The absorption and the emission spectra of the four ionic species formed by 3-, 6-, and 7-hydroxyquinoline in aqueous solution have been measured, and the constants governing the prototropic equilibria between the ionic forms of each compound in the lowest singlet excited state have been estimated theoretically from the transition energies and experimentally from the change in fluorescence spectrum with hydrogen-ion concentration. It is found that the phenolic group of the hydroxyquinolines is more acidic, and the ring nitrogen atom more basic, in the excited than in the ground electronic state, as expected from the theoretical electronic charge redistribution on excitation derived from a corresponding-carbanion model. The overall velocity of the two-stage prototropic change in the excited state from the neutral molecule to the zwitterion of the hydroxyquinolines in ethanol or neutral aqueous solution is found to be slower than the rate of emission, but the corresponding one-stage processes in acid or alkaline solution are comparable in rate or faster.

Coherent laser radiation has enabled many scientific and technological breakthroughs including Bose-Einstein condensates, ultrafast spectroscopy, superresolution optical microscopy, photothermal therapy, and long-distance telecommunications. However, it has remained a challenge to refrigerate liquid media (including physiological buffers) during laser illumination due to significant background solvent absorption and the rapid (∼ps) nonradiative vibrational relaxation of molecular electronic excited states. Here we demonstrate that single-beam laser trapping can be used to induce and quantify the local refrigeration of physiological media by >10°C following the emission of photoluminescence from upconverting yttrium lithium fluoride (YLF) nanocrystals. A simple, low-cost hydrothermal approach is used to synthesize polycrystalline particles with sizes ranging from <200 nm to >1 μm.

Sodium yttrium fluoride (β-NaYF ) nanowires (NWs) with a hexagonal crystal structure are synthesized using a low-cost hydrothermal process and are shown to undergo laser refrigeration based on an upconversion process leading to anti-Stokes (blueshifted) photoluminescence. Single-beam laser trapping combined with forward light scattering is used to investigate cryophotonic laser refrigeration of individual NWs through analysis of their local Brownian dynamics.

A theoretical model is developed here in tandem with single-beam laser trapping experiments to elucidate the effects of the numerous thermal, optical, and geometric parameters that affect internal temperature distributions within finite nanowires (NWs) during laser irradiation. Analytical solutions to the heat-transfer equation are presented to predict internal temperature distributions within individual nanowires based on numerical calculations of the internal electromagnetic heat source. Single-beam laser-trapping experiments are performed to measure photothermal heating of silicon NWs. Silicon has not been considered to date for photothermal heating applications due to its indirect band gap and low absorption coefficient in the near-infrared tissue-transparency window. We also show here that ion implantation may be used to increase the optical absorption of silicon nanowires (SiNWs), leading to significant heating to temperatures greater than 42 °C in an aqueous environment at an irradiance of 3 MW/cm2. Experimental observations of photothermal heating agree well with theoretical predictions. Calculations for comparison with amorphous carbon NWs reveal significantly greater heating effects, as well as internal radial gradients not observed for SiNWs.

Cadmium sulfide (CdS) nanostructures have attracted a significant amount of attention for a variety of optoelectronic applications including photovoltaic cells, semiconductor lasers, and solid-state laser refrigeration due to their direct bandgap around 2.42 eV and high radiative quantum efficiency. Nanoribbons (NRs) of CdS have been claimed to laser cool following excitation at 514 and 532 nm wavelengths by the annihilation of optical phonons during anti-Stokes photoluminescence. To explore this claim, we demonstrate a novel optomechanical experimental technique for microthermometry of a CdSNR cantilever using Young’s modulus as the primary temperature-dependent observable. Measurements of the cantilever’s fundamental acoustic eigenfrequency at low laser powers showed a red-shift in the eigenfrequency with increasing power, suggesting net heating. At high laser powers, a decrease in the rate of red-shift of the eigenfrequency is explained using Euler–Bernoulli elastic beam theory, considering Hookean optical-trapping force. A predicted imaginary refractive index for CdSNR based on experimental temperature measurement agrees well with a heat transfer analysis that predicts the temperature distribution within the cantilever and the time required to reach steady state (<100 μs). This approach is useful for investigating solid-state laser refrigeration of a wide variety of material systems without the need for complex pump/probe spectroscopy.