Optically-induced cooling

Abstract

An illumination source is configured to illuminate a medium, having a chromium-based dopant, with light at a wavelength selected based on an emission band of a selected absorption band of the medium. The selected absorption and emission bands being associated with an electric-dipole-allowed transition of the medium. Upon illumination by the light the medium is cooled.

Description

OPTICALLY-INDUCED COOLINGSTATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made with government support under 1947070 awarded by the National Science Foundation and HDTRA12020002 awarded by the U.S. Department of Defense, Defense Threat Reduction Agency. The government has certain rights in the invention.PRIORITY

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 539,730, filed September 21 , 2023, and titled OPTICALLY-INDUCED COOLING, which is incorporated by reference herein in its entirety.BACKGROUNDTechnical Field

[0003] The disclosure relates generally to optically-induced cooling or optical refrigeration.Brief Description of Related Technology

[0004] In recent years, laser cooling has been successfully applied to create new forms of matter (Bose-Einstein condensates), to enable new sensor technologies based on atom interferometry, to perform quantum computation, and to develop quantum memories. Laser cooling to reach cryogenic temperatures in vacuum has been confirmed by the demonstration of a solid state optical cryo-cooler that operates via anti-Stokes fluorescence on forbidden transitions. Also, radiation-balanced lasers have been operated successfully on forbidden transitions. Accordingly, continued improvements in optical cooling technologies will further open new areas of investigation.BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Figure 1 shows an example optical cooling system.

[0006] Figure 2 shows an example method for optical cooling.

[0007] Figure 3 shows example plots of cooling efficiency data.

[0008] Figure 4 shows an example plot of cooling efficiency data.DETAILED DESCRIPTION

[0009] The inventors are grateful for the financial support provided by Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq, grants #308242 / 2022- 0), Fundagao de Apoio ao Desenvolvimento do Ensino, Ciencia e Tecnologia do Estado de Mato Grosso do Sul (FUNDECT, grants #71 / 032.546 / 2022).

[0010] Laser cooling of solids has not been so widely employed in photonic device applications because the cooling rate and efficiency demonstrated to date are poorer than in vapors. In condensed matter it has not been possible to implement rapid, efficient cooling with allowed electric-dipole transitions because in general the dense environment of solids causes heating due to configuration relaxation during optical interactions. Forbidden transitions incur no extra heating due to configuration relaxation, and permit lower temperatures to be reached than by any other means to date. On the other hand, optical refrigeration based on forbidden transitions may not necessarily be fast enough for all applications and may not necessarily scale to all payloads.

[0011] In various contexts, it may be desirable to cool a target, including some cases, where forbidden-transition-based cooling alone (e.g., without combination with other technologies) may be insufficient. For example, it may be desirable to cool a sensor (or other semiconductor device), act as a coolable substrate for a semiconductor device (for example a lll-IV and / or ll-VI semiconductor device) to create a self-cooled radiation-balanced laser, to refrigerate a target to a cryogenic temperature or below, to cool a target with minimal or no induced vibration as a result of the cooling, and / or to implement other systems where increased cooling power or efficiency is desirable.

[0012] In optical cooling, heat can be removed from a target by having a laser induce excitation in the material with laser light including photons of a first energy. The excitations in the material relax over time and release photons of a second energy. If the second energy is higher than the first energy, the excitation-relaxation cycle carries heat away from the target material. To achieve the excitation in the material, an energy-level transition may be used. According to conventional wisdom, optical cooling must avoid the use of electric-dipole-allowed transitions. According to the conventional wisdom, the intense interactions of the electrons with the “cooling” light on electric-dipole-allowed transitions induces in-material vibrations due to configuration relaxation. According to the conventional wisdom, these vibrations would clearly lead to heating that would overwhelm any cooling effect achievable through use of the electric-dipole-allowed transitions.

[0013] Contrary to the conventional wisdom, various ones of the techniques and architectures discussed herein implement optical cooling using electric-dipole-allowed transitions. Electric-dipole-allowed transitions may be comparatively faster than forbidden or disallowed transitions, for example some electric-dipole-allowed transitions may have fluorescent relaxation time scales shorter than 10'7seconds. In some cases, forbidden transitions may have relaxation times longer than 10'3seconds. Accordingly, cooling via electric-dipole-allowed transitions may be able to increase the rate of cooling by factors of 103-104or more. Thus, electric-dipole-allowed transitions may have fast relaxation times, e.g., relaxation time faster than 10-4seconds or other short-time-scale relaxation times.

[0014] The short time scales of various electric-dipole-allowed transitions may be shorter than those of impurities or other parasitic heating pathways in a cooled material. Accordingly, as an unexpected result, various ones of these impurities and / or other parasitic heating pathways may be saturated with sufficient cooling illumination and be unable to relax quickly enough to compete with the cooling rate of the electric- dipole-allowed transition. Therefore, in some cases, the heating by unintended impurities through parasitic absorption may be overwhelmed and increased cooling efficiency may be achieved.

[0015] The selection of a host material, e.g., with a crystal field, and the dopant material may provide a specific environment to support dipole-transition-allowedcooling. As recognized herein a host material with a weak crystal field may be paired with a dopant, such as chromium. Due to synergy with the host material, the chromium may include trivalent ions. Moreover, the selection of a weak crystal field may pull one or more quartet transitions for the chromium below the doublet transitions. Thus, the dipole-allowed quartet transitions may have lower energy levels than the dipole- disallowed doublet transitions within the dopant chromium ions. This inversion of of the levels, recognized herein and created via the pairing of dopant and host material, allows for practical and beneficial application, e.g., laser cooling on a dipole allowed transition.

[0016] Using chromium-based dopants with one or more of the host materials, such as lithium strontium hexafluoroaluminate (LiSAF), lithium calcium hexafluoroaluminate (LiCAF), and / or other various other host materials (e.g., with a weak crystal field, a quantum efficiency (e.g., when combined with the dopant) near 1 (e.g., above about .9), and / or with low excited state absorption and / or upconverision / Auger absorption) may support the dipole-allowed cooling described herein.

[0017] In various implementations, cooling wavelengths longer than the mean fluorescence wavelength may be used for illumination of the medium to avoid non- radiative relaxation processes. The quantum efficiency at room temperature may be around 1.0 (e.g., above .9, e.g., above .95, e.g., above .99 or otherwise around 1.0) at such wavelengths for some implementations. This is an effect similar to zero phonon transitions in gamma ray spectroscopy because the excitation of the bulk crystal can be avoided while using electronic transitions of dopant ions. Thus, the quantum efficiency of the host material with the chromium-based dopant may be above that of titanium sapphire, which may have a quantum efficiency around 0.8.

[0018] Unexpectedly, the cooling efficiency of the chromium-based dopant combined with the selected host material may be greater than that of competing cooling technologies. For example, the cooling efficiency of the chromium-based dopant combined with the selected host material may be up to four times (or more) than that of Peltier coolers and / or up to five times (or more) than that of dipole-disallowed laser cooling. Moreover, the system may support cooling from ambient temperatures down to cryogenic temperatures, without necessarily using multi-stage cooling techniques. In comparison, Peltier coolers do not necessarily support cooling to cryogenictemperatures. Conversely, dipole-disallowed laser cooling may not support cooling starting at ambient temperatures. Accordingly, a target may need to be pre-cooled to support dipole-disallowed laser cooling and cooling of a target may need to be taken over from Peltier coolers once the object cools below the operational range of the Peltier. In comparison, the system may operate as a single technology cooling solution for a broad cooling range. The broad range of cooling applications that can be supported, at high comparative efficiency, by this optical cooling technology will drive commercial adoption. Nevertheless, the dipole-allowed cooling technologies described herein may optionally and readily integrated with other cooling technologies in multi-stage solutions, e.g. to leverage the efficiency advantages of dipole-allowed cooling, to cool a target below cryogenic temperatures, and / or to support other robust and flexible cooling applications.

[0019] Referring now to Figure 1 , an example optical cooling system (OCS) 100 is shown. The example OCS 100 may include a target cooling medium 102. The target cooling medium 102 may be made up of a material (e.g., with a mass) characterized by an absorption band corresponding to one or more electric-dipole-allowed transitions. In some implementations, the cooling medium 102 may include a chromium-based dopant within a host material characterized by a weak-crystal field. The absorption band may have a corresponding fluorescence spectrum (e.g., when the material is excited via illumination within the absorption band). The fluorescence spectrum may be characterized by one or more emission bands.

[0020] The OCS 100 may further include an illumination source 104. The illumination source may illuminate the medium with light at a selected wavelength within a portion of the corresponding fluorescence spectrum (including the long-wavelength tail portion). In some cases, the selected wavelength may be greater than an average fluorescence wavelength of the mass for the corresponding fluorescence spectrum. In various implementations, the illumination source 104 may provide light that is spectrally distributed. At least some of the light from the illumination source 104 may be at the selected wavelength, while other portions of the light from the illumination source 104 may be at one or more other wavelengths. Thus, the illumination source may illuminate 104 the medium 102 with light at the selected wavelength, and, in some cases, light at other wavelengths.

[0021] In various implementations, the illumination source may include a laser light source. In some cases, a low-entropy light source, such as a single-mode laser may be absorbed to cause anti-Stokes fluorescence in a dispersed form with greater entropy than that of the beam at the time of absorption. Accordingly, the light exiting the material is “hotter” (e.g., more disorganized) and more energetic photon by photon than the beam coming into the material. Hence, the light may carry heat (e.g., via disorganization) out of the material.

[0022] Various laser systems may be used as the illumination source, such as titanium sapphire lasers, indium gallium arsenide (InGaAs) lasers, other semiconductor lasers, or various other laser sources. The light source may be continuous-wave or pulsed.

[0023] In some cases, the illumination source may further be used to perform laser pumping for population inversion within the material. For example, when a lasing material, such as a Chromium-doped weak-crystal-field crystal, is used as the cooling target, the illumination source may double as a laser pump in addition to providing cooling. The combination of cooling and laser pumping may support a radiation- balanced laser. In some cases, uniform cooling (or non-uniform cooling with the same spatial profile as the pumping power) by the illumination source may mitigate thermal effects normally present due to heating by the pump laser, such as thermal lensing. In some cases, this may allow for higher pumping powers than that achievable without self-cooling or radiation balancing.

[0024] In some cases, pumping for lasing in the medium may be provided using a laser pump that is separate from the illumination source used for cooling.

[0025] Referring now to Figure 2, while continuing to refer to Figure 1 , an example method 150 for optical cooling is shown. At 152, it may be determined to cool a chromium-doped medium 102 using light at a selected wavelength. The wavelength may be selected based on absorption and / or emission bands corresponding to electric-dipole-allowed transitions.

[0026] At 154, cooling may be implemented via illumination of the medium by the illumination source 104 with light at the selected wavelength. As discussed above, illumination of the chromium-doped medium 102 via the illumination source 104 maycause excitation of particles in the material which may lead to eventual relaxation via the electric-dipole-allowed transitions. The emissions associated with the electricdipole-allowed transitions may correspond to higher energy photons than that of the light at the selected wavelength. Thus, the excitation-emission cycle may, on average, carry energy out of the medium (e.g., resulting in cooling).

[0027] At 156, the cooling may be executed in accord with a selected cooling scheme. For example, the material may be continuously and / or continually refrigerated by constant and / or repetitive exposure to the light at the selected wavelength. For example, the material may be cooled to a specific temperature and / or held within a specific temperature range. For example, the medium may be cooled to a cryogenic temperature and / or held within a cryogenic temperature range.

[0028] In various implementations, the cooling medium may include a doped host material. The host material may cause a shift within the excitation energy levels of the dopant such that a dipole-allowed transition is shifted below any disallowed transition of the dopant such that a dipole-allowed transition is shifted to a lower energy than any disallowed transition of the dopant. Accordingly, the dipole-allowed transition may originate from a lowest energy level within a grouping of levels. Thus, excitation of the dipole-allowed transition may decay without necessarily being mediated by the disallowed transition. In some implementations, the host material may cause the shift within the excitation energy levels due to a crystal field of the host material, present due to a crystal structure of the host material. In some cases, a crystal with a “weak” crystal field may be used. Nevertheless, in some cases, crystal host materials that provide the above-described relative shift in the excitation levels may be used regardless of whether the crystal field of the material is weak.

[0029] For example, the medium may be cooled without net cooling by the cooling process. For example, the cooling may be implemented to counteract (in part) heating done by the illumination source itself. For example, the medium may include a lasing medium pumped by the illumination source. In the absence of cooling, the pumping process generates net heat at a higher level than when the illumination source is also tuned to effect simultaneous cooling. Thus, cooling requirements for such a lasing system (e.g., a radiation “sub-balanced” laser) may be relaxed relative to cooling requirements for systems or applications that require net cooling.

[0030] For example, the material may be cooled in accord with specific timings and / or specific target cooling rates. In some implementations, various criteria for cooling may be set, e.g., initiate cooling when the material exceeds a threshold temperature, cease cooling when the material falls below a threshold temperature; e.g., initiate cooling when the system is exposed to solar (or other celestial) radiation; and / or other cooling criteria.

[0031] Example Implementations

[0032] Various illustrative example implementations are included in the drawing sheets (for clarity of presentation) and below. The various illustrative example implementations included in the drawing sheets should be treated as if included in the specification as indicated below. The illustrative example implementations are illustrative of the general architectures and techniques described above and in the claims below. Designations of particular features such as “key”, “critical”, “important”, “essential”, “must”, and / or other similar designations are included to clarify the relationship of that particular feature to the specific illustrative scenario / scenarios in which the particular feature is discussed. Such a relationship to the same degree may not apply without express description of such a relationship to other implementations. Nevertheless, the various features described with respect to the individual example implementations may be readily and optionally integrated with other implementations with or without various other features present in the respective example implementation.

[0033] In an illustrative example scenario, a system may perform optical refrigeration (e.g., cooling) of Cr3+:LiSAF (chromium(lll+)-doped Lithium strontium hexafluoroaluminate) on a dipole-allowed transition. This constitutes cooling on an electric-dipole-allowed transition in a bulk solid. In some cases, electric-dipole-allowed transitions may support more rapid cooling than forbidden transitions of rare earths. Further, Cr3+:LiSAF crystals may serve as a substrate material suitable for the growth of lll-V semiconductor circuits. This may support imaging arrays with improved signal- to-noise performance at cryogenic temperatures for sensing applications in outer space. In an illustrative example scenario, Cr3+:LiCAF (chromium(lll+)-doped Lithiumcalcium hexafluoroaluminate) may be used as a cooling material via dipole-allowed transitions, a lasing material, and / or a cooling substrate for various devices.

[0034] In an illustrative example, a method is disclosed for speeding up laser refrigeration in bulk, solid material based on anti-Stokes fluorescent emission by utilizing electric-dipole-allowed transitions. The method is also capable of achieving cooling efficiencies much higher than Yb-doped compounds. Cr3+:LiSaF is an example of a crystal in which the 4T energy levels of Cr3+lie below the 2E levels, giving rise to strong, broadband, allowed transitions of the transition metal dopant in the near infrared spectral region. Because the lowest-lying ground state absorption is an electric-dipole transition, anti-Stokes cooling mediated by this transition is much faster than laser cooling on forbidden transitions that have been used to date. Results of thermal lens spectroscopy in samples of 4% Cr3+:LiSaF in the wavelength range 750- 1000 nm have shown that the cooling threshold can be reached even when the limiting background absorption coefficient is as high as 10'3cm-1. The cooling performance of this material may depend on the optical polarization, propagation direction of light, and background impurity absorption. For the an example orientation of the optical interaction, a cooling efficiency of 20% is predicted in crystals of LiSAF with reduced impurity concentration yielding a background absorption of only 10-4 cm-1. Such a cooling efficiency may be more than the reported efficiency of purified Yb3+:LiYF4, which is the best optical cooling solid reported to date, and may be higher than the efficiency of Peltier coolers in general. In some case Cr3+doping of the host crystal LiCaF may outperform Cr3+:LiSaF as a cooling medium. Laser cooling is capable of reaching cryogenic temperatures in vacuum. Hence the improvement offered by this method may benefit refrigeration of computers and sensors in space as well as improve earthbound applications that previously relied solely on Peltier cooling.

[0035] Figure 3 shows example plots of cooling efficiency data 300, 350. As a working example system, cooling efficiency data 300, 350 for a Cr3+:LiSAF cooling medium for two different input laser parameter sets is shown. The cooling efficiency is shown to fit with various models. Thus, showing agreement between modeled efficiency and those achieved via operation of the system.

[0036] Figure 4 shows an example plot of cooling efficiency data 400. As a working example system, cooling efficiency data another Cr3+: LiSAF cooling medium is shown.At various input parameters, which correspond to cooling operation at over a 100nm band from ~850nm wavelengths to over ~1000nm wavelengths, the system shows data points 401 , 402, 403 indicative of net cooling. Thus, in this working example, the cooling medium may be used from refrigeration in real world applications.

[0037] Various examples have been shown. Various other examples are possible.

[0038] Table 1 shows various examples.

[0039] The present disclosure has been described with reference to specific examples that are intended to be illustrative only and not to be limiting of the disclosure. Changes, additions and / or deletions may be made to the examples without departing from the spirit and scope of the disclosure.

[0040] The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom.

Claims

What is claimed is:

1. A method includes: cooling a medium by illuminating the medium with light at a selected wavelength for an emission band of a corresponding fluorescence spectrum, the cooling medium including: a chromium-based dopant characterized by a selected absorption band of one or more electric-dipole-allowed transitions, the selected absorption band having the corresponding fluorescence spectrum where: the selected wavelength is greater than an average fluorescence wavelength of the medium for the corresponding fluorescence spectrum; and a host material for the dopant, the one or more electric dipole allowed transitions shifted below a disallowed transition, the shift caused by a crystal field of the host material, the crystal field including a weak crystal field.

2. The method of claim 1 , where the chromium-based dopant includes trivalent chromium ions.

3. The method of claim 1 , where the crystal field of the host material causes at least a dipole allowed quartet level to descend below a dipole-disallowed doublet energy level.

4. The method of claim 1 , where the host material includes a fluoroaluminate crystal.

5. The method of claim 1 , where the host material includes a lithium strontium hexafluoroaluminate crystal.

6. The method of claim 1 , where the host material includes a lithium calcium hexafluoroaluminate crystal.

7. The method of claim 1 , where the medium includes a cooling substrate for components disposed on the cooling substrate.

8. The method of claim 1 , where the medium includes a lasing medium of a radiation-balanced laser.

9. The method of claim 1 , where the method is implemented in an outer space environment.

10. The method of claim 1 , where unabsorbed and / or emitted light escapes from the medium due to a refractive index of the medium, where the refractive index of the medium is less than that of silica glass.

11. A device includes:A cooling medium including: a chromium-based dopant characterized by a selected absorption band of one or more electric-dipole-allowed transitions, the selected absorption band having a corresponding fluorescence spectrum where: a selected wavelength for an emission band of the corresponding fluorescence spectrum is greater than an average fluorescence wavelength of the cooling medium for the corresponding fluorescence spectrum; and a host material for the dopant, the one or more electric dipole allowed transitions shifted below a disallowed transition, the shift caused by a crystal field of the host material, the crystal field including a weak crystal field; and an illuminator configured to illuminate the cooling medium with light at the selected wavelength.

12. The device of claim 11 , where the chromium-based dopant includes trivalent chromium ions.

13. The device of claim 11 , where the crystal field of the host material causes at least a dipole allowed quartet level to descend below a dipole-disallowed doublet energy level.

14. The device of claim 11 , where the host material includes a fluoroaluminate crystal.

15. The device of claim 11 , where the host material includes a lithium strontium hexafluoroaluminate crystal.

16. The device of claim 11 , where the host material includes a lithium calcium hexafluoroaluminate crystal.

17. The device of claim 11 , where the cooling medium includes a cooling substrate for components disposed on the cooling substrate.

18. The device of claim 11 , where the cooling medium includes a lasing medium of a radiation-balanced laser.

19. The device of claim 11 , where the cooling medium includes a component within a spacecraft.

20. A method includes: cooling a medium by illuminating the medium with light at a selected wavelength for an emission band of a corresponding fluorescence spectrum, the cooling medium including: a chromium-based dopant characterized by a selected absorption band of one or more electric-dipole-allowed transitions, the selected absorption band having the corresponding fluorescence spectrum where: the selected wavelength is greater than an average fluorescence wavelength of the medium for the corresponding fluorescence spectrum; anda host material for the dopant, the one or more electric dipole allowed transitions shifted to be a lowest energy level of the chromium-based dopant, the shift caused by a crystal field of the host material, the crystal field including a weak crystal field.

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