Based on analyzing the condition of transmission through high density plasma and its ionization front, and the theory of detecting plasma with holographic interferometer, diagnostic experiments of ionization front and plasma ionized by high laser electric field has been reported in this paper. The Mach-Zehnder interferometer with the femosecond laser of center wavelength of 800nm and pulse duration of 58fs has been used to get interference images in CCD camera. By Abel inversion, the density map of plasma has been obtained based on the interference image. Evolution pictures of ionization front within the time or propagation distance have been reported and ionization front’s size has been estimated with the help of 4mm diameter of nozzle. One good diagnosis method for ionization front and plasma has been given within these experiments.
Research on terahertz wave transmission through plasma is significant for researches on plasma itself and transmission discipline of terahertz wave through plasma. It is possible for plasma with suitable density to be an available stealth outerwear for plane or missile in THz waveband. In this paper, plasma is gotten by ionizing inert gases such as argon and helium gases with pulsed high alternating voltage. With electro-optic pump-probe measurement, THz transmission phenomena through plasma have been studied. The experiments show that some parts of THz frequency components have been cut off by plasma, and with the density of plasma rising, the starting frequency of THz prohibited by plasma is going higher. Experiments also provide an assistant scheme for plasma diagnose with terahertz technique.
High energy terahertz pulses are produced by illuminating a biased GaAs wafer using a short pulse from a Ti:sapphire
laser with a central wavelength of 800 nm, a pulse width of 50fs (FWHM) and a repetition rate of 10 Hz. We show that
the peak THz amplitude scales with the bias voltage and thus the THz energy and intensity scales quadratically with bias
voltage for bias fields up to 3 kV/cm. For laser pulses with an energy density of 1 mJ/cm2 we observe a multiple pulse
structure. We show that the polarity of the terahertz pulses is consistent with multiple reflections from the exit face of the
GaAs slab and the boundary of a plasma slab inside the wafer produced by the laser. We use the standard Drude model
for terahertz production from the GaAs wafer to describe multiple pulse structure due to reflections from the plasma
boundary layer in the slab. The time delays between multiple pulses are consistent with a 120 μm thick slab produced by
The absorption spectra of two illicit drugs, methylenedioxyamphetarnine (MDA) and methamphetamine (MA), within
and without two conventional envelopes are studied using terahertz time-domain spectroscopy technique. The
characteristic absorption spectra of MDA and MA are obtained in the range of 0.2 THz to 2.5 THz. MDA has an obvious
absorption peak at 1.41 THz while MA has obvious absorption peaks at 1.23 THz, 1.67 THz, 1.84 THz and 2.43 THz. We
find that the absorption peaks of MDA and MA within the envelopes are almost the same as those without the envelopes
respectively although the two envelopes have some different absorption in THz waveband. This result indicates that the
type of illicit drugs in envelopes can be determined by identifying their characteristic absorption peaks, and THz
time-domain spectroscopy is one of the most powerful candidates for illicit drugs inspection.