Photomixing Technique Research Articles

The effect of nanoelectrodes number and length on enhancing the THz photomixer performance

Abstract: Despite the distinct features of the continuous wave (CW) Terahertz (THz) emitter using photomixing technique, it suffers from the relatively low radiation output power. Therefore, one of effective ways to improve the photomixer emitter performance was using nanodimensions electrodes inside the optical active region of the device. Due to the nanodimension sizes and good electrical conductivity of silver nanowires (Ag-NWs), they have been exploited as THz emitter electrodes. The excited surface plasmon polariton waves (SPPs) on the surface of nanowire enhances the incident excitation signal. Therefore, the photomixer based Ag-NW compared to conventional one significantly exhibits higher THz output signal. In this work, the effect of Ag-NWs dimensions and number on the incident optical field is investigated by utilizing the Computer Simulation Technology (CST) Studio Suite. The simulation results show that increasing Ag-NWs length to SPP propagation length ratio plays a significant role on the incident field increment due to its effect on reducing the SPP propagation losses. The increment of Ag-NWs number and length in the nanoelectrodes based photomixer can contribute to increase the electric field in the active region by 1.5 times at the longer excitation wavelength (850 nm). As a result of this increment, the THz output power and the conversion efficiency are expected to be enhanced by a factor of five

A Review on Terahertz Technologies Accelerated by Silicon Photonics.

In the last couple of decades, terahertz (THz) technologies, which lie in the frequency gap between the infrared and microwaves, have been greatly enhanced and investigated due to possible opportunities in a plethora of THz applications, such as imaging, security, and wireless communications. Photonics has led the way to the generation, modulation, and detection of THz waves such as the photomixing technique. In tandem with these investigations, researchers have been exploring ways to use silicon photonics technologies for THz applications to leverage the cost-effective large-scale fabrication and integration opportunities that it would enable. Although silicon photonics has enabled the implementation of a large number of optical components for practical use, for THz integrated systems, we still face several challenges associated with high-quality hybrid silicon lasers, conversion efficiency, device integration, and fabrication. This paper provides an overview of recent progress in THz technologies based on silicon photonics or hybrid silicon photonics, including THz generation, detection, phase modulation, intensity modulation, and passive components. As silicon-based electronic and photonic circuits are further approaching THz frequencies, one single chip with electronics, photonics, and THz functions seems inevitable, resulting in the ultimate dream of a THz electronic–photonic integrated circuit.

On-chip coherent frequency-domain THz spectroscopy for electrical transport

We developed a coherent frequency-domain terahertz (THz) spectroscopic technique on a coplanar waveguide in the ultrabroad frequency range from 200 MHz to 1.6 THz based on continuous wave (CW) laser spectroscopy. Optical beating created by mixing two frequency-tunable CW lasers is focused on photoconductive switches to generate and detect high-frequency current in a THz circuit. In contrast to time-domain spectroscopy, our frequency-domain spectroscopy enables an unprecedented frequency resolution of 10 MHz without using the complex building blocks of femtosecond laser optics. Furthermore, due to the coherent nature of the photomixing technique, we are able to identify the origin of multiple reflections in the time domain using the Hilbert analysis and inverse Fourier transform. These results demonstrate that the advantages of on-chip coherent frequency-domain spectroscopy, such as its broadband, frequency resolution, usability, and time-domain accessibility, provide a unique capability for measuring ultrafast electron transport in integrated THz circuits.

Extending the spectral region of a sub-terahertz spectroscopic system by mixing broad-area and single-mode laser beams

The photomixing technique is applied to a sub-THz spectroscopic system with a continuous wave multimode laser diode (CW-MLD) to extend its spectral range. For the MLD, a broad-area laser diode (BLD) is employed to produce broad continuous spectrum signals. By mixing the light from the BLD with light from a continuous wave single-mode laser diode, a new broad continuous component is generated in the higher frequency region (0.4–1.0 THz) of the Fourier spectrum of the signal waveform. This is achieved via a simple and low-cost experimental setup using laser diodes and fiber optics. Spectroscopic measurement of a Si wafer in the frequency range below 1 THz is demonstrated.

High-Precision Continuous-Wave Terahertz Spectroscopy Based on a Photomixing Technique for Identifying Pearls.

We present the accurate terahertz spectra of between imitation and cultured pearls using continuous-wave terahertz (CW-THz) spectroscopy. Using Fourier transform infrared (FT-IR) spectroscopy and optical coherence tomography (OCT) measurements, cultured pearls can be distinguished from imitation pearls by observing distinct absorption peaks and discriminative boundaries. The THz absorption spectra up to 0.3 THz obtained from CW-THz spectroscopy show several absorption peaks at specific frequencies with the cultured pearls but no peaks with the imitation pearls, which results from the existence of the nacre polymorph of cultured pearls. Hence, it is expected that the CW-THz system proposed herein will be applicable to fast, nondestructive spectrum analysis including pearl identification.

Extension of spectral region of sub-terahertz spectroscopic system with multimode laser diodes by simple configuration for photomixing technique

The photomixing technique is applied to a sub-terahertz spectroscopic system with continuous-wave multimode laser diodes (CW-MLDs) to extend the spectral range. I employ two CW-MLDs which have equal spectral widths and slightly different central wavelengths, where sophisticated techniques such as external cavities are not required. I observe that photomixing causes generation of new spectral components of the signal in the frequency region of about 0.5–0.65 THz, while spectral components are limited below about 0.5 THz before the photomixing technique is applied. The generation of spectral components implies that the spectral range covered by the spectroscopic system is extended to higher-frequency regions.

Optical frequency switching scheme for a high-speed broadband THz measurement system based on the photomixing technique.

This study presents an optical frequency switching scheme for a high-speed broadband terahertz (THz) measurement system based on the photomixing technique. The proposed system can achieve high-speed broadband THz measurements using narrow optical frequency scanning of a tunable laser source combined with a wavelength-switchable laser source. In addition, this scheme can provide a larger output power of an individual THz signal compared with that of a multi-mode THz signal generated by multiple CW laser sources. A swept-source THz tomography system implemented with a two-channel wavelength-switchable laser source achieves a reduced time for acquisition of a point spread function and a higher depth resolution in the same amount of measurement time compared with a system with a single optical source.

Wide-capture-range, high-precision wavelength stabilization within ±50 MHz for flexible-grid wavelength division multiplexing by photomixing technique

The lasers for the flexible-grid wavelength division multiplexing (WDM) system are required to have high precision of wavelength stability. Previously, we proposed the wavelength-controlling system of the distributed feedback laser diode (DFB-LD) with the photomixing technique and a microwave filter to precisely measure the optical frequency error from the target value. To enlarge the wavelength-capture range, we improve the system to detect the wavelength error with two different microwave filters in parallel. Experimental results show that the wavelength-capture range is extended up to 4 GHz while the wavelength error is still kept within ±50 MHz.

Doppler limited rotational transitions of OH and SH radicals measured by continuous-wave terahertz photomixing

Abstract A continuous-wave terahertz (CW-THz) source generated by photomixing has been employed to detect and quantify radicals produced in a cold plasma probing their spin-rotation transitions. Due to their dual interest for both atmospherists and astrophysicists, the hydroxyl OH and the mercapto SH radicals have been chosen. The photomixing technique which can access the largest range of THz frequencies of any known coherent source, allowed to resolve the Doppler-limited hyperfine transitions of OH in the 2.5 THz frequency region. Line profile analysis of the hyperfine components demonstrated that OH radicals have been detected in this region at a ppm level at a temperature close to 490 K. The hyperfine structure of SH has been resolved for the first time above 1 THz. Ten new frequency transitions have been measured in the 1.3–2.6 THz frequency range using the CW-THz synthesizer based on a frequency comb. With relative uncertainties better than 10 −7 , the CW-THz frequencies measured in this study are now competitive with those measured by other instruments such as frequency multiplication chains or FT-FIR spectrometers and are now capable to improve the predictions of the complete high-resolution spectra of these radicals collected in the atmospheric and astrophysical spectroscopic databases. versioncorrigeeAC 2011-07-18 17:32 © 2011 Arnaud Cuisset.

Measurement of optical-beat frequency in a photoconductive terahertz-wave generator using microwave higher harmonics

A new method for measuring optical-beat frequencies in the terahertz (THz) region using microwave higher harmonics is presented. A microwave signal was applied to the antenna gap of a photoconductive (PC) device emitting a continuous electromagnetic wave at about 1 THz by the photomixing technique. The microwave higher harmonics with THz frequencies are generated in the PC device owing to the nonlinearity of the biased photoconductance, which is briefly described in this article. Thirteen nearly periodic peaks in the photocurrent were observed when the microwave was swept from 16 to 20 GHz at a power of -48 dBm. The nearly periodic peaks are generated by the homodyne detection of the optical beat with the microwave higher harmonics when the frequency of the harmonics coincides with the optical-beat frequency. Each peak frequency and its peak width were determined by fitting a Gaussian function, and the order of microwave harmonics was determined using a coarse (i.e., lower resolution) measurement of the optical-beat frequency. By applying the Kalman algorithm to the peak frequencies of the higher harmonics and their standard deviations, the optical-beat frequency near 1 THz was estimated to be 1029.81 GHz with the standard deviation of 0.82 GHz. The proposed method is applicable to a conventional THz-wave generator with a photomixer.

편광 및 위상 부정합을 이용한 광혼합을 통하여 발생된 서브 밀리미터파 대역 연속파의 위상 잡음 특성 개선

본 논문에서는 광혼합 방식으로 서브 밀리미터 및 테라헤츠 대역 연속파 신호를 발생시키는 기법 중 가장 널리 사용되는 광반송파가 억제된 양측 대역 발생 방식(Double Sideband-Suppressed Carrier: DSB-SC)을 이용하여 발생된 연속파(Continuous Wave: CW) 신호의 위상 잡음 개선을 위하여 광신호의 편광과 위상 제어 기법을 제안하고 실험적으로 증명하였다. 광신호의 편광 및 위상 제어 기법은 일반적인 DSB-SC 신호와 DSB-SC 신호에 포함된 광반송파와 동일한 파장과 위상차를 가지며, 광반송파의 편광 성분 중 하나의 편광 성분만을 갖도록 편광제어된 광신호를 결합하여 광혼합하는 방법이다. 실험 및 측정 결과, 서브 밀리미터파 대역 CW 신호의 크기는 1.5 dB 증가하였으며, 위상 잡음 특성은 약 3 dB@10 kHz offset frequency 개선됨을 확인하였다. 따라서 본 논문의 결과는 광신호의 위상 및 편광 성분 제어만으로 광반송파를 효과적으로 억제하여 서브 밀리미터 및 테라헤르츠 대역 CW 신호의 특성을 개선함으로써 광혼합 방법을 이용한 밀리미터파 및 테라헤르츠파 대역 CW 신호 발생기의 저가화를 위한 기본적인 데이터로서 활용 가치가 높다. In this paper, we have proposed and experimentally performed a polarization and phase control method of an optical signal which has same wavelength with the optical carrier to improve phase characteristics of a continuous wave(CW) generated by the double sideband-suppressed carrier(DSB-SC) as one of the famous photomixing technique for making sub-millimeter and terahertz waves. A polarization and phase controlled optical signal has been coupled with the general DSB-SC on an optical coupler. The output of the optical coupler is then photomixed by a photomixer. From our analysis and measurement results, we have found that the amplitude of the generated sub-mm and terahertz CW signal is higher 1.5 dB and the phase noise is lower about 3 dB@10 kHz offset frequency than the general DSBSC. Consequently, since our proposed method has improved the amplitude and phase noise of CW signals in the sub-mm and terahertz bands, it can be helpful results to make low cost CW generator in sub-millimeter and subterahertz bands.

Telecommunications technology-based terahertz sources

A review is presented of continuous-wave terahertz sources based on the optical heterodyne generation (photomixing) technique, that make use of optical components and fabrication techniques originally developed for the 1550 nm optical fibre telecommunications window. The uni-travelling carrier photodiode is identified as a key component for conversion of optical to terahertz power, and the state of the art is summarised in terms of terahertz power generated at various frequencies. An approach based on phase locking the heterodyned lasers is described, which enables terahertz signals to be generated with extremely high spectral purity and frequency accuracy, and progress in developing the photonic integrated circuits required for its implementation is reported. Finally, possible future developments in the field are discussed.

Detection of Photomixing Signals With a YBCO Josephson Junction Coupled to a Coplanar Waveguide

We have fabricated a generation and detection system of photomixing signals using a photomixing technique and a Josephson junction detector. Up to now, an electromagnetic wave generated by a heterodyne mixing of two lasers was emitted from an antenna, and irradiated to a Josephson junction. In this study, we have developed the system, and the photomixing signal was introduced into a coaxial cable and a coplanar wave guide coupled to a Josephson junction. This structure is more suitable for device application since it can easily be integrated into electronic devices. With this system we can clearly observed photomixing signals from 16 GHz to 95 GHz using a uni-traveling-carrier photodiode as a photomixer as well as signals in the range of 14 GHz to 50 GHz using an InGaAs PIN photodiode.

Study on Sub-THz Signal Input for Superconducting Electronic Devices

Development of ultrafast optical interfaces that can operate in sub-terahertz region is important to apply superconducting electronic devices to the high-end systems. We have performed several fundamental researches to realize the ultrafast optical input interface for superconducting electronic devices. Firstly, we observed optical response of amorphous Ge thin films, and the results indicated that an amorphous Ge photoconductive switch could stably operate in a terahertz frequency range as an optical-to-electrical signal converter in the low-temperature region below T c of YBCO. Next, we have fabricated optical-to-electrical signal conversion system with photomixing technique, and we have demonstrated the generation and the detection of high frequency signals over 50 GHz. Finally, we have observed optical responses of a Josephson vortex flow transistor under irradiation of femtosecond laser pulses, and the results suggeste that the device has high potential as an optical interface.

Study on the optical input interface for Nb single-flux-quantum logic circuits

The authors have studied the optical input interface for Nb single-flux-quantum (SFQ) circuits and demonstrated its operation using photomixing technique. Two tunable lasers of C band were used for photomixing. We adopted a multistage toggle flip-flop (TFF) circuit as the main part of the test circuit. As the first trial, we used lasers with wavelength difference of 0.044nm at around 1550nm, which corresponds to the laser beat frequency of 5.5GHz. The output signals from the 13- and 14-stage TFFs were observed and it showed the input SFQ frequency of 5.5GHz. Thus, the authors demonstrated the conversion of the optical signal to SFQ signal and succeeded in driving the SFQ circuit.

A tunable sub-terahertz wave generation and detection system with a photomixer and ahigh-Tc Josephson junction

We have fabricated a generation and detection system of tunable sub-terahertzelectromagnetic waves using a photomixing technique combined with ahigh-Tc Josephson junction. The frequency of the electromagnetic wave generated using thephotomixing technique was monitored by a YBCO Josephson junction detector,and was obtained from the voltage interval of the Shapiro steps that appear inthe current–voltage characteristics of the junction. We succeeded in widebandoperation in a frequency range from 58 to 161 GHz with a uni-travelling-carrierphotodiode as a photomixer, and discuss the coupling efficiency between the radiationsource and the Josephson detector. Such a technique should become the basictechnology for THz applications, such as imaging systems and wireless communication.

Frequency detection of focused sub-THz waves using a high-Tc Josephson junction

We have proposed a new type of frequency detection system of sub-terahertz electromagnetic waves using a photomixing technique and a Josephson junction detector. The photomixer illuminated by two lasers with different wavelengths generates coherent electromagnetic waves that include a frequency component corresponding to the frequency difference of the two lasers. The generated electromagnetic waves were radiated from a horn antenna and focused to the Josephson junction detector with Si lens. The high-Tc Josephson junction operates as a frequency detector for electromagnetic waves. Using this system, we observed the Shapiro steps of 188μV that correspond to the frequencies of 90.8GHz.

Electronic transport properties of the μc-(Si,Ge) alloys prepared by ECR

Abstract Some μc-(Si,Ge):H alloys have been grown using low-pressure, reactive ECR plasma deposition with high H dilution and subtle (sub ppm) B-doping. Incorporating these high-quality materials into devices leads to low-gap μc-(Si,Ge) solar cells with acceptable performance. This justifies a detailed investigation of the electronic transport properties of μc-(Si,Ge):H alloys by employing the microwave photomixing technique. From the measurements of the electric field dependence of the drift mobility and lifetime, we have found strong evidence for the existence of long-range potential fluctuations in μc-(Si,Ge):H alloys. We determine the depth and range of the potential fluctuations, and subsequently the charged defect density, as a function of the deposition rate. It was found that the film transport properties do not degrade or enhance monotonically with increasing deposition rate; there exists a valley point where the strongest potential fluctuations occur as a result of a significant increase in the charged defect density. Beyond this point, the film quality increases again. The evidence indicates that it is the long-range potential fluctuations that result in the deterioration of the transport properties of μc-(Si,Ge):H alloys. Specifically, it is the increase in the depth, and a decrease in the length of the potential fluctuations, which lead to a decrease in the mobility, and consequently in the photoconductivity. Our present results demonstrate that aside from the increase of charged scattering centers, compositional disorder in the alloys play an important role with the build-up of the potential fluctuations.

Electronic transport properties of low band gap a-SiGe:H alloys prepared by PECVD technique

The electronic transport properties of a-(Si,Ge):H alloys across the range of Ge content (0–40%) grown using PECVD technique have been investigated in detail by employing the microwave photomixing technique. Strong evidence for the existence of long-range potential fluctuations in a-(Si,Ge):H alloys has been found from the measurements of electric field dependence of the drift mobility. It was observed that the film transport properties degrade monotonically with increasing Ge content, where the strongest potential fluctuations occur as a result of a significant increase in the charged defect density. The potential fluctuations whose effect is enhanced by adding Ge to the alloy system result in deterioration of the transport properties of a-(Si,Ge):H alloys. Our present results demonstrate that the increased charged scattering centers and compositional disorder upon adding Ge to the alloys play an important role in the potential fluctuations.

On the electronic transport properties of amorphous (Si,Ge) alloys: charged scattering centers and compositional disorder

The electronic transport properties of a-(Si,Ge):H alloys over the entire range of Ge content (0–100%) grown using low pressure, reactive ECR plasma deposition with high H dilution and subtle (sub-ppm) B-doping have been investigated in detail by employing the microwave photomixing technique. Strong evidence for the existence of long-range potential fluctuations in a-(Si,Ge):H alloys has been found from the measurements of electric field dependence of the drift mobility. It was observed for the first time that the film transport properties do not degrade monotonically with increasing Ge content; there exists a valley point near the middle of the composition range, where the strongest potential fluctuations occur as a result of a significant increase in the charged defect density. Beyond this point, the film quality increases again, but still much worse than that for the Si end. The effect of potential fluctuations can be enhanced by adding Ge or Si to the alloy system resulting in deterioration of the transport properties of a-(Si,Ge):H alloys. Our present results demonstrate that the increased charged scattering centers and compositional disorder upon adding Ge or Si to the alloys play an important role in the potential fluctuations.