CMOS-Compatible Room-Temperature Rectifier Toward Terahertz Radiation Detection

In this paper, we present a new rectifying device, compatible with the technology of CMOS image sensors, suitable for implementing a direct-conversion detector operating at room temperature for operation at up to terahertz frequencies. The rectifying device can be obtained by introducing some simple modifications of the charge-storage well in conventional CMOS integrated circuits, making the proposed solution easy to integrate with the existing imaging systems. The rectifying device is combined with the different elements of the detector, composed of a 3D high-performance antenna and a charge-storage well. In particular, its position just below the edge of the 3D antenna takes maximum advantage of the high electric field concentrated by the antenna itself. In addition, the proposed structure ensures the integrity of the charge-storage well of the detector. In the structure, it is not necessary to use very scaled and costly technological nodes, since the CMOS transistor only provides the necessary integrated readout electronics. On-wafer measurements of RF characteristics of the designed junction are reported and discussed. The overall performances of the entire detector in terms of noise equivalent power (NEP) are evaluated by combining low-frequency measurements of the rectifier with numerical simulations of the 3D antenna and the semiconductor structure at 1 THz, allowing prediction of the achievable NEP

Plasmonic Terahertz Detector Based on Asymmetric Silicon Field-Effect Transistor for Real-Time Terahertz Imaging System

Department of Electrical EngineeringTerahertz (THz) technology has a great potential application owing to the unique properties of THz wave that has both permeability and feature of straight. Among the various technology in THz frequency range, THz imaging technology is very promising and attractive owing to harmlessness in human body by very low energy. In particular, for real-time THz imaging detectors, field-effect transistor (FET)-based THz detectors are now being intensively developed in multi-pixel array configuration by exploiting the silicon (Si) technology advantages of low-cost and high density integration. FET-based plasmonic wave detection mechanism, which is not limited by cut-off frequency as in transit-mode, has attractive features such as enhanced responsivity (Rv) according to frequency increase in THz range and robustness to high THz input power. To analyze the operation principle of plasmonic THz detector, an analytical device model has been implemented in terms of device physics. The non-resonant and ???overdamped??? plasma-wave behaviors have been modeled by introducing a quasi-plasma electron charge box as a two-dimensional electron gas (2DEG) in the channel region only around the source side of Si FETs. Based on the coupled non-resonant plasma-wave physics and continuity equation on the technology computer-aided design (TCAD) platform, the alternate-current (ac) signal as an incoming THz wave radiation successfully induced a direct-current (dc) drain-to-source output voltage as a detection signal in a sub-THz frequency regime under the asymmetric boundary conditions between source and drain. The significant effects of asymmetric source and drain structure, channel shape on the charge asymmetry and performance enhancement have been analytically investigated based on non-resonant plasmonic THz detection theory. By designing and fabricating an asymmetric transistor integrated with antenna, more enhanced channel charge asymmetry has been obtained for enhanced detection response. Through verification of the advanced non-quasi-static (NQS) compact model, the intrinsic FET delay and total detector delay in THz plasmonic detection are successfully characterized and are small enough to guarantee a real-time operating detector. These results can provide that the real-time THz imaging of moving objects has been experimentally demonstrated based on plasmonic 1x200 array scanner by using the high/fast detecting performance asymmetric FET and multiplexer/amplifier circuits. The highly-enhanced Rv and reduced noise equivalent power (NEP) have been demonstrated by exploiting monolithic transistor-antenna device considering impedance matching between transistor and antenna. This record-high enhancement is due to antenna mismatching and feeding line loss reduction as well as the enhanced charge asymmetry in the proposed monolithic transistor-antenna device. Therefore, high-performance plasmonic THz detector based on asymmetric Si FET can compete as commercial THz detector by taking advantages of monolithic device technology for real-time THz imaging system.ope

Materials Analysis Using a THz Imaging System Based on Atomic Vapour

This thesis studies the response of the interaction between Rydberg atomic vapour and a THz frequency field. When Caesium atoms at room temperature are excited to a Rydberg state using three infrared lasers and a 0.55 THz field resonant with the 14P3/2 → 13D5/2 transition is applied, the atoms respond by emitting a green optical fluorescence corresponding to the 13D5/2 → 6P3/2 decay. This response is exploited to investigate the absorption coefficient for different polymer materials that transmit well in the THz frequency range using the Beer–Lambert law. We calibrate the system to obtain a measure of THz intensity. As the THz imaging system is highly sensitive to environmental changes, and to show that our results are consistent, we provide a comparison of results between our atomic detection method and a commercial thermal power meter. Additionally, we measure the absorption coefficient of the same materials at a frequency of 1.1 THz, and the results are compared with those measured at 0.55 THz. The THz imaging system is also used to perform some experiments in order to demonstrate its effectiveness in real-world applications. The system provides an interesting image contrast in the case of a sample containing two different polymer materials measured at two THz frequencies. The result is a proof-of-concept that multispectral THz imaging can provide additional information and is motivation to improve our THz imaging system by introducing a dual-species THz imager. We also investigate the polarisation spectroscopy of an excited-state transition of rubidium vapour at room temperature as a step towards a rubidium THz imaging system. The narrow dispersive signal produced by this spectroscopy technique is ideal for laser frequency stabilisation of excited-state transitions

Analytic and Machine Learning Based Design of Monolithic Transistor-Antenna for Plasmonic Millimeter-Wave Detectors

Department of Electrical EngineeringThis thesis reports an advanced analysis on a monolithic transistor-antenna by designing a ring-type asymmetric FET itself as a receiving antenna element which receives millimeter-waves in a loss-less manner with a plasmonic ampli fication for millimeter-wave (mmW) detectors. The proposed transistor-antenna device combines the plasmonic and the electromagnetic (EM) aspects in a single place. As a result, it can absorb the incoming mmW and transfer power directly to the ring-type asymmetric channel without any feeding line and a separate antenna element. Both the charge asymmetry in the device channel and the antenna coupling are contributing to the enhanced photoresponse. Among the two factors, the improved antenna coupling is more dominant in the performance enhancement of our proposed design. Also, our transistor-antenna device have enhanced performance with a uniformly enhanced responsivity of every pixel by characterizing its impedance exactly pursuing real-time mmW imaging. Operation principle of the proposed device is discussed, focusing on how signal transmission through the ring-type structure is available without any feeding line between the antenna and the detector. To determine the antenna geometry aiming for a desired resonant frequency, we present an efficient design procedure based on periodic bandgap analysis combined with parametric electromagnetic simulations. From a fabricated ring-type FET-based monolithic antenna device, we demonstrated the highly enhanced optical responsivity and the reduced optical noise-equivalent power, which are in comparable order with the reported state-of-the-art CMOS-based antenna integrated direct detectors. Another part of the thesis focuses on developing machine learning models to enable fast, accurate design and veri fication of electromagnetic structures. We proposed a novel Bayesian learning algorithm named as Bayesian clique learning, for searching the optimal electromagnetic design parameter by using the structural property of EM simulation data set. Along with this, we also given an inverse problem approach for designing the electromagnetic structures which suggest going in the opposite direction to determine the design parameters from characteristics of the desired output.clos

Plasmonic photoconductive terahertz focal-plane array with pixel super-resolution

Imaging systems operating in the terahertz part of the electromagnetic spectrum are in great demand because of the distinct characteristics of terahertz waves in penetrating many optically-opaque materials and providing unique spectral signatures of various chemicals. However, the use of terahertz imagers in real-world applications has been limited by the slow speed, large size, high cost, and complexity of the existing imaging systems. These limitations are mainly imposed due to the lack of terahertz focal-plane arrays (THz-FPAs) that can directly provide the frequency-resolved and/or time-resolved spatial information of the imaged objects. Here, we report the first THz-FPA that can directly provide the spatial amplitude and phase distributions, along with the ultrafast temporal and spectral information of an imaged object. It consists of a two-dimensional array of ~0.3 million plasmonic photoconductive nanoantennas optimized to rapidly detect broadband terahertz radiation with a high signal-to-noise ratio. As the first proof-of-concept, we utilized the multispectral nature of the amplitude and phase data captured by these plasmonic nanoantennas to realize pixel super-resolution imaging of objects. We successfully imaged and super-resolved etched patterns in a silicon substrate and reconstructed both the shape and depth of these structures with an effective number of pixels that exceeds 1-kilo pixels. By eliminating the need for raster scanning and spatial terahertz modulation, our THz-FPA offers more than a 1000-fold increase in the imaging speed compared to the state-of-the-art. Beyond this proof-of-concept super-resolution demonstration, the unique capabilities enabled by our plasmonic photoconductive THz-FPA offer transformative advances in a broad range of applications that use hyperspectral and three-dimensional terahertz images of objects for a wide range of applications.Comment: 62 page

Millimeter-wave and terahertz imaging techniques

This thesis presents the development and assessment of imaging techniques in the millimeterwave (mmW) and terahertz frequency bands. In the first part of the thesis, the development of a 94 GHz passive screener based on a total-power radiometer (TPR) with mechanical beamscanning is presented. Several images have been acquired with the TPR screener demonstrator, either in indoor and outdoor environments, serving as a testbed to acquire the know-how required to perform the research presented in the following parts of the thesis. In the second part of the thesis, a theoretical research on the performance of near-field passive screeners is described. This part stands out the tradeoff between spatial and radiometric resolutions taking into account the image distortion produced by placing the scenario in the near-field range of the radiometer array. In addition, the impact of the decorrelation effect in the image has been also studied simulating the reconstruction technique of a synthetic aperture radiometer. Guidelines to choose the proper radiometer depending on the application, the scenario, the acquisition speed and the tolerated image distortion are given in this part. In the third part of the thesis, the development of a correlation technique with optical processing applicable to millimeter-wave interferometric radiometers is described. The technique is capable of correlating wide-bandwidth signals in the optical domain with no loss of radiometric sensitivity. The theoretical development of the method as well as measurements validating the suitability to correlate radiometric signals are presented in this part. In the final part of the thesis, the frequency band of the imaging problem is increased to frequencies beyond 100 GHz, covering the THz band. In this case the research is centered in tomographic techniques that include spectral information of the samples in the reconstructed images. The tomographic algorithm can provide detection and identification of chemical compounds that present a certain spectral footprint in the THz frequency band.Postprint (published version

Metamaterial based CMOS terahertz focal plane array

The distinctive properties of terahertz radiation have driven an increase in interest to develop applications in the imaging field. The non-ionising radiation properties and transparency to common non-conductive materials have led research into developing a number of important applications including security screening, medical imaging, explosive detection and wireless communications. The proliferation of these applications into everyday life has been hindered by the lack of inexpensive, compact and room-temperature terahertz sources and detectors. These issues are addressed in this work by developing an innovative, uncooled, compact, scalable and low-cost terahertz detector able to target single frequency imaging applications such as stand-off imaging and non-invasive package inspection. The development of two types of metamaterial (MM) based terahertz focal plane arrays (FPAs) monolithically integrated in a standard complementary metal-oxide semiconductor (CMOS) technology are presented in this Thesis. The room temperature FPAs are composed of periodic cross-shaped resonant MM absorbers, microbolometer sensors in every pixel and front-end readout electronics fabricated in a 180 nm six metal layer CMOS process from Texas Instruments (TI). The MM absorbers are used due to the lack of natural selective absorbing materials of terahertz radiation. These subwavelength structures are made directly in the metallic and insulating layers available in the CMOS foundry process. When the MM structures are distributed in a periodic fashion, they behave as a frequency-selective material and are able to absorb at the required frequency. The electromagnetic (EM) properties are determined by the MM absorber geometry rather than their composition, thus being completely customisable for different frequencies. Single band and broadband absorbers were designed and implemented in the FPAs to absorb at 2.5 THz where a natural atmospheric transmission window is found, thus reducing the signal loss in the imaging system. The new approach of terahertz imaging presented in this Thesis is based in coupling a MM absorber with a suitable microbolometer sensor. The MM structure absorbs the terahertz wave while the microbolometer sensor detects the localised temperature change, depending on the magnitude of the radiation. Two widely used microbolometer sensors are investigated to compare the sensitivity of the detectors. The two materials are Vanadium Oxide (VOx) and p-n silicon diodes both of which are widely used in infrared (IR) imaging systems. The VOx microbolometers are patterned above the MM absorber and the p-n diode microbolometers are already present in the CMOS process. The design and fabrication of four prototypes of FPAs with VOx microbolometers demonstrate the scalability properties to create high resolution arrays. The first prototype consists of a 5 x 5 array with a pixel size of 30 μm x 30 μm. An 8 x 8 array, a 64 x 64 array with serial readout and a 64 x 64 array with parallel readout are also presented. Additionally, a 64 x 64 array with parallel output readout electronics with p-n diode microbolometers was fabricated. The design, simulation, characterisation and fabrication of single circuit blocks and a complete 64 x 64 readout integrated circuit is thoroughly discussed in this Thesis. The absorption characteristics of the MMs absorbers, single VOx and p-n diode pixels, 5 x 5 VOx FPA and a 64 x 64 array for both microbolometer types demonstrate the concept of CMOS integration of a monolithic MM based terahertz FPA. The imaging performance using both transmission and reflection mode is demonstrated by scanning a metallic object hidden in a manila envelope and using a single pixel of the array as a terahertz detector. This new approach to make a terahertz imager has the advantages of creating a high sensitivity room temperature technology that is capable of scaling and low-cost manufacture