A 7% Yb:YLF crystal is laser cooled to 131 ± 1 K from room temperature by placing it inside the external cavity of a high power InGaAs/GaAs VECSEL operating at 1020 nm with 0.15 nm linewidth. This is the lowest temperature achieved in the intracavity geometry to date and presents major progress towards realizing an all-solid-state compact optical cryocooler.
© 2014 Optical Society of America
Solid state laser cooling (also known as optical refrigeration) is based on the anti-Stokes fluorescence process [1–5]. Low entropy laser light, tuned to a wavelength slightly longer than the mean fluorescence wavelength (λf) of the material, is absorbed and reemitted as broadband fluorescence having a mean photon energy higher than that of the pump laser. The increased energy is accounted for by the absorption of lattice phonons (vibrational energy), reducing net temperature of the solid. In order to realize cooling, high purity (low parasitic absorption) and high external quantum efficiency are essential for the cooling material. Rare-earth ions doped into a pure host material with low phonon energy such as fluoride glasses and crystals satisfy these conditions.
The first experimental demonstration of laser cooling in solids was in 1995 by R. Epstein, et al. using a Yb3+-doped fluorozirconate glass Yb:ZBLAN . Recently, optical refrigeration has reached 114 K in a high-purity 10% Yb3+-doped yttrium lithium fluoride (Yb3+:YLF) crystal , utilizing a multi-pass geometry for enhancement of pump light absorption, important for low temperature operation. The achieved temperature is far below what can be approached by standard thermoelectric or Peltier coolers (approximately 170 K), thus rendering optical refrigeration as the only solid-state cooling technology capable of reaching below NIST defined cryogenic temperatures (−150 °C/123 K) .
In this work, we focus on the enhancement of pump absorption by placing a 7% Yb:YLF crystal cooling sample inside of a high-efficiency laser cavity. The absorption of the pump laser by the rare-earth ions at the cooling wavelengths is typically very low (approximately 1-2% per pass), particularly at low temperatures. This in turn requires a multi-pass geometry to fully utilize the incident laser power. A number of techniques have been proposed and demonstrated involving resonant and non-resonant cavity configurations [6, 8–10]. Thus far, the lowest achieved temperatures have been made possible using a non-resonant multi-pass cavity geometry . While currently most successful in the laboratory, this method offers a trade-off between the number of trapped laser passes and the geometrical compactness and thus is not ideal for future system applications. An external optical resonator, on the other hand, can potentially combine both high intra-cavity absorption enhancement and small size. Earlier attempts have demonstrated over a factor of 20 times enhancement  in a small footprint under critical coupling condition (optical impedance matching). Longitudinal mode instabilities of the pump laser during high power operation, however, limited the applicability of this approach for cryogenic cooling. These complications can however be avoided by placing the sample inside of a laser cavity (intra-cavity). Additionally, intra-cavity cryo-coolers can be made very compact and provide on-demand rapid cryogenic spot cooling for demanding applications with high efficiency. By combining the highest performance cooling material to date, the Yb:YLF crystal, and high efficiency semiconductor VECSELs, we have demonstrated cooling to an absolute temperature of 131 ± 1 K starting from room temperature. Current limitations and further optimization possibilities are also discussed.
2. Optical refrigeration cooling model11].
The anti-Stokes cooling process depicted in Fig. 1(a) shows a Yb3+ ion pumped from the top of the ground state manifold (E4) to the bottom of the excited state manifold (E5), which corresponds to 1020 nm. Figure 1(b) shows the temperature dependent cooling efficiency contour map (Eq. (1)) for the 7% doped Yb:YLF crystal cooled in this work. The global minimum achievable temperature (gMAT) for this specific crystal, with background absorption of 3 × 10−4 cm−1 and external quantum efficiency of 99.5% is calculated to be approximately 100 K at the optimal wavelength of 1020 nm.
3. Enhancement of absorbed power
With the crystal parameters known, low temperatures can be reached after mitigating external heat loads. The steady-state solution of the heat equation for a known laser-cooling sample is given by 10, 13]. The cooling efficiency is maximized by the choice of the pump wavelength to overlap with the E4-E5 Stark manifold transition in Yb3+ , while the actual value of the efficiency depends on the properties of each sample used for cooling and can only be improved through material synthesis . With the reduction of heat load and the crystal parameters known, increasing the pump absorption is the final step for improved cooling.
Resonant absorption of the cooling sample decreases at lower temperatures due to Boltzmann distribution of the population in the ground state manifold. Therefore, it is necessary to maintain high pump power absorption at low temperatures using multi-pass geometries in non-resonant or resonant external cavities. An efficient technique is to access high resonant enhancement of the absorption inside of a laser cavity [16–19].
For a weakly absorbing cooling sample placed inside a high-Q linear cavity (Fig. 2), the round-trip absorbed pump power is approximated by
The intracavity and therefore the absorbed power can be estimated by measuring the leakage power through one of the HR mirrors:.
Due to the nature of the resonant enhancement, the cavity can be matched to the absorption loss of the sample (optical impedance matching, see e.g .). In the context of the laser, this means that the intra-cavity loss introduced by the cooling sample should be made equal to the optimal outcoupling loss of the active resonator for maximum absorbed power. Essentially, the fluorescence of the cooling element serves as the only output coupler of the laser. The condition of optimal coupling should be satisfied at the target (low) temperature, where the (single pass) absorption is at its lowest. Intracavity techniques are therefore better suited to achieve optimal absorption compared with multi-pass external cavities, which would require more than100 passes at low temperatures.
Any high-efficiency laser with intracavity access and operating at the optimal cooling wavelength of 1020 nm (linewidth <0.5 nm) can be used to house the cooling sample. Vertical external-cavity surface-emitting lasers (VECSELs) - also known as optically pumped semiconductors lasers (OPSLs) or semiconductor disk lasers (SDLs) - are especially well suited for this purpose, since the gain profile of VECSEL can be engineered for the optimum wavelength (1020 nm) and they are capable of operating at very high powers [20–22]. The optimum coupling condition in a laser can be fulfilled by adjusting the length of the sample to result in round-trip absorption being equal to the optimal loss of the laser system at a given wavelength and temperature of the crystal. These reasons motivate the design, development, and performance characterization of the VECSEL, including determination of the optimal intra-cavity loss for the laser cooling experiments.
4. VECSEL design and performance
A VECSEL is an optically pumped semiconductor laser consisting of a multiple quantum well gain region grown on top of a distributed Bragg reflector (DBR). VECSELs have successfully combined the high power of edge-emitting lasers and excellent beam quality of surface-emitting semiconductor lasers. Additionally, the external cavity arrangement allows for inserting intra-cavity elements, such as a birefringent filters for wavelength tuning , nonlinear crystals for harmonic generation , and here laser cooling samples for optical refrigeration.
Our specific gain chip is designed for laser cooling of Yb:YLF crystal, requiring an operation wavelength of 1020 nm at high CW powers. The structure is grown by metal-organic chemical vapor deposition (MOCVD) on GaAs substrate in a “bottom emitter” geometry where the active region is grown first, followed by the DBR . This allows for the DBR to be metalized with Ti, Au, and In and subsequently soldered to a similarly coated thermal grade CVD diamond heat spreader. The GaAs substrate can then be removed by a selective wet etch, which stops at the InGaP window layer. The active region consists of 12 In0.23Ga0.77As quantum wells aligned with antinodes of the standing wave inside the GaAs0.97P0.03 sub-cavity for strain compensation, and is capped by a lattice-matched In0.51Ga0.49P window layer for carrier confinement. The DBR, which is used as the end mirror, is formed by 25 pairs of AlAs/GaAs.
The VECSEL performance was tested in a basic setup, schematically shown in Fig. 3(a). The pump laser is a fiber-coupled 808 nm laser diode with maximum output power of 75 W, focused onto the gain chip to a spot of approximately 300 μm in diameter. The diamond heat spreader was mounted to a water-cooled copper heat sink with typical cooling water temperature of 12 °C during laser operation. Figure 3(b) shows the VECSEL CW output power as a function of the absorbed 808 nm pump power with a 5% transmission output coupler. A slope efficiency of 41 ± 1% and more than 20 W of output power are achieved with no signs of thermal rollover, which shows that the output power is limited by the available pump power.
A dynamic control of the wavelength of the VECSEL is crucial for laser cooling. While a stable operation at 1020 nm is desirable in the steady-state (cold-sample) condition, to jump-start the laser with the cooling sample initially at room temperature, the laser must be tuned to longer wavelengths (~1030nm), where intracavity losses are sufficiently reduced to satisfy the lasing threshold condition. Temperature dependent absorption spectra of the 7% Yb:YLF crystal versus wavelength are provided in Fig. 4(a). Near room temperature, absorption at 1020 nm is 5 times higher than that at 1030 nm, well above the lasing threshold. As the crystal cools, absorption decreases, allowing adjustment of the lasing wavelength for highest cooling efficiency at 1020 nm.
At low temperatures, the highest cooling efficiency is achieved at 1020 nm, therefore optimum coupling (useful loss) is experimentally determined at this wavelength. With the crystal removed from the cavity, a variable coupling loss is introduced by inserting an optical-grade fused silica window, initially at the Brewster’s angle, and then incrementally rotating it to continually vary the coupling through the Fresnel reflection losses from the window. The resulting total output power (reflected out from counter propagating beams), as a function of the calculated roundtrip loss in the cavity, is illustrated in Fig. 4(b) for 40 W of incident pump power. Two points become apparent: first, that more than 15% of round-trip loss can be tolerated by the VECSEL, and second, the optimal round-trip loss is approximately 4-5% at this pumping condition. This measurement allows us to design the cooling sample for optimal coupling at 1020 nm and low temperatures.
Tuning the VECSEL is accomplished with a 4 mm thick crystalline-quartz birefringent filter (BRF), inserted in the external cavity at Brewster’s angle. The BRF allows us to tune the VECSEL from 1000 nm to 1030 nm as shown in Fig. 5(a), which is adequate to ensure room temperature operation with an optimal laser cooling sample. A home-made scanning Fabry-Perot interferometer is used to measure the full width at half maximum (FWHM) of the VECSEL linewidth to be 0.15 ± 0.02 nm at 1020 nm for 40 W of incident pump power, as shown in Fig. 5(b).
5. Intracavity laser cooling experiment and results
Laser cooling experiments are carried out by placing the cooling sample inside of the VECSEL cavity. The cooling sample is a high-purity 7 at.% doped Yb3+:YLF crystal cut for E||c pumping with Brewster orientation. A schematic diagram of the intra-cavity laser cooling experiment is depicted in Fig. 6. In order to reduce the convective heat load and hence maximize temperature drop in the cooling sample, the experiments are performed in an aluminum vacuum chamber evacuated to a pressure of below 10−5 torr using a turbo-molecular pump. The collimated beam from the 808 nm pump laser outside the vacuum chamber is sent through an anti-reflection (AR) coated vacuum-port window.
Two 5 cm and 7.5 cm focal length lenses are used to image the pump laser onto an approximately 300 μm diameter spot on the VECSEL gain chip, which is mounted on a copper heat sink water-cooled to a temperature of 12°C. Any pump light reflected by the semiconductor gain chip is redirected to the outside to avoid heat deposition inside of the vacuum chamber. At the other end of the VECSEL cavity, a 20 cm radius of curvature high-reflecting mirror is housed in a 3-axis piezo-actuated mount for fine tuning of the VECSEL alignment. A 4 mm thick quartz birefringent filter is used to tune the lasing wavelength. The Yb:YLF crystal is supported by microscope cover slips (100 µm thick) in order to reduce the conductive heat load. Non-contact temperature measurement is accomplished via the technique of differential luminescence thermometry [14, 26], where the change of luminescence spectrum is compared to a previously measured calibration, allowing for high-precision real-time temperature monitoring of the cooling sample. For this purpose fluorescence is collected by a multi-mode optical fiber positioned in the vicinity of the crystal and fed through a fiber-optic vacuum port to an outside spectrometer.
The target operation wavelength of the VECSEL is 1020 nm, corresponding to the E4-E5 Stark manifold resonance of the Yb:YLF crystal, which is achieved by rotating the BRF around the normal to its face. We note that the high absorption loss of Yb:YLF crystal at 1020 nm at room temperature (in excess of 35% per round-trip) prevents the laser operation. Therefore, at room temperature the VECSEL wavelength is tuned to approximately 1030 nm, where the round-trip loss is estimated to be around 9%, allowing for the operation of the laser (Fig. 4). Once the VECSEL starts lasing and the Yb:YLF begins to cool, we gradually tune the wavelength toward 1020 nm such that the absorbed power in the crystal is maintained despite the decreasing absorption. The thickness of the cooling sample is chosen based on its absorption coefficient to match the optimal coupling loss (~4-5%). For the optimal cooling wavelength of 1020 nm and at a temperature of 131 K, a thickness of approximately 2 mm results in an estimated round-trip loss of 6.5%, close to the optimal coupling.
The dynamics of the cooling are shown in Fig. 7(a), starting from room temperature and reaching 131 K after about 5 minutes. Fluorescence spectra collected during the experiment at different times are compared in Fig. 7(b), showing the change in spectrum used to compute the sample temperature. A clear red-shift of the mean fluorescence wavelength and dramatic narrowing of the emission peaks, together with an overall decrease of the fluorescence counts, are clear signatures of the sample cooling. A portion of the scattered intracavity laser light, also visible in the spectra, shows the active tuning of the VECSEL using the birefringent filter from 1030 nm to 1020 nm as the crystal temperature decreases over time.
The heat load on the cooling sample at 131 K is estimated based on the radiative and conductive heat loads to be approximately 20 mW . This corresponds to an absorbed power of 2.5 W assuming a cooling efficiency of 0.8% as inferred from Fig. 1(b). From the leakage laser power through the HR mirror (with a measured transmission of 0.012 ± 0.004%), an intra-cavity power of 125 W is obtained (at 1020 nm and 131 K) corresponding to an intensity of approximately 100 kW/cm2 on the cooling sample. This intensity exceeds by a factor of 2-3 the range of saturation intensities reported in the literature when scaled to λ = 1020 nm and assuming T = 130 K [27, 28]. Taking a saturated absorption, reduced by a factor of 3.5 from the small signal value of 0.1 cm−1, results in 2.5 W of absorbed power in the cooling sample in good agreement with our estimates from the heat load calculations. More careful determination of the saturation intensity and absorption coefficient at low temperatures and at the wavelength of interests is desirable. Such an investigation is currently underway in our group and the results will be exploited for future optimization of the heat lift and the overall cooling performance. The current results nonetheless suggest that lower temperatures and/or higher cooling powers may be achieved by better matching the laser mode volume to the sample (e.g. larger beam area).
Saturation also raises the minimum achievable temperature (MAT), which, for these experimental conditions, is estimated to be 128 K . The final temperature of 131 K is within 2% of the estimated MAT, pointing to the optimal cooling performance in the given experimental arrangement.
We have developed a high power narrow linewidth VECSEL around 1020 nm, optimized for laser cooling of Yb:YLF crystal. CW VECSEL output power of more than 20 W is achieved and is limited by the available pump power. Tunability in excess of 30 nm was demonstrated and lasing threshold was still reached with more than 16% round-trip loss in the cavity, while the maximum power could be extracted around 4-5% of intracavity loss. With a setup inside a vacuum chamber we were able to cool a 2 mm thick 7% doped Yb:YLF crystal from room temperature to 131 K, the lowest temperature achieved in the intra-cavity geometry to date. This demonstration is an important step toward next-generation compact device cryocoolers.
The authors wish to thank Dr. Richard Epstein for useful discussions, and AC Materials Inc. for customized sample growth and preparation. We acknowledge support provided by UNM-Science and Technology Corporation (STC) Gap Fund, AFRL contract FA94531310223, DARPA grant 10669320, and AFOSR STTR grant FA9550-13-C-0006 in collaboration with Thermodynamic Films (TDF) LLC. SDM acknowledges the support of a National Research Council Research Associateship Award at AFRL. DVS acknowledges support by the National Science Foundation under Grant No. 1160764. Sandia’s Laboratory Directed Research and Development Office provided growth of VECSEL devices. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
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