Photonic Bragg waveguide platform for multichannel resonant sensing applications in the THz range

Enhancement of wide-band trace terahertz absorption spectroscopy based on microstructures: a review

Research on the interaction between nanoscale materials and light holds significant scientific significance for the development of fields such as optoelectronic conversion and biosensing. The study of micro- and nano-optics has produced numerous outstanding research achievements by utilizing the dielectric optical coupling mechanism and plasmon effects to enhance the interaction between light and matter. These findings have demonstrated tremendous potential for applications in the field of molecular fingerprint sensing. This review focuses on a retrospective analysis of recent research studies in the enhancement of wide-band trace terahertz absorption spectroscopy. The physical mechanisms of using waveguide structures, dielectric metasurfaces/meta-gratings, and spoof surface plasmon polaritons (SSPs) to improve the interaction between light and trace-amount matters are introduced. The new approaches and methods for enhancing broad-band terahertz absorption spectroscopy of trace samples using microstructure designs are discussed. Additionally, we elucidate the scientific ideas and exploratory achievements in enhancing terahertz fingerprint spectroscopy detection. Finally, we provide an outlook on the research and development direction and potential practical applications of absorption spectroscopy enhancement detection.

Cascaded terahertz hollow-core Bragg waveguide: numerical design and experimental demonstration

A THz hollow-core Bragg waveguide with discontinuous support bridges in both radial and axial directions is proposed. The influence of the support bridges on the transmission loss of the waveguide is demonstrated numerically. The proposed waveguide shows confinement loss two orders of magnitude lower than that of the Bragg waveguide with conventional support bridges. A waveguide sample is fabricated by 3D printing technology, and the experimental results show that the transmission loss is in agreement with that of the simulation results. It is also demonstrated that the transmission loss of the fabricated waveguide is mainly determined by the large absorption loss of the waveguide material used in the experiment.

Design of a single aspheric beam homogenizer for accurate particle sizing application

Understanding, detection, and accurate monitoring of particles are of utmost importance in various industrial fields and environmental science. Optical sensors allow for real-time monitoring of particles at the single species level by analyzing the elastically scattered light intensities. Nevertheless, since most laser diodes employed for illuminating the particle generally follow a Gaussian-type intensity distribution, the non-uniform energy distribution across the aerosol channel causes considerable errors in the conversion of the scattered light intensities into the actual particle sizes. In order to achieve uniform illumination of particles across the aerosol channel and improve the particle sizing and classification accuracy, we design and customize a single aspheric lens, which efficiently converts the divergent Gaussian beam profile of a TO packaged laser diode into a one-dimensional flattop beam profile along the fast axis at the desired working distance. A beam uniformity better than 5% has been achieved. Furthermore, we demonstrate a practical sensing application using the designed lens for accurate particle sizing, and an obvious improvement in the accuracy has been achieved compared to that based on off-the-shelf aspheric lenses. The singlet beam homogenizer developed in this work has many appealing features (e.g., high uniformity and energy efficiency, compactness, and low stray light), which is especially relevant for building portable particle sensors in order to address various industrial applications where on-site or remote metrology and classification of particles are required.

Terahertz angle-independent photonic bandgap in a one-dimensional photonic crystal containing InSb-based hyperbolic metamaterials

According to the Bragg scattering theory, terahertz (THz) photonic bandgaps (PBGs) in all-dielectric one-dimensional (1-D) photonic crystals (PhCs) are strongly dependent on the incident angle. Such a strongly angle-dependent property of the PBGs not only limits the widths of omnidirectional PBGs, but also causes the strongly angle-dependent property of defect modes and optical Tamm states in multilayer structures containing all-dielectric 1-D PhCs. Until now, ways to achieve a THz angle-independent PBG have been an open problem. Herein, according to the existing phase-variation compensation theory, we achieve a THz angle-independent PBG in a 1-D PhC containing indium antimonide (InSb)-based hyperbolic metamaterials for transverse magnetic polarization. Different from conventional strongly angle-dependent PBGs, the angle-independent PBG remains almost unshifted as the incident angle changes. The relative frequency shifts of the upper and the bottom edges of the angle-independent PBG are only 1.4% and 0.4%, respectively. Besides, the angle-independent property of the PBG is robust against the disturbance of the layer thickness. The proposed 1-D PhC composes only two frequently used materials: silicon (Si) and InSb. Such a Si/InSb multilayer can be fabricated by the current ion-assisted electron beam coating or spin coating techniques. This THz angle-independent PBG would be utilized to design THz omnidirectional filters or absorbers.

Polymer Optical Waveguide Grating-Based Biosensor to Detect Effective Drug Concentrations of Ginkgolide A for Inhibition of PMVEC Apoptosis

In this work, we successfully developed a fluorinated cross-linked polymer Bragg waveguide grating-based optical biosensor to detect effective drug concentrations of ginkgolide A for the inhibition of pulmonary microvascular endothelial cell (PMVEC) apoptosis. Fluorinated photosensitive polymer SU-8 (FSU-8) as the sensing core layer and polymethyl methacrylate (PMMA) as the sensing window cladding were synthesized. The effective drug concentration range (5–10 µg/mL) of ginkgolide A for inhibition of PMVEC apoptosis was analyzed and obtained by pharmacological studies. The structure of the device was optimized to be designed and fabricated by direct UV writing technology. The properties of the biosensor were simulated with various refractive indices of different drug concentrations. The actual sensitivity of the biosensor was measured as 1606.2 nm/RIU. The resolution and detection limit were characterized as 0.05 nm and 3 × 10⁻⁵ RIU, respectively. The technique is suitable for safe and accurate detection of effective organic drug dosages of Chinese herbal ingredients.

Enhanced on-chip terahertz vibrational absorption spectroscopy using evanescent fields in silicon waveguide structures

In this study, we demonstrate on-chip terahertz absorption spectroscopy using dielectric waveguide structures. The structures' evanescent fields interact with the sample material surrounding the waveguide, enabling the absorption signature of the material to be captured. The ability of fabricated terahertz dielectric waveguide structures, based on the newly developed silicon-BCB-quartz platform, to capture the fingerprint of α-lactose powder (as an example material) at 532 GHz is examined. Enhancement of the spectroscopy sensitivity through techniques such as tapering the waveguide, confining the field in a slot dielectric waveguide, and increasing the interaction length using a spiral-shaped waveguide are investigated experimentally. The proposed on-chip spectroscopy structures outperform conventional and state-of-the-art approaches in terms of sensitivity and compactness.

Tuning transmission properties of 3D printed metal rod arrays by breaking the structural symmetry

In this work, one metallic photonic crystal waveguide composed of periodic metal rod arrays (MRAs) is experimentally and numerically demonstrated in terahertz frequencies. Such waveguides fabricated by 3D printers exhibit two resonant modes: the fundamental mode and the high-order mode, separating by a broad bandgap. Compared to the fundamental mode, the high-order mode shows higher field confinement and more sensitive to the geometry changes. By breaking the structure parameter, i.e., increasing or decreasing the metal rod interspace, the spectral positions, bandwidths, as well as the transmittances of high-order modes can be optimized. With broken symmetry in MRAs, the third resonant mode having high transmittance has emerged in the transmission spectrum. Results showing that fine-tuning in the alignment of metal rods leads to a great change in the transmission of high-order modes. These findings suggest that the transportation efficiency of THz waves through an MRA is tunable by breaking the structural symmetry.