Ultrafast laser techniques have opened up a tremendous research opportunity in studying the interaction of short pulses of light with matter. With discovering of the picosecond photoconducting hertzian dipoles and high-brightness THz beams characterized with an ultrafast detector, we have seen more and more applications of ultrafast light in non-invasive imaging. Standard methods, when applied to the measurement of thin optical materials, doesnot independently determine the material's thickness and index of refraction. The proposed method is fundamentally different from other imaging such as contrast difference in optical coherent tomography (OCT) or the peak-to-peak intensity ratio as in THz imaging to determine index of refraction and thickness. We show that the application of ultrafast techniques allows simultaneous measurements of material thickness and optical constants in optical precision from transmission measurements. Such finding invites new perspectives in imaging and other applicable disciplines such as imaging processing after recording of the THz waveform of biological samples.
This article shows the applications of ultrafast light in studying material optical properties and its application for rudimental imaging. Standard methods, when applied to the imaging, can not independently determine the material's thickness and index of refraction. The proposed method is fundamentally different from other imaging such as contrast difference in optical coherent tomography (OCT) or the peak-to-peak intensity ratio as in THz imaging to determine index of refraction and thickness. We show that the application of ultrafast techniques allows simultaneous measurements of material thickness and optical constants in optical precision from transmission measurements. Such finding invites new perspectives in imaging and other applicable disciplines.
Terahertz Time domain spectroscopy (THz-TDS) can provide the optical response of a medium in both amplitude and phase. We show that such capability can enable a detail analysis of optical properties of biological sample. Such study is important for standoff detection of presence of biological sample, where a detail analysis is difficult if not possible due to a complicated system involved and multiple effects involved. We proposed a transfer function study of the response of such system.
Terahertz Time domain spectroscopy (THz -TDS ) can provide the optical response of a medium in both amplitude and phase. We show that such capability can enable a detail analysis of optical properties around a resonance regime. Such study is important for standoff detection of explosive material where numerous absorption peaks exist. We proposed a model where one can synthesize the optical properties with THz-TDS around the resonance regimes.
Terahertz Time domain spectroscopy (THz -TDS ) can provide the optical response of a medium in both amplitude and phase. We show that such capability can enable a detail analysis of optical properties of RDX sample. Such study is important for standoff detection of presence of RDX sample, where a detail analysis is difficult if not possible due to a complicated system involved and multiple effects involved. We proposed a match filter method for detection of RDX inside or behind a barrier.
AOM-based pulse shaping permits precise spectrum slicing, which is useful for DWDM-based architectures. In essence, this method uses microsecond-duration radiofrequency pulses to completely control the spectra of femtosecond laser pulses, hence achieves dramatic temporal data compression. A multiple color sources from a mode-locked laser simplify the system design and operation. We demonstrated the AOM pulse shaper as the modulator and a CCD camera with 256 pixels as the receiver. The spectrum of a 200 fs Erbium Doped Fiber Laser (EDFL) pulse was dispersed across the AOM’s aperture (FWHM was 35 nm) and was then modulated in a conventional pulse shaper. We tested 87 channels with channel-spacing of 0.41 nm using a 518-MHz modulator, and 120 channels with channel-spacing of 0.29 nm using a 148-MHz modulator (in each case a 0.1 nm guard band was used). Starting from the original pulses, this modulation creates time slots of 43 ps and 63.4 ps respectively. The equivalent speed of the transmission will be 2.0 Tb/s and 1.9 Tb/s in a highly multiplexed system. The spectral efficiencies achieved in this experiment were ~46%, approaching the theoretical limit of 50% for On-Off Keying (OOK) modulation. A benchmark image was successfully sent over the test bed.
A non-invasive means to detect and characterize concealed agents of mass destruction in near real-time with a wide field-of-view is under development. The method employs spatial interferometric imaging of the characteristic transmission or reflection frequency spectrum in the Terahertz range. However, the successful (i.e. low false alarm rate) analysis of such images will depend on correct distinction of the true agent from non-lethal background signals. Neural networks are being trained to successfully distinguish images of explosives and bioagents from images of harmless items. Artificial neural networks are mathematical devices for modeling complex, non-linear relationships. Both multilayer perceptron and radial basis function neural network architectures are used to analyze these spectral images. Positive identifications are generally made, though, neural network performance does deteriorate with reduction in frequency information. Internal tolerances within the identification process can affect the outcome.
We have conducted visible pump-THz probe experiments on single wall carbon nanotubes (SWCNTs) deposited on quartz substrates. Our results suggest that the photoexcited nanotubes exhibit localized transport due to Lorentz-type photo-induced localized states. These experiments were repeated for ion-implanted, 3-4nm Si nanoclusters in quartz for which a similar behavior was observed.
We have conducted visible pump-THz probe experiments on single wall carbon nanotubes (SWCNTs) on quartz substrates. Our results suggest an upper limit to the carrier-lifetime, which is on the order 1.5ps, limited only by the THz pulse duration. These experiments were repeated for ion-implanted, 3-4nm Si nanoclusters in quartz for which the carrier lifetime was also assessed at 1.5ps. THz time-domain spectroscopy (THz-TDS) of SWCNTs revealed that the THz pulse peak transmission changed under optical illumination.
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