No abstract

The extension of an earlier monolithic photopyroelectric spectroscopic model [A. Mandelis et al., Phys. Rev. B 48, 6808 (1993)] to include surface and bulk optical absorptions and the subsequent optical-to-thermal (nonradiative) energy conversion efficiencies associated with these sites has allowed the self-consistent determination of visible spectra of these quantities, using a very sensitive noncontact experimental configuration to obtain in-phase and quadrature photopyroelectric spectra of Ti:sapphire crystals of two thicknesses and figures of merit.

A rigorous analytical treatment of nonradiative deexcitation and thermal-conduction transient evolution in solid-state laser media, resulting from intense optical pumping with rectangular pulses from a time-gated laser source, is presented. This situation arises in rate-window photothermal detection from laser rods with bulk and surface absorptions, the latter being due to polishing during the manufacturing of the rod. Numerical simulations of the theory show that the surface nonradiative ͑optical-to-thermal͒ energy conversion term is likely to dominate even at absorptances on the order of 1%-2%. Therefore, polishing optimization appears to be necessary in order to minimize laser losses at the surfaces when laser rods are active in a resonator cavity. The present theory also provides physical insights into the very different nature of the bulk-and surface-originating heat-conduction transients, as well as on the profile of the superposition photothermal wave form and its dependence on the optical, thermal, and metastable-state relaxation parameters. As a prelude for input to the photothermal theory, a treatment of the excited-state dynamics of a typical laser manifold pumped by an intense laser beam, away from or near saturation, is developed, and the luminescence and photothermal energy source profiles are calculated analytically.

The understanding of the problem of nonradiative energy conversion in solid-state laser materials is a key factor in improving the overall efficiency of solid-state lasers. Furthermore, the reduction of the heat generated in an optically pumped laser crystal can lead to several new applications of solid-state lasers, especially in the high-power region. To improve the quality of grown crystals, laser crystal growers require accurate techniques to perform the quality control that is so vital to improving the growth process.Using a time-domain approach and a time-domain theoretical treatment of the IR radiative emission signal, it was determined that one may probe nonradiative surface and bulk processes by monitoring different time ranges. Our results show that photothermal radiometry can be used as a single-ended technique to evaluate both the bulk and surface nonradiative energy conversion rates in a solid-state laser material. This technique was compared to the standard laser cavity technique and it was concluded that photothermal radiometry can provide additional information to the standard technique by identifying the sources of heat generation as either surface-or bulk-originating.

Single-gate boxcar-integrator time-domain photothermal radiometry (PTR) is proposed as a noncontact remote technique with a high signal-to-noise ratio that can evaluate the optical quality of the surface in a solid-state laser-gain medium such as Ti:sapphire. It was found that immediately after the boxcar-averaged laser-pulse cutoff, the PTR signal is dominated by the laser metastable-level transition lifetime. A PTR theoretical model was formulated to account for this effect and to deconvolute its contribution to the signal from surface absorption of the laser radiation. With the theoretical model, the surface contribution to experimental boxcar PTR signals was deconvoluted and the surface quality was quantified in terms of the surface nonradiative energy generation rate, as the percentage of the input optical power that is converted to heat.