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.
