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Laser cooling of a semiconductor by 40 kelvin
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Optical irradiation accompanied by spontaneous anti-Stokes emission can lead to cooling of matter, in a phenomenon known as laser cooling, or optical refrigeration, which was proposed by Pringsheim in 19291. In gaseous matter, an extremely low temperature can be obtained in diluted atomic gases by Doppler cooling2, and laser cooling of ultradense gas has been demonstrated by collisional redistribution of radiation3. In solid-state materials, laser cooling is achieved by the annihilation of phonons, which are quanta of lattice vibrations, during anti-Stokes luminescence. Since the first experimental demonstration in glasses doped with rare-earth metals4, considerable progress has been made, particularly in ytterbium-doped glasses or crystals: recently a record was set of cooling to about 110 kelvin from the ambient temperature, surpassing the thermoelectric Peltier cooler5, 6. It would be interesting to realize laser cooling in semiconductors, in which excitonic resonances dominate7, 8, 9, rather than in systems doped with rare-earth metals, where atomic resonances dominate. However, so far no net cooling in semiconductors has been achieved despite much experimental10, 11, 12 and theoretical7, 8, 9, 13, 14 work, mainly on group-IIICV gallium arsenide quantum wells. Here we report a net cooling by about 40 kelvin in a semiconductor using group-IICVI cadmium sulphide nanoribbons, or nanobelts, starting from 290 kelvin. We use a pump laser with a wavelength of 514 nanometres, and obtain an estimated cooling efficiency of about 1.3 per cent and an estimated cooling power of 180 microwatts. At 100 kelvin, 532-nm pumping leads to a net cooling of about 15 kelvin with a cooling efficiency of about 2.0 per cent. We attribute the net laser cooling in cadmium sulphide nanobelts to strong coupling between excitons and longitudinal optical phonons (LOPs), which allows the resonant annihilation of multiple LOPs in luminescence up-conversion processes, high external quantum efficiency and negligible background absorption. Our findings suggest that, alternatively, group-IICVI semiconductors with strong excitonCLOP coupling could be harnessed to achieve laser cooling and open the way to optical refrigeration based on semiconductors.
作 者： Jun Zhang, Dehui Li, Renjie Chen & Qihua Xiong
期刊名称： Nature
期卷页： 23 January 2013
第493卷 第7433期 ~页
学科领域：工程材料 & 无机非金属材料 & 无机非金属类光电信息与功能材料
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Physicists in Singapore are the first to create a refrigerator that cools a piece of semiconductor using light, using their technique to cool a room-temperature sample of cadmium sulphide by some 40&#160;K. Although a similar technique has previously been used to chill glasses doped with rare-earth elements, this latest work could lead to practical optical refrigeration devices for use in satellites, or even "self-cooling" lasers.
First developed in the 1980s, laser cooling has opened up the new and incredibly fruitful study of ultracold atomic gases. The technique involves firing counter-propagating laser beams at an atomic gas, with the atoms absorbing and emitting photons in such a way that the net effect is to reduce the average motion of the atoms, and thus lower the temperature of the gas.
Removing phonons
The laser cooling of solids is somewhat different because heat is stored in a solid in the form of quantized lattice vibrations called phonons, which do not interact directly with light. In the case of rare-earth-doped glasses, energy is removed from the phonons when a single atom in the glass undergoes an "anti-Stokes" transition. This involves a photon being absorbed by an atom before emitting a higher-energy photon &#8211; with the extra energy coming from phonons.
From a technological point of view, it would be much more useful to be able to laser-cool a more conventional material, such as a semiconductor, than a doped glass. Last year, Eugene Polzik and colleagues at the University of Copenhagen managed to use a laser to cool an extremely thin sheet of semiconductor that was stretched like a drumhead. Rather than being a general refrigeration method, however, the optomechanical technique focused on damping out a specific subset of drum-like phonon modes in the sheet.
Chilly nanobelts
What Qihua Xiong and colleagues at Nanyang Technological University in Singapore have now demonstrated is a more general technique that uses lasers to cool an extremely thin ribbon (or "nanobelt") of the semiconductor cadmium sulphide (CdS). The method also relies on an anti-Stokes process, but in this case the transition involves an absorbed photon being converted into an electron&#8211;hole pair. This "exciton" annihilates and the semiconductor emits a higher-energy photon &#8211; with the extra energy coming from the annihilation of phonons. As a result, the sample loses phonons and cools.
Xiong told < that his team stumbled upon the effect by accident when doing laser-based Raman-spectroscopy experiments on the CdS nanobelts &#8211; materials that have a particularly strong anti-Stokes photoluminescence. The nanobelts were about 3&#160;&#956;m wide and about 100&#160;nm thick, and were draped across a silicon-oxide substrate that was peppered with holes that were about 4&#160;&#956;m across. Measurements were made on the portions of the nanobelts that were suspended over the holes.
Strong coupling
The experiment involved firing "pump" laser pulses at the nanobelt to create excitons, with the laser energy adjusted so that the exciton energy plus the energy of several phonons equals the energy of a photon emitted in an anti-Stokes transition. Each photon emitted in this way therefore takes a significant amount of heat energy. Indeed, Xiong says that more than 100&#160;meV of energy is removed per pump photon &#8211; the very high efficiency being because excitons and phonons in CdS nanobelts couple very strongly.
The team began the cooling process with the nanobelt at room temperature (290&#160;K) and then reduced the temperature to about 250&#160;K in about 40&#160;min. This corresponds to a cooling power of 180&#160;&#956;W. The temperature of the sample was measured using a technique called pump-probe luminescence thermometry, which involves firing a second "probe" laser pulse at the sample.
Sensors in space
According to Xiong, the cooling technique could be used to cool tiny devices. As well as being relatively straightforward to miniaturize, laser cooling does not involve mechanical refrigeration &#8211; which can introduce unwanted vibration &#8211; or cryogenic liquids. One application that Xiong says is "particularly appealing" is the cooling of sensors used on satellites and other space missions. He also says that the technique could be used to cool a laser by using some of its own light.
While Xiong says that there are several challenges that must be overcome to make the technique work on larger samples of semiconductor, it is, in principle, possible. Polzik described this latest cooling technique as "a very interesting result" adding that, in principle, the technique could be used to remove heat from semiconductor devices.
The cooling method is described in
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