In this paper, we present a new compact and versatile spectrometer system operating at 1.55 μm for research and industrial application. The system is capable of testing solid, powder, thin film, gas, and liquid samples for material sensing and characterization applications. A high efficient system with bandwidth up to 1.2 THz is realized by using a fiber coupled terahertz chip packaging technology. The key components are the fiber-coupled THz transmitter and receiver modules, where the laser beam is directly coupled to the THz chip using optical fibers to provide stable and movable transmitter and receiver heads. The antennas are excited by 100 fs optical pulses at 1550 nm telecom wavelength and average power of 10mW. As femtosecond pulses are required on the antenna, the linear dispersion and nonlinear effect resulting from the propagation of the high power optical pulse along the fiber are taken into account and compensated using dispersion compensation fiber. A fast scan optical delay module is employed to realize real-time THz signal and spectrum measurement. The optical delay module also has a long delay scan unit to allow the user to adjust the distance between the transmitter and receiver heads by up to 1m to use the system for characterization of materials in different industrial applications.
Following the development of efficient THz devices operating at 1550 nm based on low temperature (LT) grown semiconductor compounds, the effect of the substrate of such devices in the generated THz radiation is investigated, a new compact, portable and reconfigurable fiber based THz spectrometer is built and a pair of THz devices are evaluated in the spectrometer. The key findings are firstly the transparency of the InP substrate to THz radiation, which implies that the generated THz signal from these devices is not affected by the substrate, and secondly the development of a THz spectrometer at 1550 nm laser excitation, which can be used for high quality measurements for various material sensing and characterization applications.
This paper presents the use of Terahertz (THz) SPR near-field sensor to characterize materials such as PMMA and those
used in organic light emitting diode (OLED). The SPR device contains 2D periodic circular or square hole array in 500
nm Al on an 5 mm-thick intrinsic silicon, and was fabricated by photolithography and wet etching. For THz spectrum
measurement, the SPR device with and without thin (PMMA) film on it is placed at the focus of the THz beam in
transmission THz Time Domain Spectroscopy (TDS), where the spectrum is obtained from the Fourier-transformed
sample and reference THz pulses. The transmission is obtained from the ratio between the sample spectrum and
reference spectrum, whereas the phase change is the phase difference between the two spectra. To avoid overlap with
water absorption lines, the optimal SPR device design has a period of 320 μm and square holes of 150 μm side length.
The theoretical SPR frequencies in the THz range are determined for the metal-silicon modes and metal-air modes
(0.9375 THz for mode (0, 1) at the metal-air interface). The measurement results confirmed the theoretical SPR
frequencies for metal-silicon mode and demonstrate a shift to 0.9211 THz due to 2 μm of PMMA layer on the surface.
In this paper, we conduct transmission and reflection mode terahertz time-domain spectroscopy (THz-TDS) measurements of organic semiconductors such as ALQ3 and TBADN. THz-TDS is effective for determining the purity of the organic semiconductors based on the refractive index and spectral signatures in THz range. In order to prepare the sample for a custom built sample holder, the powder samples are pressed into pellets of 13 mm diameter and a thickness of 2 mm using a hydraulic press. The organic semiconductor, for example ALQ3 sample, is prepared as a 70% ALQ3 and 30% polyethylene (PE) concentration pellet by mixing ALQ3 and PE. The ALQ3 pellet is measured in a chamber purged with dry nitrogen to avoid the effect of water vapor absorptions in ambient air. The absorption coefficient and index of refraction are measured from the spectra of the reference THz pulse and the THz pulse after transmission through the sample. The THz spectrum is obtained by applying a fast Fourier transform to the THz waveform. Further studies were conducted by reducing the concentration of the organic semiconductor from 70% to 10% ALQ3. We also obtained the spectral signature and absorption coefficient for 50% TBADN 50% PE pellet. The spectral signatures of ALQ3 were found to be at 0.868 THz, 1.271 THz and 1.52 THz, while spectral signature of TBADN was found to be at 1.033 THz.
An integrated continuous-wave (CW) terahertz biosensor is proposed based on an edge-coupled terahertz photomixer source with guided-wave optical excitation scheme. In this device, two laser beams are guided inside an optical dielectric waveguide structure and being gradually absorbed by an overlying ultra-fast photoabsorbing layer, wherein a terahertz signal is generated due to photomixing phenomenon. The generated THz signal is guided by a coplanar-stripline (CPS) and is coupled to an integrated CPS resonator, which acts as a sample carrier and transducer. After interaction by bio-sample, terahertz wave is guided by a CPS line to a wide-band antenna and is detected by a THz power detector. Our performance analysis for the proposed CW terahertz biosensor supports the feasibility of the whole idea very well. The proposed device is attractive for system-on-a-chip terahertz sensors and spectrometers.
Continues-wave photomixing phenomenon in ultra-fast photoconductors
and high-temperature superconductors (HTS) is studied and photomixing efficiencies of these materials are investigated.
Photocurrent distributions in both photoconductor and superconductor
based photomixers are calculated and their common characteristics
are compared in detail.
A new CW photoconductive integrated photomixer/antenna THz source is presented. A THz signal is generated in the DC-biased photoconductive strip by employing optical heterodyne photomixing, and at the same time the size of the photoconductive strip on the grounded dielectric substrate is designed to have an efficient broadside radiation. Analytical expressions for the photo-induced current as well as the radiation power are calculated in detail, which make it possible to evaluate the performance of the structure made by different photoconductive materials. The typical μW output power can be obtained by mW laser pump power for frequencies up to 1 THz.