We report a theoretical scheme for realizing efficient and enhanced laser cooling in Ho3+-doped solids, based on ground-state absorption (GSA) of I58→I57 at around 2.07 μm and excited-state absorption (ESA) of I55→F55 at around 2.28 μm. The upconversion-quenching-induced heat that usually is found in Ho3+ optical refrigeration can be significantly alleviated by using the copumping scheme, so the cooling power density and the cooling efficiency are greatly improved. The scheme works at temperatures above 133 K, and can shorten the time needed to reach the minimum attainable temperature of the cooling system. Besides, the dopant concentration is allowed to be promoted to around 1%. The copumping scheme can also be applied to other multilevel (such as Tm3+ and Er3+) optical refrigeration systems to improve the cooling performance.
We report a theoretical scheme for realizing efficient and enhanced laser cooling in Ho3+-doped solids, based on ground-state absorption (GSA) of I58→I57 at around 2.07 μm and excited-state absorption (ESA) of I55→F55 at around 2.28 μm. The upconversion-quenching-induced heat that usually is found in Ho3+ optical refrigeration can be significantly alleviated by using the copumping scheme, so the cooling power density and the cooling efficiency are greatly improved. The scheme works at temperatures above 133 K, and can shorten the time needed to reach the minimum attainable temperature of the cooling system. Besides, the dopant concentration is allowed to be promoted to around 1%. The copumping scheme can also be applied to other multilevel (such as Tm3+ and Er3+) optical refrigeration systems to improve the cooling performance.
To overcome the ETU-induced heating and realize efficient and enhanced laser cooling in Ho3+-doped solids, in Section 4.3, we will provide a modified excitation scheme based on ground-state absorption (GSA) of 5I8 Æ 5I7 at around 2.07 μm and ESA of 5I5 Æ 5F5 at around 2.28 μm. Compared with the conventional pumping scheme, the co-pumping scheme ameliorates the cooling performance significantly through alleviating the upconversion-quenching-induced heat [12], that is, the heat-producing 5I5 Ho3+ions are pumped to the 5F5 state by the laser at 2.28 μm. Fortunately, the 5F5 Ho3+ ions barely transit to the lower nearby state and tend to decay radiatively. Therefore, major heat produced in the conventional laser cooling of Ho3+-doped solids is avoided, and both cooling efficiency and cooling power density can be improved. In Section 4.4, we broaden the research domain of optical refrigeration to rare earth ion co-doped systems. Specifically, when Ho3+–Tm3+ co-doped YLiF4 crystals are pumped at around 2.07 μm, Ho3+ ions will populate at the 5I7 manifold. Meanwhile, Tm3+ ions will also be sensitized by absorbing energy transferred from 5I7 Ho3+ ions. Since the average energy of fluorescence photons generated by Tm3+3F4 Æ 3H6 radiative transitions is higher than that generated by Ho3+5I7 Æ 5I8 radiative transitions, energy transfer (ET) from Ho3+ to Tm3+ ions will induce annihilation of host phonons, and bring enhancement in both the cooling efficiency and the cooling power. Therefore, this cooling mechanism is referred next as energy transfer-enhanced laser cooling (ETLC) of solids [11]. 4.2 Conventional Laser Cooling of Holmium- The main upconversion process and the anti-Stokes fluorescence cooling (ASFC) processes of Ho3+-doped fluoride crystals are illustrated in Fig. 4.1. The ground-state (5I8) Ho3+ ions are pumped to the first excited state (5I7) with laser excitations of energy hν (solid red arrow on the left) and then relax thermally among the Stark levels in a timescale of picoseconds with host phonon absorption. Part of the Ho3+5I7 ions de-excite by emitting fluorescence photons of mean energy hν10 (solid blue arrow, ν10 > ν), which accounts for the ASFC process. The rest of the 5I7 Ho3+ ions de-excited and transfer their energy to the neighboring 5I7 Ho3+ ions, causing population accumulation at the third excited state (5I5) with host phonon absorption (solid green arrows). Comparison between the energy mismatch and the host phonon energy (for YLiF4 crystal host, ħω ≈ 450 cm-1) indicates that about two host phonons are annihilated for each ETU process (see Table 4.1).
We report the first demonstration of solid-state optical refrigeration of a Ho-doped material. A 1 mol% Ho-doped Yttrium Lithium Fluoride (YLF) crystal is cooled by mid-IR laser radiation, and its external quantum efficiency and parasitic background absorption are evaluated. Using detailed temperature-dependent spectroscopic analysis the minimum achievable temperature of a 1% Ho:YLF sample is estimated. Owing to its narrower ground- and excited-state manifolds, larger absorption cross-section, and coincidence of optimum cooling wavelength of 2070 nm with commercially available high-power and highly efficient Tm-fiber lasers, Ho^3+-doped crystals are superior to Tm^3+-doped systems for mid-IR optical refrigeration. With further improvement in material purity and increased doping concentration, they offer great potential towards enhancing the cooling efficiency nearly two-fold over the best current Yb:YLF systems, achieving lower temperatures as well as for realization of eye-safe mid-IR high-power radiation balanced lasers.
We report a theoretical scheme for laser cooling of solids based on energy transfer usually found in rare-earth codoped materials. The cooling scheme enables a large enhancement in the cooling efficiency with regard to the standard anti-Stokes fluorescence cooling. A Ho3+ and Tm3+-codoped low-phonon crystal (LiYF4) sample is investigated to find that the cooling efficiency increases, and then decreases with the increasing of the resonant absorption. The optimal cooling efficiency is predicted to exceed 5%. The maximum cooling power density could be promoted greatly by applying the codoped cooling scheme. The cooling scheme is also valid for other rare-earth (for example, Tm3+ and Er3+, or Er3+ and Yb3+) codoped materials.
A novel method for modeling optical refrigeration of H o 3 + -doped fluoride crystals is presented to obtain a laser-induced thermal modulation spectroscopy (LITMoS)-compatible fitting expression of cooling efficiency in a simple way. The quantum-yield frequency that corresponds to the output energy of a specific transition channel is introduced to simplify theoretical derivation. This model takes into account excitation dependency of external quantum efficiency and can be applied in theoretical analysis and experimental data fitting of optical refrigeration systems with multiple 4 f manifolds.
Since the successful realization of laser cooling of solids in 19951, the optical cooling techniques2-5 have made fast development, and laser cooling experiments have obtained some important progress. However, how to improve the absorption of pump laser and enhance the temperature drops so as to realize the practical optical refrigeration, it has become a hot research point in applied physics. As early as 2002, the Heeg et. al proposed an extraor intra-cavity enhancement scheme to improve the laser cooling effect of solids2, and experimentally demonstrated their intra-cavity enhancement scheme in 2004, and found that there is no large temperature drops due to the experimental parameters were not optimized3. In 2003, Hoyt proposed a multiple-pass scheme to enhance the absorption of pump laser and did a series of experimental researches about the multiple-pass scheme4. As expected, a greater temperature drop was obtained. However, due to some limitation in the early work, it is difficulty to cool the sample to the cryogenics temperature, such as low absorption of cooling materials5, the coating of the sample surfaces3, the purity of the sample6, the load from the ambient1,7, etc. Therefore, people have done much work to improve the record of temperature drop. For example, to avoid the coating absorption deposited on the sample surface, they cut the sample with a Brewster angle along with the cavity axis8, or improve the purity of cooling material9-11, and so on. Mansoor. Seletskiy used the multi-pass scheme to do some meaningful experiments and obtain a milestone progress, that is, they cool a high pure Yb3+-doped YLF to cryogenic temperature12 in this year. To obtain an efficient absorption, another scheme was proposed to insert a Brewster-cut sample into a resonant cavity and make the cavity to be resonated with the pump laser, and then the maximum absorption of the pump laser can be achieved 2, 13-18. Seletskiy demonstrated this scheme in the experiment13. However, this scheme depends on the power of the pump laser. Fortunately, the intra-cavity scheme proposed by Heeg can solve this problem in the case of low absorption2. He also suggested that inserting a cooling sample into the resonant cavity of a laser with a gain (so-called intra-cavity enhancement scheme) to realize the maximum absorption of the pump laser2, 14, 19 and carried out it in the experiment3. Besides of enhancement laser cooling of bulk solids using resonant cavity, we propose another new scheme to optically cool some film materials (i.e., filmier solids) by using a high fineness planar waveguide, which is similar to the cavity enhanced cooling scheme. In our laser cooling scheme, the solid film as a high-index gap waveguide is separated from a prism by a low-index gap one, and an incident laser beam is resonantly coupled into a
Optical refrigeration of solids with anti-Stokes fluorescence has been widely explored as a vibration-free cryogenic cooling technology. A minimum temperature of 87 K has been demonstrated with rare-earth-ion-doped crystals using optical refrigeration. However, the depletion of the upper-lying energy levels in the ground-state manifold hinders further cooling to below the liquid nitrogen ( LN 2 ) temperatures, restricting its applications. In this work, we introduce a Purcell-enhanced optical refrigeration method to circumvent this limitation. This approach enhances the emission of high-energy photons by coupling the emitters to an optical cavity, blueshifting the mean emission wavelength. Such Purcell-enhanced emission facilitates cooling starting from a lower-energy level in the ground-state manifold, which exhibits a higher occupation below the LN 2 temperatures. Using experimentally measured optical coefficients, our theoretical analysis predicts a minimum achievable internal temperature of about 38 K for a Yb 3 + : YLiF 4 nanocrystal near a cavity under realistic conditions. The proposed method is applicable to other rare-earth-ion-doped materials and semiconductors and will have applications in creating superconducting and other quantum devices through solid-state cooling.