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

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

<ol>
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The 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). ...

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

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9.1K Views.  West Virginia University. The overall goal of this protocol is to achieve resonant excitation of a quantum dot and simultaneous fluorescence detection using orthogonal excitation and detection modes. The realization of resonant excitation with fluorescence detection is essential for studying light matter interactions in low dimensional nano structures and it has been used to illustrate quantum optical effects such as dress states. The main advantage of this technique is that it uses orthogonality between the excitatio...