dephasing of Triplet-Sideband Optical Emission of a Resonantly Driven InAs/GaAs Quantum Dot inside a Microcavity.</a> Phys. Rev. Lett. 106 (24), 247402 (2011).</li><li>Atature, M., Dreiser, J., Badolato, A., Hogele, A., Karrai, K., Imamoglu, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantum-dot+spin-state+preparation+with+near-unity+fidelity.">Quantum-dot spin-state preparation with near-unity fidelity.</a> Science. 312 (5773), 551-553 (2006).</li><li>Kroner, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Resonant+two-color+high-resolution+spectroscopy+of+a+negatively+charged+exciton+in+a+self-assembled+quantum+dot.">Resonant two-color high-resolution spectroscopy of a negatively charged exciton in a self-assembled quantum dot.</a> Phys. Rev. B. 78 (7), 075429 (2008).</li><li>Press, D., Ladd, T. D., Zhang, B., Yamamoto, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Complete+quantum+control+of+a+single+quantum+dot+spin+using+ultrafast+optical+pulses.">Complete quantum control of a single quantum dot spin using ultrafast optical pulses.</a> Nature. 456 (7219), 218-221 (2008).</li><li>Sun, S., Waks, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-shot+optical+readout+of+a+quantum+bit+using+cavity+quantum+electrodynamics.">Single-shot optical readout of a quantum bit using cavity quantum electrodynamics.</a> Physical Review A. 94 (1), 012307 (2016).</li><li>Metcalfe, M., Solomon, G. S., Lawall, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Heterodyne+measurement+of+resonant+elastic+scattering+from+epitaxial+quantum+dots.">Heterodyne measurement of resonant elastic scattering from epitaxial quantum dots.</a> Appl. Phys. Lett. 102 (23), 231114 (2013).</li><li>Nguyen, H. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optically+Gated+Resonant+Emission+of+Single+Quantum+Dots.">Optically Gated Resonant Emission of Single Quantum Dots.</a> Phys. Rev. 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Quantum Elect. 34 (9), 1612-1631 (1998).</li><li>K&#246;hler, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ein+neues+Beleuchtungsverfahren+f&#252;r+mikrophotographische+Zwecke.">Ein neues Beleuchtungsverfahren f&#252;r mikrophotographische Zwecke.</a> Zeitschrift f&#252;r wissenschaftliche Mikroskopie und f&#252;r Mikroskopische Technik. 10 (4), 433-440 (1893).</li><li>K&#246;hler, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=New+method+of+illumination+for+photomicrographical+purposes.">New method of illumination for photomicrographical purposes.</a> Journal Royal Microscopical Society (Great Britain). 14, 261-262 (1894).</li><li>He, Y. -. 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The authors have nothing to disclose.

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

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

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

resonance fluorescence of an ingaas quantum dot in a planar cavity using orthogonal excitation and detection

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

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

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

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

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

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Research

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Engineering

9.1K Views.  West Virginia University. Resonant excitation of a single self-assembled quantum dot can be achieved using an excitation mode orthogonal to the fluorescence collection mode. We demonstrate a method using the waveguide and Fabry-Perot modes of a planar microcavity surrounding the quantum dots. The method allows complete freedom in the detection polarization.

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

Compact Quantum Dots for Single-molecule Imaging

Single-molecule imaging is an important tool for understanding the mechanisms of biomolecular function and for visualizing the spatial and temporal heterogeneity of molecular behaviors that underlie cellular biology 1-4. To image an individual molecule of interest, it is typically conjugated to a fluorescent tag (dye, protein, bead, or quantum dot) and observed with epifluorescence or total internal reflection fluorescence (TIRF) microscopy. While dyes and fluorescent proteins have been the mainstay of fluorescence imaging for decades, their fluorescence is unstable under high photon fluxes necessary to observe individual molecules, yielding only a few seconds of observation before complete loss of signal. Latex beads and dye-labeled beads provide improved signal stability but at the expense of drastically larger hydrodynamic size, which can deleteriously alter the diffusion and behavior of the molecule under study.
Quantum dots (QDs) offer a balance between these two problematic regimes. These nanoparticles are composed of semiconductor materials and can be engineered with a hydrodynamically compact size with exceptional resistance to photodegradation 5. Thus in recent years QDs have been instrumental in enabling long-term observation of complex macromolecular behavior on the single molecule level. However these particles have still been found to exhibit impaired diffusion in crowded molecular environments such as the cellular cytoplasm and the neuronal synaptic cleft, where their sizes are still too large 4,6,7.
Recently we have engineered the cores and surface coatings of QDs for minimized hydrodynamic size, while balancing offsets to colloidal stability, photostability, brightness, and nonspecific binding that have hindered the utility of compact QDs in the past 8,9. The goal of this article is to demonstrate the synthesis, modification, and characterization of these optimized nanocrystals, composed of an alloyed HgxCd1-xSe core coated with an insulating CdyZn1-yS shell, further coated with a multidentate polymer ligand modified with short polyethylene glycol (PEG) chains (Figure 1). Compared with conventional CdSe nanocrystals, HgxCd1-xSe alloys offer greater quantum yields of fluorescence, fluorescence at red and near-infrared wavelengths for enhanced signal-to-noise in cells, and excitation at non-cytotoxic visible wavelengths. Multidentate polymer coatings bind to the nanocrystal surface in a closed and flat conformation to minimize hydrodynamic size, and PEG neutralizes the surface charge to minimize nonspecific binding to cells and biomolecules. The end result is a brightly fluorescent nanocrystal with emission between 550-800 nm and a total hydrodynamic size near 12 nm. This is in the same size range as many soluble globular proteins in cells, and substantially smaller than conventional PEGylated QDs (25-35 nm).

We describe the preparation of colloidal quantum dots with minimized hydrodynamic size for single-molecule fluorescence imaging. Compared to conventional quantum dots, these nanoparticles are similar in size to globular proteins and are optimized for single-molecule brightness, stability against photodegradation, and resistance to nonspecific binding to proteins and cells.

Single-molecule imaging is an important tool for  understanding the mechanisms of biomolecular function and for visualizing the  spatial and temporal ...

Rejection of Fluorescence Background in Resonance and Spontaneous Raman Microspectroscopy

Raman spectroscopy is often plagued by a strong fluorescent background, particularly for biological samples. If a sample is excited with a train of ultrafast pulses, a system that can temporally separate spectrally overlapping signals on a picosecond timescale can isolate promptly arriving Raman scattered light from late-arriving fluorescence light. Here we discuss the construction and operation of a complex nonlinear optical system that uses all-optical switching in the form of a low-power optical Kerr gate to isolate Raman and fluorescence signals. A single 808 nm laser with 2.4 W of average power and 80 MHz repetition rate is split, with approximately 200 mW of 808 nm light being converted to &lt; 5 mW of 404 nm light sent to the sample to excite Raman scattering. The remaining unconverted 808 nm light is then sent to a nonlinear medium where it acts as the pump for the all-optical shutter. The shutter opens and closes in 800 fs with a peak efficiency of approximately 5%. Using this system we are able to successfully separate Raman and fluorescence signals at an 80 MHz repetition rate using pulse energies and average powers that remain biologically safe. Because the system has no spare capacity in terms of optical power, we detail several design and alignment considerations that aid in maximizing the throughput of the system. We also discuss our protocol for obtaining the spatial and temporal overlap of the signal and pump beams within the Kerr medium, as well as a detailed protocol for spectral acquisition. Finally, we report a few representative results of Raman spectra obtained in the presence of strong fluorescence using our time-gating system.

We discuss the construction and operation of a complex nonlinear optical
system that uses ultrafast all-optical switching to isolate Raman from fluorescence
signals. Using this system we are able to successfully separate Raman and fluorescence
signals utilizing pulse energies and average powers that remain biologically safe.

Raman spectroscopy is often plagued by a strong fluorescent background, particularly for biological samples. If a sample is excited with a train of ...

Bioengineering

Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing

This paper discusses fluorescent core microcavity-based sensors that can operate in a microfluidic analysis setup. These structures are based on the formation of a fluorescent quantum-dot (QD) coating on the channel surface of a conventional microcapillary. Silicon QDs are especially attractive for this application, owing in part to their negligible toxicity compared to the II-VI and II-VI compound QDs, which are legislatively controlled substances in many countries. While the ensemble emission spectrum is broad and featureless, an Si-QD film on the channel wall of a capillary features a set of sharp, narrow peaks in the fluorescence spectrum, corresponding to the electromagnetic resonances for light trapped within the film. The peak wavelength of these resonances is sensitive to the external medium, thus permitting the device to function as a refractometric sensor in which the QDs never come into physical contact with the analyte. The experimental methods associated with the fabrication of the fluorescent-core microcapillaries are discussed in detail, as well as the analysis methods. Finally, a comparison is made between these structures and the more widely investigated liquid-core optical ring resonators, in terms of microfluidic sensing capabilities.

Fluorescent-core microcavity sensors employ a high-index quantum-dot coating in the channel of silica microcapillaries. Changes in the refractive index of fluids pumped into the capillary channel cause shifts in the microcavity fluorescence spectrum that can be used to analyze the channel medium.

This paper discusses  fluorescent core microcavity-based sensors that can operate in a microfluidic  analysis setup. These structures are based on the ...

Nanofabrication of Gate-defined GaAs/AlGaAs Lateral Quantum Dots

A quantum computer is a computer composed of quantum bits (qubits) that takes advantage of quantum effects, such as superposition of states and entanglement, to solve certain problems exponentially faster than with the best known algorithms on a classical computer. Gate-defined lateral quantum dots on GaAs/AlGaAs are one of many avenues explored for the implementation of a qubit. When properly fabricated, such a device is able to trap a small number of electrons in a certain region of space. The spin states of these electrons can then be used to implement the logical 0 and 1 of the quantum bit. Given the nanometer scale of these quantum dots, cleanroom facilities offering specialized equipment- such as scanning electron microscopes and e-beam evaporators- are required for their fabrication. Great care must be taken throughout the fabrication process to maintain cleanliness of the sample surface and to avoid damaging the fragile gates of the structure. This paper presents the detailed fabrication protocol of gate-defined lateral quantum dots from the wafer to a working device. Characterization methods and representative results are also briefly discussed. Although this paper concentrates on double quantum dots, the fabrication process remains the same for single or triple dots or even arrays of quantum dots. Moreover, the protocol can be adapted to fabricate lateral quantum dots on other substrates, such as Si/SiGe.

This paper presents a detailed fabrication protocol for gate-defined semiconductor lateral quantum dots on gallium arsenide heterostructures. These nanoscale devices are used to trap few electrons for use as quantum bits in quantum information processing or for other mesoscopic experiments such as coherent conductance measurements.

A quantum computer is a computer composed of quantum bits (qubits) that takes advantage of quantum effects, such as superposition of states and ...

Measurement of Coherence Decay in GaMnAs Using Femtosecond Four-wave Mixing

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, spin-flip scattering) is described, using experiments on GaMnAs as a prototype III-Mn-V system.&#160; Spectrally-resolved and time-resolved experimental configurations are described, including the use of zero-background autocorrelation techniques for pulse optimization.&#160; The etching process used to prepare GaMnAs samples for four-wave mixing experiments is also highlighted.&#160; The high temporal resolution of this technique, afforded by the use of short (20 fsec) optical pulses, permits the rapid spin-flip scattering process in this system to be studied directly in the time domain, providing new insight into the strong exchange coupling responsible for carrier-mediated ferromagnetism.&#160; We also show that spectral resolution of the four-wave mixing signal allows one to extract clear signatures of (s,p)-d hybridization in this system, unlike linear spectroscopy techniques.&#160;&#160; This increased sensitivity is due to the nonlinearity of the technique, which suppresses defect-related contributions to the optical response. This method may be used to measure the time scale for coherence decay (tied to the fastest scattering processes) in a wide variety of semiconductor systems of interest for next generation electronics and optoelectronics.

The technique of femtosecond four-wave mixing is described, including spectrally-resolved and time-resolved configurations. We illustrate the utility of this technique for the investigation of crucial physical properties in the III-V diluted magnetic semiconductors, afforded by its nonlinearity and high temporal resolution.

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, ...

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 ...

Quantum State Engineering of Light with Continuous-wave Optical Parametric Oscillators

Engineering non-classical states of the electromagnetic field is a central quest for quantum optics1,2. Beyond their fundamental significance, such states are indeed the resources for implementing various protocols, ranging from enhanced metrology to quantum communication and computing. A variety of devices can be used to generate non-classical states, such as single emitters, light-matter interfaces or non-linear systems3. We focus here on the use of a continuous-wave optical parametric oscillator3,4. This system is based on a non-linear &#967;2 crystal inserted inside an optical cavity and it is now well-known as a very efficient source of non-classical light, such as single-mode or two-mode squeezed vacuum depending on the crystal phase matching. 
Squeezed vacuum is a Gaussian state as its quadrature distributions follow a Gaussian statistics. However, it has been shown that number of protocols require non-Gaussian states5. Generating directly such states is a difficult task and would require strong &#967;3 non-linearities. Another procedure, probabilistic but heralded, consists in using a measurement-induced non-linearity via a conditional preparation technique operated on Gaussian states. Here, we detail this generation protocol for two non-Gaussian states, the single-photon state and a superposition of coherent states, using two differently phase-matched parametric oscillators as primary resources. This technique enables achievement of a high fidelity with the targeted state and generation of the state in a well-controlled spatiotemporal mode.

We describe the reliable generation of non-Gaussian states of traveling optical fields, including single-photon states and coherent state superpositions, using a conditional preparation method operated on the non-classical light emitted by optical parametric oscillators. Type-I and type-II phase-matched oscillators are considered and common procedures, such as the required frequency filtering or the high-efficiency quantum state characterization by homodyning, are detailed.

Engineering non-classical states of the electromagnetic field is a central quest for quantum optics1,2. Beyond their fundamental significance, such states ...

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 ...

Determination of the Excitation and Coupling Rates Between Light Emitters and Surface Plasmon Polaritons

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising from metallic periodic arrays without involving time-resolved techniques. We have formulated the rates by quantities that can be measured by simple optical measurements. The instrumentation based on angle- and polarization-resolved reflectivity and photoluminescence spectroscopy will be described in detail here. Our approach is intriguing due to its simplicity, which requires routine optics and several mechanical stages, and thus is highly affordable to most of the research laboratories.

This protocol describes the instrumentation for determining the excitation and coupling rates between light emitters and Bloch-like surface plasmon polaritons arising from periodic arrays.

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising ...

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 ...