The authors would like to acknowledge Allan Bracker and Daniel Gammon at the Naval Research Laboratory for providing the samples being studied. This work was supported (in part) by the Defense Threat Reduction Agency, Basic Research Award # HDTRA1-15-1-0011, to University of California-Merced.

Note: The methodology described is specific to a particular software, although other software packages may be used instead.
1. Sample Preparation and Cool Down
<ol>
	<li>Fabricate the sample.
	<ol>
		<li>Grow the sample, using the Stranski-Krastanov growth method via molecular beam epitaxy creating two vertically-stacked self-assembled InAs/GaAs QDs that are separated by a 4 nm tunnel barrier as previously described.16 Embed the QDs in an electric field effect structure (i.e.<...

<ol>
	<li>Germanis, 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=Piezoelectric+InAs/GaAs+quantum+dots+with+reduced+fine-structure+splitting+for+the+generation+of+entangled+photons.">Piezoelectric InAs/GaAs quantum dots with reduced fine-structure splitting for the generation of entangled photons.</a> Phys. Rev. B. 86, 1-4 (2012).</li><li>Valenti, J. A., Fischer, D. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spectroscopic+Properties+of+Cool+Stars+(SPOCS).+I.+1040+F,+G,+and+K+Dwarfs+from+Keck,+Lick,+and+AAT+Planet+Search+Programs.">Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs.</a> ApJ. 159, 141-166 (2005).</li><li>Oetiker, B., 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=Searching+for+Companions+to+Late+Type+M+Stars.">Searching for Companions to Late Type M Stars.</a> .Astro. Soc. Pac. Conf. Ser. 212, (2000).</li><li>Seguin, R., Rodt, S., Schliwa, A., Potschke, K., Pohl, U. W., Bimberg, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Size-dependence+of+anisotropic+exchange+interaction+in+InAs/GaAs+quantum+dots.">Size-dependence of anisotropic exchange interaction in InAs/GaAs quantum dots.</a> Phys. Status Solidi B. 243 (15), 3937-3941 (2006).</li><li>Belhadj, T., 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=Controlling+the+Polarization+Eigenstate+of+a+Quantum+Dot+Exciton+with+Light.">Controlling the Polarization Eigenstate of a Quantum Dot Exciton with Light.</a> Phys. Rev. Lett. 103 (1-4), (2009).</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=Control+of+single+quantum+dot+emission+characteristics+and+fine+structure+by+lateral+electric+fields.">Control of single quantum dot emission characteristics and fine structure by lateral electric fields.</a> Phys. Status Solidi B. 246 (2), 302-306 (2009).</li><li>Vamivakas, A. N., 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=Observation+of+spin-dependent+quantum+jumps+via+quantum+dot+resonance+fluorescence.">Observation of spin-dependent quantum jumps via quantum dot resonance fluorescence.</a> Nature. 467, 297-300 (2010).</li><li>Poem, E., 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=Polarization+sensitive+spectroscopy+of+charged+quantum+dots.">Polarization sensitive spectroscopy of charged quantum dots.</a> Phys. Rev. B. 76, (2007).</li><li>Flagg, E. B., 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=Resonantly+driven+coherent+oscillations+in+a+solid-state+quantum+emitter.">Resonantly driven coherent oscillations in a solid-state quantum emitter.</a> Nature Phys. 5, 203-207 (2009).</li><li>Scheibner, M., Bacher, G., Forchel, A., Passow, T., Hommel, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spin+Dynamics+in+CdSe/ZnSe+Quantum+Dots:+Resonant+vesus+Nonresonant+Excitation.">Spin Dynamics in CdSe/ZnSe Quantum Dots: Resonant vesus Nonresonant Excitation.</a> J. Supercond. Nov. Magn. 16 (2), 395-398 (2003).</li><li>Faelt, S., Atature, M., Tureci, H. E., Zhao, Y., Badolato, A., 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=Strong+electron-hole+exchange+in+coherently+coupled+quantum+dots.">Strong electron-hole exchange in coherently coupled quantum dots.</a> Phys. Rev. Lett. 100, 1-4 (2008).</li><li>Vamivakas, A. N., 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=Strong+Extinction+of+a+Far-Field+Laser+Beam+by+a+Single+Quantum+Dot.">Strong Extinction of a Far-Field Laser Beam by a Single Quantum Dot.</a> Nano Letters. 7 (9), 2892-2896 (2007).</li><li>Scheibner, M., Economou, S., Ponomarev, I. V., Jennings, C., Bracker, A., Gammon, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two-Photon+Absorption+by+a+Quantum+Dot+Pair.">Two-Photon Absorption by a Quantum Dot Pair.</a> Phys. Rev. B. 92, (2015).</li><li>Palik, E. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Handbook of Optical Constants of Solids. Vols. I and II. , (1985).</li><li>Kerfoot, M. L., 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=Optophononics+with+Coupled+Quantum+Dots.">Optophononics with Coupled Quantum Dots.</a> Nat. Commun. 5, 1-6 (2013).</li><li>Scheibner, M., Bracker, A. S., Kim, D., Gammon, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Essential+concepts+in+the+optical+properties+of+quantum+dot+molecules.">Essential concepts in the optical properties of quantum dot molecules.</a> Solid State Commun. 149, 1427-1435 (2009).</li><li>Bracker, A. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Engineering+electron+and+hole+tunneling+with+asymmetric+InAs+quantum+dot+molecules.">Engineering electron and hole tunneling with asymmetric InAs quantum dot molecules.</a> Appl. Phys. Lett. 89, 1-3 (2006).</li><li>Doty, M. F., 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=Electrically+Tunable+g+Factors+in+Quantum+Dot+Molecular+Spin+States.">Electrically Tunable g Factors in Quantum Dot Molecular Spin States.</a> Phys. Rev. Lett. 97, 1-4 (2006).</li><li>Stinaff, E. A., 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=Optical+Signatures+of+Coupled+Quantum+Dots.">Optical Signatures of Coupled Quantum Dots.</a> Science. 311, 636-639 (2006).</li><li>Tkachenko, N. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Optical Spectroscopy: Methods and Instrumentations. , (2006).</li><li>Hecht, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Optics. , (2014).</li><li>O'Donnell, K. P., Chen, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Temperature+dependence+of+semiconductor+band+gaps.">Temperature dependence of semiconductor band gaps.</a> Appl. Phys. Lett. 58, 2924-2926 (1991).</li><li>Stinaff, E. A., 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=Polarization+dependent+photoluminescence+of+charged+quantum+dot+molecules.">Polarization dependent photoluminescence of charged quantum dot molecules.</a> Phys. Stat. Sol. (c). 5 (7), 2464-2468 (2008).</li><li>Jelezko, F., Wrachtrup, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single+defect+centres+in+diamond:+A+review.">Single defect centres in diamond: A review.</a> Phys. Stat. Sol. (a). 203 (13), 3207-3225 (2006).</li><li>Doherty, M. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+nitrogen-vacancy+colour+centre+in+diamond.">The nitrogen-vacancy colour centre in diamond.</a> Physics Reports. 528 (1), 1-45 (2013).</li></ol>

The authors have nothing to disclose.

The above instructions demonstrate the phonon-assisted quasi-resonance spectroscopy method. By exciting into a QD discrete state, one can monitor the phonon emission line, achieving high resolutions. In the example provided, by using phonons it is even possible to resolve the lifetime-limited linewidth of the neutral exciton visible in experiments. The method is easy to incorporate into existing PL spectroscopy setups. As mentioned, once the energy of the desired transition line is identified via non-resonant spectroscop...

High resolution is the key to unlocking new knowledge. With this knowledge, new technologies can be developed such as better sensors, more precise manufacturing tools, and more efficient computational devices. Generating this key, however, often comes at a high cost of resources, time or both. This issue is omnipresent across all scales from the atomic physics of resolving the lifted degeneracies of electron spins to astronomy where a small spectral shift can lead to detection of planets next to distant stars.1,2,3 
The focus of this work is on using a standard spectrometer setup and showing how it can resolve spectral features below its resolution limit, especially with regard to the field of semiconductor optics. The example presented is that of anisotropic electron-hole (e-h) exchange splitting in InAs/GaAs quantum dots (QDs), which is on the order of a few &#181;eV.4 The resolution limit of the spectrometer can be overcome by combining standard PL and laser spectroscopy techniques. This method of quasi-resonance fluorescence has the added benefit of achieving laser limited resolution using a commonplace single-stage spectrometer.
A standard optical spectroscopy system for single QD PL spectroscopy consists of a single-stage 0.3-0.75 m monochromator and a charge coupled device (CCD) detector along with an excitation laser source and optics. Such a system is at best capable of resolving 50 &#181;eV in the near-infrared spectrum around 950 nm. Even with the use of statistical and deconvolution techniques, such a single monochromator setup is not capable of resolving less than 20 &#181;eV in PL measurements.5 This resolution can also be improved by using a triple spectrometer, in triple additive mode, where the spectrum is successively dispersed by all three gratings. The triple spectrometer has the advantage of increased resolution, capable of resolving around 10 &#181;eV. In an alternative configuration, triple subtractive mode, the first two gratings behave as a band pass filter, giving the added feature of being able to separate the excitation and detection by less than 0.5 meV. The drawback of the triple spectrometer is that it is a costly system.
Before presenting the method of interest, we briefly discuss other experimental approaches that, with added complexity, achieve better spectral resolution and are able to resolve the fine structure of single QDs. Elements of these methods are relevant to the presented method. One such method is adding a Fabry-Perot interferometer (FPI) in the detection path of a single spectrometer setup.6 Using this method the resolution is set by the finesse of the FPI. Thus, the spectrometer&#39;s resolution is improved to 1 &#181;eV, at the cost of added complexity and lower signal intensity.7 The interferometer method also changes the general operation of the spectrometer with the CCD camera, effectively becoming a single point detector, and the tuning through various energies is achieved by adjusting the FPI cavity itself.
Resonance fluorescence (RF) spectroscopy, another method where a single optical transition is both excited and monitored also offers the promise of high-resolution spectroscopy. The spectral resolution is only limited by the laser linewidth and keeps the CCD as a multi-channel detector, where not just one sensor is detecting the signal but a number of CCD pixels. This multichannel detection is advantageous in terms of signal averaging. The challenge in RF spectroscopy is separating the PL signal from the larger background of the scattered laser light, especially when measuring at the single QD level. A number of techniques can be used to lower the ratio of signal to scattered laser light, which involve either polarization8, spatial9 or temporal separation10 of the excitation and detection. The first is to use high extinction polarizers to suppress the scattered light, but this method has the unfavorable outcome of losing polarization information from the PL.8 Another possible method to obtain resonance fluorescence is to engineer semiconductor systems that are coupled to optical cavities where the excitation and detection paths are spatially separated. This eliminates the issue of having to resolve the PL signal from the large laser background. However, this method is limited to intricate sample fabrication which is in general resource intensive.9
Another class of methods that is also able to resolve minute energy differences is that of pure laser spectroscopy, such as differential transmission, which has the benefit of achieving laser-limited resolution with complete polarization information. This method typically requires lock-in detection to observe miniscule changes in the transmission signal compared to that of the large laser background.11 Lately, advances in nanofabrication have led to a boost of the fraction of laser light that interacts with the QD(s) to values up to 20%, by either using index-matched solid immersion lenses or embedding the dots in photonic crystal waveguides.12
Even though these methods have the capability of achieving high energy resolution, they come at the cost of expensive equipment, complex sample fabrication and loss of information. The method in this work combines elements from these three methods without adding complexity in instrumentation or sample fabrication to a regular PL setup.
Recent work has shown that with a triple spectrometer system in subtractive mode, it is possible to visualize the singlet-triplet fine structure in the two-photon transition spectrum of a quantum dot molecule (QDM).13 The involved energy splitting on the order of a few to tens of &#956;eV were resolved using a triple subtractive mode, which allowed to excite the transitions resonantly and detect within less than a meV. The spectral information was extracted by monitoring below the transition using acoustic phonons and other lower-lying exciton transitions. This method can also be applied to resolve the anisotropic e-h exchange splitting and even the lifetime-limited linewidth of the exciton transition of 8 &#181;eV and 4 &#181;eV, respectively as seen in Figure 1. Similar to this result, this paper will focus on a simple spectrometer setup that will incorporate many of the advantages that the other high resolution methods possess. Additionally the CCD will remain as a multi-channel detector. The experimental setup can also be kept fairly inexpensive relative to other high-resolution spectroscopy methods and has the added benefit of being easily modified to achieve single point correlation measurements. Unlike the result using acoustic phonons and a triple spectrometer, the underlying key is to make use of the LO-phonon satellite associated with the semiconductors and related alloys that make up semiconductor samples. The energy separation between LO-phonon satellite and the zero-phonon line (ZPL) is on the order of tens of meV for such samples, allowing the use of a single-stage spectrometer.14 This energy separation allows for use of the proposed quasi-resonance spectroscopy method by resonantly driving a transition and monitoring below the excitation by an energy equal to one LO phonon. This technique is analogous to that of PL excitation where one excites into an excited transition and monitors the ground state transition.15 The separation between the transition being excited and that of the LO-phonon satellite allows for the use of edge pass filters to suppress the elastically scattered light. This method of using the phonon satellite allows for laser linewidth limited resolution, since resonantly exciting the transition is typically the only time that the LO-phonon satellite emission becomes visible.

<table border="0" cellpadding="0" cellspacing="0" width="1542">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">Tunable Diode Laser DL pro</td>
			<td style="width:399px;">Toptica Photonics</td>
			<td style="width:459px;">DL Pro</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">Closed Cycle Cryogen Free Refrigerator System for Microscopy</td>
			<td style="width:399px;">Cryo Industries of America Inc.</td>
			<td style="width:459px;">Cryocool G2</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">Sourcemeter</td>
			<td style="width:399px;">Keithley&#160;</td>
			<td style="width:459px;">2611a</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">50x Mitutoyo Plan Apo NIR Infinity-Corrected Objective</td>
			<td style="width:399px;">Mitutoyo America Corporation</td>
			<td style="width:459px;">378-825-5</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">Turbo pump</td>
			<td style="width:399px;">Pfeiffer Vacuum</td>
			<td style="width:459px;">HiPace 80</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">NIR coated Mirrors&#160;</td>
			<td style="width:399px;">Thor labs</td>
			<td style="width:459px;">BB1-E03</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">Polarizers&#160;</td>
			<td style="width:399px;">Thorlabs</td>
			<td style="width:459px;">LPNIR050-MP</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">200mm AR coated Achromatic lens&#160;</td>
			<td style="width:399px;">Thorlabs</td>
			<td style="width:459px;">AC254-200-B-ML</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">100mm AR coated Achromatic lens&#160;</td>
			<td style="width:399px;">Thorlabs</td>
			<td style="width:459px;">AC254-100-B-ML</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">960 Long pass filter</td>
			<td style="width:399px;">Thorlabs</td>
			<td style="width:459px;">960aelp</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">960 Short pass filter</td>
			<td style="width:399px;">Thorlabs</td>
			<td style="width:459px;">960aesp</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">Liquid Crystal Variable Retarder</td>
			<td style="width:399px;">Meadowlark Optics</td>
			<td style="width:459px;">LVR-100</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">0.75m Spectrometer Acton SpectraPro</td>
			<td style="width:399px;">Princeton Instruments</td>
			<td style="width:459px;">Trivista</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">Liquid Nitrogen Cooled Camera&#160;</td>
			<td style="width:399px;">Princeton Instruments</td>
			<td style="width:459px;">7508-0002</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">External Camera</td>
			<td style="width:399px;">Watec</td>
			<td style="width:459px;">Wat-902H Ultimate</td>
			<td>Optional</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">Ostoalloy</td>
			<td style="width:399px;">Lake Shore Cryotronics</td>
			<td style="width:459px;">Ostalloy 158</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">Gold wire (40 gauge)</td>
			<td style="width:399px;">Surepure Chemetals</td>
			<td style="width:459px;">Au-Wire-03-02</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">Silver Epoxy</td>
			<td style="width:399px;">A.I. Technology</td>
			<td style="width:459px;">Prima-Solder EG8020</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:517px;">Program Software&#160;</td>
			<td style="width:399px;">National Instruments&#160;</td>
			<td style="width:459px;">LabView</td>
			<td></td>
		</tr>
	</tbody>
</table>

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 results presented in the figures show the high resolution capabilities of using phonons to assist in the PL measurement. The schematic (Figure 2) shows that, with the exception of the edge pass filters on both excitation and detection, the experimental setup remains a standard spectroscopy setup, with the optional addition of polarization control. Comparison with a single and triple spectrometer (Figure. 3) portrays the phonon-assisted method&#39;s gr...

Watch this Scientific Journal Video about High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy at JoVE.com

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy

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 ...

The overall goal of Phonon-assisted Quasi-resonance Florescence Spectroscopy is to achieve laser linewidth limited resolution of optical spectra of atom-like semiconductor structures with a standard photoluminescent Spectroscopy setup. This method facilitates study in semiconductor optical Spectroscopy related to zero dimensional systems, such as quantum dots, nitrogen-vacancy centers in diamond, and defects in silicon carbide. The main advantage of this technique is that it has the ability to obtain high-resolution optical spectra of energy features less than 10 micro-electron volts, while retaining polarization information.The implications of this technique extend the power of optical Spectroscopy due to it's ability to obtain high-resolution optical spectra. Individuals new to the method will struggle with finding the signal. The greatest hurdle is using a single spectrometer and extinction filter to eliminate stray laser light.Start with the material or device that will be used in the experiment. This device has quantum dot molecules mounted onto a chip. An overview of the quantum dot molecules is provided in this schematic.There are two indium arsenide quantum dot layers that are embedded between the gallium arsenide layers. Each quantum dot can be optically addressed. The quantum dot molecules are in the intrinsic region of a Schottky diode, allowing application of an electric field.The first step is to mount the device onto the cryostat cold finger for cool down. Before mounting the chip on the cold finger, apply indium foil for the chip to rest on. Next, put the chip in position on top of the foil.Use screws and washers at each end of the device to firmly tighten it to the cold finger and ensure good thermal contact. Continue by attaching the devices electrode wires to the contacts in the cryostat that connect to a source meter. Now mount the cryostat on an XYZ Translation Stage and make the sample accessible to optical probes.In this case, the translation stage is already in the experiment's optical path. Evacuate the cryostat before cooling it down to the desired temperature. Once the sample has cooled, begin to work with the optical elements.For photoluminescence, have a long working distance objective and collimation lens in place to capture light from the sample. From there, use mirrors to direct the light onto a single-stage spectrometer. Employ a nitrogen cooled CCD camera to detect the spectra.In this experiment, we use the last stage of a triple spectrometer. For alignment purposes, mount a white light source near the sample. Illuminate the sample with the source before continuing.Employ a separate camera to capture the image of the sample in the spectrometer. On the bench, work to properly align the collimating and focusing lenses to sharpen the image of the sample. Conclude alignment efforts only when a clean, focused image appears in the camera.After removing the light source, prepare a laser to excite the sample. Use a Tunable Diode Laser set to an energy above the ground state transitions. Direct the laser onto the sample at an oblique angle to reduce detecting scattered light.Employ a lens to focus the beam spot to the smallest size possible on the region captured by the camera. An overview of the setup at this point is given in this schematic. Note the oblique angle of the laser light onto the sample for noise reduction.Use the laser to excite a higher, non-resonant energy, and run the spectrum acquisition software in focus mode to observe the signal. Use the sample Housing Translation Stage to scan the sample through the laser spot. When the CCD captures the discreet lines of the ground state transitions, stop the scan.Concentrate on one quantum dot molecule, and optimize the signal by fine-tuning the laser beam's position. The next step is to generate a Bias map using lap control software. The software first applies a Bias potential across the sample electrodes with a source meter.It then records the spectrum associated with that Bias value. The map is generated by incrementing the Bias potential at regular intervals, recording the energy spectrum at each Bias value, and using the data to produce this representation. Given a Bias map, identify the transition that will be excited.Note the transition Energy and the Bias range of interest. In this figure, the transition is stepped through by scanning the temperature of the sample. A detection signal will be present when the laser is resonant with the transition.Use the chosen transition to determine edge filters for the optical setup. The transition energy justifies a 960 nanometer short pass filter for the excitation branch. A longitudinal optical phonon emission energy for the semiconductor alloy justifies a 960 nanometer long pass filter for the detection branch.The type of filter in the setup can make later steps easier. Interference cut-off filters are ideal because they can be tuned by adjusting the angle. At the computer, set the laser excitation energy and the minus one longitudinal optical phonon center frequency.Start the CCD, collecting spectra in continuous mode. The signal may be hidden by scatter at this point. In order to maximize the signal, return to the bench.There, tune the short pass filter by adjusting it's angle to have the proper wavelength cut-off. Monitor the signal for feedback to determine the best angle. In the lab control software, set the Laser energy to scan a range centered on the transition energy, and set the range of Bias voltages to scan over.This Bias map was created by setting the laser energy, varying the Bias over it's range to collect a spectrum, then repeating this over the entire range of laser energies. By collecting data on the background concurrently, the two can be used during post-processing to remove spurious signals and create an improved image. As seen in these schematics, the experimental setup for the Phonon-assisted Spectroscopy method is almost the same as for standard Spectroscopy.The only difference is the presence of edge filters on both the detection and excitation beam paths in the Phonon-assisted method. To compare this technique with others, consider the resolution of a neutral exciton. This data is from a single spectrometer with non-resident excitation around 918 nanometers.The spectral resolution is about 26 micro-electron volts per pixel, and the electron-hole exchange splitting cannot be resolved. When the same spectral region is studied with a spectrometer in triple additive mode, the resolution is about 10 micro-electron volts. The electron-hole splitting is beginning to be resolved.Using the Quasi-resonant Phonon-assisted Spectroscopy method in the same spectral region, the resolution is laser limited, giving better than one micro-electron volt resolution, and the two peaks are well resolved. The red curve is the result of a double-Lorentzian fit. It suggests an isotropic electron-hole exchange splitting of about 23.3 micro-electron volts.Once mastered, this technique can be done in the matter of a couple of hours. While attempting this procedure, it's important to remember that after obtaining photoluminescence signal, the key to a good spectrum is being able to separate the laser from the photoluminescence. Following this procedure, other additions can be made to the setup, such as adding polarization analyzers to both the excitation and detection path.After it's development, this technique paved the way for researchers in the field of Semiconductor Physics to further explore zero dimensional systems, such as quantum dots. After watching this video, you should have a very good understanding of how to use this Phonon-assisted Quasi-resonance Fluorescence Spectroscopy method in order to obtain very high-resolution optical spectra of atom-like solid state systems. Don't forget that working with lasers can be extremely hazardous, and precautions such as proper safety gear should be worn at all times when performing these experiments.

high-resolution-phonon-assisted-quasi-resonance-fluorescence

Research

JoVE Journal

Engineering

7.4K Views.  University of California, Merced. The overall goal of Phonon-assisted Quasi-resonance Florescence Spectroscopy is to achieve laser linewidth limited resolution of optical spectra of atom-like semiconductor structures with a standard photoluminescent Spectroscopy setup. This method facilitates study in semiconductor optical Spectroscopy related to zero dimensional systems, such as quantum dots, nitrogen-vacancy centers in diamond, and defects in silicon carbide. The main advantage of this technique is that it has the ability t...