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Open AccessArticle

Real-Time High Resolution THz Imaging with a Fiber-Coupled Photo Conductive Antenna and an Uncooled Microbolometer Camera

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Sensors 2021, 21(11), 3757; https://doi.org/10.3390/s21113757 - 28 May 2021

Cited by 4 | Viewed by 3173

Abstract

We present a real-time THz imaging method using a commercial fiber-coupled photo conductive antenna as the THz source and an uncooled microbolometer camera for detection. This new combination of state-of-the-art components is very adaptable due to its compact and uncooled radiation source, whose [...] Read more.

We present a real-time THz imaging method using a commercial fiber-coupled photo conductive antenna as the THz source and an uncooled microbolometer camera for detection. This new combination of state-of-the-art components is very adaptable due to its compact and uncooled radiation source, whose fiber coupling allows for a flexible placement. Using a camera with high sensitivity renders real-time imaging possible. As a proof-of-concept, the beam shape of a THz Time Domain Spectrometer was measured. We demonstrate real time imaging at nine frames per second and show its potential for practical applications in transmission geometry covering both material science and security tasks. The results suggest that hidden items, complex structures and the moisture content of (biological) materials can be resolved. We discuss the limits of the current setup, possible improvements and potential (industrial) applications, and we outline the feasibility of imaging in reflection geometry or extending it to multi-spectral imaging using band pass filters. Full article

(This article belongs to the Special Issue Terahertz Imaging and Sensors)

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Open AccessArticle

Antenna-Coupled Titanium Microbolometers: Application for Precise Control of Radiation Patterns in Terahertz Time-Domain Systems

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Sensors 2021, 21(10), 3510; https://doi.org/10.3390/s21103510 - 18 May 2021

Cited by 5 | Viewed by 1919

Abstract

An ability of lensless titanium-based antenna coupled microbolometers (Ti-

μ

bolometers) operating at room temperature to monitor precisely radiation patterns in terahertz time-domain spectroscopy (THz-TDS) systems are demonstrated. To provide comprehensive picture, two different THz-TDS systems and Ti-

μ

bolometers coupled with three [...] Read more.

An ability of lensless titanium-based antenna coupled microbolometers (Ti-

μ

bolometers) operating at room temperature to monitor precisely radiation patterns in terahertz time-domain spectroscopy (THz-TDS) systems are demonstrated. To provide comprehensive picture, two different THz-TDS systems and Ti-

μ

bolometers coupled with three different antennas—narrowband dipole antennas for 0.3 THz, 0.7 THz and a log-periodic antenna for wideband detection—were selected for experiments. Radiation patterns, spatial beam profiles and explicit beam evolution along the propagation axis are investigated; polarization-sensitive properties under various THz emitter power ranges are revealed. It was found that the studied Ti-

μ

bolometers are convenient lensless sensors suitable to discriminate and control THz radiation pattern features in various wideband THz-TDS systems. Full article

(This article belongs to the Special Issue Terahertz Imaging and Sensors)

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Open AccessArticle

Subcarrier Frequency-Modulated Continuous-Wave Radar in the Terahertz Range Based on a Resonant-Tunneling-Diode Oscillator

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Sensors 2020, 20(23), 6848; https://doi.org/10.3390/s20236848 - 30 Nov 2020

Cited by 10 | Viewed by 1717

Abstract

We introduce a new principle for distance measurement in the terahertz-wave range using a resonant-tunneling-diode (RTD) oscillator as a source at 511 GHz and relying on the frequency-modulated continuous-wave (FMCW) radar technique. Unlike the usual FMCW radar, where the sawtooth frequency modulation is [...] Read more.

We introduce a new principle for distance measurement in the terahertz-wave range using a resonant-tunneling-diode (RTD) oscillator as a source at 511 GHz and relying on the frequency-modulated continuous-wave (FMCW) radar technique. Unlike the usual FMCW radar, where the sawtooth frequency modulation is applied to the carrier, we propose applying it to a subcarrier obtained by amplitude modulation; this is advantageous when the source cannot be controlled precisely in oscillation frequency, but can easily be modulated in amplitude, as is the case of the RTD oscillator. The detailed principle and a series of proof-of-concept experimental results are presented. Full article

(This article belongs to the Special Issue Terahertz Imaging and Sensors)

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Open AccessArticle

Passive Detection and Imaging of Human Body Radiation Using an Uncooled Field-Effect Transistor-Based THz Detector

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Dovilė Čibiraitė-Lukenskienė

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Sensors 2020, 20(15), 4087; https://doi.org/10.3390/s20154087 - 22 Jul 2020

Cited by 22 | Viewed by 4326

Abstract

This work presents, to our knowledge, the first completely passive imaging with human-body-emitted radiation in the lower THz frequency range using a broadband uncooled detector. The sensor consists of a Si CMOS field-effect transistor with an integrated log-spiral THz antenna. This THz sensor [...] Read more.

This work presents, to our knowledge, the first completely passive imaging with human-body-emitted radiation in the lower THz frequency range using a broadband uncooled detector. The sensor consists of a Si CMOS field-effect transistor with an integrated log-spiral THz antenna. This THz sensor was measured to exhibit a rather flat responsivity over the 0.1–1.5-THz frequency range, with values of the optical responsivity and noise-equivalent power of around 40 mA/W and 42 pW/

\sqrt{Hz}

, respectively. These values are in good agreement with simulations which suggest an even broader flat responsivity range exceeding 2.0 THz. The successful imaging demonstrates the impressive thermal sensitivity which can be achieved with such a sensor. Recording of a 2.3 × 7.5-cm

^{2}

-sized image of the fingers of a hand with a pixel size of 1 mm

^{2}

at a scanning speed of 1 mm/s leads to a signal-to-noise ratio of 2 and a noise-equivalent temperature difference of 4.4 K. This approach shows a new sensing approach with field-effect transistors as THz detectors which are usually used for active THz detection. Full article

(This article belongs to the Special Issue Terahertz Imaging and Sensors)

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Open AccessArticle

A Versatile Illumination System for Real-Time Terahertz Imaging

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Jean-Baptiste Perraud

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Sensors 2020, 20(14), 3993; https://doi.org/10.3390/s20143993 - 17 Jul 2020

Cited by 9 | Viewed by 3165

Abstract

Terahertz technologies are attracting strong interest from high-end industrial fields, and particularly for non-destructive-testing purposes. Currently lacking compactness, integrability as well as adaptability for those implementations, the development and commercialisation of more efficient sources and detectors progressively ensure the transition toward applicative implementations, [...] Read more.

Terahertz technologies are attracting strong interest from high-end industrial fields, and particularly for non-destructive-testing purposes. Currently lacking compactness, integrability as well as adaptability for those implementations, the development and commercialisation of more efficient sources and detectors progressively ensure the transition toward applicative implementations, especially for real-time full-field imaging. In this work, a flexible illumination system, based on fast beam steering has been developed and characterized. Its primary goal is to suppress interferences induced by the coherence length of certain terahertz sources, spoiling terahertz images. The second goal is to ensure an enhanced signal-to-noise ratio on the detector side by the full use and optimized distribution of the available power. This system provides a homogeneous and adjustable illumination through a simplified setup to guarantee optimum real-time imaging capabilities, tailored to the sample under inspection. Working toward industrial implementations, different illumination process are conveniently assessed as a result of the versatility of this method. Full article

(This article belongs to the Special Issue Terahertz Imaging and Sensors)

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Review

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Open AccessEditor’s ChoiceReview

Roadmap of Terahertz Imaging 2021

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Sensors 2021, 21(12), 4092; https://doi.org/10.3390/s21124092 - 14 Jun 2021

Cited by 99 | Viewed by 9666

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Terahertz Imaging and Sensors)

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Open AccessEditor’s ChoiceReview

Sensitivity of Field-Effect Transistor-Based Terahertz Detectors

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Sensors 2021, 21(9), 2909; https://doi.org/10.3390/s21092909 - 21 Apr 2021

Cited by 45 | Viewed by 4393

Abstract

This paper presents an overview of the different methods used for sensitivity (i.e., responsivity and noise equivalent power) determination of state-of-the-art field-effect transistor-based THz detectors/sensors. We point out that the reported result may depend very much on the method used to determine the [...] Read more.

This paper presents an overview of the different methods used for sensitivity (i.e., responsivity and noise equivalent power) determination of state-of-the-art field-effect transistor-based THz detectors/sensors. We point out that the reported result may depend very much on the method used to determine the effective area of the sensor, often leading to discrepancies of up to orders of magnitude. The challenges that arise when selecting a proper method for characterisation are demonstrated using the example of a

2 \times 7

detector array. This array utilises field-effect transistors and monolithically integrated patch antennas at 620 GHz. The directivities of the individual antennas were simulated and determined from the measured angle dependence of the rectified voltage, as a function of tilting in the E- and H-planes. Furthermore, this study shows that the experimentally determined directivity and simulations imply that the part of radiation might still propagate in the substrate, resulting in modification of the sensor effective area. Our work summarises the methods for determining sensitivity which are paving the way towards the unified scientific metrology of FET-based THz sensors, which is important for both researchers competing for records, potential users, and system designers. Full article

(This article belongs to the Special Issue Terahertz Imaging and Sensors)

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Open AccessEditor’s ChoiceReview

Machine Learning Techniques for THz Imaging and Time-Domain Spectroscopy

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Hochong Park

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Joo-Hiuk Son

Sensors 2021, 21(4), 1186; https://doi.org/10.3390/s21041186 - 08 Feb 2021

Cited by 40 | Viewed by 4958

Abstract

Terahertz imaging and time-domain spectroscopy have been widely used to characterize the properties of test samples in various biomedical and engineering fields. Many of these tasks require the analysis of acquired terahertz signals to extract embedded information, which can be achieved using machine [...] Read more.

Terahertz imaging and time-domain spectroscopy have been widely used to characterize the properties of test samples in various biomedical and engineering fields. Many of these tasks require the analysis of acquired terahertz signals to extract embedded information, which can be achieved using machine learning. Recently, machine learning techniques have developed rapidly, and many new learning models and learning algorithms have been investigated. Therefore, combined with state-of-the-art machine learning techniques, terahertz applications can be performed with high performance that cannot be achieved using modeling techniques that precede the machine learning era. In this review, we introduce the concept of machine learning and basic machine learning techniques and examine the methods for performance evaluation. We then summarize representative examples of terahertz imaging and time-domain spectroscopy that are conducted using machine learning. Full article

(This article belongs to the Special Issue Terahertz Imaging and Sensors)

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Open AccessFeature PaperReview

The Magic of Optics—An Overview of Recent Advanced Terahertz Diffractive Optical Elements

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Agnieszka Siemion

Sensors 2021, 21(1), 100; https://doi.org/10.3390/s21010100 - 26 Dec 2020

Cited by 20 | Viewed by 4445

Abstract

Diffractive optical elements are well known for being not only flat but also lightweight, and are characterised by low attenuation. In different spectral ranges, they provide better efficiency than commonly used refractive lenses. An overview of the recently invented terahertz optical structures based [...] Read more.

Diffractive optical elements are well known for being not only flat but also lightweight, and are characterised by low attenuation. In different spectral ranges, they provide better efficiency than commonly used refractive lenses. An overview of the recently invented terahertz optical structures based on diffraction design is presented. The basic concepts of structure design together with various functioning of such elements are described. The methods for structure optimization are analysed and the new approach of using neural network is shown. The paper illustrates the variety of structures created by diffractive design and highlights optimization methods. Each structure has a particular complex transmittance that corresponds to the designed phase map. This precise control over the incident radiation phase changes is limited to the design wavelength. However, there are many ways to overcome this inconvenience allowing for broadband functioning. Full article

(This article belongs to the Special Issue Terahertz Imaging and Sensors)

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