Light at wavelengths in the near-infrared (NIR) region allows for deep penetration and minimal absorption through high scattering tissue media. NIR light has been conventionally used through the first NIR optical tissue window with wavelengths from 650 to 950 nm. Longer NIR wavelengths had been overlooked due to major water absorption peaks and a lack of NIR-CCD detectors. The second NIR spectral window from 1100 to 1350 nm and a new spectral window from 1600 to 1870 nm, known as the third NIR optical window, were investigated. Optical attenuation measurements from thin tissue slices of normal and malignant breast and prostate tissues, pig brain, and chicken tissue were obtained in the spectral range from 400 to 2500 nm. Optical images of chicken tissue overlying three black wires were also obtained using the second and third spectral windows. Due to a reduction in scattering and minimal absorption, longer attenuation lengths and clearer optical images could be seen in the second and third NIR optical windows compared to the conventional first NIR optical window. A possible fourth optical window centered at 2200 nm was noted.
The observation that welding strength as a function of wavelength follows the absorption bands of water suggests that absorption of light by water plays a significant role in laser tissue welding.
A novel technique in time resolved luminescence spectroscopy called population mixing using a subpicosecond cw mode-locked dye laser has been developed and applied to p-type GaAs at low temperatures. Using this technique the relaxation lifetime for electron recombination was measured to be 39±7 ps for p-type GaAs with Zn at 6×1018 cm−3 hole concentration. This is comparable to the relaxation time measured by a streak camera.
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