Richard Dudley

Methodologies for determining the dynamic ranges and signal-to-noise ratios of terahertz time-domain spectrometers

We discuss procedures for calculating the signal-to-noise ratios and the dynamic ranges of terahertz time-domain spectrometers. We also offer recommendations for standardized calculations for time-domain and frequency-domain data.

Linearity calibration of amplitude and power measurements in terahertz systems and detectors.

We describe a device consisting of a set of silicon plates to be employed for calibrating the linearity of amplitude/power measurements of terahertz systems and detectors. The device is used to test a time-domain spectrometer, a pyroelectric sensor, and a Golay cell. The results demonstrate that the linearity of amplitude/power measurement cannot be assumed but must be experimentally verified.

Frequency calibration of terahertz time-domain spectrometers

We present three techniques of calibrating frequency and spectral profile measurements of terahertz time-domain spectrometers. The first utilizes the etalon effect generated by multiple reflections in thin nonabsorbing samples. The second employs a CO gas cell to provide multiple narrow absorption lines at known frequencies. The third aims to use a grating monochromator to produce an independently measured comparable spectrum.

Characterization of Terahertz Beam Profile and Propagation

Two techniques for imaging a terahertz (THz) beam profile are described and employed in characterizing the THz beam propagation in a time-domain spectrometer. Aperture scanning yields a 2-D cross-sectional map of field amplitude at selected frequencies that is used to track beam evolution along its path. The Hartmann test produces a 2-D topographical image of the wavefront, also at selected frequencies. The results are compared with theory, and the respective advantages of the two methods are discussed.

Optimization of apodization functions in terahertz transient spectrometry

A quadratic programming optimization procedure for designing asymmetric apodization windows tailored to the shape of time-domain sample waveforms recorded using a terahertz transient spectrometer is proposed. By artificially degrading the waveforms, the performance of the designed window in both the time and the frequency domains is compared with that of conventional rectangular, triangular (Mertz), and Hamming windows. Examples of window optimization assuming Gaussian functions as the building elements of the apodization window are provided. The formulation is sufficiently general to accommodate other basis functions.

Terahertz reflectivities of metal-coated mirrors

Terahertz (THz) reflectivities of a variety of metal-coated mirrors were measured. Gold, silver, and aluminum coatings were tested and compared. It was found that all types of optical metal-coated mirrors perform equally well as THz reflectors.

Line strengths and self-broadening of pure rotational lines of carbon monoxide measured by terahertz time-domain spectroscopy

Pure rotational lines of carbon monoxide were observed using terahertz time-domain spectroscopy. Measurements were carried out at elevated pressures of 0.7–5.1 bar. Line intensities and self-broadening parameters of transitions from J′=3←2 to J′=22←21 were obtained. Good agreement with the HITRAN database was found.

Orientational dependence of THz optical constants, birefrigence and dichroism of liquid crystals BL037 and GT3-23001

The optical constants, birefringence and dichroism of liquid crystals BL037 and GT3-23001 are investigated in the frequency range 0.4-4 THz using time-domain spectroscopy. A specially designed bias cell is described which allows the orientational dependence of optical properties to be observed.

A simple fluid cell for the study of aqueous solutions using THz time-domain spectroscopy

We describe a fluid cell for the measurement of aqueous solutions of biomolecules adapted particularly for the requirements of THz time-domain spectroscopy. The design is simple, requires small-volume samples, avoids cross-contamination and is inexpensive.

Control of terahertz pulse generation by optical pulse shaping

We present the use of a programmable optical pulse-shaper based on a liquid crystal amplitude and phase modulator to control the production of terahertz pulses in a ZnTe crystal. A genetic algorithm is used to optimize the pump pulse shape for a desired outcome, avoiding the need for prior knowledge of the required optical pulse shape. As an example we demonstrate the introduction of spectral holes at selected frequencies. This spectral application provides a purely optical means of emulating a physical terahertz filter.