THz Photomixer Research Articles

Determination of Thermal Damage Threshold in THz Photomixers Using Raman Spectroscopy

The increase of device lifetime and reliability of THz photomixers will play an essential role in their possible future application. Therefore, their optimal work conditions/operation range, i.e., the maximal incident optical power should be experimentally estimated. We fabricated and tested THz photomixer devices based on nitrogen-implanted GaAs integrated with a Bragg reflector. Raman spectroscopy was applied to investigate the material properties and to disclose any reversible or irreversible material changes. The results indicate that degradation effects in the photomixer structures/material could be avoided if the total optical power density does not exceed levels of about 0.7 mW/µm2 for 100 min of operation. Furthermore, the investigations performed during 1000 min of optical exposure on the photomixer devices’ central region comprising interdigitated metal-semiconductor-metal (MSM) structures suggest a reversible “curing” mechanism if the power density level of ~0.58 mW/µm2 is not exceeded. Long-term operation (up to 1000 h) reveals that the photomixer structures can withstand an average optical power density of up to ~0.4 mW/µm2 without degradation when biased at 10 V. Besides the decrease of the position of the A1g (LO) Raman mode from ~291 cm−1 down to ~288 cm−1 with increasing optical power density and operation time, broad Raman modes evolve at about 210 cm−1, which can be attributed to degradation effects in the active photomixer/MSM area. In addition, the performed carrier lifetime and photomixer experiments demonstrated that these structures generated continuous wave sub-THz radiation efficiently as long as their optimal work conditions/operation range were within the limits established by our Raman studies.

Plasmonic hybrid terahertz photomixer of graphene nanoantenna and nanowires

Due to their attractive properties, silver nanowires (Ag-NWs) are newly used as nanoelectrodes in continuous wave (CW) THz photomixer. However, since these nanowires have small contact area, the nanowires fill factor in the photomixer active region is low, which leads to reduce the nanowires conductivity. In this work, we proposed to add graphene nanoantenna array as nanoelectrodes to the silver nanowires-based photomixer to improve the conductivity. In addition, the graphene nanoantenna array and the silver nanowires form new hybrid nanoelectrodes for the CW-THz photomixer leading to improve the device conversion efficiency by the plasmonic effect. Two types of graphene nanoantenna array are proposed in two separate photomixer configurations. These are the graphene nanodisk (GND) array and the graphene bow-tie nanoantenna (GNA) array. The photomixer active region is simulated using the computer simulation technology (CST) Studio Suite<sup>®</sup> for three optical wavelengths: 780 nm, 810 nm, and 850 nm. From the results, we found that the electric field in the active region is enhanced by 4.2 and 4.8 times for the aforementioned configurations, respectively. We also showed that the THz output power can be enhanced by 310 and 530 times, respectively.

Design and analysis of improved configuration for antenna-less unbiased THz photomixer emitters

We propose an antenna-less unbiased CW terahertz photomixer comprising an array of recessed contact electrodes. Each unit of the recessed contact electrodes array embedded in the photoconductive layer includes two pairs of unsymmetrical metals making up heterogeneous Schottky contacts. As compared to the analogous array emitters of the same size with planar electrodes, by optimizing the geometrical and physical properties of the recessed contact electrodes in the proposed emitter to excite the appropriate resonant modes in the larger volume of the active region, it is possible to benefit from the having of high optical absorption and so terahertz photo-current, which in turn leads to enhance the obtainable THz power by more than three orders of magnitude. This is also more than an order of magnitude larger THz power than that is radiated from the similar antenna-coupled emitter with planar contact electrodes. Furthermore, the calculation results show the proposed emitter has a frequency tuning capability in a broad range of 0.1–5 THz.

Branchlike nano-electrodes for enhanced terahertz emission in photomixers

Branchlike nano-electrode structures were found to improve the THz emission intensity of a photomixer by approximately one order of magnitude higher than that of a photomixer with one row of nano-electrodes separated by the same 100 nm gap. The enhancement is attributed to a more efficient collection of generated carriers, which is in turn due to a more intense electric field under the branchlike nano-electrodes’ structures. This is coupled with an increased number of effective areas where strong tip-to-tip THz field enhancements were observed. The optical-to-THz conversion efficiency of the photomixers with the new branchlike nano-electrodes was found to be 10 times higher. The more efficient THz photomixer will greatly benefit the development of continuous-wave THz imaging and spectroscopy systems.

Ultrawide-Bandwidth Single-Channel 0.4-THz Wireless Link Combining Broadband Quasi-Optic Photomixer and Coherent Detection

Free space communications with huge data capacity have become a key point for the development of mobile access, services, and network technologies convergence. Wireless links using emerging terahertz technologies, also referred to as T-ray communications, have become an intensive research field within last years since T-rays at the millimeter/submillimeter frontier remain more robust in terms of scintillation, dielectric obstacles, and fog compared with near-infrared signals. Using a passive THz hot-spot which can be fed by fiber optic networks, we investigate up bit rates up to 46 Gbps for a THz wireless transmission system at 400-GHz carrier frequency. Using a THz photomixer integrated with a specific broadband antenna and heterodyne electronic detection, we investigate the eye diagrams using 1-μW received power, highlighting the high sensitivity and ultra-wideband behavior of the whole THz system.

Folded dipole antenna for power enhancement of a terahertz photomixer

A folded dipole antenna is proposed to increase the input impedance of an antenna for power enhancement of a terahertz integrated photomixer/antenna because the impedance of a teraherz photomixer is very high compared to the impedance of an antenna. The antenna was optimized using an electromagnetic simulator, and its resistance was increased to 2750 Ω, which is about 40 times as high as the impedance of a self-complementary broadband antenna. The optimized antenna was fabricated on low-temperature-grown GaAs (LTG-GaAs) and measured using a liquidhelium-cooled silicon bolometer. Two distributed-feedback tunable diode lasers were scanned for photomixing at frequencies from 100 to 1300 GHz in steps of 1 GHz. For stable frequency scans, the two lasers were controlled by using a feedback loop system. The measured THz output power of the proposed folded dipole antenna integrated with a THz photomixer showed resonant characteristics and was larger at the resonant frequency bands.

Selected polymers on CW and pulsed THz investigations

AbstractThe paper gives the results of refractive indexes for three plastic materials: polyethylene, polymethyl methacrylate and polyvinyl chloride, which are measured and compared in two arrangements - a CW THz photomixer and a pulsed THz spectrometer.

Frequency aspects of the THz photomixer

In the paper methods of the terahertz photomixer operational frequency estimation are considered. Three methods are investigated — estimation of the frequency via direct measurements of the laser heterodyne compounds, via calculations from datasheets, and via calculations from raw collected data obtained from the photomixer system.

Four-leaf-clover-shaped antenna for a THz photomixer

To improve the output power of a photomixer used as a THz source, we propose a four-leaf-clover-shaped antenna structure composed of a highly resonant radiation element and a stable DC feed element. The resonance characteristics of the proposed structure were first investigated on a half-infinite substrate as a simplified radiation environment to reduce the computation time. Based on the antenna characteristics on that half-infinite substrate, the antenna structure was designed to have a maximum total efficiency and a maximum directivity on an extended hemispherical lens. The input resistance of this structure was six times that of a full-wavelength dipole, significantly improving the mismatch efficiency between a photomixer and the antenna. The terahertz output power from this structure is expected to be 2.7 times that of a full-wavelength dipole.

THz laser study of self-pressure and temperature broadening and shifts of water vapor lines for pressures up to [formula omitted

The self-broadened linewidths of pure rotational lines of water vapor have been investigated in the temperature range 263– 340 K over a pressure range from 30– 1400 Pa using an optically pumped THz photomixer spectrometer having a relative precision of ≈10 −5 cm −1 . The lineshapes of several lines between 12 and 52 cm −1 were well fit by Lorentzian profiles over the spectral interval spanning ±5 FWHM. The Lorentzian FWHM linewidths varied between 6.6×10 −6 and 1.06×10 −6 cm −1 Pa −1 (0.67 and 1.07 cm −1 atm −1 ) and were found to depend on the J, K a and K c quantum numbers as well as temperature. The observed pressure-induced shifts, δν( P), ranged from 1.3×10 −7– 4.5×10 −7 cm −1 Pa −1 (0.013– 0.046 cm −1 atm −1) . The measured linewidths also exhibited a ( T 0/ T) n power law dependence on temperature. The best fit exponent n varied from 0.56 to 0.81, dependent on rotational line. A decrease in δν( T) with increasing temperature is also observed, varying between 2×10 −8 and 5×10 −8 cm −1 Pa −1 (2×10 −3 and 5×10 −3 cm −1 atm −1 ) over the 300– 340 K temperature range.

A traveling-wave THz photomixer based on angle-tuned phase matching

A traveling-wave THz photomixer based on a free-space optical-THz phase-matching scheme is proposed. A dc-biased coplanar strip line fabricated on low-temperature-grown GaAs serves as the active area of the device, and is illuminated by two noncollinear laser beams which generate interference fringes that are accompanied by THz waves. The device with the laser-power-handling capability over 300 mW and a 3-dB bandwidth of 1.8 THz was experimentally demonstrated. The results show that traveling-wave photomixers have the potential to surpass small-area designs.