Laser cooling of solids has advanced immensely in recent years and temperatures well below 100 K have been demonstrated in Yb:YLF crystals. We will discuss our progress towards developing a functional all-solid-state cryocooler based on this principle. We present data and analysis concerning laser coupling efficiency, thermal link between the cooling crystal and the cold-finger, shielding the load from the fluorescence, and overall thermal load management. Considerations for building a cooler prototype for specific applications will also be discussed.
Optical refrigeration of solids requires crystals with exceptional qualities. Crystals with external quantum efficiencies (EQE) larger than 99% and background absorptions of 4×10-4cm-1 have been cooled to cryogenic temperatures using non resonant cavities. Estimating the cooling efficiency requires accurate measurements of the above mentioned quantities. Here we discuss measurements of EQE and background absorption for two high quality Yb:YLF samples. For any given sample, to reach minimum achievable temperatures heat generated by fluorescence must be removed from the surrounding clamshell and more importantly, absorption of the laser light must be maximized. Since the absorption coefficient drops at lower temperatures the only option is to confine laser light in a cavity until almost 100% of the light is absorbed. This can be achieved by placing the crystal between a cylindrical and spherical mirror to form an astigmatic Herriott cell. In this geometry light enters through a hole in the middle of the spherical mirror and if the entrance angle is correct, it can make as many round trips as required to absorb all the light. At 120 K 60 passes and 150 passes at 100K ensures more than 95% absorption of the laser light. 5 and 10% Yb:YLF crystals placed in such a cell cool to sub 90K temperatures. Non-contact temperature measurements are more challenging for such a geometry. Reabsorption of fluorescence for each pass must be taken into account for accurate temperature measurements by differential luminescence thermometry (DLT). Alternatively, we used part of the spectrum that is not affected by reabsorption.
Cooling rare-earth-doped crystals to the lowest temperature possible requires enhanced resonant absorption and high-purity crystals. Since resonant absorption decreases as the crystal is cooled, the only path forward is to increase the number of roundtrips that the laser makes inside the crystal. To achieve even lower temperatures than previously reported, we have employed an astigmatic Herriott cell to improve laser absorption at low temperatures. Preliminary results indicate improvement over previous designs. This cavity potentially enables us to use unpolarized high-power fiber lasers, and to achieve much higher cooling power for practical applications.
We present a study of cooling enhancement in optical refrigerators by the implementation of advanced non-resonant
optical cavities. Cavity designs have been studied to maximize pump light-trapping to improve absorption and thereby
increase the efficiency of optical refrigeration. The approaches of non-resonant optical cavities by Herriott-cell and totalinternal-
reflection were studied. Ray-tracing simulations and experiments were performed to analyze and optimize the
different light-trapping configurations. Light trapping was studied for laser sources with high quality beams and for
beams with large divergences, roughly corresponding to the output from fiber lasers and from diode lasers, respectively.
We present a trade-off analysis between performance, reliability, and manufacturability.
We present our recent work in developing a robust and versatile optical refrigerator. This work focuses on minimizing
parasitic energy losses through efficient design and material optimization. The cooler’s thermal linkage system and
housing are studied using thermal analysis software to minimize thermal gradients through the device. Due to the
extreme temperature differences within the device, material selection and characterization are key to constructing an
efficient device. We describe the design constraints and material selections necessary for thermally efficient and durable
We report the first observation of laser cooling in 1% doped Tm:YLF by 0.5 K and in 0.8% doped Ho:YLF crystals by
0.1 K starting from room temperature in air. To achieve this, we designed and constructed a high power, broadly tunable
(1735 nm-2086 nm) continuous wave singly-resonant optical parametric oscillator. (OPO). The cooling experiments were
performed at ambient pressure, and temperature changes were measured using a thermal camera.
We present a study of non-resonant optical cavities for optical refrigerators. Designs have been studied to maximize
pump light-trapping to improve absorption and thereby increase the efficiency of optical refrigeration. The approaches of
non-resonant optical cavities by Herriott-cell and total-internal-reflection were studied. Ray-tracing simulations and
experiments were performed to analyze and optimize the different light-trapping configurations. We present a trade-off
analysis between performance, reliability, and manufacturability.
Laser cooling of solids has great potential to achieve an all-solid-state optical cryo-cooler. The advantages of compactness, no vibrations, no moving parts or fluids, and high reliability have motivated intensive research. Increasing the pump power absorption is essential to reach lower temperatures. Here, using a high power broadly tunable InGaAs/GaAs vertical external-cavity surface-emitting laser (VECSEL) we demonstrate how we have increased the pump power absorption in an intra-cavity geometry cooling a 10% Yb:YLF crystal. We also discuss the progress, advantages, and challenges of laser cooling inside a VECSEL cavity, including the VECSEL active region design, cavity design, and cooling sample choice for optimal cooling. A novel method to increase the absorption of the pump power in the crystal has also been proposed.
Laser cooling of Yb:YLF crystal to 131 K from room temperature has been demonstrated in an active intracavity arrangement for enhanced pump absorption. The laser is a high-power, broadly-tunable InGaAs/GaAs MQW VECSEL capable of producing 20 Watts at 1020 nm, directly at the E4-E5 transition of the Yb-ion. This is the coldest temperature achieved to date in an intracavity geometry and without sophisticated heat load management of the crystal. This progress presents a significant advancement towards an all-solid-state compact cryocooler.
We report on the use of a high power InGaAs quantum well vertical external-cavity surface-emitting laser (VECSEL) emitting at a wavelength of 1020 nm for intra-cavity cooling of a 5% Yb-doped YLF crystal to 148 K from room temperature. Similar crystals have now reached temperatures below the NIST-defined cryogenic temperature of 123 K when pumped outside a laser cavity. We discuss the progress, advantages, and challenges of laser cooling inside a VECSEL cavity, including the VECSEL active region design, cavity design, and cooling sample choice for optimal cooling.
Laser cooling of solids to 148 K has been demonstrated in a Yb:YLF crystal using intracavity absorption enhancement in
an InGaAs MQW VECSEL at 1020 nm. This is the lowest temperature achieved in the intracavity geometry to date and
presents a significant advancement towards an all-solid-state compact cryocooler.
Noncontact temperature measurements with large (thermal) dynamic range are desirable in many applications. Aside
from interferometric techniques, fluorescence intensity and spectral shape have been exploited in the past for sensitive
thermometry in luminescent materials. Here, we present a novel method that utilizes the polarization-sensitive reflection
and/or transmission of light from (through) an optical material without relying on any fluorescence. A balanced
photodetector will measure the difference signal corresponding to two orthogonal polarization states with high singal-tonoise
ratio. Temperature resolution of 5 mK have been demonstrated.
We demonstrate application of the thermal reflectance measurement in a balanced detector arrangement to resolve
laser induced temperature shifts in ytterbium-doped yttrium-lithium-fluoride (Yb:YLF) during optical refrigeration
experiments. Definite signature of cooling versus heating allows for rapid screening of the performance of the laser