William P. Acker

Laser diagnostics for droplet characterization: Application of morphology dependent resonances

A micron-sized liquid droplet acts as a very high-quality factor optical cavity. The cavity modes are referred to as morphology-dependent resonances (MDRs), which are sensitive to the droplet size, shape, and inclusions. The MDRs greatly lower the input intensity needed to generate detectable amounts of linear and nonlinear optical radiation and exhibit sharp peaks in the spectra of elastic scattering, fluorescence, lasing, and stimulated Raman scattering. Novel spectroscopic-based techniques are now available for droplet diagnostics of its physical, chemical, and thermal properties. In particular, we will review how MDR related spectroscopy can be used to deduce the droplet size, shape, evaporation rate, surface tension, viscosity, near-surface temperature, species, and species concentration in multicomponent droplets.

Two-dimensional imaging of sprays with fluorescence, lasing, and stimulated Raman scattering

Two-dimensional fluorescence, lasing, and stimulated Raman scattering images of a hollow-cone nozzle spray are observed. The various constituents of the spray, such as vapor, liquid ligaments, small droplets, and large droplets, are distinguished by selectively imaging different colors associated with the inelastic light-scattering processes.

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.

Stimulated raman scattering of fuel droplets

The strong stimulated Raman scattering (SRS) from diesel fuel droplets has the potential of providing the relative concentration of multicomponent fuel and the absolute size of individual droplets. The morphology-dependent resonances (MDRs) of a sphere cause the droplet to act as an optical resonator which greatly lowers the SRS threshold. The number density, quality factor, and frequency shift of several MDRs are calculated as a function of the ratio of the index of refraction of the liquid and the surrounding gas, which approaches unity at the thermodynamic critical condition for the fuel spray. The SRS spectra of monodispersed droplets of toluene, pentane, Exxon-Aromatic-150, and Mobil D-2 are presented. The exponential growth region of the SRS intensity I1S as a function of the input laser intensity Iinput is investigated for the toluene carbon ring breathing mode v2 and the pentane C-H stretching region. The I1S ratio of toluene and pentane is measured as a function of the ratio of the toluene and pentane concentration for monodispersed droplets. The reduced fluctuation in I1S when Iinput is changed from multimode to single-mode is displayed as a histogram of the I1S of the v2 mode of toluene droplets.

Fine structures in the angular distribution of stimulated Raman scattering from single droplets

The angular distribution of the stimulated Raman scattering (SRS) from ethanol droplets is observed to be sinusoidal with n peaks, which are consistent with the mode number n of the morphology-dependent resonances (MDRs). Two distinct values of n depend on the illumination geometry and the linewidth of the input laser radiation. Angular fine structures of the SRS can be used to identify the n of the MDR.

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.

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.

Quadrupole mode in the extinction and elastic-scattering spectra of gold microstructures

The quadrupole mode and the retardation-broadened, wavelength-shifted dipole mode have been observed in the extinction and elastic-scattering spectra of two-dimensional arrays of Au microstructures immersed in air, water, and toluene. The electron-beam lithographically produced Au microstructures, resembling hemispheres with radius ~100 nm, were arranged in a square lattice with a center-to-center separation of 500 nm and situated on a 5-nm-thick layer of Cr coated on an optically flat quartz substrate.