Efficient low-loss nanometallic suspended waveguide for terahertz generation

A novel, low loss and tunable source of continuous terahertz (THz) wave radiation based on frequency conversion in GaAs is presented. The tunability of the device can be achieved by varying the distance between quartz slabs (air gap distance). The output THz power at 1.3 THz for different air gap distances and various waveguide widths has been calculated. By tuning the air gap distance in the range of 290 to 320 nm for a device with 4 cm long at 1.3 THz, a relatively high output power of 3 to 9.5 µW is predicted.

This paper presents a novel nanosource of continuous terahertz (THz) wave radiation based on difference frequency generation (DFG). Both the optical and THz waves are confined in a nanoscale plasmonic waveguide. The results of theoretical investigations indicate that the appropriate geometry of the plasmonic waveguide has a relatively long optical and THz propagation distance (on the order of several tens of wavelengths). Phase matching is obtained using the dispersive properties of the waveguide for generating 0.8 THz at the central optical wavelength of 1.52 μm. Our numerical calculation shows that the conversion efficiency in this waveguide structure can be achieved up to 5 times as large as the micron scale dielectric waveguides and almost 10 times as large as the metallic micron scale waveguides.

Interests on low-loss terahertz (THz) waveguides are increasing due to their remarkable applications in various fields. Since most the materials are highly absorbent to THz waves therefore it is an ongoing challenge to obtain a low-loss waveguide. This paper presents a novel porous-core square lattice photonic crystal fiber (PCF) for efficient transmission of THz waves. The guiding properties of the proposed fiber are characterized by using finite element method (FEM) with circular perfectly matched layer (PML) boundary conditions. It is demonstrated that the designed PCF shows very low effective material loss (EML) of 0.076 cm-1 at 1.0 THz that indicates about 62 % reduction of bulk absorption loss of the background material. In addition to this, the proposed fiber exhibits low confinement loss of 8.96 × 10-3 dB/cm and low flattened dispersion of 0.96 ± 0.086 ps/THz/cm for the optimal design parameters. Other important propagation characteristics such as single mode propagation, power fraction, and bending loss are also investigated thoroughly. A porous-core PCF is an efficient mechanism for the transmission of THz waves. The proposed low-loss and low-dispersion PCF can find numerous applications in THz regime.

In this paper, the interaction between a gold nanotip and a silver substrate under the terahertz (THz) wave radiation is simulated by a finite difference time domain method and theoretically analyzed from the quasi-static charges model. Results from simulations and theoretical analysis show that with the increase in the distance between the nanotip and the substrate, the maximum electric field intensity on the substrate prominently decreases. With the increase in radius of the nanotip, diameter of the focused THz spot almost linearly increases and the intensity of THz spot also has a slight increase. These results are in agreement with the physical principles of the tip-enhanced near-field optical microscopy (TENOM), which indicates the TENOM technique is also suitable for the THz frequency range.

Laser pumped terahertz (THz) wave up-conversion detection with high sensitivity, fast responsivity and wide frequency band is achieved at room temperature, based on home-made organic nonlinear crystals 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST). Green laser pulses pumped KTiOPO<sub>4</sub> optical parametric oscillators are utilized as the sources of dual-wavelength near-infrared (NIR) beams (1.3–1.6 μm, for THz-wave difference frequency generation (DFG)) and a single NIR beam (1.2–1.4 μm, for up-conversion detection). The nonlinear medium for both THz-DFG and detection is DAST (grown by CETC-46). A nanosecond-time-resolved THz pulse is obtained with an InGaAs p-i-n photo-diode. The spectrum of the up-converted NIR light is acquired, which allows us to measure the THz frequency indirectly. The sensitivity (also at room temperature) is 4 orders better at 19 THz than the sensitivity of a commercial thermal detector (Golay Cell). The wide frequency band operation is realized with different sets of band-pass filters, which cover the entire range from 3.15 to 29.82 THz except 8.4 THz of the strong absorption peak of DAST. The dynamic range of a THz source based on DFG can be commonly improved by 2–3 orders, by changing the traditional thermal detector with the up-conversion detection. The presented technology can promote the applications of DFG THz source in the fields of high-resolution spectroscopy and imaging.

Generation of continuous-wave (CW) terahertz (THz) radiation with a power of 2µW is demonstrated. Two CW laser beams of wavelengths around 800nm are photomixed in a large-area photomixer to generate a THz radiation of 1.2THz. The utilized photomixer has interdigitated contacts with desirable radiation characteristics in the far-field region. We show that the radiated THz beam is steerable by changing the incidence angles of the CW pump beams. In our experimental setup, the emission direction of the large-area photomixer can be steered by rotating the beam splitter used for beam combining. Using this setup, we are able to steer the THz beam in a range of −30 to +30 degrees.

Long-range spoof surface plasmons on the doubly corrugated metal surfaces

Terahertz (THz) wave science and technology have been found countless applications in biomedical imaging, security screening, and non-destructive testing as they approach maturity. However, due to the challenge of high ambient moisture absorption, the development of remote open-air broadband THz spectroscopy technology is lagging behind the compelling need that exists in homeland security, astronomy and environmental monitoring. Furthermore, the underlying physical mechanisms behind the interaction between the THz wave and laserinduced plasma which responds strongly to electromagnetic waves have not been fully understood. This review aims to explain the light-plasma interaction at THz frequencies within a semiclassical framework along with experimental study of the femtosecond-laserinduced nitrogen plasma fluorescence under the illumination of single-cycle THz pulses. The results indicate that THz-radiation-enhanced-emission-of-fluorescence (THz-REEF) is dominated by electron kinetics in the THz field and the electron-impact excitation of gas molecules/ions. The information of the time-dependent THz field can be recovered from the measured time-resolved THz-REEF from single-color laser induced plasma with the help of the bias as local oscillator. The calculations and experimental verification lead to complete understanding of the science behind these effects and push forward to extend their capabilities in related applications such as remote THz sensing, plasma diagnostics and ultrafast photoluminescence modulation. Systematic studies in selected gases including neon, argon, krypton, xenon, methane (CH4), ethane (C2H6), propane (C3H8), and n-butane (C4H10) gases were performed to obtain an improved understanding of the THz-REEF. The dependences of the enhanced fluorescence on the THz field, laser excitation intensity, gas pressure, and intrinsic atomic properties were experimentally characterized. Both narrow line emission and broad continuum emission of the gas plasma were enhanced by the THz field. Their fluorescence enhancement ratios and time-resolved enhanced fluorescence were largely dependent on the scattering cross section and ionization potential of atoms. For the first time, we demonstrated a novel ‘all-optical’ technique of broadband THz wave remote sensing by coherently manipulating the fluorescence emission from asymmetrically ionized gas plasma that interacted with THz waves. By studying the ultrafast electron dynamics under the single cycle THz radiation, we found that the fluorescence emission from laser-induced air plasma was highly dependent on the THz electric field and the symmetry of the electron drift velocity distribution created by two-color laser fields. The time-resolved THz-REEF can be tailored by switching the relative two-color phase and laser polarizations. Owing to the high atmospheric transparency and omni-directional emission pattern of fluorescence, this technique can be used to measure THz pulses at standoff distances with minimal water vapor absorption and unlimited directionality for optical signal collection. The coherent THz wave detection at a distance of 10 m had been demonstrated. The combination of this method and previously demonstrated remote THz generation would eventually make remote THz spectroscopy available. We also introduced a unique plasma diagnostic method utilizing the THz-wave-enhanced fluorescence emission from the excited atoms or molecules. The electron relaxation time and plasma density were deduced through applying the electron impact excitation/ionization and electron-ion recombination processes to the measured time-delay-dependent enhanced fluorescence. The electron collision dynamics of nitrogen plasma excited at different gas pressures and laser pulse energies were systematically investigated. This plasma diagnostic method offers picosecond temporal resolution and is capable of omnidirectional optical signal collection. The ultrafast quenching dynamics of laser-pulse-induced photoluminescence in semiconductors under the radiation of single-cycle THz pulses was studied. It was found that the quenching in both cadmium telluride (CdTe) and gallium arsenide (GaAs) was linearly proportional to the intensity of incident THz waves and reaches up to 17% and 4% respectively at the peak intensity of 13 MW/cm2. The THz-wave-induced heating of the carriers and lattice and the subsequent decreased efficiency of photocarrier generation and recombination were most likely to be responsible for the quenching. This is potentially useful for the applications of a non-invasive ultrafast light modulator for photoluminescence devices with picoseconds switching time in the fields of the light-emitting devices and optical communication.

Research in the area of terahertz (THz) waveguides has seen a rapid progress recently and has led to demonstration of THz waveguides with transmission losses comparable to losses in air. We will discuss dielectric-lined hollow metallic waveguides, in which THz waves propagate with attenuation as low as 1 dB/m and dispersion of 6 ps/(THz·m), and compare them to other low-loss THz waveguides. As a key technique for THz waveguide research, we will discuss the application of THz near-field microscopy in combination with THz time-domain spectroscopy for waveguide characterization. This technique allows us to map spatial profiles of normal modes and to measure transmission loss and dispersion spectra for individual modes within the THz range.

A compact efficient high-repetition-rate doubly-resonant dual-wavelength KTP optical parametric oscillator (OPO), with output power up to 3.65 W and tuning ranges of 2.088-2.133 μm/2.171-2.122 μm for signal/idler waves, was deployed for terahertz (THz) generation in a GaSe crystal. Based on difference frequency generation (DFG), the THz wave was continuously tunable from 730.9 μm (0.41 THz) to 80.8 μm (3.71 THz), believed to be the first report of a compact high-repetition-rate widely-tunable THz source. The maximum THz average power reached 1.2 μW at 1.54 THz and the corresponding DFG efficiency was 7.8 × 10-7, entirely suitable for portable applications. The utility of the THz source was also demonstrated through spectroscopy and imaging experiments.

The ever-growing number of interconnected devices and the emergence of high-bandwidth technologies such as holographic communications in mixed reality are driving an exponential growth in the demand for terabits-per-second (Tbps) data rates with sub-milliseconds latency. For this reason, the sixth-generation (6G) wireless communications are moving towards the terahertz (THz) frequency band because of its wider spectral bandwidth availability to enable higher channel capacity and data rates according to the Shannon-Hartley theorem. However, the efficiency of wireless THz signal transmission is reduced by the increased path loss and atmospheric attenuation due to water vapor absorption at THz frequencies, which significantly decreases the signal-to-noise ratio and the spectral efficiency of THz wireless communications. The realization of THz band communications is further impeded by the limited power output of existing compact THz sources due to the extremely low conversion efficiencies. Therefore, it is crucial to develop broadband THz integrated circuits with waveguiding and beamforming functionalities for supporting high data-rate on-chip communications and to provide sufficient gains to counteract the path loss and limited power of THz wireless transmissions. This thesis aims to improve the spectral efficiency of 6G wireless communications via the spatiotemporal shaping of THz waves using silicon photonics. Spatiotemporal light shaping is the basis of nearly all THz photonic devices, ranging from THz wavefront manipulation using metasurfaces to THz planar waveguiding using photonic crystal waveguides. Silicon photonics is a promising material platform for THz wave shaping due to the low-loss propagation of THz waves in high-resistivity silicon and the prospect of a monolithic integrated circuit due to the complementary metal-oxide semiconductor (CMOS) fabrication process compatibility. Silicon metasurfaces allow the wavefront modulation of incident THz radiation by exploiting the generalized Snell’s law of refraction and reflection to beamform highly directional THz waves in a dynamic multi-user communication environment for enhancing the spectral efficiency. Likewise, broadband silicon photonic crystal waveguides allow the in-plane and lossless transport of THz waves to support on-chip high data-rate THz communications that can also seamlessly integrate multiple THz photonic devices in a THz integrated circuit. In addition, silicon electron emitters and silicon gratings potentially allow the on-chip generation of highly intense and broadband THz radiation to counteract the significant THz attenuation. In this thesis, spatiotemporal wave shaping techniques using silicon photonics for enhancing the spectral efficiency of THz band communications is presented. First, a self-adaptive and deep reinforcement learning model for THz beamforming is proposed, which can beamform multiple highly directional THz radiation beam in real-time to serve a dynamically changing multi-user communication environment and improve the signal-to-noise ratio of THz wireless transmission while potentially reducing the communication latency. Second, a broadband THz topological silicon waveguide designed based on valley photonic crystals is proposed for the robust and lossless transport of broadband THz waves for the construction of THz integrated circuits with broadband channel capacity for enabling higher data-rate on-chip communication. Last, the versatility of free-electron radiation from silicon gratings is investigated for the generation of highly intense spatiotemporal wave packets with user-designed group velocities and tunable intensity profile, with the potential to realize integrated high-power THz sources. The deep learning beamforming model, the broadband topological waveguide designs, and the free-electron radiation intensity shaping techniques presented in this thesis extends beyond the THz regime and are scalable across the entire electromagnetic spectrum. The CMOS process compatibility of silicon metasurfaces, silicon planar waveguides, along with silicon electron emitters and silicon gratings offers the opportunity to realize a broadband THz photonic integrated circuit with waveguiding, wave generation and beamforming functionalities for enhancing the spectral efficiency of THz communications in 6G wireless networks.

A compact cascaded optical parametric device generating a coherent pulse terahertz (THz) wave is demonstrated. The terahertz parametric oscillator (TPO) and the difference frequency generation (DFG) are designed for cascaded operation use with two outputs producing tunable THz wavelengths. From the first optical parametric device, a TPO with a MgO: LiNbO3 crystal pumped by a Q switch laser of 1.064 μm, 1.8 mJ idler pulse and 5.5 mJ residual pump pulse is obtained. Both of the two beams are employed as the pump and signal beams in the second optical parametric device DFG with a GaSe crystal. More than 0.6 μJ and about 2.1 ns THz pulse at 183 μm is achieved from the DFG. A tunable THz source in the range 104–226 μm via tuning the external phase matching (PM) angles of the TPO and the DFG flexibly under room temperature is obtained. The observed tunable THz wavelengths from the DFG are the same as those from the TPO.

Terahertz (THz) emission by difference frequency generation (DFG) technique has attracted vast interests recently. Quantum dot (QD) system is proposed to be the best candidate for THz emission by DFG technique due to its large optical nonlinearity. For efficient QD THz emission, the QD transition energies must lie within the THz range (0.4-41 meV). However, QD transition energies generally fall outside the critical energy difference, i.e. more than 41 meV. In this work, we investigated the effects of QD structures and growth conditions on the energy states. Consequently, we determined the required QD structures and growth conditions needed to obtain energy difference in the THz range.

It is a great pleasure for me to introduce this Special Issue of IET Optoelectronics, which is focussed on semiconductor optical devices. Once a year, a Special Issue on this topic is published in association with the Semiconductor and Integrated OptoElectronics (SIOE) conference held in Cardiff, UK. In 2016 SIOE celebrated its 30th anniversary and as always, featured an exciting programme with a very high standard of technical talks, some of which feature as extended versions in this issue. Semiconductor optoelectronics is constantly evolving with the emphasis on integration continuing to grow as a key theme, in both materials and device functions, and this is largely to meet the demand of technological trends. The push for photonic integrated systems and ‘smart’ devices is setting the scene for research and such systems are predominately designed with energy efficiency in mind. In the context of this field, so called ‘green photonics’ has a great importance for society and naturally promotes high impact for research proposals, which is particularly beneficial given the competitive nature of funding. In this issue, we showcase some of the key papers featured at SIOE, giving you a flavour of the subject areas of the conference sessions, many of which relate to the trends identified above. On the topic of green photonics, there are two papers concerning the development of solar cells: Wang, Q. et al. make use of a detailed balance model to investigate enhancing the efficiency of intermediate band solar cells by introducing carrier losses and compare this with the effect of using alloying and biaxial strain. Cappelluti, F. et al. present research on open-circuit voltage recovery in quantum dot solar cells. A numerical study is used to investigate the effect of wetting layer states and doping on the photovoltage loss in InAs/GaAs solar cells.This includes quantum mechanical simulations to analyse how the reduction of the wetting layer by overgrowth can influence the quantum dot states. It is an exciting and busy time for researchers in the area of terahertz (THz) technology, this relatively untapped region of the electromagnetic spectrum promises a plethora of applications, in medicine, quality-control and detection of explosives, to name a few. Low-cost, energy efficient ways to emit THz radiation using semiconductor optical devices is fast becoming a realistic prospect for commercial applications and will surely be a ubiquitous technology in the near future. THz research featured prominently at the conference with two papers included in the Special Issue: firstly Gwaro, J. et al. report on the implementation of a cost-effective compact THz source, which features a monolithically integrated dual-wavelength distributed Bragg reflector (DBR) semiconductor laser diode, making use of photo-conductive antennas. Secondly, Wang, Y. et al. describe their research on THz photoconductive antennas using low-temperature grown III-V compounds which incorporated DBRs to enhance the THz peak signal. Characterisation of novel laser devices and materials always forms a large part of the SIOE conference, through both experimentation and modelling. Modulation and tuneability of lasers formed a key topic area, as well as mode-locking and dynamic behaviour. Dugzul, O. et al. investigated the effect of optical-injection-locking on modulation dynamics of tuneable laser diodes and a travelling wave approach, making use of an integrated VPI, and PICS3D commercial software was used to assess the dynamic response of a three-section InGaAsP/InP tuneable laser. Tawfieq, M. et al. discuss the concept of numerical simulations of a widely tuneable 4-section sample-grating laser, emitting at 976 nm, and present data on the accessible wavelength range, the threshold current behaviour and gain characteristics. Experimental results, from monolithic colliding-pulse mode-locked lasers, containing chirped double-quantum-well active regions are reported by Prziwarka T. and co-workers. They discuss factors such as gain current, absorber length/voltage, and compromises made to achieve short (picosecond) pulses with relatively low repetition frequencies. Finally, our invited paper by Kellehur, B. et al. theoretically investigates an experimental phenomenon observed between mutually coupled quantum dot lasers, which exhibit a greatly reduced modal linewidth when mutually phase-locked. I thank the authors who contributed fully expanded papers for the Special Issue and I would also like to thank the referees for their time spent reviewing manuscripts and forming constructive comments. Now I shall leave you to enjoy reading this selection of papers and hope to see you all at future SIOE conferences. Dr. Samuel Shutts is a Research Associate in the School of Physics and Astronomy at Cardiff University. He is involved with the design, fabrication and characterisation of semiconductor light emitting devices. His current research interests include quantum dot lasers structures grown on silicon substrates; InP/AlGaInP quantum dot lasers; dual-wavelength sources; distributed Bragg reflectors and distributed feedback lasers and THz emission via difference frequency generation.

This paper presents a comparative study between a porous core kagome lattice photonic crystal fiber (PCF) and a air filled Hollow core kagome lattice PCF (HC-PCF), which acts as a low loss and polarization maintaining tera hertz (THz) waveguide. Proposed PCFs are simulated using finite element method (FEM), including the effective material loss (EML), confinement loss and single mode propagation. Cyclic olefin copolymer, also known as TOPAS material have been chosen for the air micro structured inhibited coupled design of PCF which has the lowest bulk material absorption loss of 0.2 cm-1. Expressions are given to asses this optimization and the result are shown for the variation of core diameter from 425 μm to 600 μm for both the Hollow core and porous core PCFs and for the comparison with porous core the optimized porosity taken as 60% and 70% along with the frequency of 1THz. The analysis results approximately 80% to 90% improvement in losses in case of passing THz waves through HC-PCF rather than Porous core PCF. By varying core parameter such as core porosity and core diameter and strut width at 1THz, the goal is to show an optimized solution consisting of low EML and confinement loss for both of the wave guidance and comparing the results to analyze and hence finding out a better solution.