Hexagonal microcavities have been proposed for a wide range of applications including microlasers, levitated optomechanics, quantum information science, and biosensors that make use of both Fabry-Perot and whispering gallery mode cavity resonances. Photothermal heating impacts a number of optical and mechanical properties of hexagonal microcavities based on the temperature dependence of quantities such as radiative lifetime, Young’s modulus, optical index of refraction, and the corresponding wavelength of cavity mode resonances. This talk will present recent results in both analytical and numerical modeling of photothermal heating in hexagonal cavities inspired by recent optomechanical levitation experiments in both aqueous and high-vacuum environments.
Recent advances in the synthesis of inorganic materials have enabled a wide range of control of particle size, composition, phase, and morphology that can be tuned in the design of future levitated optomechanical sensors. This talk will describe recent results in the hydrothermal synthesis and characterization of two-dimensional microprisms of hexagonal sodium yttrium fluoride crystals based on novel molecular ligands. Hexagonal cavities with high aspect with high aspect ratios of ~50 are observed with potential applications in the detection of high frequency gravitational waves, radiation balanced microlasers, and the solid-state laser cooling of quantum sensors.
Negatively charged nitrogen-vacancy (NV-) centers in diamond have a plethora of potential applications in quantum systems, including sensing and computing1-3. Photothermal heating can limit the utility of NV- center nanodiamonds, especially under high laser irradiances4-6. A composite of nanodiamonds with NV- defects and ytterbium-doped cubic sodium yttrium fluoride (Yb:α-NaYF4 or NaYF) could offset the photothermal heating of nanodiamonds by the anti-Stokes fluorescence cooling of Yb3+ ions7. We present a novel preparation method for generating a NV- diamond NaYF composite material based on a hydrothermal synthesis approach. Particle size was determined to be 230 ± 90 nm by SEM, and DLS data show a permanent connection between nanodiamonds and NaYF. Nanodiamonds are observed on the surfaces of NaYF materials. Nanodiamonds may also be incorporated within the body of individual NaYF grains, however the question of whether nanodiamonds are fully incorporated into the host NaYF material remains to be answered. The temperatures of host material and NV- defects are accessed using mean fluorescence wavelength shifts and Debye-Waller factor thermometry respectively. The obtained temperature changes with increasing 1020 nm irradiance show good agreement. Two data sets showed photothermal heating of around 10 and 13 K at 6.3 MW/cm2. Increased particle smoothness and sizes could lead to coolable composite materials.
Microlaser designs based on the coupling of whispering gallery modes (WGMs) with the upconversion processes which take place within lanthanide-doped nanoparticles (UCNPs) have been demonstrated and shown to have many valuable qualities, such as high Q factors and low lasing thresholds. One obstacle that these microlaser designs still face is the challenges caused by photothermal heating of the gain medium, which could be solved through the design of a radiation balanced microlaser. In this work, WGM microresonators composed of 5 μm diameter polystyrene spheres are fabricated with a layer of Yb3+-doped NaYF4 UCNPs in order to test if the anti-Stokes cooling properties of the UCNPs can cool the microresonator and its environment under laser irradiation. We find via calibrated mean fluorescence spectroscopy that the UCNPs can cool their local environment by as much as 23 °C and significantly reduce the heating of the aqueous environment surrounding the microresonator, showing promise for inclusion in a design for a radiation balanced microlaser.
We present the first experimental optical trapping of ytterbium-doped sodium yttrium fluoride (Yb:NaYF4) hexagonal microdisks with a dual-beam dipole trap. These high-aspect-ratio hexagonal microdisks exhibit reduced photon recoil heating due to light scattering while allowing for 10s of kHz mechanical frequencies. These features make them good candidates as force sensors for the Levitated Sensor Detector (LSD) project, which detects high-frequency gravitational waves above the region previously probed by LIGO. We discuss motional dynamics of these microdisks by showing their motional spectra in comparison with analytical and numerical models and the recent progress of 1-meter LSD prototype that is under development at Northwestern University.
The H3 center in diamond has been shown to exhibit several promising characteristics for laser cooling applications including a neutral charge state, high radiative quantum yield, and efficient anti-Stokes photoluminescence. In this work, we show that upon excitation with a 532 nm laser, bulk diamond crystals doped with H3 centers emit efficient up-conversion photoluminescence and also show significantly reduced photothermal heating relative to crystals doped with NV centers. These results encourage future exploration of techniques for H3 enrichment in diamonds at high-pressure, high-temperature conditions for the simultaneous anti-Stokes fluorescence cooling and radiation balanced lasing in semiconductor materials.
Heat engines for space applications and radiation balanced lasers need to withstand radiation damage. The lasere cooling function of Yb:YLF microcrystals after X-ray radiation was studied. F-centers stable at room temperature were induced after X-ray irradiation. Such point defects were confirmed with electron paramagnetic resonance spectroscopy and thermal luminescence. The cooling performance of the YLF microcrystals deteriorated after irradiation, which is caused by F-center induced increased background absorption and/or higher non-radiative relaxation rate. The absorption spectrum and Yb3+ excited state lifetime were measured to investigate the failure of optical refrigeration in X-ray irradiated Yb:YLF microcrystals.
Hexagonal sodium yttrium fluoride (β-NaYF) is a promising material for optical refrigeration due to the narrow crystal field splitting of the Yb(III) ion. However, growing single crystals of β-NaYF remains a challenge due to thermal expansion stresses during melt growth. We demonstrate a hydrothermal synthesis of β-NaYF with widely tunable aspect ratios that match computationally predicted cavity resonances. The β-NaYF microcrystals contain 10% Yb(III) cations and are used to build optomechanical laser-refrigeration cantilever devices. Laser refrigeration of these devices shows cooling up to 12.5°C, which is measured using the cantilever’s fundamental eigenfrequency and photoluminescence from the Yb(III) ions.
Optically levitated nanoparticles provide excellent systems to sense minute forces and explore quantum effects in a large system. However, optically levitated nanoparticles are prone to heating and require cooling and temperature stabilization to reach sensitivities necessary to study small forces and quantum effects. This problem can be solved by trapping nanocrystals doped with rare-earth ions that can be anti-Stokes cooled by tens of degrees. The efficiency of the anti-Stokes depends on gas pressure and can counter heating due to optical absorption. Cooling nanocrystals allows for thermally stabilizing nanocrystal systems to measure minute forces and quantum effects.
KEYWORDS: Spherical lenses, Temperature metrology, Solid state lasers, Silica, Nanoparticles, Systems modeling, Solid state physics, Sensors, Sensor performance, Optical spheres
Microlasers based on lanthanide-doped upconverting nanoparticles (UCNPs) have been demonstrated but radia- tion balanced lasing (RBL) remains a challenge. We present a novel design of radiation-balanced microlaser using microspheres coated with UCNPs. The model is tested using nitrogen vacancy doped nanodiamonds (NV:NDs) coated on silica microspheres. High quality factor enhancement of selective bands in the NV spectral features due to coupled whispering gallery modes (WGMs) was measured. The temperature of NDs on silica spheres was measured experimentally using Debye-Waller factor thermometry and analyzed using a novel analytical heat transfer model for spherical coordinates with localized sources.
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