Multilayer Extraction of Complex Refractive Index in Broadband Transmission Terahertz Time-Domain Spectroscopy

In terahertz spectroscopy, multi-layered samples often need to be measured, for instance in a liquid flow cell, and this complicates the extraction of material parameters. We present a spectroscopic parameter extraction algorithm for multilayer samples that can also be used to extract the thickness of an unknown sample laye

Probing temperature- and solvent-dependent protein dynamics using terahertz time-domain spectroscopy

The effect of temperature on the terahertz-frequency-range material properties of lyophilized and single-crystal hen egg-white lysozyme has been measured using terahertz time-domain spectroscopy, with the results presented and discussed in the context of protein and solvent dynamical and glass transitions. Lyophilized hen egg-white lysozyme was measured over a temperature range from 4 to 290 K, and a change in the dynamical behaviour of the sample at around 100 K was observed through a change in the terahertz absorption spectrum. Additionally, the effect of cryoprotectants on the temperature-dependent absorption coefficient is studied, and it is demonstrated that terahertz time-domain spectroscopy is capable of resolving the true glass transition temperature of single-crystal hen egg-white lysozyme at 150 K, which is in agreement with literature values measured using differential scanning calorimetry

Estimation of Spectroscopic Uncertainty and Correlation in Terahertz Time Domain Spectroscopy

We present a method of calculating the measurement variance-covariance matrix of a spectroscopic sample’s complex refractive index from time-domain statistics in order to estimate uncertainty of a measurement. We compare this method to a numerical analysis and previously derived methodology, and show that our time-based estimate is both accurate and adaptable to complex extraction models

Accurate Parameter Extraction From Liquids Measured Using On-chip Terahertz Spectroscopy

We introduce a method for estimating the permittivity of liquid samples measured using integrated microfluidic/planar Goubau line terahertz waveguides, in which simulation results are incorporated with measurement data to enable accurate frequency-dependent analysis

THz-TDS of liquids in a temperature-controlled transmission flowcell

Precise temperature-control is necessary for many spectroscopic measurements. We present the temperature-dependent complex THz refractive index of liquid samples measured in a flowcell, analysed by a numerical method that allows simultaneous extraction of the sample's thickness to verify the temperature-stability of the cell

Discrete Vernier tuning in terahertz quantum cascade lasers using coupled cavities

Terahertz-frequency quantum cascade lasers (THz QCLs) are compact solid-state sources of coherent radiation in the 1–5 THz region of the electromagnetic spectrum . The emission spectra of THz QCLs typically exhibit multiple longitudinal modes characteristic of Fabry–Pérot (FP) cavities. However, widely-tunable (single-mode) THz QCLs would be ideally suited to many THz-sensing applications, such as trace gas detection, atmospheric observations , and security screening . Here we demonstrate discrete Vernier tuning using a simple two-section coupled-cavity geometry. A monolithic THz QCL ridge cavity was etched using focused ion beam milling to create two coupled FP cavities separated by an air gap. In this scheme, one of the two sections (the ‘lasing section’) is electrically driven above the lasing threshold, while the other is driven below threshold and acts as a ‘tuning section’. The lengths of the two sections and the air gap were designed such that the longitudinal FP modes of the respective sections coincide at a selected (‘resonant’) frequency. The dominant lasing mode of the coupled cavity occurs at this frequency owing to the reduction in threshold . A small perturbation to the frequency of the modes in either section of the device will detune the resonance, causing the dominant mode of the coupled-cavity to ‘hop’ to a different frequency, in a manner analogous to the Vernier effect. The longitudinal modes of the tuning section are controlled by perturbing its refractive index through current-induced heating

Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter

We report on large-area photoconductive terahertz (THz) emitters with a low-temperature-grown GaAs (LT-GaAs) active layer fabricated on quartz substrates using a lift-off transfer process. These devices are compared to the same LT-GaAs emitters when fabricated on the growth substrate. We find that the transferred devices show higher optical-to-THz conversion efficiencies and significantly larger breakdown fields, which we attribute to reduced parasitic current in the substrate. Through these improvements, we demonstrate a factor of ~8 increase in emitted THz field strength at the maximum operating voltage. In addition we find improved performance when these devices are used for photoconductive detection, which we explain through a combination of reduced parasitic substrate currents and reduced space-charge build-up in the device