Broadband high-resolution terahertz single-pixel imaging

High-throughput terahertz imaging: progress and challenges

Many exciting terahertz imaging applications, such as non-destructive evaluation, biomedical diagnosis, and security screening, have been historically limited in practical usage due to the raster-scanning requirement of imaging systems, which impose very low imaging speeds. However, recent advancements in terahertz imaging systems have greatly increased the imaging throughput and brought the promising potential of terahertz radiation from research laboratories closer to real-world applications. Here, we review the development of terahertz imaging technologies from both hardware and computational imaging perspectives. We introduce and compare different types of hardware enabling frequency-domain and time-domain imaging using various thermal, photon, and field image sensor arrays. We discuss how different imaging hardware and computational imaging algorithms provide opportunities for capturing time-of-flight, spectroscopic, phase, and intensity image data at high throughputs. Furthermore, the new prospects and challenges for the development of future high-throughput terahertz imaging systems are briefly introduced.

Real-time single-pixel imaging using a system on a chip field-programmable gate array

Unlike conventional imaging, the single-pixel imaging technique uses a single-element detector, which enables high sensitivity, broad wavelength, and noise robustness imaging. However, it has several challenges, particularly requiring extensive computations for image reconstruction with high image quality. Therefore, high-performance computers are required for real-time reconstruction with higher image quality. In this study, we developed a compact dedicated computer for single-pixel imaging using a system on a chip field-programmable gate array (FPGA), which enables real-time reconstruction at 40 frames per second with an image size of 128 × 128 pixels. An FPGA circuit was implemented with the proposed reconstruction algorithm to obtain higher image quality by introducing encoding mask pattern optimization. The dedicated computer can accelerate the reconstruction 10 times faster than a recent CPU. Because it is very compact compared with typical computers, it can expand the application of single-pixel imaging to the Internet of Things and outdoor applications.

Linear scanning system for THz imaging

A linear scanning system utilizing constant wave 280 GHz radiation has been developed and characterized. The system comprises a linear array of detectors based on a unique plasma wave approach in terahertz sensing, an impact ionization avalanche transit-time-diode signal generator coupled to a frequency multiplier and an optical system. The performed tests allowed us to estimate the resolution of the system reaching the value of 2.3 mm and to determine the dynamic range of the system to be around 200. The imaging capabilities of the scanner were tested in realistic cases of non-destructive testing and security screening.

Speckle patterns formed by broadband terahertz radiation and their applications for ghost imaging

Speckle patterns can be very promising for many applications due to their unique properties. This paper presents the possibility of numerically and experimentally formation of speckle patterns using broadband THz radiation. Strong dependence of the statistical parameters of speckles, such as size and sharpness on the parameters of the diffuser are demonstrated: the correlation length and the mean square deviation of the phase surface inhomogeneity. As the surface correlation length is increasing, the speckle size also increases and its sharpness goes down. Alternatively, the magnification of the standard deviation of the surface height leads to the speckle size diminishing and growth of the speckle sharpness. The dimensions of the experimentally formed speckles correspond to the results of numerical simulation. The possibility of utilizing formed speckle patterns for the implementation of the ghost imaging technique has been demonstrated by methods of numerical modeling.

Single-pixel camera based on a spinning mask

Single-pixel imaging (SPI) has been intensively studied in recent years for its capacity to obtain 2D images using a non-pixelated detector. However, the traditional modulation modality using an iteratively refreshed spatial light modulator has significantly restricted its imaging speed, which is a primary barrier to its widespread application. In this work, we propose and demonstrate a new, to the best of our knowledge, SPI scheme using a spinning mask for modulation. An annular binary mask is designed and spun to perform fast spatial modulation, neglecting the iterative modulation modality that limits SPI speed. A multi-spectral SPI system at 100 frames per second is demonstrated, covering a wide range of spectra, from ultraviolet to short-wave infrared light. We believe that this elegant and low-cost scheme will enable SPI to pave its way for practical application.

Ultra-broadband terahertz bandpass filter with dynamically tunable attenuation based on a graphene-metal hybrid metasurface

We propose an ultra-broadband terahertz bandpass filter with dynamically tunable attenuation based on a graphene-metal hybrid metasurface. The metasurface unit cell is composed of two metal stripes enclosed with a graphene rectangular ring. Results show that when the metasurface is normally illuminated by a terahertz wave polarized along the metal stripes, it can act as an ultra-broadband bandpass filter over the spectral range from 1.49 THz to 4.05 THz, corresponding to a fractional bandwidth of 92%. Remarkably, high transmittance above 90% covering the range from 1.98 THz to 3.95 THz can be achieved. By changing the Fermi level of graphene, we find that the attenuation within the passband can be dynamically tuned from 2% to 66%. We expect that the proposed ultra-broadband terahertz bandpass filter with tunable attenuation will find applications in terahertz communication and detection and sensing systems.

Roadmap of Terahertz Imaging 2021

In this roadmap article, we have focused on the most recent advances in terahertz (THz) imaging with particular attention paid to the optimization and miniaturization of the THz imaging systems. Such systems entail enhanced functionality, reduced power consumption, and increased convenience, thus being geared toward the implementation of THz imaging systems in real operational conditions. The article will touch upon the advanced solid-state-based THz imaging systems, including room temperature THz sensors and arrays, as well as their on-chip integration with diffractive THz optical components. We will cover the current-state of compact room temperature THz emission sources, both optolectronic and electrically driven; particular emphasis is attributed to the beam-forming role in THz imaging, THz holography and spatial filtering, THz nano-imaging, and computational imaging. A number of advanced THz techniques, such as light-field THz imaging, homodyne spectroscopy, and phase sensitive spectrometry, THz modulated continuous wave imaging, room temperature THz frequency combs, and passive THz imaging, as well as the use of artificial intelligence in THz data processing and optics development, will be reviewed. This roadmap presents a structured snapshot of current advances in THz imaging as of 2021 and provides an opinion on contemporary scientific and technological challenges in this field, as well as extrapolations of possible further evolution in THz imaging.

High-resolution fast mid-wave infrared compressive imaging

In the mid-wave infrared (MIR) band, large detector arrays are extremely costly and technically difficult to be manufactured. Thus, it is difficult to obtain high-resolution images for a conventional MIR camera. Spatial compressive imaging can improve resolution. However, system errors due to misalignment or optical aberrations degrade reconstruction quality significantly. Another common issue for compressive imaging is the slow imaging speed, which is caused by slow measurement collection and reconstruction processes. To deal with the two issues, we use an imaging calibration method to improve reconstruction quality and a sliding window measurement collection strategy plus a reconstruction algorithm accelerated by parallel computing to fasten the speed. We build a prototype of a compressive imaging camera with an angular resolution 1.17 lp/mrad. A four-bar target is used as an object. We reconstruct a moving scene of size $1280 \times 1024$ with a frame rate 20 frames per second.