Prof. Hakan Altan
Prof./Lecturer at Middle East Technical Univ
SPIE Involvement:
Conference Program Committee | Author
Proceedings Article | 5 October 2018
Understanding the detection mechanism of mm-wave radiation in glow discharge detectors
Cemre Kusoglu Sarikaya, Demiral Akbar, Marco Ribeiro, Hakan Altan
Proc. SPIE. 10800, Millimetre Wave and Terahertz Sensors and Technology XI
KEYWORDS: Particles, Electrons, Ions, Lamps, Ionization, Terahertz radiation, Neon, Plasma
Read Abstract +
Sub-mm and mm waves are used in many applications such as spectroscopy and imaging, detection of weapons concealed under clothing, detection of diseases and non-destructive product inspection. By developing the THz detection mechanisms, the quality of this usability can be enhanced considerably. The commercial detectors currently used to detect THz waves range from Shottky diodes to bolometers, Golay cells and pyroelectric detectors. However, many of these detectors have limitations in terms of speed and responsivity. In addition, they have a common disadvantage of being expensive. For these reason, glow discharge detectors (GDDs) can be a good alternative since they are cheap and can detect mm-wave or THz radiations without the aforementioned limitations. To obtain optimal detection parameters, one needs to understand the interaction of the radiation with the plasma particles. In the literature, efforts to explain this interaction has focused on qualitative descriptions with analytical models. However, these theories have not been tested with real discharge parameters. For that reason, in this study, the plasma in the GDD is simulated by using previously developed parallel 1d3v Particle in Cell/Monte Carlo Collision (PIC/MCC) code to obtain the plasma parameters and determine the gas mixture ratios. Initial results show that the discharge current approximates the current measured through the home built glow discharge detector. Using these results an accurate simulation of the GDD discharge parameters has been performed. This platform will allow for understanding the effect of mm sub-mm radiation when added to the simulation which can aid in understanding optimum detection parameters.
TRay: active THz imaging autonomous biometric secured access control and tracking gate system
İ. Ozan Yıldırım, Ali Telli, Tolga Çolak, Cengiz Güray, Burak Demirsoy, Tugay Osmanoğlu, İhsan Ünal, Gökhan Beltekin, Asaf Behzat Şahin, Hakan Altan
KEYWORDS: Biometrics, Facial recognition systems, Visualization, Imaging systems, X-rays, Inspection, Control systems, Terahertz radiation, Information security
Recent security concerns worldwide have made it necessary to put a large number of people through detailed security inspections in critical infrastructure and border entrances. It is desired to implement inspection processes, which will enhance security while providing improvements in several aspects, such as answering the high throughput at crowded places, standardizing control processes, minimizing operator errors and increasing the feel of security, without making individuals uncomfortable as possible. To answer these requirements, Active Terahertz Imaging Autonomous Biometric Security Access Control and Tracking Gate System, or TRay, integrates various security technologies. TRay offers epassport control, biometric identity check (fingerprint, face etc.) and THz imaging for concealed object detection. During the inspection process, the person using the gate is guided through e-passport control and biometric identity check steps by following a set of instructions, without requiring the assistance of an operator. The person will also be scanned for concealed objects by a THz imaging system. Locations of possible threat objects and contraband, made of metal and, in addition, ceramic, plastic, liquids etc. are identified using object detection algorithms. The system is able to generate a warning in case of a detected object, for further inspection of the location of the body part by security personnel. THz imaging system operates at a standoff distance, which allows scanning to be performed at any point during the inspection process, eliminating the need to use confined spaces for body scanning. Less human intervention and misunderstandings, decreased waiting time at passport/identification checkpoints, improved public satisfaction, standardized control duration, optimized use of space, easy tracking of passenger transitions, generating alarms in case of undesired transitions, 24/7 operation, forged document usage prevention and decreased workload on the security personnel are further advantages of the system, which is unique in terms of the features it combines.
SPIE Journal Paper | 10 April 2018
Enhancing the image resolution in a single-pixel sub-THz imaging system based on compressed sensing
Ümit Alkuş, Esra Sengün Ermeydan, Asaf Behzat Sahin, Ilyas Cankaya, Hakan Altan
OE Vol. 57 Issue 04
KEYWORDS: Imaging systems, Reconstruction algorithms, Image quality, Image resolution, Image enhancement, Compressed sensing, Spatial light modulators, Resolution enhancement technologies, Terahertz radiation, Detection and tracking algorithms
Compressed sensing (CS) techniques allow for faster imaging when combined with scan architectures, which typically suffer from speed. This technique when implemented with a subterahertz (sub-THz) single detector scan imaging system provides images whose resolution is only limited by the pixel size of the pattern used to scan the image plane. To overcome this limitation, the image of the target can be oversampled; however, this results in slower imaging rates especially if this is done in two-dimensional across the image plane. We show that by implementing a one-dimensional (1-D) scan of the image plane, a modified approach to CS theory applied with an appropriate reconstruction algorithm allows for successful reconstruction of the reflected oversampled image of a target placed in standoff configuration from the source. The experiments are done in reflection mode configuration where the operating frequency is 93 GHz and the corresponding wavelength is λ = 3.2 mm. To reconstruct the image with fewer samples, CS theory is applied using masks where the pixel size is 5 mm × 5 mm, and each mask covers an image area of 5 cm × 5 cm, meaning that the basic image is resolved as 10 × 10 pixels. To enhance the resolution, the information between two consecutive pixels is used, and oversampling along 1-D coupled with a modification of the masks in CS theory allowed for oversampled images to be reconstructed rapidly in 20 × 20 and 40 × 40 pixel formats. These are then compared using two different reconstruction algorithms, TVAL3 and ℓ1-MAGIC. The performance of these methods is compared for both simulated signals and real signals. It is found that the modified CS theory approach coupled with the TVAL3 reconstruction process, even when scanning along only 1-D, allows for rapid precise reconstruction of the oversampled target.
Investigation of radiant millimeter wave/terahertz radiation from low-infrared signature targets
B. Aytaç, Ü. Alkuş, M. Sivaslıgil, A. B. Şahin, H. Altan
Proc. SPIE. 10439, Millimetre Wave and Terahertz Sensors and Technology X
KEYWORDS: Target detection, Optical filters, Polyurethane, Cameras, Black bodies, Terahertz radiation, Infrared signatures, Temperature metrology
Millimeter (mm) and sub-mm wave radiation is increasingly becoming a region of interest as better methods are developed to detect in this wavelength range. The development of sensitive focal plane array (FPA) architectures as well as single pixel scanners has opened up a new field of passive detection and imaging. Spectral signatures of objects, a long standing area of interest in the Short Wave Infrared (SWIR), Mid-Wave (MWIR) and Long Wave-IR (LWIR) bands can now be assessed in the mm-wave/terahertz (THz) region. The advantage is that this form of radiation is not as adversely affected by poor atmospheric conditions compared to other bands. In this study, a preliminary experiment in a laboratory environment is performed to assess the radiance from targets with low infrared signatures in the millimeter wave/terahertz (THz) band (<1 THz). The goal of this approach is to be able to model the experimental results to better understand the mm-wave/THz signature of targets with low observability in the IR bands.
Super-resolution image reconstruction applied to an active millimeter wave imaging system based on compressive sensing
Ümit Alkuş, Esra Şengün Ermeydan, Asaf Behzat Şahin, İlyas Çankaya, Hakan Altan
KEYWORDS: Super resolution, Detection and tracking algorithms, Imaging systems, Sensors, Image resolution, Spatial light modulators, Reconstruction algorithms, Target acquisition, Resolution enhancement technologies, Compressed sensing
The development of passive and active millimeter wave imaging systems is progressing rapidly fueled by the need for many applications in the area of security and defense. Imaging schemes that may either utilize array detectors or single detectors in scan architectures offer suffer from poor resolution due to the longer wavelengths used and the limits of the optical system in terms of lens and mirror dimensions. In order to overcome this limit, super-resolution techniques can be employed to enhance the resolution of the imaging system. Here, a form of this technique based on oversampling is applied to reconstruct the image of a target which is acquired using compressive sensing based on scanning the image plane using randomly patterned masks with fixed pixel sizes. The mm-wave stand-off imaging system uses a 93 GHz center frequency source and heterodyne sub-harmonic receiver place in a bi-static configuration to image a target in reflection mode. The image of the target is projected onto a mechanically scanned spatial light modulator (SLM), which is a patterned two-dimensional mask that is translated along one axis. In order to improve the resolution of the image, the masks are shifted by half the pixel size (2.5mm). To enhance the resolution of the image, the patterns are shifted by smaller steps, thereby each pixel is oversampled and the resulting new pattern and detected intensity is fed into the CS algorithm to reconstruct the image of the target. After the image reconstruction process, sharper edges are observed for a circular object of 12mm diameter compared to the image acquired by whole pixel step scanning.
Proceedings Article | 1 August 2017
Terahertz pulsed imaging for the monitoring of dental caries: a comparison with x-ray imaging
Burcu Karagoz, Kıvanc Kamburoglu, Hakan Altan
Proc. SPIE. 10417, Medical Laser Applications and Laser-Tissue Interactions VIII
KEYWORDS: Dental caries, Medical imaging, Terahertz radiation, X-ray imaging, 3D image processing
Dental caries in sliced samples are investigated using terahertz pulsed imaging. Frequency domain terahertz response of these structures consistent with X-ray imaging results show the potential of this technique in the detection of early caries.
Proceedings Article | 27 January 2017
340 GHz imaging system for detection of concealed threat objects at 5 meters stand-off distance (Conference Presentation)
Ihsan Ozan Yildirim, Vedat Ali Özkan, Taylan Takan, Asaf Behzat Şahin, Hakan Altan
Proc. SPIE. 9993, Millimetre Wave and Terahertz Sensors and Technology IX
KEYWORDS: Target detection, Image fusion, Transceivers, Imaging systems, Detector development, Heterodyning, Diodes, Terahertz radiation, Antennas, Directional couplers
A THz active scanned imaging system is developed for detection of concealed threat objects at a stand-off distance of 5 meters. Single pixel, active imaging system utilizes a continuous wave transceiver unit operating at 340 GHz, based on RF components and Schottky diode rectifiers. The transceiver has a heterodyne detection geometry and has 7 mW total power output which is derived from a 3dB directional coupler (25 dB directivity) and a horn antenna. 2D opto-mechanical scanning is performed using two mirror coupled galvanometer scanners to scan 50x50 cm2 field of view at 5 meters stand-off distance. 2 cm resolution is achieved on the target plane. Based on the opto-mechanical scan speed, frame rate is 2 Hz. 340 GHz provides penetration to common barrier objects such as clothing for detection of concealed threats. Here we present imaging capability of the system and its penetration abilities. Effect of barrier objects to the dynamic range is also investigated. To provide real time screening of the target scene for potential threat objects, its visual image is combined with the acquired THz image of the field of view by using image fusion technique.
WATCH PRESENTATION
Transmission and detection of terahertz radiation in a weakly ionized plasma (Conference Presentation)
Taylan Takan, Namig Alasgarzade, Ihsan Yildirim, Ilker Uzun-Kaymak, Asaf Behzat Sahin, Hakan Altan
KEYWORDS: Modulation, Polarization, Electrodes, Gases, Lamps, Ionization, Terahertz radiation, Neon, Millimeter wave sensors, Plasma
Plasma, used as a terahertz (THz) detection medium has promising features. Several studies for mm-wave/THz radiation detection using various kind of methods for plasma creation such as neon indicator lamps [1], gas cells [2] and laser-induced air plasma [3] have been conducted. The interaction between the plasma and various frequency EM waves are still being investigated and in the mm-wave/terahertz range the interaction mechanism is still not well understood. In this study a home-built gas chamber with variable electrode separation is studied using a continuous wave mm-wave/THz measurement systems. A breakdown is induced in gas mixture by applying a bias DC voltage to the electrodes and under sufficient conditions the modulated incident radiation can generate variations in the plasma current which can be measured electronically. The main mechanism of detection is the addition of the electric field of the incident EM radiation to the DC bias field, increasing the total electric field thus excitation collisions. Therefore the EM field is expected to effect the rate of ionization and excitation collisions at most at the regions with maximum total electron energy that is around the cathode dark space [4]. Depending on the orientation of its polarization, the incident EM radiation can also diffuse the signal electrons to the walls of the chamber giving rise to a negative change in bias current, decreasing the signal. Therefore the internal signal gain depends on the electrode geometry and polarization of radiation besides other parameters of the plasma. Several parameters, such as gas pressure, gas species, discharge current, electrode spacing and electrode geometry effect the plasma-THz interaction thus changing the responsivity of the device. The plasma – THz interaction is studied here using a VDI multiplied source (WR2.8AMC). Driven by a frequency tunable Yttrium Iron Garnett (YIG) oscillator the source was modulated electronically providing a frequency tunable output in the 82-125 GHz and 246-375 GHz frequency range by use of a passive tripler. For the gas chamber different gases and gas mixtures are used. Using a Penning mixture, which is a mixture of one type of another gas with miniscule amount of another gas which has a lower ionization voltage than the main gas, a breakdown voltage lower than that of both gases can be obtained. Measurement of changes in the plasma current are carried out for different incident radiation frequencies, different electrode geometries, various gas mixtures and different modulation frequencies.
Proceedings Article | 7 March 2016
Monitoring of tryptophan as a biomarker for cancerous cells in Terahertz (THz) sensing
Hakan Altan, Nihal Simsek Ozek, Seher Gok, Ipek Ozyurt, Feride Severcan
Proc. SPIE. 9703, Optical Biopsy XIV: Toward Real-Time Spectroscopic Imaging and Diagnosis
KEYWORDS: Biosensing, Cancer, Breast cancer, Tissues, Blood, Glasses, Spectroscopy, Terahertz radiation, Colon, Picosecond phenomena, Sodium, Terahertz spectroscopy, Liquids
Tryptophan is an extremely important amino acid for a variety of biological functions in living organisms. Changes in the concentration of this amino acid can point to identification of cancerous tissues or even confirm symptoms of depression in patients. Therefore it is extremely important to identify and quantify tryptophan concentrations in human blood as well as in in-vivo diagnostic studies. Here a reflection based terahertz pulsed spectroscopy system was used to study the interaction of THz pulses with cancerous cells to gauge the possibility of using L-tryptophan as a biomarker for THz sensing of diseases. Initial measurements were performed on human colon adenocarcinoma cells and human breast cancer cells cultivated on glass slides. The glass slides utilized in the growth process limited the measurements not only to reflection based geometries but also limited the analysis of the samples in the frequency domain due to the highly absorbing nature of glass in the THz region. The useful bandwidth was limited to frequencies below 0.6THz which prohibited us from investigating the effects of L-tryptophan in these samples. Even with the limited frequency range the measurements show that there are slight differences in the transmission of the THz pulse through different samples.
Proceedings Article | 21 October 2015
Detection of the mm-wave radiation using a low-cost LWIR microbolometer camera from a multiplied Schottky diode based source
Basak Kebapci, Firat Tankut, Hakan Altan, Tayfun Akin
Proc. SPIE. 9651, Millimetre Wave and Terahertz Sensors and Technology VIII
KEYWORDS: Staring arrays, Microbolometers, Long wavelength infrared, Infrared cameras, Signal to noise ratio, Infrared imaging, Cameras, Sensors, Diodes, Terahertz radiation
This paper presents the measurement and analysis method for detection of the mm-wave signal at 96 GHz, by using a low-cost microbolometer infrared (IR) camera with 70 μm pixel-pitch optimized for detection in the 8-12 μm (LWIR) range. The mm-wave beam derived from a multiplied Schottky diode based source is detected within ~ 65 % of the whole area of a 160×120 pixel focal plane array microbolometer sensor. Under ~73 mW incident power, responsivity is measured as 7.3 V/W, and the average noise for the measurement is determined as 12 μV, which includes both detector and readout electronics contribution. From the measured parameters, the integrated Noise Equivalent Power (NEP) is calculated as 1.63 μW within the 7.8 kHz readout bandwidth. By using a simple setup, it is shown that a low-cost microbolometer camera which is designed for LWIR range can detect a distinct mm-wave beam at 96 GHz.
Modulation and frequency response of GDDs in the millimeter wave/THz region
N. Alasgarzade, T. Takan, I. Uzun-Kaymak, A. Sahin, H. Altan
KEYWORDS: Modulation, Sensors, Lamps, Diodes, Terahertz radiation, Neon, Signal detection, Dielectric polarization, Plasma, Lead
New methods are being developed for efficient detection of terahertz waves. While many detection techniques show promise their commercial development is still limited due to the overall complexity and cost of the imaging system. Using commercially available neon indicator lamps the interaction mechanism between the glow plasma and the millimeter / THz wave is investigated in detail as a function of the device speed, sensitivity to frequency and polarization of the light. A lock-in amplifier was used to measure the response up to 90kHz when the GDD was placed at the focus of a 113GHz center frequency reconfigured Dielectric Resonating Oscillator (DRO) driven multiplied Schottky diode source. In addition the polarization sensitivity of the GDD was tested for two different scenarios whereby rotating the GDD the detected signal is observed to agree well with Malus's Law for one particular orientation. Furthermore, the frequency dependent GDD-THz interactions are investigated using a 240-380 GHz tunable continuous wave radiation source. Employing both systems allow us to understand the response of GDDs with respect to modulation frequency, RF frequency and polarization orientation. Resonance effects, frequency sensitivity and geometrical structures of GDDs are studied for the purpose of obtaining better performance in THz-GDD interaction for applications including general THz wave detection and imaging.
Terahertz modulation using a bandpass filter combined with a graphene supercapacitor
Taylan Takan, Mehmet Nebioglu, Nurbek Kakenov, Osman Balci, Coskun Kocabas, Hakan Altan
KEYWORDS: Metamaterials, Modulation, Graphene, Wave propagation, Signal processing, Terahertz radiation, Aluminum, Terahertz technology, FSS based metamaterials, Bandpass filters
Graphene is proving to be an efficient medium for the control of mm-wave/THz radiation. Its electrical and dielectric properties allows it to be incorporated into various existing device architectures. One such application is in the modulation of the amplitude of the propagating THz radiation. Due to its electrical properties this interaction is typically broadband in nature. To make this frequency selective we propose the use of metamaterials or frequency selective surfaces. Generally, these structures perform the frequency filtering by modifying the propagation of the input wave with respect to changing structural parameters of the device itself. By fabricating a frequency selective surface based on a periodic circular hole array on an aluminum sheet we show that the transmission of a narrow band of THz radiation can be modulated when the sheet is combined with a highly efficient graphene based supercapacitor device. The modulation depth of the device was 15% in the frequency region of interest. The simple structure of the device coupled with the obtained performance shows that graphene based devices have great potential for the development of THz technologies.
Proceedings Article | 15 July 2015
Terahertz pulsed imaging study of dental caries
Burcu Karagoz, Hakan Altan, Kıvanç Kamburoglu
Proc. SPIE. 9542, Medical Laser Applications and Laser-Tissue Interactions VII
KEYWORDS: Beam splitters, Dental caries, Teeth, Reflection, Imaging systems, X-rays, 3D metrology, Terahertz radiation, 3D image processing
Current diagnostic techniques in dentistry rely predominantly on X-rays to monitor dental caries. Terahertz Pulsed Imaging (TPI) has great potential for medical applications since it is a nondestructive imaging method. It does not cause any ionization hazard on biological samples due to low energy of THz radiation. Even though it is strongly absorbed by water which exhibits very unique chemical and physical properties that contribute to strong interaction with THz radiation, teeth can still be investigated in three dimensions. Recent investigations suggest that this method can be used in the early identification of dental diseases and imperfections in the tooth structure without the hazards of using techniques which rely on x-rays. We constructed a continuous wave (CW) and time-domain reflection mode raster scan THz imaging system that enables us to investigate various teeth samples in two or three dimensions. The samples comprised of either slices of individual tooth samples or rows of teeth embedded in wax, and the imaging was done by scanning the sample across the focus of the THz beam. 2D images were generated by acquiring the intensity of the THz radiation at each pixel, while 3D images were generated by collecting the amplitude of the reflected signal at each pixel. After analyzing the measurements in both the spatial and frequency domains, the results suggest that the THz pulse is sensitive to variations in the structure of the samples that suggest that this method can be useful in detecting the presence of caries.
Compressive sensing imaging through a drywall barrier at sub-THz and THz frequencies in transmission and reflection modes
Taylan Takan, Vedat Özkan, Fırat İdikut, Ihsan Ozan Yildirim, Asaf Şahin, Hakan Altan
Proc. SPIE. 9244, Image and Signal Processing for Remote Sensing XX
KEYWORDS: Extremely high frequency, Imaging systems, Metals, Image resolution, Image quality, Image transmission, Diodes, Terahertz radiation, Reconstruction algorithms, Compressed sensing
In this work sub-terahertz imaging using Compressive Sensing (CS) techniques for targets placed behind a visibly opaque barrier is demonstrated both experimentally and theoretically. Using a multiplied Schottky diode based millimeter wave source working at 118 GHz, metal cutout targets were illuminated in both reflection and transmission configurations with and without barriers which were made out of drywall. In both modes the image is spatially discretized using laser machined, 10 × 10 pixel metal apertures to demonstrate the technique of compressive sensing. The images were collected by modulating the source and measuring the transmitted flux through the apertures using a Golay cell. Experimental results were compared to simulations of the expected transmission through the metal apertures. Image quality decreases as expected when going from the non-obscured transmission case to the obscured transmission case and finally to the obscured reflection case. However, in all instances the image appears below the Nyquist rate which demonstrates that this technique is a viable option for Through the Wall Reflection Imaging (TWRI) applications.
Image reconstruction and optimization using a terahertz scanned imaging system
İhsan Ozan Yıldırım, Vedat Özkan, Fırat İdikut, Taylan Takan, Asaf Şahin, Hakan Altan
Proc. SPIE. 9250, Electro-Optical Remote Sensing, Photonic Technologies, and Applications VIII; and Military Applications in Hyperspectral Imaging and High Spatial Resolution Sensing II
KEYWORDS: Transceivers, Transmitters, Point spread functions, Mirrors, Imaging systems, Metals, Image restoration, Receivers, Image quality, Terahertz radiation
Due to the limited number of array detection architectures in the millimeter wave to terahertz region of the electromagnetic spectrum, imaging schemes with scan architectures are typically employed. In these configurations the interplay between the frequencies used to illuminate the scene and the optics used play an important role in the quality of the formed image. Using a multiplied Schottky-diode based terahertz transceiver operating at 340 GHz, in a stand-off detection scheme; the effect of image quality of a metal target was assessed based on the scanning speed of the galvanometer mirrors as well as the optical system that was constructed. Background effects such as leakage on the receiver were minimized by conditioning the signal at the output of the transceiver. Then, the image of the target was simulated based on known parameters of the optical system and the measured images were compared to the simulation. By using an image quality index based on &chi;<sup>2</sup> algorithm the simulated and measured images were found to be in good agreement with a value of &chi;<sup>2</sup> = 0 .14. The measurements as shown here will aid in the future development of larger stand-off imaging systems that work in the terahertz frequency range.
Proceedings Article | 21 May 2014
Comparison of terahertz technologies for detection and identification of explosives
René Beigang, Sandra Biedron, Slawomir Dyjak, Frank Ellrich, Magnus Haakestad, Daniel Hübsch, Tolga Kartaloglu, Ekmel Ozbay, Frank Ospald, Norbert Palka, Uroš Puc, Elżbieta Czerwińska, Asaf Sahin, Aleksander Sešek, Janez Trontelj, Andrej Švigelj, Hakan Altan, Arthur van Rheenen, Michał Walczakowski
Proc. SPIE. 9102, Terahertz Physics, Devices, and Systems VIII: Advanced Applications in Industry and Defense
KEYWORDS: Electronics, Sensors, Spectroscopy, Crystals, Terahertz radiation, Antennas, Absorbance, Explosives, Spectroscopes, Absorption
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.
THz transmission and detection through glow discharge detectors
K. Çınar, H. Altan, A. B. Şahin
Proc. SPIE. 8716, Terahertz Physics, Devices, and Systems VII: Advanced Applications in Industry and Defense
KEYWORDS: Transmitters, Principal component analysis, Modulation, Sensors, Electrodes, Resistance, Ionization, Terahertz radiation, Terahertz spectroscopy, Plasma
The capability of low cost glow discharge detectors (GDDs) to detect terahertz (THz) radiation has drawn much attention recently. In order to employ them in applications such as THz imaging these studies have typically focused on the response of the GDD at specific frequencies. To better understand the spectral behavior of glow discharges, we have not only examined the response of the GDD at a specific frequency of 118 GHz, but also we examined the interaction mechanism of GDDs with THz radiation using terahertz time domain spectroscopy (THz-TDS) in a broader range of frequencies between 0.05 THz - 0.5 THz. These results show that in addition to THz induced oscillations in the plasma charge density, the structure of the GDD itself plays an important role in the detection mechanism as supported by the large response observed at a specific frequency. By increasing the bias voltage on the gap, not only is the transmission greatly reduced at this specific frequency, the results suggest that it can also be tuned. Furthermore, measurements done at 118GHz show that the GDD structure has a varying response dependent on the modulation frequency. With increasing bias voltage the reponsivity of the GDD also increases which supports previous measurements that the change in current through the plasma is due to the sub-mm wave radiation.
Proceedings Article | 8 May 2009
Mine detection using time-domain THz spectroscopy
H. Altan
Proc. SPIE. 7303, Detection and Sensing of Mines, Explosive Objects, and Obscured Targets XIV
KEYWORDS: Soil science, Scattering, Particles, Rayleigh scattering, Terahertz radiation, Mie scattering, Antennas, Mass attenuation coefficient, Land mines, Absorption
The use of broadband techniques in the Terahertz (THz) region, has allowed us to probe the effectiveness of far-infrared waves to assess the penetration depths required for anti-personnel mine detection. Using THz-Time Domain Spectroscopy (THz-TDS) based methods we have probed the effective penetration depth of broadband THz pulses through soil samples collected locally in Ankara, Turkey. It was found that certain frequencies within the spectral range from 0.01 to 1THz were severely attenuated by both absorption and scattering effects dependent on the geometry and size of the particles inside the soil samples. For the most extreme case, the rocky soil sample exhibited severe attenuation of the frequencies above 0.1 THz. The measurements were repeated for wetted soil samples as well. From these results we have gained insight into the boundaries imposed by soil conditions on the detection of mines buried near and far beneath the surface. We found that the effective penetration depth is strongly affected by scattering effects and the dominant scattering mechanism is explained by Mie scattering for the soil samples in consideration rather than Rayleigh scattering.
Proceedings Article | 27 February 2004
Characteristics of nanoscale composites by THz spectroscopy
Hakan Altan, Feng Huang, John Federici, Aidong Lan, Haim Grebel
Proc. SPIE. 5268, Chemical and Biological Standoff Detection
KEYWORDS: Semiconductors, Visible radiation, Data modeling, Quartz, Spectroscopy, Dielectrics, Silicon, Terahertz radiation, Picosecond phenomena, Absorption
We have conducted visible pump-THz probe experiments on single wall carbon nanotubes (SWCNTs) deposited on quartz substrates. Our results suggest that the photoexcited nanotubes exhibit localized transport due to Lorentz-type photo-induced localized states. These experiments were repeated for ion-implanted, 3-4nm Si nanoclusters in quartz for which a similar behavior was observed.
Characteristics of nanoscale composites by terahertz spectroscopy
Proc. SPIE. 5070, Terahertz for Military and Security Applications
KEYWORDS: Visible radiation, Silica, Quartz, Spectroscopy, Composites, Ions, Silicon, Terahertz radiation, Picosecond phenomena, Absorption
We have conducted visible pump-THz probe experiments on single wall carbon nanotubes (SWCNTs) on quartz substrates. Our results suggest an upper limit to the carrier-lifetime, which is on the order 1.5ps, limited only by the THz pulse duration. These experiments were repeated for ion-implanted, 3-4nm Si nanoclusters in quartz for which the carrier lifetime was also assessed at 1.5ps. THz time-domain spectroscopy (THz-TDS) of SWCNTs revealed that the THz pulse peak transmission changed under optical illumination.
Showing 5 of 20 publications
Conference Committee Involvement (5)
Millimetre Wave and Terahertz Sensors and Technology XII
9 September 2019 | Strasbourg, France
Millimetre Wave and Terahertz Sensors and Technology
10 September 2018 | Berlin, Germany
11 September 2017 | Warsaw, Poland
29 September 2016 | Edinburgh, United Kingdom
22 September 2015 | Toulouse, France
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