Junwei Meng

Astigmatic Herriott cell for optical refrigeration

Abstract. 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.

Bridgman growth of fluoride crystals for radiation-balanced lasers (Conference Presentation)

Radiation-balanced lasers (RBL) combine solid-state optical refrigeration and lasing in one material to enable a net zero thermal load that allows for favorable scaling to high laser powers. A high-performance RBL material, therefore, has to first qualify as a high-performance laser-cooling material. This necessitates exquisite material purity in order to achieve the required near-unity external quantum efficiency and low background absorption. Solvent extraction, ion exchange, and electrochemical treatment of aqueous solutions or melts are some of the techniques available for the purification of starting materials used in the growth of RBL crystals. Scaling these methods to the 100s of gram scale needed for traditional Czochralski crystal growth while maintaining parts-per-billion level impurity concentrations however has proven challenging in several past efforts. In contrast, we have previously shown solvent extraction and electrochemical treatment to be effective on the several gram scale. This creates a need for exploring alternative methods for growing optical-cooling-grade fluoride crystals on the small scale. We will present results on growing Yb-doped YLiF4 (YLF) and LuLiF4 (LLF) single crystals using the vertical Bridgman method. The external quantum efficiency and background absorption of these samples will be reported and discussed in the context of RBL.

Investigation of temperature dependence of quantum efficiency and parasitic absorption in rare-earth doped crystals (Conference Presentation)

Optical refrigeration of rare-earth doped crystals has exceptional qualities that can be used for building a compact and vibration-free all-solid-state optical cooler. Estimating the lowest achievable temperature and cooling power of such a device requires accurate measurements of external quantum efficiency, mean fluorescence wavelength, and parasitic absorption. Here we discuss temperature dependent measurements of these parameters for a high quality Yb:YLF sample by performing a LITMoS test (Laser Induced Temperature Modulation Spectrum) combined with contact-free differential luminescence thermometry. These measurements are challenging at low temperatures, but by integrating these two methods, we can perform LITMoS test at any temperature.

Investigation of radiation-balanced disk lasers

Employing large surface-area-to-volume ratio gain, thin-disk lasers have shown great potential in power scaling. But thermal management for these devices is still challenging. One possible approach is to balance the heat load generated by the lasing process with cooling power from the anti-Stokes cooling process, forming radiation balanced lasers (RBLs). Compared to bulk RBLs, thin-disk RBLs can be better thermally balanced with reduced thermal gradients, promising higher output power and better beam quality. In this paper, we analyze and investigate radiation balanced disk lasers with Yb:YAG and Yb:YLF crystals in different pumping configurations.

First demonstration of an all-solid-state optical cryocooler

Solid-state optical refrigeration uses anti-Stokes fluorescence to cool macroscopic objects to cryogenic temperatures without vibrations. Crystals such as Yb3+-doped YLiF4 (YLF:Yb) have previously been laser-cooled to 91 K. In this study, we show for the first time laser cooling of a payload connected to a cooling crystal. A YLF:Yb crystal was placed inside a Herriott cell and pumped with a 1020-nm laser (47 W) to cool a HgCdTe sensor that is part of a working Fourier Transform Infrared (FTIR) spectrometer to 135 K. This first demonstration of an all-solid-state optical cryocooler was enabled by careful control of the various desired and undesired heat flows. Fluorescence heating of the payload was minimized by using a single-kink YLF thermal link between the YLF:Yb cooling crystal and the copper coldfinger that held the HgCdTe sensor. The adhesive-free bond between YLF and YLF:Yb showed excellent thermal reliability. This laser-cooled assembly was then supported by silica aerogel cylinders inside a vacuum clamshell to minimize undesired conductive and radiative heat loads from the warm surroundings. Our structure can serve as a baseline for future optical cryocooler devices.Optical Refrigeration: an all-solid-state cryocooler becomes a realityThe demonstration that an arbitrary load can be successfully cooled by optical refrigeration bodes well for the future for a new breed of solid-state optical cryocoolers. Markus Hehlen and coworkers from Los Alamos National Laboratory and the University of New Mexico built the cooler from a crystal of YLF:Yb that was pumped by a fiber laser. The device cools due to anti-Stokes fluorescence from the crystal, and a HgCdTe sensor was attached to the cooling crystal via a thermal link made from undoped YLF and a copper cold-finger. Upon application of 47W of 1020 nm pump laser light, the optical cryocooler reduced the temperature of the sensor to 135 K from room temperature. The YLF:Yb crystal was placed inside a Herriott cell optical cavity while the entire cooler was placed inside a vacuum chamber in order to control heat flows.

Optical refrigeration inches toward liquid-nitrogen temperatures

Superconductivity, longand mid-wave IR detectors, and ultrastable laser cavities that operate in the 77–150K temperature range can all benefit from vibration-free cooling.1 Currently, such low temperatures can only be achieved using cryogenic gases or liquids, solid cryogens, or mechanical refrigerators. Unfortunately, these coolers require regular attention, introduce vibrational noise, and are subject to mechanical wear over time. Many space-based applications (particularly ultra-stable laser cavities) cannot tolerate these drawbacks. All-solid-state cryocoolers are therefore desirable because of their inherent vibration-free operation and potentially long lifetime. Optical refrigeration (i.e., anti-Stokes fluorescence cooling) is the only solid-state cooling technology capable of reaching cryogenic temperatures. Anti-Stokes cooling—in which a doped crystal is excited by a laser with a wavelength that is longer than the average wavelength of the resulting fluorescence, thus leading to cooling of the crystal—was first suggested by Peter Pringsheim almost 90 years ago.2 It was not actually observed, however, until years after the invention of lasers and the availability of high-purity host materials. The first demonstration of optical refrigeration, reported in 1995, used a fluorozirconate glass doped with ytterbium (Yb). The resulting material is known as Yb3C: ZBLANP.3 Cooling occurs when low-entropy laser light (tuned to a slightly lower energy than the mean fluorescence of a material) is absorbed, thus giving rise to efficient fluorescence generation and escape. On average, each pump photon removes vibrational energy (i.e., phonons) from the cooling sample after being absorbed and re-emitted. Figure 1. Schematic of our astigmatic Herriott cell. The geometry of the cell enables laser light (red) to be trapped inside of the crystal, ensuring more than 95% absorption. R1x;y D 50cm, R2x D 1, and R2y D 50cm, where R1 and R2 are the radii of curvature of the spherical and cylindrical mirrors, respectively. x;y : Launching angle. W : Crystal length, width, and height (Wx D Wy D W ).

Enhanced cooling of Yb:YLF using astigmatic Herriott cell (Conference Presentation)

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.

Recent advances in optical refrigeration of a load (Conference Presentation)

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.