Although the output power of current commercial fiber lasers has been reported to exceed 500kW, the heat generated within fiber gain-media has limited the generation of higher laser powers due to thermal lensing and melting of the gain -media at high temperatures. Radiation balanced fiber lasers promise to mitigate detrimental thermal effects within fiber gain- media based on using upconverted, anti-Stokes photoluminescence to extract heat from the optical fiber core. In this manuscript, we demonstrate that Yb(III) ions within YLiF<sub>4</sub> (YLF) crystals are capable of cooling both the core and cladding of optical fibers. We also demonstrate a novel radiation balanced fiber laser design using a composite fiber cladding material that incorporates YLF nanocrystals as the active photonic heat engine. YLF crystals are mixed with the cladding material to mitigate thermal gradients within the core and cladding. Analytical models of heat transfer within the fiber are presented where the electric- field amplitude within the fiber core is responsible for both the heating of the core, and also the excitation of Yb(III) ions for anti-Stokes laser refrigeration in the cladding.
Laser radiation has conventionally been used to cool the mechanical amplitude of oscillators with approaches based on electronic feedback and cavity-induced radiation pressure. However, the direct laser refrigeration of an optomechanical oscillator has remained a challenge. Optically refrigerating the lattice of an optical resonator promises to impact several fields including the development of radiation balanced lasers. In this work, we demonstrate laser refrigeration of a hydrothermally synthesized 10 % ytterbium (Yb<sup>3+</sup>) doped lithium yttrium fluoride (YLF) crystal placed at the free end of a cadmium sulfide nanoribbon (CdSNR). An incident 1020 nm laser is used to cool the crystal and the back-scattered up-converted Yb<sup>3+</sup> emission is analyzed using two-band differential luminescence thermometry (DLT) to monitor the temperature of the YLF crystal. A temperature drop of 23.6 K below room temperature is recorded at a focused laser power of 40.1 mW. Lastly, a combination of finite element wave optics and heat transfer calculations were used to estimate the imaginary part of the refractive index of the YLF crystal.
Recent experimental breakthroughs in the laser-refrigeration-of-solids (LRS) have demonstrated that cryogenic temperatures can now be achieved opening up a range of promising applications using compact, vibration-free optical cryocoolers. These results also have stimulated significant interest in the development of new material designs for applications in radiation balanced lasing (RBL). The development of practical host materials for RBLs requires the understanding of how both spontaneous emission rates and non-radiative decay rates change under a wide range of thermal conditions and dielectric host environments. In this work the photoluminescence lifetime of 4S3/2 transitions from Er(III) ions within co-doped Yb3+/Er3+-codoped hexagonal sodium-yttrium-fluoride (beta-NaYF4) nanostructures is presented as a rapid, low-cost, spatially resolved method of quantifying the temperature of within RBL materials. Lifetime measurements from single nanostructures are made using single-beam laser-traps, where the focal plane of the trapping laser is used to control the spacing between single nanowires and dielectric chamber surfaces that are supported by a temperature-controlled piezo-stage. The lifetime of Er(III) ions is observed to change significantly based on the distance between emitting dipoles and nearby dielectric interfaces and also as a function of chamber temperature. Lifetime measurements are also presented for measuring the temperature within polydimenthylsiloxane-polymer/nanocrystal composite materials that serve as a model system for future optical-fiber cladding materials. Lastly, ratiometric photoluminescence and lifetime measurements will be presented for Yb(III):YLiF4 microcrystals supported on cadmium sulfide nanoribbon cantilevers, indicating the potential for hybrid semiconductor/RE-fluoride composite structures for future RBL applications.
Cadmium sulfide (CdS) has been studied for decades due to a variety of applications from photovoltaic cells to solid state lasers owing to its direct bandgap of 2.42 eV (512 nm) and a high radiative quantum efficiency. Recently it has been considered to be a potential candidate for radiation balanced lasing. In particular, nanoribbons (NRs) of CdS have been claimed to laser cool at 514 and 532 nm wavelengths, due to annihilation of phonons to produce antistokes fluorescence near the bandgap. In an effort to verify the claim, we demonstrate a novel optomechanical experimental technique for micro-thermometry of a single CdS-NR where the material’s Young’s modulus is the primary temperature-dependent observable. The eigenfrequency of individual cantilevers is measured as a function of the laser irradiance by processing the time-dependent photovoltage of an avalanche photodiode. We observe a red-shift in the cantilever’s eigenfrequency with increasing laser power, suggesting net heating at low laser irradiances. However, a heating effect combined with possible laser trapping forces has been hypothesized for higher powers based on a modified Euler-Bernoulli elastic-beam model. The experimental cantilever heating results are supported by a heat transfer analysis to obtain the temperature distribution in the cantilever and the time required to reach steady state (<1ms). This thermometry technique can be used to probe the effects of laser irradiation on CdS cantilevers fabricated from thin films grown by pulsed laser deposition.