Plasmonics enhanced photomixing for generating quasi-continuous-wave frequency-tunable terahertz radiation

Evaluation of high-stability optical beats in laser chaos by plasmonic photomixing

The stability of optical beats in a chaotically oscillating laser is compared to that of a free-running continuous-wave laser using a highly efficient plasmonic photomixer. Using a chaotically oscillating laser diode, stable optical beats are observed over an operation current range of 60-90 mA. The optical spectra are stable even with frequent mode hopping. In contrast, optical beats in a free-running continuous-wave laser are not stable compared to those of a chaotically oscillating laser, because of intermittent hopping of the laser modes. The high stability of chaotically oscillating lasers makes these lasers promising candidates for optical pump sources in terahertz time-domain spectroscopy systems.

Heterodyne terahertz detection through electronic and optoelectronic mixers

The high sensitivity detection of terahertz radiation is crucial for many chemical sensing, biomedical imaging, security screening, nondestructive quality control, high-data-rate communication, atmospheric, and astrophysics sensing applications. Among various terahertz detection techniques, heterodyne detection is of great interest for applications that require high spectral resolution. Heterodyne detection involves mixing the received terahertz radiation with a reference terahertz signal provided by a local oscillator and then down-converting it to an intermediate frequency for detection. The frequency of the intermediate frequency signal is usually chosen to be in the radio frequency regime, so that it can be accurately analyzed by well-developed radio frequency electronics, including amplifiers, filters, and spectrometers, for further processing. Heterodyne terahertz detection offers two major advantages over direct terahertz detection. First, the detected terahertz radiation is effectively enhanced by the reference local oscillator signal through the mixing process, thereby enabling the detection of very weak terahertz signals. Second, the detected noise power is effectively reduced by limiting the detected spectral bandwidth to the bandwidth of the intermediate frequency electronics. In this article, we present a broad overview of various types of heterodyne terahertz receivers, which utilize different electronic and optoelectronic techniques to down-convert the received terahertz signal to a radio frequency signal. We describe how the inherent nonlinearity of a Schottky diode, superconductor-insulator-superconductor junction, hot electron bolometer, and field-effect transistor can be utilized to mix the received terahertz radiation with a reference local oscillator signal from a gas laser, quantum cascade laser, photomixer, Gunn diode, IMPATT diode, and frequency multiplier and then down-convert it to a radio frequency signal. The down-converted radio frequency signal can be subsequently detected and analyzed by various backend spectrometers, including filter bank, acousto-optical, autocorrelator, fast Fourier transform, and chirp transform spectrometers. We also discuss how a photomixer pumped by a heterodyning optical beam can be used to down-convert the received terahertz radiation to a radio frequency signal with far fewer bandwidth constraints than conventional techniques. The advantages and disadvantages of different heterodyne receivers in terms of their noise performance, operation frequency, operation bandwidth, and operation temperature are discussed in detail.

High-efficiency ultrafast optical-to-electrical converters based on InAs nanowire-plasmonic arrays

There has been a growing interest in developing high-efficiency ultrafast optical-to-electrical converters for advanced imaging and sensing applications. Here, we propose a three-dimensional (3D) plasmonic platform based on InAs nanowire arrays with self-assembled gold gratings, which converts a telecom-wavelength (1550 nm) optical beam to sub-picosecond current pulses with quantum efficiency up to 18.3%, while operating in photovoltaic mode, i.e., at zero bias. Using a comprehensive 3D photoresponse model, we reveal that the incident photons form tightly confined fields near the gratings at nanowire tips, and thus a majority of the photogenerated carriers are efficiently routed to the metal within a few tens of nanometers distance, resulting in ultrafast current pulses. In addition, we show that the amplitude of current pulses is robust to the nanowire surface quality and can be effectively tuned by varying the doping levels in nanowires. This work paves a way to realizing a low-power, highly compact, and low-cost device scheme for ultrafast pulse generation.

Improvement of Terahertz Photoconductive Antenna using Optical Antenna Array of ZnO Nanorods

An efficient terahertz (THz) photoconductive antenna (PCA), as a major constituent for the generation or detection of THz waves, plays an essential role in bridging microwave-to-photonic gaps. Here, we propose an impressive approach comprising the use of arrayed zinc oxide nanorods (ZnO NRs) as an optical nanoantenna over an anti-reflective layer (silicon nitride) in the antenna gap to boost the photocurrent and consequently the THz signal. The numerical approach applied in investigating the optical behavior of the structure, demonstrates a significant field enhancement within the LT-GaAs layer due to the optical antenna performing simultaneously as a concentrator and an antireflector which behaves as a graded-refractive index layer. ZnO NRs have been fabricated on the PCA gap using the hydrothermal method as a simple, low cost and production compatible fabrication method compared to other complex methods used for the optical nanoantennas. Compared to the conventional PCA with a traditional antireflection coating, the measured THz power by time domain spectroscopy (TDS) is increased more than 4 times on average over the 0.1–1.2 THz range.

Nanostructure-Enhanced Photoconductive Terahertz Emission and Detection

Photoconductive antennas are commonly used for terahertz wave generation and detection. However, their relatively low radiation power and detection sensitivity often place limitations on the signal-to-noise ratio and operation bandwidth of terahertz imaging and spectroscopy systems. Several different techniques are attempted to address these limitations. The most promising ones take advantage of the unique tools provided by nanotechnology. In this review, the recent nanotechnology-enabled advances in photoconductive antennas, which use nanostructures, such as optical nanoantennas, plasmonic structures, and optical nanocavities, to increase the interaction of the optical pump beam with the photoconductive semiconductor, are discussed. All of these techniques are experimentally demonstrated to be efficient tools for enhancing the performance of photoconductive antennas for terahertz wave generation and detection.

A polarization-insensitive plasmonic photoconductive terahertz emitter

We present a polarization-insensitive plasmonic photoconductive terahertz emitter that uses a two-dimensional array of nanoscale cross-shaped apertures as the plasmonic contact electrodes. The geometry of the cross-shaped apertures is set to maximize optical pump absorption in close proximity to the contact electrodes. The two-dimensional symmetry of the cross-shaped apertures offers a polarization-insensitive interaction between the plasmonic contact electrodes and optical pump beam. We experimentally demonstrate a polarization-insensitive terahertz radiation from the presented emitter in response to a femtosecond optical pump beam and similar terahertz radiation powers compared to previously demonstrated polarization-sensitive photoconductive emitters with plasmonic contact electrode gratings at the optimum optical pump polarization.

Nanoslit cavity plasmonic modes and built-in fields enhance the CW THz radiation in an unbiased antennaless photomixers array

A new generation unbiased antennaless CW terahertz (THz) photomixer emitters array made of asymmetric metal-semiconductor-metal (MSM) gratings with a subwavelength pitch, operating in the optical near-field regime, is proposed. We take advantage of size effects in near-field optics and electrostatics to demonstrate the possibility of enhancing the THz power by 4 orders of magnitude, compared to a similar unbiased antennaless array of the same size that operates in the far-field regime. We show that, with the appropriate choice of grating parameters in such THz sources, the first plasmonic resonant cavity mode in the nanoslit between two adjacent MSMs can enhance the optical near-field absorption and, hence, the generation of photocarriers under the slit in the active medium. These photocarriers, on the other hand, are accelerated by the large built-in electric field sustained under the nanoslits by two dissimilar Schottky barriers to create the desired large THz power that is mainly radiated downward. The proposed structure can be tuned in a broadband frequency range of 0.1-3 THz, with output power increasing with frequency.

Tunable terahertz wave generation through a bimodal laser diode and plasmonic photomixer

We demonstrate a compact, robust, and stable terahertz source based on a novel two section digital distributed feedback laser diode and plasmonic photomixer. Terahertz wave generation is achieved through difference frequency generation by pumping the plasmonic photomixer with two output optical beams of the two section digital distributed feedback laser diode. The laser is designed to offer an adjustable terahertz frequency difference between the emitted wavelengths by varying the applied currents to the laser sections. The plasmonic photomixer is comprised of an ultrafast photoconductor with plasmonic contact electrodes integrated with a logarithmic spiral antenna. We demonstrate terahertz wave generation with 0.15-3 THz frequency tunability, 2 MHz linewidth, and less than 5 MHz frequency stability over 1 minute, at useful power levels for practical imaging and sensing applications.

Spectral characteristics of terahertz radiation from plasmonic photomixers

We present a comprehensive analysis of spectral characteristics of terahertz radiation from plasmonic photomixers. We fabricate plasmonic photomixer prototypes with plasmonic contact electrode gratings on a low temperature grown GaAs substrate and characterize the spectral properties of the generated terahertz radiation by use of a heterodyne detection scheme. Our analysis shows that linewidth, stability, and frequency tuning range of the generated terahertz radiation are directly determined by linewidth, stability, and wavelength tuning range of optical pump beam and not affected by device geometry, substrate properties, optical pump power level and other operational settings. Our study indicates the crucial role of optical sources in realizing high performance terahertz spectroscopy and wireless communication systems based on plasmonic photomixers.

Bias field tailored plasmonic nano-electrode for high-power terahertz photonic devices

Photoconductive antennas with nano-structured electrodes and which show significantly improved performances have been proposed to satisfy the demand for compact and efficient terahertz (THz) sources. Plasmonic field enhancement was previously considered the dominant mechanism accounting for the improvements in the underlying physics. However, we discovered that the role of plasmonic field enhancement is limited and near-field distribution of bias field should be considered as well. In this paper, we clearly show that the locally enhanced bias field due to the size effect is much more important than the plasmonic enhanced absorption in the nano-structured electrodes for the THz emitters. Consequently, an improved nano-electrode design is presented by tailoring bias field distribution and plasmonic enhancement. Our findings will pave the way for new perspectives in the design and analysis of plasmonic nano-structures for more efficient THz photonic devices.