1. Measuring the Anti-Stokes Luminescence of CdSe/ZnS Quantum Dots for Laser Cooling Applications
aNaval Surface Warfare Center, Carderock Division, 9500 MacArthur Blvd, West Bethesda, MD 20817
bU.S. Naval Academy, Physics Department, Annapolis, MD 21402
Distribution A : Approved for Public Release

2. Laser Cooling of Solids
Anti-Stokes Emission
Pump Laser
In 1929, German physicist Peter Pringsheim proposed that anti-Stokes emission can lead to the cooling of bulk matter.
Anti-Stokes emission results in photons of shorter wavelength (higher photon energy) than that of the exciting light due to thermal absorption.

3. Phonons
In solids, the thermal energy is stored as phonons, i.e., vibrational quanta of heat.
Two types of phonons:
Longitudinal: Phonons vibrate in the direction of the propagation
Transverse: Phonons vibrate normal to the propagation direction
If the two atoms carry opposite charges, it is possible to excite optical modes of lattice vibrations.
Longitudinal Phonon
Transverse Phonon

4. Requirements for Laser Cooling
Optical quantum efficiency must be close to 1.
Strong phonon-electron coupling.
Minimum reabsorption.

5. Rare-Earth Ions for Cooling
Laser cooling has been mostly limited to glasses and crystals doped with rare-earth elements.
Nonradiative decay caused by multiphonon emission is suppressed due to the large energy gap between the ground and excited bands.
Theoretical limit of 100 K.

6. Semiconductors Research has expanded to semiconductors owing to their:
More efficient pump light absorption
Potential for lower temperatures (10 K is possible)
Ability to directly integrate the material into electronic and photonic devices
In direct band gap semiconductors, the probability of radiative recombination is very large due to conservation of momentum during electronic transitions. However, there is a very high probability that it will absorb its own emission
In indirect band gap semiconductors nonradiative recombination competes with radiative recombination

7. Laser Cooling of Direct Bandgap Semiconductors
Using lasers to cool a direct bandgap semiconductor, a sample is irradiated with laser light (photons) near the band gap (the long-wavelength tail of the absorption spectrum).
The photon creates an electron-hole pair. The electron and hole absorbs energy from a phonon. When the electron and hole recombine across the band gap a blue-shifted photon is emitted.
By removing these phonons, the material is cooled.

8. Core-Shell Quantum Dots
To improve the efficiency and lower the nonradiative recombination, quantum dots (semiconductors that tightly confine electrons or holes in all three spatial dimensions).
The shells passivate surface trap sites and reduce non-radiative recombination.
To ensure that the emission from the core escapes the QD, the shell material must have a wider bandgap than the core.
CdSe/ZnS can exhibit a near unity quantum efficiency.

9. Group II-VI Semiconductors
Group II-VI semiconductors have strong electron-phonon coupling, which leads to enhanced anti-Stokes upconversion.
Zhang et al. successfully demonstrated 40 K of cooling of CdS nanoribbons.
Group III-V suffer from Weak electron-phonon coupling strengths and low anti-Stokes photon escape owing to the large refractive index

10. Objective/Benefits
Laser cool a direct bandgap semiconducting quantum dot using anti-Stokes optical emission.
Benefits of Laser cooling:
All-solid-state cryocooler that is compact, contains no moving parts, has a high reliability, and does not require the use of cryogenic fluids
Noncontact cooling.

11. Stokes Spectrum of CdSe/ZnS
Stokes spectra were obtained using
Ocean Optics JAZ spectrometer
405-nm LED light source
Bifurcated fiber placed next to the sample.
Medium does not significantly alter the Stokes spectrum.
The only notable difference in the spectra is the phonon sidebands near 680 nm.
The CdSe/ZnS powder produces the most intense phonon sidebands, while CdSe/ZnS in toluene has none.
The results were normalized to the colloidal spectrum.
633 nm emission with 25 nm FWHM

12. CdSe/ZnS Colloidal Anti-Stokes Spectra
CdSe/ZnS colloidal anti-Stokes spectra used a 2 mg/mL solution of CdSe/ZnS in toluene.
SPEX 1680B double monochromator was used for the experiment. The entrance and exit slit widths were set to 1.5 mm.
637, 640, 647, 660, or 685 nm OBIS laser with maximum powers of 140, 100, 120, 100, or 40 mW, respectively, were used to produce anti-Stokes luminescence.
A Hamamatsu HC135 photomultiplier tube (PMT) was used to measure the luminescent spectrum.
Typical settings for all anti-Stokes runs were: integration time of 3 s, wavelength range of nm, laser power of 20 mW, and step size of 0.2 nm.

13. Anti-Stokes Results The optimum laser excitation wavelength is 647 nm.
According to Baranov et al., the longitudinal optical phonon (LO) frequencies of CdSe are near 200 and 400 cm–1.
The difference between the peak frequency of the anti-Stokes emission (near 630 nm) and the pump photon frequency is equal to the frequency of 2LO phonons of CdSe when the pump laser wavelength is 647 nm.

14. Anti-Stokes Process for CdSe/ZnS
When we irradiate the material with a laser, the following sequence of events occurs.
The laser excites CdSe level from 0 to 1.
Thermal absorption leads to a transition from level 1 to 2.
The excitons then emit from state 2 back down to level 0 to complete the cycle, i.e., by emitting anti-Stokes fluorescence.
400 cm-1 2
2LO
15973 cm-1 1
Laser
Anti-Stokes
0 cm-1
Based on the emission peak of 630 nm, the optimal laser wavelength would be cm–1 (15973 – 400 cm–1), which converts to nm, overlapping the 647-nm OBIS laser.
Discuss this slide with someone who knows about spectroscopy of semiconductors. It’s not clear. You mix up the words levels and states. The font size isn’t consistent. You don’t explain what these three levels actually are. The diagram has only 3 levels whereas you had 4 levels back on slide 7. The wavenumbers are inconsistently labeled (level 2 has less energy than level 1?). Unless you get these ideas straightened out, you’re going to remain confused about the project. It’s your responsibility to sort it out. (John Check this). I was going to just say this is a possibility and I do not know exactly where the 2LO level is.

15. Laser Cooling Measurements
Laser cooling measurements were performed using the 647-nm OBIS laser at 25 mW.
Two 4 mL samples were investigated.
Pure toluene
Colloidal solution containing 2 mg/mL of CdSe/ZnS to toluene.
The temperature was measured using a Type T thermocouple placed inside the solution.
Measurements were recorded every 20 minutes.

16. Laser Cooling Results
There is no net laser effect for the pure toluene; the upward drift arises from ambient temperature variations in the laboratory.
QD colloid shows that there is a cooling trend over time which could be attributed to anti-Stokes cooling.
After the first hour, the temperature of the solution systematically drops by 2.3°C over the course of the next three hours.
The temperature then equilibrates to within the ambient drift.
Why do you mention evaporation? Explain. Also the font size is inconsistent. I will talk about evaporation during the talk.
Evaporation of 0.1 mL toluene per hour (or 2.5% of the volume per hour) was observed for all solutions

17. Why does it Take so Long to Cool Down?
The reason for the long laser cooling time may be convection within the solution.
The laser beam has a diameter of 4 mm; therefore, the total volume of cooling is only 0.13 mL, i.e., 3% of the entire solution.
As a small volume of solution is cooled, it sinks and a warmer volume rises into the laser path. This creates a convection current, which slowly circulates the solution around the cuvette.
Because the particles in the colloid are undergoing Brownian motion, they are traveling in and out of the laser path randomly.

18. Polymerization of QDs
The desired quantity of quantum dots (between 1 and 8 mg) is dissolved in 1 mL of toluene.
In a separate test tube, 3 mL of methyl methacrylate (MMA) and 0.1% AIBN by weight are thoroughly mixed.
Then, the quantum dot colloidal solution is poured into the test tube containing MMA and AIBN.
This solution is thoroughly mixed and placed in an oven at 90°C for 20 min.
The oven is reduced to 60°C, and the sample is left to polymerize for 20 h.

19. Next Steps Measured the anti-Stokes spectrum of a polymerized QD.
Due to the intense laser reflection and weak anti-Stokes emission, a notch filter is required to reduce the laser emission.
Use noncontact methods of temperature measurements such as photothermal techniques to perform on the polymerized QDs.
Replicate the laser cooling results with toluene and QD colloids with a split laser beam and simultaneous temperature measurement.

20. Conclusions
Anti-Stokes emission of CdSe/ZnS in toluene has been measured.
The optimum laser wavelength for absorption of optical phonons was found to be 647 nm, which corresponds to thermal excitation of the 2LO phonon of CdSe.
The intense anti-Stokes emission from the quantum dots as well as the preliminary 2.3°C reduction in the QD colloidal temperature indicate that this material is a viable candidate for laser cooling.

21. Selected References
Epstein, R. and Sheik-Bahae, M., [Optical Refrigeration: Science and Applications of Laser Cooling of Solids], Wiley-VCH, Weinheim, Germany, Chapter 1 Optical Refrigeration in Solids: Fundamentals and Overview, 1-32 (2009)
Nemova, G. and Kashyap, R., “Laser cooling of solids,” Reports on the Progress of Physics, 73, (2010)
Sheik-Bahae, M. and Epstein, R.I., “Optical refrigeration,” Nature Photonics, 1, (2007)
Edwards, B.C., Buchwald, M.I., and Epstein, R.I., “Development of the Los Alamos solid-state optical refrigerator,” Review of Scientific Instruments, 69(5), (1998)
Zhang, J., Li, D., Chen, R., and Xiong, Q., “Laser cooling of a semiconductor by 40 kelvin”, Nature, 43, (2013).
Pang, L., Shen, Y., Tez, K., and Fainman, Y., “PMMA quantum dots composites fabricated via use of pre-polymerization,” Optics Express, 13(1), (2005)
Baranov, A.V., Rakovich, Y.P., Donegan, J.F., Perova, T.S., Moore, R.A., Talapin, D.V., Rogach, A.L., Masumoto, Y., and Naviev, I., “Effect of ZnS shell thickness on the phonon spectra in CdSe quantum dots,” Physical Review B, 68, (2003)