Device design for unitraveling-carrier photodiodes (UTC-PDs) and their derivative structures is reconsidered from the point of view of terahertz (THz) applications. A key design procedure for maximizing their bandwidth is optimization by incorporating hybrid absorbers. The effect of quasi-field in p-type absorber is carefully examined. It has been shown that the initial velocity transient must be taken into account to evaluate the effective average velocity. Photomixers integrating a hybrid-absorber UTC-PD and a bow-tie antenna were fabricated and characterized. THz-wave generation by the photomixers in a frequency range of up to around 2.5 THz was confirmed. The observed THz-wave output exhibits significant changes with bias voltage, where the decrease in the output with increasing negative bias voltage is more pronounced at higher frequencies. This output behavior is due to the change in electron velocity in the diode depletion layer associated with the overshoot effect. From the dependence of the output power on frequency, effective electron velocity is found to be as high as $6 \times 10^{7}$ cm/s at optimum bias voltage of -0.4 V.

Unitraveling-Carrier Photodiodes for Terahertz Applications

We propose and experimentally demonstrate a W-band seamlessly integrated fiber-wireless-fiber transmission system enabled by photonic millimeter-wave generation and demodulation techniques, in which up to 128-Gb/s (16-Gbd) polarization division multiplexing 16-ary quadrature amplitude modulation (16QAM) signal can be first transmitted over 50-km single-mode fiber-28 (SMF-28), then delivered over 1-m × 2 multiple-input multiple-output wireless link at 100 GHz and finally transmitted over another 50-km SMF-28 with a bit-error ratio less than the third-generation soft-decision forward-error-correction threshold of 2 × 10
 -2
. The proposed system can be well compatible with high-level QAM and is suited to large-capacity high-spectral-efficiency fiber-wireless integration communication.

Fiber-Wireless-Fiber Link for 100-Gb/s PDM-QPSK Signal Transmission at W-Band

This paper reviews optical injection locking (OIL) of semiconductor lasers and its application in optical communications and signal processing. Despite complex OIL dynamics, we attempt to explain the operational principle and main features of the OIL in an intuitive way, aiming at a wide understanding of the OIL phenomenon and its associated techniques in the optic and photonic communities. We review and compare different control techniques that enable robust OIL in practical systems. The applications are reviewed with a focus on new developments in the past decade, under the categories of ‘High Speed Directly Modulated Lasers’ and ‘Optical Carrier Recovery’. Finally, we draw our vision for future research directions.

/pdf/optical-injection-locking-from-principle-to-applications-32kzhq2anw.pdf

Optical Injection Locking: From Principle to Applications

Rectifiers have been used to detect electromagnetic waves with very low photon energies In these rectifying devices, different methods have been utilized, such as adjusting the bandgap and the doping profile, or utilizing the contact potential of the metal-semiconductor junction to produce current flow depending on the direction of the electric field In this paper, it is shown that the asymmetric application of nano-electrodes to a metal-semiconductor-metal (MSM) structure can produce such rectification characteristics, and a terahertz (THz) wave detector based on the nano-MSM structure is proposed Integrated with a receiving antenna, the fabricated device detects THz radiation up to a frequency of 15 THz with responsivity and noise equivalent power of 108 V/W and [Formula: see text] respectively, estimated at 03 THz The unidirectional current flow is attributed to the thermionic emission of hot carriers accelerated by the locally enhanced THz field at the sharp end of the nano-electrode This work not only demonstrates a new type of THz detector but also proposes a method for manipulating ultrafast charge-carrier dynamics through the field enhancement of the nano-electrode, which can be applied to ultrafast photonic and electronic devices

Terahertz rectifier exploiting electric field-induced hot-carrier effect in asymmetric nano-electrode.

In the past year, fifth-generation (5G) wireless technology has seen dramatic growth, spurred on by the continuing demand for faster data communications with lower latency. At the same time, many researchers argue that 5G will be inadequate in a short time, given the explosive growth of machine connectivity, such as the Internet-of-Things (IoT). This has prompted many to question what comes after 5G. The obvious answer is sixth-generation (6G), however, the substance of 6G is still very much undefined, leaving much to the imagination in terms of real-world implementation. What is clear, however, is that the next generation will likely involve the use of terahertz frequency (0.1–10 THz) electromagnetic waves. Here, we review recent research in terahertz wireless communications and technology, focusing on three broad topic classes: the terahertz channel, terahertz devices, and space-based terahertz system considerations. In all of these, we describe the nature of the research, the specific challenges involved, and current research findings. We conclude by providing a brief perspective on the path forward.

https://www.mdpi.com/2227-7080/7/2/43/pdf

A Perspective on Terahertz Next-Generation Wireless Communications

We present a review of recent developments in THz coherent systems based on photonic local oscillators. We show that such techniques can enable the creation of highly coherent, thus highly sensitive, systems for frequencies ranging from 100 GHz to 5 THz, within an energy efficient integrated platform. We suggest that such systems could enable the THz spectrum to realize its full applications potential. To demonstrate how photonics-enabled THz systems can be realized, we review the performance of key components, show recent demonstrations of integrated platforms, and give examples of applications.

Coherent terahertz photonics.

High-speed photodiodes are a key element of numerous photonic systems. With the development of potential applications in the THz range such as sensing, spectroscopy, and wireless transmission, devices with integrated antenna covering the frequency range from 0.1 to 3 THz will become essential. In this paper, we discuss the development of uni-traveling carrier photodiodes with integrated antennas to address that need. In particular we develop the key elements to present a simple design tool for the efficient integration of the device with an antenna. We also present fabricated device results that show the highest figure of merit to date for photonic THz emitters. When integrated with well-matched antennas the devices have achieved record level of power up to 1 THz compared to other published photomixers. We also show that these devices can be used as receivers at frequencies up to 560 GHz with conversion losses of the order of 30 dB.

Antenna Integrated THz Uni-Traveling Carrier Photodiodes

We present a comprehensive study of uni-travelling carrier photodiode impedance and frequency photo-response supported by measurements up to 110 GHz. The results of this investigation provide valuable new information for the optimisation of the coupling efficiency between UTC-PDs and THz antennas. We show that the measured impedance cannot be explained employing the standard junction-capacitance/series-resistance concept and propose a new model for the observed effects, which exhibits good agreement with the experimental data. The achieved knowledge of the photodiode impedance will allow the absolute level of power emitted by antenna integrated UTCs to be predicted and ultimately maximised.

Accurate equivalent circuit model for millimetre-wave UTC photodiodes.

We have modelled the experimental system based on the sub-wavelength aperture probe employed in our previous work for terahertz (THz) surface plasmon wave imaging on a bowtie antenna. For the first time we demonstrate the accuracy of the proposed interpretation of the images mapped by the probe. The very good agreement between numerical and experimental results proves that the physical quantity detected by the probe is the spatial derivative of the electric field normal component. The achieved understanding of the near-field probe response allows now a correct interpretation of the images and the distribution of the electric field to be extracted. We have also carried out the first assessment of the probe invasiveness and found that the pattern of the surface plasmon wave on the antenna is not modified significantly by the proximity of the probe. This makes the experimental system an effective tool for near-field imaging of THz antennas and other metallic structures.

Modelling of surface waves on a THz antenna detected by a near-field probe

We determine the output impedance of uni-travelling carrier (UTC) photodiodes at frequencies up to 400 GHz by performing, for the first time, 3D full-wave modelling of detailed UTC photodiode structures. In addition, we demonstrate the importance of the UTC impedance evaluation, by using it in the prediction of the absolute power radiated by an antenna integrated UTC, over a broad frequency range and confirming the predictions by experimental measurements up to 185 GHz. This is done by means of 3D full-wave modelling and is only possible since the source (UTC) to antenna impedance match is properly taken into account. We also show that, when the UTC-to-antenna coupling efficiency is modelled using the classical junction-capacitance/series-resistance concept, calculated and measured levels of absolute radiated power are in substantial disagreement, and the maximum radiated power is overestimated by a factor of almost 7 dB. The ability to calculate the absolute emitted power correctly enables the radiated power to be maximised through optimisation of the UTC-to-antenna impedance match.

Michele Natrella

Papers

Coherent terahertz photonics.

Antenna Integrated THz Uni-Traveling Carrier Photodiodes

Accurate equivalent circuit model for millimetre-wave UTC photodiodes.

Modelling of surface waves on a THz antenna detected by a near-field probe

Modelling and measurement of the absolute level of power radiated by antenna integrated THz UTC photodiodes.