Photoconductive Antenna Arrays Research Articles

Synthetic Photoconductive Antenna Array for Terahertz 3-D Imaging and Spectroscopy

We propose a 3D imaging and spectroscopy approach in the terahertz (THz) domain based on the synthetic aperture (SA) imaging technique achieved using a time domain spectroscopy system (TDS). The proposed system enables the imaging and spectroscopy of an object in a stationary position in transmission configuration. We demonstrate the reconstruction of 3D amplitude images with a sub-millimeter resolution of a multi-material object from 0.6 to 1 THz using the back-projection algorithm. The reconstructed data is further used to characterize the optical properties of the substrates forming the object by calculating the refractive index and the absorption coefficient of its constituents in the reconstruction frequency band. We demonstrate the use of the SA not only for imaging, but also for material characterization. Consequently, we are able to reconstruct 2D high-resolution spectral maps of the real refractive index at different THz frequencies of the imaged object.

New Antenna for Detecting Polarization States of Terahertz

The polarization measurement of terahertz (THz) waves is indispensable in THz time-domain spectroscopy (THz-TDS) applications to detect material properties. The rapid development of accurate THz wave polarization-sensitive detectors will greatly promote THz-TDS applications. A new type of photoconductive antenna array detector is proposed in this paper. The antenna is composed of two vertical 1*2 arrays, which respectively detect the orthogonal component of the terahertz pulse in any direction, while quickly and accurately detecting the polarized THz waves. Rotating the detector to measure the THz electric field at different angles shows the reliability of the detector for THz wave polarization measurement. Its polarization detection accuracy is 0.2°. At the same time, we use the response matrix to analyze the symmetry of the antenna array.

A High Performance Terahertz Photoconductive Antenna Array Detector With High Synthesis Efficiency

A 2 × 2 terahertz photoconductive antenna (PCA) array detector with high efficiency synthesis characteristic that improves the signal-to-noise ratio (SNR) of the detected signals has been reported in this paper. By processing the substrate material through a special micromachining process, the current signal generated by the adjacent antenna elements as opposed to that generated by the antenna gap is eliminated. Experiments show that the amplitude of the current signal output by the PCA array detector is consistent with the amplitude of the synchronous superposition of the current signals output by antenna elements, and the synthesis efficiency of the device achieves 93.7%. At the same time, the antenna array detector has low current noise, and its highest SNR is 62 dB under the excitation of different light energy, which is related to the number of antenna array elements.

Improvement of terahertz photoconductive antennas array using crossfingers structure

The purpose of this research is to improve terahertz photoconductive antenna arrays to enhance their output power. The improved arrays were designed based on crossfingers structure as a high power terahertz generator. The finite difference time domain method was used for modelling and simulating of the generator. The characteristics of the improved array and its emphasis points and key parameters were studied. This structure demonstrated that the terahertz antenna array can generate drastically increased terahertz field amplitudes compared to other structures. The output power of the improved array enhanced as large as 74.1 µW. The improved array remains in an unsaturated region of operation, whereas the optical pumping with high power level and beam focus was used for its excitation. The improved array enhances the terahertz output power for use in extensive terahertz science and technology applications.

Radiation characteristics of terahertz GaAs photoconductive antenna arrays

A GaAs photoconductive antenna is one of the important radiation sources of terahertz electromagnetic waves. Antenna arrays can increase the radiation intensity of terahertz waves. Therefore, photoconductive antennas and arrays have attracted much attention for a long time. In this study, CST Microwave Studio is used to conduct a simulation calculation of the characteristics of a photoconductive antenna array radiating terahertz electromagnetic waves. Using the current transient model, the pulsed photocurrents generated when the laser is incident on the GaAs photoconductive antenna are calculated. With the pulsed photocurrents serving as an excitation source, a simulation calculation of the radiation performance of photoconductive antenna is conducted, and the effects of antenna structure and substrate material on the radiation of terahertz waves are analyzed. Based on this, the far-field radiation of terahertz wave radiated by the GaAs photoconductive antenna array is calculated. The simulation results show that the photoconductive antenna array radiates terahertz waves with stronger directivity. The width of main lobe is reduced, and its far-field radiation conforms to the multiple relationships of electric field superposition. A 1 × 2 GaAs photoconductive antenna array is developed, and the experimental results are consistent with the simulation conclusions, thereby laying a theoretical and experimental basis for fabricating the multielement terahertz photoconductive antenna arrays.

Fabrication and Characterization of an 8 × 8 Terahertz Photoconductive Antenna Array for Spatially Resolved Time Domain Spectroscopy and Imaging Applications

Terahertz (THz) technology is promising in several applications such as imaging, spectroscopy and communications. Among several methods in the generation and detection of THz waves, a THz time-domain system that is developed using photoconductive antennas (PCA) as emitter and detector presents several advantages such as simple alignment, low cost, high performance etc. In this work, we report the design, fabrication and characterization of a 2-D PCA array that is capable of detecting both the amplitude and phase of the THz pulse. The PCA array is fabricated using LT-GaAs and has 8 channels with 64 pixels ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8 \times 8$ </tex-math></inline-formula> ). A novel approach using a spatial light modulator (SLM) to steer and focus the infrared probe beam towards pixels of the PCA array is presented. Each channel records the photocurrent generated by the THz signal (amplitude and phase) separately and frequencies up to 1.4 THz can be detected. Furthermore, the parameters such as directional time delay of the THz pulse, crosstalk between the channels etc., were characterized. Finally, we show that the proposed 2D PCA array design is flexible and can be used for accelerated THz spectral image acquisition.

High power T-ray generation using a new array of photoconductive antennas

In this work, a new array of photoconductive antennas is proposed as a high power T-ray source. The array was modeled and simulated by the finite difference time domain method. The structure and characteristics of the T-ray source were studied. The potential advantages of the proposed array include enhanced efficiency of the broadband THz radiation and confined radiated THz pulse. Also, an asymmetry in the design of the array allows optimizing high optical-to-terahertz conversion efficiencies. These potential advantages of the proposed array can be exploited in satellite telecommunications such as geostationary terahertz satellite links. Moreover, the array is capable of employing radiation-band shaping technique for generation of tunable and narrow-band THz radiation.

Design of terahertz photoconductive antenna arrays based on defective photonic crystal substrates

In this paper, arrays of two different terahertz (THz) antennas based on defective photonic crystal (DPC) substrates are proposed and their radiation characteristics are investigated by using full-wave simulation. We show that employing DPC substrates in THz photoconductive antenna (PCA) arrays reduces the amount of power coupled to the substrate surface waves and, as a result, decreases the mutual coupling between the elements of the antenna array and increases the directivity in the boresight direction. By studying 1 × 2 arrays of bowtie and four-leaf-clover-shaped antennas, we show that either the improvement of directivity in a broad bandwidth or the enhancement of photomixer antenna impedance at a resonant frequency can be considered in the design of the DPC substrate. Therefore, the proposed approach can significantly improve the efficiency and radiation performance of THz photomixer antennas resulting in an improved effective THz radiated power in the desirable direction. The proposed DPC substrate can be considered as a way to realize sub-millimeter-sized dielectric resonator antennas at terahertz frequencies. In addition, this cost-effective technique can reduce the necessity of exploiting hyper-hemispherical Si lenses by overcoming the adverse effects of the thick substrate which is of prime importance in PCA arrays from a practical viewpoint.

Toward real-time terahertz imaging

Terahertz (THz) science and technology have greatly progressed over the past two decades to a point where the THz region of the electromagnetic spectrum is now a mature research area with many fundamental and practical applications. Furthermore, THz imaging is positioned to play a key role in many industrial applications, as THz technology is steadily shifting from university-grade instrumentation to commercial systems. In this context, the objective of this review is to discuss recent advances in THz imaging with an emphasis on the modalities that could enable real-time high-resolution imaging. To this end, we first discuss several key imaging modalities developed over the years: THz transmission, reflection, and conductivity imaging; THz pulsed imaging; THz computed tomography; and THz near-field imaging. Then, we discuss several enabling technologies for real-time THz imaging within the time-domain spectroscopy paradigm: fast optical delay lines, photoconductive antenna arrays, and electro-optic sampling with cameras. Next, we discuss the advances in THz cameras, particularly THz thermal cameras and THz field-effect transistor cameras. Finally, we overview the most recent techniques that enable fast THz imaging with single-pixel detectors: mechanical beam-steering, compressive sensing, spectral encoding, and fast Fourier optics. We believe that this critical and comprehensive review of enabling hardware, instrumentation, algorithms, and potential applications in real-time high-resolution THz imaging can serve a diverse community of fundamental and applied scientists.

Design of highly directive lens-less photoconductive dipole antenna array with frequency selective surface for terahertz imaging applications

In this paper, a highly directive photoconductive dipole array antenna is presented for imaging system operating at terahertz (THz) frequencies. The array antenna is formed on a single photoconductive substrate to make it compact and suitable for THz imaging applications. The presented photoconductive dipole array antenna improves the gain as well as directivity and therefore, is useful to enhance the imaging capabilities to address the considerations such as limited depth-of-field (DoF) that is the distance over which an object is considered in focus and size-weight-and-power (SWaP) of THz source for imaging applications. These are important considerations for applications like stand-off imaging and surveillance of moving targets where the high angular resolution as well as extended DoF is the key for successful detection of hidden explosives and illicit drugs. In addition to this, a technique of using frequency selective surface with photoconductive array antenna is also presented to further enhance the directivity of the radiating structure to yield high image resolution. Moreover, by using this technique, the radiation energy has been confined to the desired frequency band rather than spreading over a wide spectrum range.

Optical Scanning Techniques for Characterization of Terahertz Photoconductive Antenna Arrays

Optical scanning techniques for characterizing radiation properties of terahertz photoconductive antenna arrays are proposed, and as an example, a 14-element equally spaced array structure made on low-temperature grown GaAs is characterized using the proposed methods.

Terahertz Radiation-Band Engineering Through Spatial Beam-Shaping

We propose a terahertz (THz)-band engineering technique based on generation of specifically designed time-domain radiation. The desired time-domain radiation is formed by combining the time-delayed outputs of an array of photoconductive antennas. We have employed this technique to generate tunable narrowband THz radiation. By using an array of identical photoconductive antennas with identical spacings, we have demonstrated a 12.5-mW output peak-power over 0.05-THz bandwidth and a tuning range of 0.65-1.4 THz. Considering the 2.5-THz bandwidth of each individual photoconductive antenna, 50 times higher power efficiency is achieved as a result of radiation-band shaping.

Terahertz line detection by a microlens array coupled photoconductive antenna array

We present THz ultrashort pulse detection by a photoconductive antenna array consisting of 16 photoconductive antennas. The efficient excitation of the photoconductive antennas has been realized by a microlens array which generates 16 single spots from the exciting fs-laser beam. This combination of optoelectronics and microoptics improves the detection efficiency by an order of magnitude in comparison to an excitation by a line focus.

Characterization of low temperature GaAs antenna array terahertz emitters

We present a fabrication concept for photoconductive terahertz antenna arrays based on substrate-transferred thin films of low-temperature-grown GaAs semiconductor material. Adjacent array elements are physically decoupled by removing completely the photoconductive material in between. In contrast to former array devices based on intrinsic bulk GaAs substrates, this method allows the use of arbitrary carrier substrates with enhanced transmission properties. The emission characteristics of the device are investigated in terms of bandwidth, directivity, and saturation caused by charge-carrier induced field-screening effects. Screening-free operation is experimentally observed for an average optical power density below 2.2×10−4mW∕μm2.

Metallic slit waveguide for dispersion-free low-loss terahertz signal transmission

Transmission characteristics of a metallic waveguide are presented combining negligible dispersion, very low attenuation, and two-dimensional mode confinement. The waveguide is fabricated by sawing a 270μm wide slit through a 40mm wide and 300μm thick silicon slab and subsequent metalization of all faces. Single-mode terahertz pulses propagating through the slit are excited at one end of the waveguide using a photoconductive antenna array and detected with a freely positionable photoconductive probe tip. Low-loss propagation with negligible group velocity dispersion is observed in the 0.1–1.0THz frequency range. Compared to former approaches considerably stronger mode confinement is achieved.

Optically Induced THz Electromagnetic Radiation From Planar Photoconducting Structures

We describe a novel optoelectronic technique to generate and detect THz electromagnetic waves by using large-aperture planar photoconductive antennas and antenna arrays. This approach is an effective method of producing directional and steerable sub-millimeter wave pulses in free space.