Aleksander Sešek

Spectroscopic Terahertz Imaging at Room Temperature Employing Microbolometer Terahertz Sensors and Its Application to the Study of Carcinoma Tissues

A terahertz (THz) imaging system based on narrow band microbolometer sensors (NBMS) and a novel diffractive lens was developed for spectroscopic microscopy applications. The frequency response characteristics of the THz antenna-coupled NBMS were determined employing Fourier transform spectroscopy. The NBMS was found to be a very sensitive frequency selective sensor which was used to develop a compact all-electronic system for multispectral THz measurements. This system was successfully applied for principal components analysis of optically opaque packed samples. A thin diffractive lens with a numerical aperture of 0.62 was proposed for the reduction of system dimensions. The THz imaging system enhanced with novel optics was used to image for the first time non-neoplastic and neoplastic human colon tissues with close to wavelength-limited spatial resolution at 584 GHz frequency. The results demonstrated the new potential of compact RT THz imaging systems in the fields of spectroscopic analysis of materials and medical diagnostics.

Comparison of terahertz technologies for detection and identification of explosives

We present results on the comparison of different THz technologies for the detection and identification of a variety of explosives from our laboratory tests that were carried out in the framework of NATO SET-193 “THz technology for stand-off detection of explosives: from laboratory spectroscopy to detection in the field” under the same controlled conditions. Several laser-pumped pulsed broadband THz time-domain spectroscopy (TDS) systems as well as one electronic frequency-modulated continuous wave (FMCW) device recorded THz spectra in transmission and/or reflection.

A high performance room temperature THz sensor

Resonant THz antenna-coupled micro-bolometers are considered as a potential candidates for room temperature THz imaging, as well as spectroscopic applications. Micromachining technology is found to be well-suitable to fabricate a micro-meter bolometer sensor suitable for MEMS implementation. The sensitivity of the sensor is determined to be up to 1000V/W and the noise equivalent power (NEP) – is down to 5pW /√Hz. The sensor parameters are designed to be easily implemented with a low cost standard preamplifier array which increases the pixel sensitivity to 106V/W without compromising the noise equivalent power.

Micro-machined Millimeter Wave Sensor Array for FM Radar Application

The objective of this work was to create a low cost sensor array that operates at room temperature for millimeter wave applications and could be used for FM radars and various heterodyne receivers. The selected technology was silicon wafer micromachining allowing the creation of microstructures on silicon membranes using different metal layers. The technology used allowed submicron dimensions for a photolithography pattern and thin membranes down to a few micrometers. One of the most critical requirements for the sensor was to achieve a high signal-to-noise ratio and a high bandwidth for a mixed frequency. The sensor is a titanium-based micro-bolometer connected to the micro-antenna which is integrated with the bolometer. The results are very promising. The measured NEP is below 5pW/√Hz and the sensitivity is close to 1000 V/W. In the paper the antenna - bolometer sensor microstructure is analyzed. Theoretical analysis and design guidelines for the bolometer itself are discussed. Simulation results of the bolometer and antenna show very close matching to the measured results. Characterization measurements were performed, and thermal behavior of microbolometer structure was simulated and measured. The measurement results are presented for THz FM radar different targets, and a technology demonstrator is also described.

THz-vision system with extended functionality

A near real-time THz-vision system is presented in the paper. The most important part of it is the THz sensors focal plane array operating at the room temperature, featuring low NEP (5pW/√Hz) and high sensitivity (1e6 V/W). Its architecture allows direct digital processing of the output signal. The system performance is upgraded with large parallel processing of up to 64 channels. The second important building block is the FM THz source used for illumination. A wide FM range, of up to ±10% of the central frequency allows using the system for various applications. The THz source is a solid-state source using a GHz range frequency synthesizer followed by frequency multipliers and microwave amplifiers. Such a compact THz source can cover the lower region of the THz spectrum, i.e. below 1THz using different frequency bands. The band selection depends on the application. Three different areas of applications are discussed in the paper: 3D imaging of hidden objects as one of the most attractive features of the presented system, an accurate range finder with the resolution within a fraction of the wave length and a narrow band CW spectrometer operating in the FM range of the source.

Electronic terahertz imaging for security applications

A sophisticated THz system with 3D imaging and narrow band spectroscopy capability is presented in the paper. The key system components are the THz source, THz detector/mixer array, scanning optics, and the signal processing unit. The system is all electronic and is portable. A battery operation option allows several hours of autonomy. The most important parameters of the THz source are output power, illumination beam size and directivity, frequency modulation range, and maximal modulation frequency. The low phase noise is also a very important parameter. Optimization of these parameters is discussed in the paper. The THz source is all solid state, composed of a phase-locked oscillator, an amplifier, and frequency multipliers. The most important element of the THz system is its sensor, which performs both signal detection and at the same time mixing of the LO signal and received signal from the target. The sensor is antenna coupled nanobolometer fabricated in a linear array of eight pixels. The sensors are suspended in the vacuum to achieve an excellent signal-to-noise ratio. The quadratic characteristic of the nano-bolometer extends over six decades allowing a large dynamic range and very high LO signal levels. The scanning mirror integrated into the system allows imaging of 1024 to 8162 pixels in the x and y dimensions that are expanded to the third dimension with a resolution of few micrometers.

THz remote sensing with μm resolution

In the paper a frequency modulated THz system is presented. The system is constructed with a solid state THz source and is modulated approx. ±10% of central frequency of 0.3THz. The detector is room temperature sensor array with a square low characteristic allowing a mixer operation between a portion of transmitted signal from the beam splitter and the received signal. Due to this heterodyne approach a very good signal to noise ratio has been achieved, allowing accurate and repeatable signal analysis. The phase of the received signal is very stable and can be used for fine position measurements with the resolution well below 1μm. In this paper the focus is on measurements of thin foil thickness. Various experiments set-ups and measured results are presented.

Fertiliser characterisation using optical and electrical impedance methods

Abstract This paper addresses the problem of fertiliser characterisation using optical and electrical impedance methods. Comparative analysis was performed to estimate the methods effectiveness for quantitative and qualitative characterisation of water diluted fertiliser. Characterisation using optical method within the deep ultraviolet range indicates the variability of features that was not observed when using the impedance method. The combination of both methods showed potential for more accurate qualitative analysis than each method alone. Finally, both methods showed good sensitivity to fertiliser concentration variation that was possible to fit with a linear function for optical spectroscopy ( R 2 = 0.95 ) and an exponential function for the impedance method ( R 2 = 0.99 ).

A Microbolometer System for Radiation Detection in the THz Frequency Range with a Resonating Cavity Fabricated in the CMOS Technology

BACKGROUND The THz sensors using microbolometers as a sensing element are reported as one of the most sensitive room-temperature THz detectors suitable for THz imaging and spectroscopic applications. Microbolometer detectors are usually fabricated using different types of the MEMS technology. The patent for the detection system presented in this paper describes a method for microbolometer fabrication using a standard CMOS technology with advanced micromachining techniques. The measured sensitivity of the sensors fabricated by the patented method is 1000 V/W at an optimal frequency and is determined by the performance of a double-dipole antenna and quarter-wavelength resonant cavity. METHOD The paper presents a patented method for fabrication of a microbolometer system for radiation detection in the THz frequency range (16). The method is divided into several stages regarding the current silicon micromachining process. Main stages are fabrication of supporting structures for micro bridge, creation of micro cavities and fabrication of Aluminum antenna and Titanium microbolometer. Additional method for encapsulation in the vacuum is described which additionally improves the performance of bolometer. The CMOS technology is utilized for fabrication as it is cost effective and provides the possibility of larger sensor systems integration with included amplification. At other wavelengths (e.g. IR region) thermistors are usually also the receivers with the sensor resistance change provoked by self-heating. In the THz region the energy is received by an antenna coupled to a thermistor. Depending on the specific application requirement, two types of the antenna were designed and used; a narrow-band dipole antenna and a wideband log-periodic antenna. RESULTS With method described in the paper, the microbolometer detector reaches sensitivities up to 500 V/W and noise equivalent power (NEP) down to 10 pW/√Hz. Additional encapsulation in the vacuum improves its performance at least by a factor of 2, therefore the sensitivity reaches approximately 1000 V/W and NEP down to 5 pW/√Hz. The thermal response time of bolometer is 0.5 µs. The thermistor biasing current drops with its resistance (defined by microbolometer active area), but the sensitivity rises. Typical value of biasing current is 300 µA at 680 Ω of resistance, where the sensitivity reaches highest level. Air pressure decrease highly influences the sensitivity due to lower thermal dissipation to surrounding air. The sensitivity is therefore doubled when packaged in the high vacuum (0.1Pa). CONCLUSION The main advantage of the presented approach is that the detection devices can be fabricated by a standard silicon micromachining process. Their overall dimension is defined by the receiving antenna and they do not need any additional optic source for the operation. They are robust and appropriate for mass production and can be easily embedded or merged with other vision system in use. The developed microbolometer is highly sensitive, its noise is low and it operates at a room temperature with no additional cooling system at a normal atmospheric pressure. The output of the THz detector connected to a discrete low-noise amplifier increases the total sensitivity up to 106 V/W with no impact on the noise equivalent power of 5 pW/√HZ.

A compact THz imaging system

The objective of this paper is the development of a compact low cost imaging THz system, usable for observation of the objects near to the system and also for stand-off detection. The performance of the system remains at the high standard of more expensive and bulkiest system on the market. It is easy to operate as it is not dependent on any fine mechanical adjustments. As it is compact and it consumes low power, also a portable system was developed for stand-off detection of concealed objects under textile or inside packages. These requirements rule out all optical systems like Time Domain Spectroscopy systems which need fine optical component positioning and requires a large amount of time to perform a scan and the image capture pixel-by-pixel. They are also almost not suitable for stand-off detection due to low output power. In the paper the antenna - bolometer sensor microstructure is presented and the THz system described. Analysis and design guidelines for the bolometer itself are discussed. The measurement results for both near and stand-off THz imaging are also presented.