The development of terahertz sources and their applications

High-Density Polyethylene Custom Focusing Lenses for High-Resolution Transient Terahertz Biomedical Imaging Sensors

Transient terahertz time-domain spectroscopy (THz-TDS) imaging has emerged as a novel non-ionizing and noninvasive biomedical imaging modality, designed for the detection and characterization of a variety of tissue malignancies due to their high signal-to-noise ratio and submillimeter resolution. We report our design of a pair of aspheric focusing lenses using a commercially available lens-design software that resulted in about 200 × 200-μm2 focal spot size corresponding to the 1-THz frequency. The lenses are made of high-density polyethylene (HDPE) obtained using a lathe fabrication and are integrated into a THz-TDS system that includes low-temperature GaAs photoconductive antennae as both a THz emitter and detector. The system is used to generate high-resolution, two-dimensional (2D) images of formalin-fixed, paraffin-embedded murine pancreas tissue blocks. The performance of these focusing lenses is compared to the older system based on a pair of short-focal-length, hemispherical polytetrafluoroethylene (TeflonTM) lenses and is characterized using THz-domain measurements, resulting in 2D maps of the tissue refractive index and absorption coefficient as imaging markers. For a quantitative evaluation of the lens effect on the image resolution, we formulated a lateral resolution parameter, R2080, defined as the distance required for a 20-80% transition of the imaging marker from the bare paraffin region to the tissue region in the same image frame. The R2080 parameter clearly demonstrates the advantage of the HDPE lenses over TeflonTM lenses. The lens-design approach presented here can be successfully implemented in other THz-TDS setups with known THz emitter and detector specifications.

A 220 GHz-1.1 THz continuous frequency and polarization tunable quasi-optical electron paramagnetic resonance spectroscopic system

Here, we describe a custom-designed quasi-optical system continuously operating in the frequency range 220 GHz to 1.1 THz with a temperature range of 5-300 K and magnetic fields up to 9 T capable of polarization rotation in both transmitter and receiver arms at any given frequency within the range through a unique double Martin-Puplett interferometry approach. The system employs focusing lenses to amplify the microwave power at the sample position and recollimate the beam to the transmission branch. The cryostat and split coil magnets are furnished with five optical access ports from all three major directions to the sample sitting on a two-axes rotatable sample holder capable of performing arbitrary rotations with respect to the field direction, enabling broad accessibility to experimental geometries. Initial results from test measurements on antiferromagnetic MnF₂ single crystals are included to verify the operation of the system.

Spectroscopic Studies of BA Class Liquid Crystals in the 6-15 THz Range Using the Fourier Transform Infrared (FT-IR) Method

Fourier transform infrared spectroscopy was performed for two structurally similar liquid crystals: 4-octyloxybenzoic acid and 4-decyloxybenzoic acid in 6-15 THz range. Density functional theory modeling at PBE0/def2-TZVPP level was used to assign the vibrational modes to each observed absorption peak. We observed six peaks that are common for both liquid crystals, assigned to the same vibrational modes and appearing at similar energies. Each of the samples also had additional peaks unique to itself. A strong absorption peak appears at about 280 cm^-1 for both samples; however, it corresponds to different vibrational modes for the two samples. This work shows that the spectroscopy in this often-neglected frequency range can easily distinguish structurally similar compounds.

Biological applications of terahertz technology based on nanomaterials and nanostructures

Terahertz (THz) technology is now drawing increasing attention around the world; it has been considered as an efficient non-destructive, non-contact and label-free optical method for biological detection. In this field, nanomaterials and nanostructures have been constantly advancing the development of THz technology. Here, we proposed some latest applications of nanotechnology to improve THz biological detection capability for providing progressive THz systems, thus enabling outstanding detection performance utilizing THz spectroscopy and imaging; these will encourage broader interest in various fields. The uniqueness, limitations, and future prospects of THz biological applications based on nanomaterials and nanostructures will also be reviewed in light of recent developments.

Improvement of Terahertz Photoconductive Antenna using Optical Antenna Array of ZnO Nanorods

An efficient terahertz (THz) photoconductive antenna (PCA), as a major constituent for the generation or detection of THz waves, plays an essential role in bridging microwave-to-photonic gaps. Here, we propose an impressive approach comprising the use of arrayed zinc oxide nanorods (ZnO NRs) as an optical nanoantenna over an anti-reflective layer (silicon nitride) in the antenna gap to boost the photocurrent and consequently the THz signal. The numerical approach applied in investigating the optical behavior of the structure, demonstrates a significant field enhancement within the LT-GaAs layer due to the optical antenna performing simultaneously as a concentrator and an antireflector which behaves as a graded-refractive index layer. ZnO NRs have been fabricated on the PCA gap using the hydrothermal method as a simple, low cost and production compatible fabrication method compared to other complex methods used for the optical nanoantennas. Compared to the conventional PCA with a traditional antireflection coating, the measured THz power by time domain spectroscopy (TDS) is increased more than 4 times on average over the 0.1–1.2 THz range.

Fletching-shaped Bi4Te3–ZnTe heterostructure nanowires

We investigated Bi(4)Te(3)-ZnTe axial heterostructure nanowires grown by physical vapor transport at the substrate temperature of 450 °C, utilizing ZnTe as the source and Bi(2)Te(3) as the catalyst. The temperatures of the source and catalyst materials were individually controlled by separating the source and catalyst boats. The axial heterostructure nanowires consisted of ZnTe and Bi(4)Te(3) segments. The Bi(4)Te(3) segment had an interesting fletching shape with three wings surrounded by a ZnO shell. A systematic redshift in the nanowire's photoluminescence was observed as the excitation laser spot was moved from the heterojunction toward the root of the ZnTe segment, attributed to the formation of deep defect states under the Te-rich environment that resulted from using Bi(2)Te(3) as the catalyst instead of Bi.

Extraction of optical constants using multiple reflections in the terahertz emitter-sample hybrid structure

To extract optical constants of nontransmitted samples in the terahertz (THz) spectral region, we employ a THz emitter-sample hybrid structure where THz pulses are generated at the emitter surface and multiply reflected at the interface between the THz emitter and the sample. Since each THz electric field profile appears well separated in a time domain, we could obtain the amplitude and phase spectra for each pulse from the Fourier transform, and determine the optical constants of the sample numerically based on the Fresnel equations. We applied this technique for doped semiconductors, and found that obtained optical constants are in good agreement with the values determined by using other conventional THz techniques.

Low-loss flexible bilayer metamaterials in THz regime

Low insertion-loss single-layer and bilayer metamaterial filters in terahertz (THz) frequency regime were demonstrated on top of low cost flexible Scotch tape by utilizing pattern transfer method. The transmittance of the flexible 51-μm-thick Scotch tape was found out to be higher than 0.85 in the range of 0.2 to 3 THz, which is excellent for the substrate materials for THz applications. Free standing filters exhibited record low insertion loss of 0.6 dB and band rejection ratio as high as 30 dB. The resonance reflection characteristics of the bilayer filters were maintained when they were attached on top of curved PET bottle or metallic surfaces, providing promising application in THz identifications.

Leaf water dynamics of Arabidopsis thaliana monitored in-vivo using terahertz time-domain spectroscopy

The declining water availability for agriculture is becoming problematic for many countries. Therefore the study of plants under water restriction is acquiring extraordinary importance. Botanists currently follow the dehydration of plants comparing the fresh and dry weight of excised organs, or measuring their osmotic or water potentials; these are destructive methods inappropriate for in-vivo determination of plants' hydration dynamics. Water is opaque in the terahertz band, while dehydrated biological tissues are partially transparent. We used terahertz spectroscopy to study the water dynamics of Arabidopsis thaliana by comparing the dehydration kinetics of leaves from plants under well-irrigated and water deficit conditions. We also present measurements of the effect of dark-light cycles and abscisic acid on its water dynamics. The measurements we present provide a new perspective on the water dynamics of plants under different external stimuli and confirm that terahertz can be an excellent non-contact probe of in-vivo tissue hydration.

Nanotechnology-supported THz medical imaging

Over the last few decades, the achievements and progress in the field of medical imaging have dramatically enhanced the early detection and treatment of many pathological conditions. The development of new imaging modalities, especially non-ionising ones, which will improve prognosis, is of crucial importance. A number of novel imaging modalities have been developed but they are still in the initial stages of development and serious drawbacks obstruct them from offering their benefits to the medical field. In the 21 (st) century, it is believed that nanotechnology will highly influence our everyday life and dramatically change the world of medicine, including medical imaging. Here we discuss how nanotechnology, which is still in its infancy, can improve Terahertz (THz) imaging, an emerging imaging modality, and how it may find its way into real clinical applications. THz imaging is characterised by the use of non-ionising radiation and although it has the potential to be used in many biomedical fields, it remains in the field of basic research. An extensive review of the recent available literature shows how the current state of this emerging imaging modality can be transformed by nanotechnology. Innovative scientific concepts that use nanotechnology-based techniques to overcome some of the limitations of the use of THz imaging are discussed. We review a number of drawbacks, such as a low contrast mechanism, poor source performance and bulky THz systems, which characterise present THz medical imaging and suggest how they can be overcome through nanotechnology. Better resolution and higher detection sensitivity can also be achieved using nanotechnology techniques.

Optimization of single-cycle terahertz generation in LiNbO3 for sub-50 femtosecond pump pulses

We compare different tilted-pulse-front pumping schemes for single-cycle THz generation in LiNbO(3) crystals both theoretically and experimentally in terms of conversion efficiency. The conventional setup with a single lens as an imaging element has been found to be highly inefficient in the case of sub-50 fs pump pulses, mainly due to the resulting chromatic aberrations. These aberrations are avoided in the proposed new setup, which employs two concave mirrors in a Keplerian telescope arrangement as the imaging sequence. This partially compensates spherical aberrations and results in a ca. six times higher conversion efficiency in the case of 35-fs optical pump pulse duration compared to the single-lens setup. A THz field strength of 60 kV/cm was obtained using 0.5 mJ pump pulses. The divergence of the THz beam has been found experimentally to depend on the pump imaging scheme employed.

Limitations of the tunability of dual-crystal optical parametric oscillators

We demonstrate what is believed to be the first continuous-wave dual-crystal optical parametric oscillator (D-OPO) for generation of waves with a frequency difference of some terahertz. It is based on two magnesium-doped periodically poled lithium niobate crystals pumped with near-infrared light at 1030 nm. By changing the temperature difference of the crystals we achieve a difference-frequency tuning. This ranges from an initially unexpected lower threshold of 1.3 THz up to higher frequencies. The linewidth of the considered signal waves is smaller than 5 GHz. Smaller difference frequencies were not achievable. Our model, describing the dual-crystal parametric gain, explains the observed lower tuning limit by showing that the assumption of independent oscillation conditions in a D-OPO is not necessarily valid.

Terahertz pulse generation via optical rectification in photonic crystal microcavities

Using a three-dimensional fully vectorial nonlinear time-domain analysis, we numerically investigate generation of terahertz radiation by pumping a photonic crystal microcavity out of resonance. High quality factors and a quadratic susceptibility lead to few-cycle terahertz pulses via optical rectification. Material dispersion as well as linear and nonlinear anisotropy is fully accounted for.