The transmission of THz, near-infrared (1030 nm), and green (515 nm) pulses through Eisenia andrei body wall is studied, which consists of epithelial layer and circular and longitudinal muscles. Samples with the full-body cross-section were also investigated. The transmitted power for the green pulses followed the Beer-Lambert law of exponential attenuation for all thicknesses and tissue structures. Different body wall and body center absorption coefficients were found in case of infrared pulses. In the THz range, the body wall absorption coefficient steadily increases from about 80 cm–1 at 0.2 THz to about 273 cm–1 at 2.5 THz. Numerical estimation indicates that THz pulses of 5-μJ energy and 1-kHz repetition rate (5-mW average power) cause only a small temperature increase of about 0.4 K, suggesting that heating has minor contribution to biological effectiveness.
We present designs of semiconductor contact grating high energy terahertz pulse sources pumped by femtosecond pulses in the 1 to 5 µm wavelength range. Nearly wavelength-independent diffraction efficiencies as high as 69% and 75% in the
±
1
st diffraction orders in the transverse electric field polarization state were predicted in GaAs and GaP, respectively, based on a rectangular grating. Numerical simulations—including, for the first time, to our knowledge, the effects of both a nonlinear refractive index and free carrier absorption—were performed to investigate the possible advantage of using longer pumping wavelengths to suppress the two- to seven-photon absorption. Conversion efficiency larger than 1.0% is predicted for both crystals. We also recognized that the nonlinear refractive index and the wavelength-dependent optical parametric amplifier efficiency can significantly reduce the overall terahertz generation efficiency; thus, optimum pump wavelengths exist for the highest conversion efficiency, which are 2 and
3
µ
m
for GaP and GaAs, respectively.
Simulations of contact grating design parameters in the wavelength range of 1.76-3.9 µm are presented for gallium arsenide and gallium phosphide. Diffraction efficiencies as high as 80% and 90% have been achieved in GaAs and GaP respectively.
We report a semiconductor contact grating terahertz source design based on a rectangular profile for phase-matched terahertz generation in the long infrared pump wavelength range. The calculations show that the best diffraction efficiency can be achieved by a filling factor significantly smaller than 50%. Furthermore, the possibility of diffraction efficiency enhancement was investigated by applying three different antireflective coating structures. Numerical simulations have indicated that at 2.06 μm and 3.0 μm pump wavelength, diffraction efficiencies greater than 91% and 89% can be achieved by adding an appropriate antireflective coating to the GaP and GaAs contact grating structure, respectively. In addition, numerical simulations were performed to investigate the influence of wall angles on diffraction efficiency. The results reveal that the wall angle does not significantly affect the diffraction efficiency: while keeping the wall angle deviation from the vertical below 25 degrees, the efficiency drop remains below 5% for otherwise optimal grating parameters.
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