Raman-active Crystal Research Articles

Quantum Nondemolition Measurements for Phonons by Using Squeezing Stokes Light

A scheme of quantum nondemolition (QND) measurement is proposed for phonons in a Raman-active crystal by using a beam of squeezed Stokes light. The results of the theoretical calculation show that the QND measurements for the quadrature component of the phonons can be performed.

Methods for increasing the efficiency of solid Raman lasers

It is shown theoretically that an efficient solid Raman laser capable of withstanding the action of high power radiation can be built by satisfying certain conditions, which follow from the theory of stimulated Raman scattering and from studies of the optical strength of Raman-active crystals. A prototype calcite Raman laser was found to have an efficiency of 37% in the conversion of the energy of the pump radiation pulses (λ = 0.53μ) into the first three Stokes components. The total power of all three components was 13.5 MW, the beam divergence was 11', and the duration of the output pulses was 20 nsec. The elements of this laser withstood several hundreds of shots without any sign of damage.

Theory of laser-materials damage by enhanced stimulated Raman scattering

An analysis indicates that Raman-active crystals fail at intensities If which are greater than the enhanced stimulated Raman-scattering threshold intensity IR by an amount IT that is generally of the order of or less than IR. A typical value of IR at the ruby frequency is a few gigawatts per square centimeter, which is less than other intrinsic-mechanism thresholds. At intensities I≳IR, the excess intensity I−IR is converted into Stokes radiation and phonons in a distance l≪d, where d is the thermal diffusion distance for a 10-nsec pulse. The temperature rise from the rapidly thermalized phonons in the volume x<d is sufficient to cause material failure when I≳If.

Stimulated Raman scattering: Enhanced Stokes gain and effects of anti-Stokes and parametric phonon processes

It is shown that the previous parametric-instability explanation of the jump in the Stokes intensity ${I}_{S}$ as a function of laser intensity ${I}_{L}$ in stimulated Raman scattering experiments is valid for many solids, liquids, and gases (with high optical dispersion $\frac{\mathrm{dn}}{d\ensuremath{\lambda}}$ and high Raman frequency ${\ensuremath{\omega}}_{f}$), while a few materials (low $\frac{\mathrm{dn}}{d\ensuremath{\lambda}}$ and ${\ensuremath{\omega}}_{f}$) should show an enhanced gain, ${I}_{S}\ensuremath{\sim}\mathrm{exp}{I}_{L}^{2}$, which is greater than the usual stimulated-Raman gain but not as great as the jump result. The previously anomalous experimental results of Grun, McQuillan, and Stoicheff and of others, which show the jump in ${I}_{S}$, and of Hagenlocker, Minck, and Rado, which show both types of behavior, are explained. The instability is expected to be important in laser damage of Raman-active crystals and possibly in determining the limiting diameter of self-focused beams. The transient solution for the case of high dispersion indicates that the steady state is not reached until a time much greater than the Raman phonon-relaxation time. It is also shown that a phonon parametric instability studied previously in another context can reduce the Stokes intensity. Experiments to detect the phonon instability and measure the magnitude and temperature dependence of half-frequency phonons $Q$ (with frequencies ${\ensuremath{\omega}}_{Q}+{\ensuremath{\omega}}_{\ensuremath{-}Q}={\ensuremath{\omega}}_{f}$) are suggested.