P. P. Ho

Ballistic 2-D Imaging Through Scattering Walls Using an Ultrafast Optical Kerr Gate

An ultrafast optical shutter was used to image ultrasmall objects hidden behind scattering walls by a procedure that selects in time the ballistic component and rejects the scattered diffusive light. Scattering walls used in this experiment included human breast tissue, chicken breast tissue, and a water suspension of polystyrene particles with scattering coefficients up to 21.7. Submillimeter resolution was achieved for two-dimensional ballistic images of a single point, a double-point fluorescence source, and a bar test chart in or behind these different turbid media.

Third-order optical nonlinearity in polycondensed thiophene-based polymers and polysilane polymers

The relaxation kinetics and size of the third-order nonlinear susceptibility χ(3) of a homologous series of polycondensed thiophene-based polymers and polysilane polymers were determined by ultrafast nonlinear laser techniques. The wavelength dependence over 530–1060 nm of χ(3) was measured for polycondensed thiophene-based polymers. The resonant χ(3) was determined to be of the order of 10−9 esu. For polysilane polymers, the measured nonresonant χ(3) value was ~10−12 esu. The response time of χ(3) for both polymers was determined to be less than the resolution of optical pulse durations, <3 psec for polysilane and <12 psec for polycondensed thiophene-based polymers.

Optical harmonic generation from animal tissues by the use of picosecond and femtosecond laser pulses.

Second- and third-harmonic generations of femtosecond and picosecond laser pulses have been measured from chicken skin, muscle, and fat tissues. The magnitude of the harmonic signals showed a strong structural dependence with the signal from skin interface being the strongest. The polarization dependence of the signal was also measured and found to be consistent with the fact that the tissue samples were highly scattering random media. The second-harmonic- and third-harmonic-generation conversion efficiencies were found to be in the range of ~10(-7) to ~10(-10).

Kerr - Fourier imaging of hidden objects in thick turbid media.

Test bar charts hidden in a 5.5-cm-thick 2.5% Intralipid solution were imaged as a function of phantom depth and size with steady-state Fourier and picosecond Kerr–Fourier imaging systems. With time and space gating, a series of 250-μm bars placed in a thick highly scattering medium were resolved at a signal level of ~10−10 of the illumination intensity.

Time-resolved Fourier spectrum and imaging in highly scattering media

Time and spatial-gated Fourier spectra and imaging were measured and analyzed. A picosecond Kerr-Fourier gate was used to image objects by selecting the spatial frequencies of objects illuminated by a laser pulse passing through a thick turbid medium. The earlier arriving ballistic/snake light and most of the later scattered light were spatially filtered and temporally separated to form an image. The image contrast and the signal-to-noise ratio of hidden objects in turbid media were greatly improved with the addition of Fourier spatial filtering.

Time-resolved imaging of translucent droplets in highly scattering turbid media.

The spatial distribution of small translucent droplets inside a 50-millimeter-thick Intralipid solution was imaged with a picosecond time and spatial-gated Kerr-Fourier imaging system at a signal level of about 10–10 of the incident illumination intensity.

Fourier spatial filter acts as a temporal gate for light propagating through a turbid medium

The temporal profiles of ultrashort light pulses propagating through turbid media of different lengths were measured with a streak camera. Various Fourier spatial filters were used to select the spatial frequencies of the scattered pulses. The temporal profiles of the pulses scattered by a 0.4% Intralipid solution in a cell of 5-cm thickness were signif icantly narrowed because of the removal of the higher-frequency components by a Fourier spatial filter.

Ultrafast resonant optical Kerr effect in 4-butoxycarbonylmethylurethane polydiacetylene

The resonant optical Kerr effect in the isotropic phase of 4-butoxycarbonylmethylurethane polydiacetylene was measured to determine the third-order optical nonlinearity χ3 and the relaxation time.

Two-photon excitation of fluorescence from chicken tissue

We have measured UV fluorescence excited through two-photon absorption from native chicken tissue, using 600-nm, 500-fs pulses from a R6G dye laser. The observed emission signal was found to depend quadratically on the excitation intensity. The two-photon excitation-induced fluorescence spectrum is attributed to tryptophan residues in proteins.

Double-stage picosecond Kerr gate for ballistic time-gated optical imaging in turbid media

We demonstrate that use of a picosecond double-stage Kerr gate system results in a 3-orders-of magnitude improvement in signal-to-noise ratio and a threefold improvement in shutter speed comparedwith those of a single-stage Kerr gate.