Terahertz time-domain spectroscopy of edible oils.

Chemical degradation of edible oils has been studied using conventional spectroscopic methods spanning the spectrum from ultraviolet to mid-IR. However, the possibility of morphological changes of oil molecules that can be detected at terahertz frequencies is beginning to receive some attention. Furthermore, the rapidly decreasing cost of this technology and its capability for convenient, in situ measurement of material properties, raises the possibility of monitoring oil during cooking and processing at production facilities, and more generally within the food industry. In this paper, we test the hypothesis that oil undergoes chemical and physical changes when heated above the smoke point, which can be detected in the 0.05-2 THz spectral range, measured using the conventional terahertz time-domain spectroscopy technique. The measurements demonstrate a null result in that there is no significant change in the spectra of terahertz optical parameters after heating above the smoke point for 5 min

Dual-polarized highly sensitive plasmonic sensor in the visible to near-IR spectrum

We propose and numerically characterize the optical characteristics of a novel photonic crystal fiber (PCF) based surface plasmon resonance (SPR) sensor in the visible to near infrared (500–2000 nm) region for refractive index (RI) sensing. The finite element method (FEM) is used to design and study the influence of different geometric parameters on the sensing performance of the sensor. The chemically stable plasmonic material gold (Au) is used to produce excitation between the core and plasmonic mode. On a pure silica (SiO2) substrate, a rectangular structured core is used to facilitate the coupling strength between the core and the surface plasmon polariton (SPP) mode and thus improves the sensing performance. By tuning the geometric parameters, simulation results show a maximum wavelength sensitivity of 58000 nm/RIU (Refractive Index Unit) for the x polarization and 62000 nm/RIU for the y polarization for analyte refractive indices ranging from 1.33 to 1.43. Moreover, we characterize the amplitude sensitivity of the sensor that shows a maximum sensitivity of 1415 RIU−1 and 1293 RIU−1 for the x and y polarizations, respectively. To our knowledge, this is the highest sensitivity for an SPR in published literature, and facilitates future development of sensors for accurate and precise analyte measurement. The sensor also attains a maximum figure of merit (FOM) of 1140 and fine RI resolution of 1.6 × 10−6. Owing to strong coupling strength, high sensitivity, high FOM and improved sensing resolution, the proposed sensor is suited for real-time, inexpensive and accurate detection of biomedical and biological analytes, biomolecules, and organic chemicals.This work is supported by Australian Research Council (grant no. DP170104984). We gratefully acknowledge their support

A stabilized master laser system for differential absorption LIDAR.

In this thesis, we present a prototype water vapour DIfferential Absorption Lidar (DIAL) instrument with accurate and precise wavelength control of master diode lasers. This stabilization system design has a number of novel elements that work towards a robust and low-cost autonomous DIAL observatory. With two continuous wave optical wavelengths stabilized, a pulse is formed using an Acousto-Optic Modulator (AOM) to switch light out of each control system to form the transmitted pulse. The control systems employ synchronous reference signal detection that suppresses system perturbations due to the optical switching, facilitating the use of deep dither modulation that aids in accurate stabilization to weak absorption lines. Furthermore, ratiometric detection in the control loop suppresses interference caused by back reflections in optical fiber components, as well as amplitude modulation of the laser diode due to injection current. In our system, the first laser is stabilized to an absorption line of a water vapour cell, while the second is beat-frequency stabilized relative to the first using a passive 16 GHz bandpass filter. This technique can be expanded to stabilize any number of reference lasers with respect to each other and to an absolute optical standard. The prototype DIAL uses a Tapered optical Amplifier (TA) to form 1 μs 500 mW optical pulses with a repetition rate of >3 kHz for atmospheric transmission. Fourteen observation experiments were conducted over two years, with water vapour measurements obtained using a calibrated humidity sensor, using three saturated salt solutions as humidity references. The measured pulse extinction was used to calculate the effective absorption cross-section of the transmitter, and therefore used to calculate quantitative water vapour measurements from the DIAL observation data. It is hoped that this work will be useful to the further development and commercialization of this unique and powerful remote sensing technique.Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 201

Terahertz Signal Classification Based on Geometric Algebra

This paper presents an approach to classification of substances based on their terahertz spectra. We use geometric algebra to provide a concise mathematical means for attacking the classification problem in a coordinate-free form. For the first time, this allows us to perform classification independently of dispersion and, hence, independently of the transmission path length through the sample. Finally, we validate the approach with experimental data. In principle, the coordinate-free transformation can be extended to all types of pulsed signals, such as pulsed microwaves or even acoustic signals in the field of seismology. Our source code for classification based on geometric algebra is publicly available at: https://github.com/swuzhousl/Shengling-zhou/blob/geometricalgebra-classifier/GAclassifier/

Terahertz Sensing in a Hollow Core Photonic Crystal Fiber

A terahertz sensor based on a hollow core photonic crystal fiber has been proposed in this paper for chemical analyte detection in the terahertz frequency range. The Zeonex-based asymmetrical hollow core is filled with an analyte and surrounded by a number of asymmetrical rectangular air holes bounded by a perfectly matched layer with absorbing boundary conditions. The performance of the proposed sensor is numerically investigated by using finite element method-based COMSOL software. It is found that a hollow core provides a high relative sensitivity as well as low transmission loss. Moreover, simplicity in design facilitates manufacturability, making it practical for a number of different biological and industrial applications

Terahertz Hollow Core Antiresonant Fiber with Metamaterial Cladding

A hollow core antiresonant photonic crystal fiber (HC-ARPCF) with metal inclusions is numerically analyzed for transmission of terahertz (THz) waves. The propagation of fundamental and higher order modes are investigated and the results are compared with conventional dielectric antiresonant (AR) fiber designs. Simulation results show that broadband terahertz radiation can be guided with six times lower loss in such hollow core fibers with metallic inclusions, compared to tube lattice fiber, covering a single mode bandwidth (BW) of 700 GHz