Guest Editorial

We have experimentally demonstrated thermal evaporated group IV Ge1-xSnx-on-Si terahertz (THz) photoconductive antennas (PCA) pumped by an Er-doped femtosecond laser for broadband THz generation. The Ge1-xSnx THz PCAs, free from material epitaxial growth methods, can offer comparable material properties in photocarrier generation, transportation, recombination, and the collection as group III-V THz PCAs. At the optical pumping power of 90 mW and a bias voltage of 40V, the Ge1-xSnx THz PCAs have achieved a broadband spectrum over 1.5 THz with a 40 dB signal-to-noise ratio (SNR). This CMOS-compatible group IV THz source can be monolithically integrated on the Si photonic platform, paving the way toward THz system-on-chip (SoC) for many on-site applications in the non-destructive evaluation, biomedical imaging, and industrial inspections.

Terahertz (THz) photoconductive dipole antennas are used in a THz time domain setup and their radiation efficiency is compared with the radiation efficiency of aperture antennas. In the case of two identical dipole antennas used as the transmitter and receiver elements, the THz power spectrum is enhanced well beyond the frequency limit of the dipoles due to the resonance between the two antennas.

Quantum dot (QD) lasers epitaxially grown on Si have already been demonstrated to show record low threshold, high temperature tolerance, and low feedback sensitivity. When grown on the silicon photonic chip and integrated with Si waveguides (WGs), QD lasers offer considerable economical and foundry‐scalable solutions to on‐chip light sources. Yet, a technology that enables both growth and integration of QD lasers on a silicon photonic chip has not been demonstrated. Herein, a novel device platform which enables integration of the QD active region with passive WG structures is designed. By doing so, complex and high‐performance lasers such as distributed Bragg reflector lasers, mode‐locked lasers, and sampled grating distributed Bragg reflector tunable lasers are demonstrated in this platform. The same laser epitaxial stack can be easily grown on the substrate of a silicon photonic chip to allow light coupling from QD laser cavities to the silicon WGs.

Room-temperature terahertz (THz) quantum cascade laser sources with intracavity difference-frequency nonlinear mixing are electrically pumped monolithic semiconductor laser sources operating in the 0.6–6 THz spectral range. We report widely tunable, low-frequency THz quantum cascade laser sources using a lens-coupled Cherenkov waveguide scheme. Based on a watt-class high-power, λ ∼ 13.7 μm quantum cascade laser, the monolithic THz source is strongly coupled with a high-resistivity silicon lens, which causes a major increase in the THz coupling efficiency and demonstrates significant performance improvements. A room-temperature 1.5 THz device produces a 0.2 mW peak output power with a high-quality beam pattern. Improved THz outcoupling efficiency using the lens-coupled scheme enabled the demonstration of a high-performance external-cavity semiconductor THz source that is tunable from 420 GHz to 2 THz. The external-cavity, lens-coupled device configuration can technically be assembled into a butterfly-style package for a thumb-sized, widely frequency tunable THz semiconductor source.

Graphene nano-optics at terahertz (THz) frequencies (ν) is theoretically anticipated to feature extraordinary effects. However, interrogating such phenomena is nontrivial, since the atomically thin graphene dimensionally mismatches the THz radiation wavelength reaching hundreds of micrometers. Greater challenges happen in the THz gap (0.1-10 THz) wherein light sources are scarce. To surpass these barriers, we use a nanoscope illuminated by a highly brilliant and tunable free-electron laser to image the graphene nano-optical response from 1.5 to 6.0 THz. For ν < 2 THz, we observe a metal-like behavior of graphene, which screens optical fields akin to noble metals, since this excitation range approaches its charge relaxation frequency. At 3.8 THz, plasmonic resonances cause a field-enhancement effect (FEE) that improves the graphene imaging power. Moreover, we show that the metallic behavior and the FEE are tunable upon electrical doping, thus providing further control of these graphene nano-optical properties in the THz gap.

We present a high performance top emitting room temperature monolithic terahertz (THz) quantum cascade laser based on difference frequency generation (DFG) and a buried modulated distributed feedback grating. Using a semi-insulating InP:Fe substrate combined with an index-coupled grating could minimize the internal losses for the THz mode. The laser consists of two separate transversely superimposed gratings. The first grating provides selective feedback for the mid-infrared pump modes and is realized in an index-coupled DFG grating designed for single-mode operation for two distinct MIR wavelengths. The second metallic grating for THz extraction is carefully aligned to the spatial distribution of the MIR modes. The resulting device lases pulses up to 150 °C in the single mode. The maximum THz output power at RT is 100 μW measured at 3.37 THz. A THz wavelength tuning of around 0.065 nm/K was achieved through thermal tuning of the MIR pump lasers.

The area of Terahertz (THz) is one of the fastest growing research fields in recent years. THz antennas based upon photoconduction techniques are the most common devices in THz systems. However, the radiated THz power from these devices and the efficiency are very low. Also, different antenna excitation and current generation process in THz antennas, as compared to microwave antennas, requires new analyses approaches. Therefore, the motivation of this thesis is to theoretically establish why the THz antenna is inefficient; from which, general methods to improve the performance of such antennas are explored and validated. These investigations are essential to gain a better understanding of THz photoconductive antenna performance. In this research a new equation for the source conductance of a THz antenna is firstly developed. This is a prerequisite for further antenna radiated power analysis. Next, a new equivalent circuit, modelling the underlying physical behaviour of the device through the use of a lumped-element network, is developed. Through this model, various factors which affect the radiated power and efficiency of the THz photoconductive antenna are examined and compared with measurement results. This model can be applied to maximize the optical-to-THz conversion efficiency. Also, temporal voltage behaviour of the antenna can be predicted more realistically. Furthermore, a computational simulation procedure, solving both optoelectronic and electromagnetic problems, is proposed and validated by measurement results. This approach facilitates prediction of THz photoconductive antenna performance before antenna fabrication. In addition, considering the requirement of high THz power and good SNR devices for various THz applications, a new top loaded THz antenna embedded on a conical horn with the trapezoidal photomixer is proposed. The generation of THz photocurrent, impedance matching and coupling of the THz wave to air (the necessary factors for power enhancement) are improved. Moreover, the new trapezoidal photomixer is examined and the measurement results show that it has better radiated THz power and SNR than the bare gap and rectangular photomixers.

Breakthroughs in nanomaterials and nanoscience enable the development of novel photonic devices and systems ranging from the automotive sector, quantum cryptography to metropolitan area and access networks. Geometrical architecture presents a design parameter of device properties. Self-organization at surfaces in strained heterostructures drives the formation of quantum dots (QDs). Embedding QDs in photonic and electronic devices enables novel functionalities, advanced energy efficient communication, cyber security, or lighting systems. The recombination of excitons shows twofold degeneracy and Lorentzian broadening. The superposition of millions of excitonic recombinations from QDs in real devices leads to a Gaussian envelope. The material gain of QDs in lasers is orders of magnitude larger than that of bulk material and decoupled from the index of refraction, controlled by the properties of the carrier reservoir, thus enabling independent gain and index modulation. The threshold current density of QD lasers is lowest of all injection lasers, is less sensitive to defect generation, and does not depend on temperature below 80°C. QD lasers are hardly sensitive to back reflections and exhibit no filamentation. The recombination from single QDs inserted in light emitting diodes with current confining oxide apertures shows polarized single photons. Emission of ps pulses and date rates of 10 10 +bit upon direct modulation benefits from gain recovery within femtoseconds. Repetition rates of several 100 GHz were demonstrated upon mode-locking. Passively mode-locked QD lasers generate hat-like frequency combs, enabling Terabit data transmission. QD-based semiconductor optical amplifiers enable multi-wavelength amplification and switching and support multiple modulation formats.

Improving unbiased terahertz photoconductive antenna based on dissimilar Schottky barriers using plasmonic mode excitation

Improving the efficiency and directivity of THz photoconductive antennas by using a defective photonic crystal substrate

We engineered the photocarrier dynamics of the complementary metal–oxide–semiconductor (CMOS)-compatible material germanium tin (GeSn) through controlled phosphorus ion implantation. The incorporation of Sn atoms into the Ge lattice reduces the bandgap, thereby red-shifting the absorption edge into the short-wave infrared (SWIR) region while concurrently enhancing carrier mobility. To overcome the inherently slow carrier recombination associated with the indirect bandgap nature of GeSn, phosphorus implantation was introduced to enable the ultrafast photoresponse in terahertz (THz) photoconductive antenna (PCA) detectors without substantially compromising electrical resistivity or carrier mobility. The ion-implanted detector achieves a broadened detection bandwidth of up to 2.5 THz, a signal-to-noise ratio (SNR) above 65 dB, and a 4-fold increase in response compared with the intrinsic GeSn device counterpart. These findings highlight GeSn as a promising group-IV material for ultrafast THz detection. They also demonstrate a CMOS-compatible THz photonic integrated circuit (PIC) pathway operating at telecommunication wavelengths.

We investigate properties of MBE grown photoconductive terahertz (THz) antennas based on the InGaAs/InAlAs/InP material system aimed for an excitation wavelength of approx. 1060 nm. Therefore, we analyze several different approaches concerning growth parameters, layer and material compositions as well as doping. The carrier dynamics are probed via transient white-light pump-probe spectroscopy as well as THz Time Domain Spectroscopy (TDS) measurements. We find that the electron capture probability is reduced for higher electron energies. By adjusting the material band gap this can be resolved and lifetimes of 1.3 ps are obtained. These short lifetimes enable the detection of THz TDS spectra with a bandwidth exceeding 4 THz.

It is our pleasure to introduce this special issue of IET Optoelectronics, focussed on Semiconductor Optoelectronics. An issue on this theme is published once a year and is linked to the Semiconductor and Integrated OptoElectronics (SIOE) conference held in Cardiff. SIOE 2014 was the 28th SIOE conference and featured an exciting programme that demonstrates the continuing evolution of Semiconductor Integrated Optoelectronics. A key trend in semiconductor optoelectronics that continues to develop is the increasing drive towards integration of materials and integration of functions. While much work still focusses on device performance, it is now almost always done in the context of the integrated system. Energy efficiency or “green photonics” also now features prominently and sometimes the two trends are combined. For example, both are used in the case of silicon photonics or for photonics integrated with III-V electronics where heat dissipation is the critical and limiting factor. Both of these trends are a result of focussing on societal issues and, at a time when research funding is in short supply generally and proposals must demonstrate impact, we are very fortunate as a community to be participating in a field with such a demonstrable importance for society at large. In this issue we attempt to give you a flavour of SIOE with examples of most of the important topic areas. You will notice that many of these address the trends identified above. In green photonics, recent developments in quantum dot (QD) based solar cells, covering material growth and the impact of QD carrier dynamics on device performance are addressed. In integration, overcoming the issue of crosstalk in coupled ring resonators, investigation of QD-in-a-well structures for semiconductor optical amplifiers, and laser diodes with low far-field divergence for applications in on-chip sensors are discussed. Finally, our invited paper which describes work with integrated functionality, covers some interesting physics and reports on the design and characterisation of innovative laser-active non-linear sources. The devices described make use of parametric down-conversion for producing radiation in the near-IR and difference frequency generation between two near-IR whispering-gallery modes for THz emission. We would like to thank all of the authors who submitted papers for the special issue, which works to a relatively short timescale in the manner of a conference proceedings but which insists upon fully expanded journal papers that have passed comprehensive scrutiny. We also thank the referees for their careful and constructive comments. We will leave you to enjoy this selection of papers with the hope that we will see you at SIOE in 2015. Sam 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 Si, dual-wavelength sources, InGaN optical devices and THz emission via difference frequency generation. Peter Smowton is Professor of Semiconductor Optoelectronics in the School of Physics and Astronomy at Cardiff University. His interests include the design, fabrication and characterisation of optoelectronic devices. Current research topics include quantum dot lasers, integrated optoelectronics and the physics of InGaN light emitting devices.

1st Spring Biophotonics Conference in Porto.

A preliminary design for a broadband, multiple quantum well (MQW), distributed Bragg reflector (DBR), electronically tunable laser diode array has been developed for implementation into optical memory configurations. The proposed MQW DBR diode array and a waveguide hologram with a blazed grating and/or holographic diffraction grating will be integrated into a photonic random optical memory access (PROMAC) device. PROMAC is a low inertia, low power, sub-nanosecond, parallel (e.g. 64 bit wide), wavelength addressing device which exploits free space optical interconnects. The MQW DBR laser diode array will enhance the performance of PROMAC by offering advantages in speed, spectral resolution, holographic separation and scan efficiency. An increase in speed as well as a decrease in power consumption can also be realized. Thus, increased storage densities and faster access rates can be achieved.