David H. Leach

Photon lifetime within a droplet: temporal determination of elastic and stimulated Raman scattering

Time profiles of the elastically scattered and stimulated Raman scattered radiation from single ethanol droplets illuminated by 100-psec mode-locked pulses were measured with a streak camera. The Q factor of the droplet, which acts as an optical cavity, was deduced from the decay time of the internally trapped radiation. Based on the intensity dependence of time profiles, it is also deduced that the photon lifetime is limited by the depletion of the internal intensity in generating nonlinear-optical radiation.

Third-order optical sum-frequency generation in micrometer-sized liquid droplets.

A series of discrete emission peaks is detected from single micrometer-sized droplets. The series starts at the blue third harmonic of the input IR radiation and extends to the red as a result of four-wave mixing of the input radiation and the multiorder stimulated-Raman-scattering radiation. The detected third-harmonic intensity from a single liquid droplet is several orders of magnitude larger than that from an optical cell containing the same liquid.

The use of a charge‐coupled device and position sensitive resistive anode detector for multiorder spontaneous Raman spectroscopy from silicon

Two state‐of‐the‐art two‐dimensional photon detectors, a high quantum efficiency charge‐coupled device, and a microchannel plate photomultiplier with a position sensitive resistive anode (Mepsicron) have been used to observe spontaneous Raman scattering from multiorder phonon modes of Si in the backscattering geometry. These two detectors and an intensified linear photodiode array are compared using the multiorder spontaneous Raman signal from Si as a weak optical source. An attempt is made to assign the observed Raman scattering peaks in the first‐ through fourth‐order Raman shift regions to known peaks in the density of states of the appropriate Si phonon branches.

Precession of morphology-dependent resonances in nonspherical droplets

Temporally and spatially resolved measurements of stimulated Raman scattering from flowing ethanol droplets are presented. The observed temporal oscillations of stimulated Raman scattering from two segments of the droplet rim are 180° out of phase and dependent on the azimuthal mode number of the morphology-dependent resonance (MDR). The observed precession of the MDR about the symmetry axis of an oblate droplet is consistent with the angular momentum of the MDR, n, and with perturbation predictions of the frequency splitting of a (2n + 1)-degenerate MDR of a perfect sphere.

Stokes and anti-Stokes hyper-Raman scattering from benzene, deuterated benzene, and carbon tetrachloride

Abstract Hyper-Raman spectra from liquid benzene, deuterated benzene, and carbon tetrachloride are observed using a cw pumped Q -switched mode-locked Nd:YAG laser and a synchronously gated two-dimensional single-photon-counting detector. Both Stokes and anti-Stokes peaks are observed and assigned to hyper-Raman active vibrational modes, some of which are forbidden in the Raman and infrared spectra of benzene and deuterated benzene. The effect of the stimulated Raman process in populating the vibrational levels and modifying the hyper-Raman spectra is discussed.

Stimulated anti-Stokes Raman scattering in microdroplets.

Stimulated anti-Stokes Raman scattering (SARS) generated by one input beam is observed from CCI(4), ethanol, and water droplets. The first-order SARS intensity is approximately 10(4) times lower than the first-order stimulated-Raman-scattering (SRS) intensity for ethanol droplets. Simultaneous detection of SARS and SRS for water droplets shows an occasional lack of correlation between the SARS and SRS spectra.

Time-resolved shadowgraphs of large individual water and ethanol droplets vaporized by a pulsed CO 2 laser

Carbon dioxide laser-induced explosive vaporization of water and ethanol droplets at high laser fluence has been observed with time-resolved shadowgraphs. The asymmetry seen in the droplet vaporization can be qualitatively explained by comparison to the internal-field intensity distribution. A central green spot observed in the shadowgraph is attributed to the near-field distribution just outside the shadow face of the droplet when the droplet is illuminated by a visible laser. This spot can be used to probe the shape deformation and optical inhomogeneity of the droplet. The energy dependence of the explosive vaporization of water was also studied. Increasing the CO(2) laser fluence increases the rate of explosive vaporization.

Phonon-retention effects on stimulated Brillouin scattering from micrometer-sized droplets illuminated with multiple short laser pulses.

When micrometer-sized CS2 and CCl4 droplets are illuminated with multiple 100-ps-duration green laser pulses, which are separated by 13.2 or 171.6 ns, the first few scattered green pulses are weaker than the subsequent pulses. Our data indicate that the Brillouin amplification during a given input-laser pulse is affected by the retention of acoustic phonon amplitude that is generated by the preceding input-laser pulses. Our results suggest the retention of long-lived acoustic phonons generated by the near-forward stimulated Brillouin scattering in droplets.

Effect of the phase velocity and spatial overlap of spherical resonances on sum-frequency generation in droplets.

The spectrum of the third-order sum-frequency generation (TSFG) that results from the electric fields of the laser E(omega(L)) and of the first-order Stokes stimulated Raman scattering E(omega(1s)) is dependent on the droplet radius. In addition to the amplitude and spatial overlap of the three generating E(omega)s and of the generating and resultant waves, phase matching between the generating and TSFG waves is an important parameter. We discuss the range of phase velocities of the generating and TSFG waves, which must be tuned to coincide with spherical resonances in order to be guided waves circumnavigating the droplet rim.

Effect of the Goos–Hänchen shift on the geometrical-optics model for spherical-cavity mode spacing

In the geometrical-optics framework, the internal fields near morphology-dependent resonances (MDR) of dielectric spheres are represented by rays undergoing total internal reflection at the sphere surface. The round-trip path length of rays circumnavigating the sphere is used to compute the mode spacing of MDR’s. The Goos–Hanchen shift of the total internally reflected rays at the sphere surface is included in the ray picture to explain the qualitative behavior of the MDR frequency spacing in the Lorentz-Mie formalism for the entire size-parameter (circumference/wavelength) range. The MDR’s are characterized by a radial distance rm′. A connection between the ray picture and the Mie theory is established, based on rm′.