With the ever growing use of refrigeration in today’s civilization at one hand and efficiency, cost and environmental concern with its (refrigeration) existing vapour compression refrigeration model using environmental non-friendly refrigerants on the other hand have always motivated researchers and technologists towards the development of alternative refrigeration technologies e.g., solid state refrigeration, magnetic refrigeration and optical refrigeration etc. Along with different advantages and disadvantages being associated with alternative refrigeration systems, their role in future is inevitable. This article is focused on the science and technology behind optical refrigeration technology. Efforts are being made to bring out the latest developments and future scope of the technology.
Principle of optical refrigeration
Optical refrigeration (also called laser refrigeration or anti-Stokes fluorescent cooling) is a technique for cooling a macroscopic crystal (or a piece of glass) with a laser beam. The idea of cooling a solid-state optical material by simply shining a laser beam onto it may seem counterintuitive, but this is rapidly becoming a promising technology for future cryocoolers. Here, we discuss the evolution of the science of optical refrigeration in rare-earth-doped solids and semiconductors from its origins through to the present day. The crystal must be doped, e.g., with ytterbium or thulium ions, which are excited by the laser beam. The laser wavelength is chosen such that it is longer than the average wavelength of the resulting fluorescence. This means that the energy of the absorbed photons is lower than the average energy of the emitted photons, so that energy is removed from the crystal. Of course, it is essential that the quantum efficiency of the fluorescence is high, and that nearly all fluorescence light can leave the crystal without being absorbed, e.g., by impurities: a single absorbed photon would offset the cooling effect of many other photons.
History of optical refrigeration
Laser cooling of solids is also known as optical refrigeration, proposed in 1929 by the German physicist Peter Pringsheim – nearly 30 years before the laser was invented. In this process, the material to be cooled is irradiated by red-detuned (lower-energy) monochromatic light; as a result, spontaneous emission of higher-energy luminescence (a process called luminescence upconversion or anti-Stokes luminescence) can be excited. Lattice vibrations (or, in the language of quantum mechanics, phonons) provide the necessary energy and momentum to satisfy the conservation laws. As such, the lattice vibrations are annihilated – that is, the heat is converted into light. There was some argument whether an optical refrigerator violates the second law of thermodynamics, until Russian scientist Lev Landau assigned entropy to optical irradiation in 1946.
Based on the second law of thermodynamics, work must be done to reduce the entropy of the system in order to cool it. In optical refrigeration, the pump laser is a monochromatic source of light exhibiting low entropy, while the spontaneous emission has a broadband nature, thus having high entropy. It is the pump laser that does the work to drive the cooling; therefore, the second law of thermodynamics is obeyed.
This principle proposed by Pringsheim was later used to cool dilute atomic gases based upon the well-known Doppler effect. Dilute atomic gases can be cooled to very low temperatures – in fact, down to the nano-Kelvin regime. At such low temperatures, the quantum ground state is accessible; this has led to the discovery of many exciting phenomena, including Bose-Einstein condensation, quantum manipulation, and entanglement of single atoms. Laser cooling of solids has a similar physical origin, but with a substantial difference: the atoms in solids have no translational kinetic energy and momentum, as the atoms are locked in a crystalline lattice. However, solids have abundant lattice vibrations that provide the extra energy for upconversion. Thirty years after Pringsheim proposed the concept of laser cooling, French physicist Alfred Kastler suggested that rare-earth-doped solid material may have the potential for laser cooling of solids. Since then, there have been many failed attempts at finding workable laser cooling materials-at least until 1995, when Richard Epstein and his group at Los Alamos National Laboratory (Los Alamos, NM) realised cooling on the order of 0.3 K in a high quality Yb3+-doped glass. Just recently, the Mansoor Sheik-Bahae group at the University of New Mexico (Albuquerque, NM) has cooled Yb3+-doped LiYF4 crystals from room temperature through a net temperature drop of 190 K (allowing, for example, indirect cooling of semiconductor ‘payloads’). This is the cooling limit for rare-earth-doped materials; below this temperature, similar materials cannot efficiently absorb the pump photon because the atomic resonances follow the Boltzmann distribution and the upper levels of ground state manifold become depopulated.
Methodology involved
Optical cooling of solids, which is also known as optical refrigeration, is based on anti-Stokes fluorescence. Consider a two-multiplet system, such as the ground – and the first excited-state manifolds of a rare-earth ion, in a solid host. The sub-levels within each manifold are coupled by lattice vibrations of the host atoms, or phonons. The cooling cycle is as follows: a coherent, low entropy light source (i.e., a laser) tuned to the red tail of the absorption spectrum will only excite electrons from the top of the ground state to the bottom of the excited state. Electrons at the bottom of the excited state manifold will have to gain energy towards establishing a quasi-thermal equilibrium. They draw this extra energy from the phonons, thus cooling the lattice host in the process. A similar process simultaneously occurs during the equilibration of the ground state.
In the final stage of the cooling cycle, the heat is carried away by fluorescence when the ions decay back to the ground state by emitting photons having a mean (fluorescence) energy higher than the absorbed laser photons. This phenomenon is called anti-Stokes fluorescence, and was predicted by Peter Pringsheim in 1929. Pump light is efficiently generated by a semiconductor laser diode and carried to the mirrored cooler element by an optical fibre. The laser enters the cooler through a pinhole in one mirror and is trapped by the mirrors until it is absorbed. Isotropic fluorescence escapes the cooler element and is absorbed by the coated vacuum casing. The load to be cooled, for example an infrared detector, is connected in the shadow region of the second mirror.
Technological developmental status
From daily experience, we know that matter is heated when it absorbs light; in just one example, lasers can be used for industrial machining and cutting. The idea that laser light can cool rather than heating matters is counterintuitive; but in fact, laser light has been used to cool diluted atomic gases and certain rare-earth-element-doped glasses and crystals for the past 30 years. Now, researchers have demonstrated that a red-detuned laser can cool a semiconductor from room temperature down to -20°C. This achievement is a substantial step toward all-solid-state semiconductor optical cryocoolers that are vibrationless, cryogen-free, compact, highly efficient, and possibly even directly integratable into electronic and optoelectronic devices.
Optical refrigeration has reached a new stage in which semiconductors can be substantially cooled by laser light. Cooling a piece of ZBLAN glass in a ‘laser fridge’ from room temperature down to 208 K has been demonstrated, and 110 K have been achieved with Yb:LiYF4 (Yb:YLF). In theory, even temperatures of the order of 77 K (liquid nitrogen) should be reachable. Certain ytterbium-doped crystal materials, particularly tungstates such as Yb:KGW = Yb:KGd(WO4)2, appear to be suitable for this purpose. However, direct-bandgap semiconductors have a higher cooling efficiency and a lower theoretical cooling limit of about 10 K, because excitonic levels dominate the absorption band and the carriers follow the Fermi-Dirac distribution.
If laser cooling of semiconductors could reach 10 K, it could potentially be used to replace liquid-helium cooling for many applications. Also, semiconductor optical cryocoolers could possibly be integrated with electronic and optoelectronic devices. Although researchers have devoted much theoretical and experimental effort toward laser cooling of semiconductors using group III-V gallium arsenide (GaAs) quantum wells, no net laser cooling has ever been achieved. This is because group III-V semiconductor materials have a weak electron-phonon coupling strength, a high reabsorption effect of luminescence due to the large refractive index, and a large surface-recombination velocity.
A research group has discovered that II-VI group semiconductor cadmium sulfide (CdS) nanoribbons exhibit strong exciton longitudinal optical (LO) phonon coupling, leading to a strong luminescence upconversion facilitated by annihilation of multiple LO phonons. They have further demonstrated the first successful laser cooling in CdS nanoribbons to about 40 K from 290 K – when pumped by a 514 nm green laser. They showed that about 15 K of cooling can be realised even when starting at a low temperature of 100 K when pumped by a 532 nm laser; at the same time, the temperature of the sample is measured by monitoring the Stokes photoluminescence peak shift with an additional laser emitting at 473 nm at a very low power, with its light focused at the same spot as the pump-laser light. The success of this laser-cooling experiment can be attributed to two factors. The first is a strong electron-phonon coupling, which makes it possible to resonantly annihilate more than one LO phonon during each upconversion cycle – and thus more effectively remove the heat from the CdS nanoribbons. Secondly, the luminescence escape efficiency of the upconverted photons approaches unity because the nanoribbon thickness used in the experiment is less than half the wavelength of the fluorescence photons; this prevents the luminescence reabsorption and recycling that usually generate heat. This work opens the way to a search for other laser-cooling materials with strong electron-phonon coupling strengths, and also implies that other II-VI semiconductors may potentially be laser-cooled experimentally.
Applications of optical refrigeration
This process could become practical for cooling electronics and optoelectronics, possibly leading to athermal lasers. Possible applications of laser refrigeration are the replacement of Stirling coolers and devices like those (avoiding moving parts, vibrations, etc.), but also radiation-balanced lasers, where the internal heat generation is essentially compensated by optical refrigeration. Laser cooling of semiconductors has many potential uses. One possibility is the cooling of high-sensitivity sensors and detectors in space. Currently, most space cryocoolers are mechanical or cryogen-based; in contrast, optical refrigerators have no moving parts or cryogens. Another possible application is on-chip coolers for electronic or optoelectronic devices, including superconductor electronics. In addition, our research results suggest that a radiation-balanced laser (a so-called athermal laser) is possible – if a suitable gain-medium mechanism can be established in the material that is being laser-cooled. Of course, the current challenge is in scaling up laser-cooled devices to a practical size – for instance: to establish laser cooling in a bulk crystal.
Possible applications of laser refrigeration are the replacement of Stirling coolers and devices alike (avoiding moving parts, vibrations, etc.), but also radiation-balanced lasers, where the internal heat generation is essentially compensated by optical refrigeration. It is instructive to consider entropy changes associated with laser refrigeration. The reduction in thermal entropy of the cooled device is more than compensated by the increase in entropy that arises from the conversion of narrow-band focused laser light into fluorescence light, which has a much higher entropy due to the many spatial modes and different frequencies involved in the emission.
Required conditions
One essential requirement for a material to exhibit net cooling in this process is that it must have a high external quantum efficiency. This simply means a high probability that an excited ion will decay by emitting a photon that escapes the system. In other words, the non-radiative (phonon emitting) decay rate must be few orders of magnitude lower than the radiative rate. Rare-earth ions (such Yb, Tm and Er) are known to satisfy this condition due to their particular atomic orbitals.
However, what prevented scientists from observing net cooling for more than six decades after Pringsheim’s prediction was the presence of parasitic impurities. Apart from rare-earth doped glasses and crystals, any other material that may potentially satisfy the purity as well as quantum efficiency conditions should be considered as a candidate for laser cooling.
A prime example is direct-gap semiconductors such as GaAs, as theories predict lower temperatures as well as higher cooling power densities than obtained in rare-earth systems.
Technical challenges
The most demanding technical challenge remains in the area of materials. Researchers have recently reported a new temperature record and achieved the first cryogenic operation by cooling aYb-doped yttrium lithium fluoride (YLF) crystal to 155 K from room temperature. While it should be possible to achieve cooling down to 120 K in this crystal, cooling to lower temperatures requires crystals with a higher ratio of resonant absorption (Yb concentration) to the parasitic absorption.
Therefore, the main challenge is to grow extremely pure crystals with higher doping concentrations without degrading the quantum efficiency, which is caused by the lifetime quenching phenomena. Similarly, for semiconductors, growing high-purity heterostructures while mitigating the surface recombination velocity to extremely low levels is the paramount challenge.
Future scope
Fundamentally, there appears to be no hard limit on the lowest temperature that can be achieved in optical refrigeration – if the parasitic absorption were to be eliminated. However, like thermoelectric devices, It is ultimately the cooling efficiency, and consequently the overall heat lift that would determine the practicality of such devices. Following simple thermal statistics of phonons and electrons, cooling efficiency as well as the resonant absorption are known to drop with temperature.
Analysis shows that rare-earth doped systems should remain practical for temperatures down to 70 K or perhaps slightly lower. We and others have chased cooling semiconductors for many years – simply because they can potentially be used to cool to as low as 10 K with much higher cooling power densities than in rare-earth doped materials.
The immediate application of optical refrigeration technology is an all-solid-state cryocooler without any moving parts. This becomes particularly attractive for certain IR imaging applications (e.g., space-based imagers) where microphonic noise due to mechanical vibrations of the cooler lead to image degradation.
Also, in fibre-coupled geometries, the cooling element itself can be very lightweight, thus lending itself to high agility and gimbaled applications. Optical refrigeration has the unique capability to be scaled down to wavelength-size dimensions, thus realising compact or spatially selective micro-coolers. Future advances are also expected to render this technology suitable for cooling superconducting electronics and sensors.
It has also been suggested that using photovoltaic converters, the fluorescence waste can be recycled to enhance the efficiency towards Carnot limit.
