Direct observation of low frequency confined acoustic phonons in silver nanoparticles: Terahertz time domain spectroscopy

Terahertz time domain spectroscopy has been used to study low frequency confined acoustic phonons of silver nanoparticles embedded in poly (vinyl alcohol) matrix in the spectral range of 0.1-2.5 THz. The real and imaginary parts of the dielectric function show two bands at 0.60 and 2.12 THz attributed to the spheroidal and toroidal modes of silver nanoparticles, thus demonstrating the usefulness of terahertz time domain spectroscopy as a complementary technique to Raman spectroscopy in characterizing the nanoparticles. (C) 2010 American Institute of Physics. [doi:10.1063/1.3456372]

Terahertz Time Domain Spectroscopy to Detect Low-Frequency Vibrations of Double-Walled Carbon Nanotubes

We have measured the frequency-dependent real index of refraction and extinction coefficient (and hence the complex dielectric function) of a free-standing double-walled carbon nanotube film of thickness 200 nm by using terahertz time domain spectroscopy in the frequency range 0.1 to 2.5 THz. The real index of refraction and extinction coefficient have very high values of approximately 52 and 35, respectively, at 0.1 THz, which decrease at higher frequencies. Two low-frequency phonon modes of the carbon nanotubes at 0.45 and 0.75 THz were clearly observed for the first time in the real and imaginary parts of the complex dielectric function along with a broad resonance centred at around 1.45 THz, the latter being similar to that in single-walled carbon nanotubes assigned to electronic excitations. Our experiments bring out a possible application of double-walled carbon nanotube films as a neutral density filter in the THz range.

Electrical and optical characterization of conducting polymers by THz time-domain spectroscopy

The absorption and index of refraction of polypyrrole (PPy) and poly-3-methylthiophene (PMeT), from low frequencies up to 4 THz, have been measured by tera-Herz (THz) time-domain spectroscopy. The complex conductance was obtained over this range of frequency. Highly conducting metallic samples follow the Drude model, whereas less conducting ones fit the localization-modified Drude model. The carrier scattering time and mobility in conducting polymers can be directly determined from these measurements.

Terahertz time domain spectroscopy of amino acids and sugars

A time-domain spectrometer for use in the terahertz (THz) spectral range was designed and constructed. Due to there being few existing methods of generating and detecting THz radiation, the spectrometer is expected to have vast applications to solid, liquid, and gas phase samples. In particular, knowledge of complex organic chemistry and chemical abundances in the interstellar medium (ISM) can be obtained when compared to astronomical data. The THz spectral region is of particular interest due to reduced line density when compared to the millimeter wave spectrum, the existence of high resolution observatories, and potentially strong transitions resulting from the lowest-lying vibrational modes of large molecules.

The heart of the THz time-domain spectrometer (THz-TDS) is the ultrafast laser. Due to the femtosecond duration of ultrafast laser pulses and an energy-time uncertainty relationship, the pulses typically have a several-THz bandwidth. By various means of optical rectification, the optical pulse carrier envelope shape, i.e. intensity-time profile, can be transferred to the phase of the resulting THz pulse. As a consequence, optical pump-THz probe spectroscopy is readily achieved, as was demonstrated in studies of dye-sensitized TiO₂, as discussed in chapter 4. Detection of the terahertz radiation is commonly based on electro-optic sampling and provides full phase information. This allows for accurate determination of both the real and imaginary index of refraction, the so-called optical constants, without additional analysis. A suite of amino acids and sugars, all of which have been found in meteorites, were studied in crystalline form embedded in a polyethylene matrix. As the temperature was varied between 10 and 310 K, various strong vibrational modes were found to shift in spectral intensity and frequency. Such modes can be attributed to intramolecular, intermolecular, or phonon modes, or to some combination of the three.

Terahertz time-domain spectroscopy characterization of vertically aligned carbon nanotube films

Terahertz time-domain spectroscopy measurements were made for vertically aligned multi-walled carbon nanotube (VACNT) films. We obtained the frequency dependent complex permittivity and conductivity (on the assumption that permeability μ = 1) of several samples exhibiting Drude behaviour for lossy metals. The obtained material properties of VACNT films provide information for potential microwave and terahertz applications. © 2011 Elsevier Ltd. All rights reserved.

Characteristics in frequency domain by inductively coupled plasma atomic emission spectrometry with charge injection detection

The charactesistics of two-dimension spectra obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES) with charge injection detection (CID) in frequency domain were studied in the present paper. The measurement spectra were Fourier transformed and the frequency distribution of the spectra was obtained. Results showed that the spectra in frequency domain could he divided into two parts:high frequency and low frequency signals. The later stood for measurement spectra and the former for background and noises. However, the high frequecny signals could not be smoothed simply to reduce noises because the background was deteriorated even though the spectral signal did not change significantly.

Frequency Domain Characterization Of Sliding Mode Control Of An Inverter Used In DSTATCOM Application

In this paper, the commonly used switching schemes for sliding mode control of power converters is analyzed and designed in the frequency domain. Particular application of a distribution static compensator (DSTATCOM) in voltage control mode is investigated in a power distribution system. Tsypkin's method and describing function is used to obtain the switching conditions for the two-level and three-level voltage source inverters. Magnitude conditions of carrier signals are developed for robust switching of the inverter under carrier-based modulation scheme of sliding mode control. The existence of border collision bifurcation is identified to avoid the complex switching states of the inverter. The load bus voltage of an unbalanced three-phase nonstiff radial distribution system is controlled using the proposed carrier-based design. The results are validated using PSCAD/EMTDC simulation studies and through a scaled laboratory model of DSTATCOM that is developed for experimental verification