Taylan Takan

Graphene-enabled electrically controlled terahertz spatial light modulators

In this Letter, we demonstrate a broadband terahertz (THz) spatial light modulator using 5×5 arrays of large area graphene supercapacitors. Our approach relies on controlling spatial charge distribution on a passive matrix array of patterned graphene electrodes. By changing the voltage bias applied to the rows and columns, we were able to pattern the THz transmittance through the device with high modulation depth and low operation voltage. We anticipate that the simplicity of the device architecture with high contrast THz modulation over a broad spectral range could provide new tools for THz imaging and communication systems.

Observation of Gate-Tunable Coherent Perfect Absorption of Terahertz Radiation in Graphene

We report experimental observation of electrically-tunable coherent perfect absorption (CPA) of terahertz (THz) radiation in graphene. We develop a reflection-type tunable THz cavity formed by a large-area graphene layer, a metallic reflective electrode and an electrolytic medium in between. Ionic gating in the THz cavity allows us to tune the Fermi energy of graphene up to 1eV and to achieve critical coupling condition at 2.8 THz with absorption of 99%. With the enhanced THz absorption, we were able to measure the Fermi energy dependence of the transport scattering time of highly doped graphene. Furthermore, we demonstrate flexible active THz surfaces that yield large modulation in the THz reflectivity with low insertion losses. We anticipate that the gate-tunable CPA will lead efficient active THz optoelectronics applications.

Compressive sensing imaging through a drywall barrier at sub-THz and THz frequencies in transmission and reflection modes

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

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 χ2 algorithm the simulated and measured images were found to be in good agreement with a value of χ2 = 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.

Investigating Glow Discharge Detectors as a Millimeter-Wave/Terahertz Radiation Detection Tool

Compared to other forms of electromagnetic radiation, Terahertz (THz) radiation is considered safe for imaging and detection purposes due to their non-invasive and non-destructive nature. Novel techniques and methods are constantly being evaluated for efficient detection and measurements of THz waves, yet their commercial success is limited due to cost and overall complexity of these systems. Commercially available Glow Discharge Detectors (GDDs) are proven to detect microwave and higher frequency radiation. Although, the plasma generated inside the GDDs enables the detection of the EM radiation, the interaction mechanism is not fully understood. In this study, we investigate various types of GDDs that are commercially available, and characterize their overall behaviour in response to both CW and modulated EM radiation. In addition, the response from various GDDs to the mm-wave/THz radiation are surveyed for the range of 260–380 GHz. Measurements show that response signal at certain frequencies within the range are attenuated suggesting a resonance frequency possibly based on the electrode structure.

Compressive Sensing Imaging at Sub-THz Frequency in Transmission Mode

Due to lack of widespread array imaging techniques in the THz range, point detector applications coupled with spatial modulation schemes are being investigated using compressive sensing (CS) techniques. CS algorithms coupled with innovative spatial modulation schemes which allow the control of pixels on the image plane from which the light is focused onto single pixel THz detector has been shown to rapidly generate images of objects. Using a CS algorithm, the image of an object can be reconstructed rapidly. Using a multiplied Schottky diode based multiplied millimeter wave source working at 113 GHz, a metal cutout letter F, which served as the target was illuminated in transmission. The image is spatially discretized by laser machined, 10 × 10 pixel metal apertures to demonstrate the technique of spatial modulation coupled with compressive sensing. The image was reconstructed by modulating the source and measuring the transmitted flux through the metal apertures using a Golay cell. Experimental results were compared to reference image to assess reconstruction performance using χ2 index. It is shown that a satisfactory image is reconstructed below the Nyquist rate which demonstrates that after taking into account the light intensity distribution at the image plane, compressive sensing is an advantageous method to be employed for remote sensing with point detectors.

An indium tin oxide metasurface filter for terahertz applications: Design, fabrication, and characterization

In this paper, using a cross-shaped complementary Indium Tin Oxide (ITO)-based metasurface design, the transmission of THz radiation is shown to be filtered within the 3 dB level from maximum in the frequency range of interest ( ∼333 GHz). Various metasurface structures primarily composed of cross-shaped openings with a 400 micron unit cell size are patterned on top of 1750 micron thick fused silica substrates. They are patterned using UV lithography methods after the films were grown using DC sputtering. The fabricated structures were characterized using Terahertz Time Domain Spectroscopy (THz-TDS) measurement technique. The measured transmission agrees well with the simulation of the structure for four different samples with different geometries. These results suggest that metasurface and/or metamaterial patterns based on ITO in visibly transparent media can be utilized for filtering of frequencies in the long wavelength spectrum. These types of filters can be very useful in the near future for THz communication and security applications.

Electrically controlled terahertz spatial light modulators with graphene arrays

Gate-tunable high-mobility electrons on atomically thin graphene layers provide a unique opportunity to control electromagnetic waves in a very broad spectrum. In this paper, we describe an electrically-controlled multipixel terahertz light modulators. The spatial light modulator is fabricated using two large-area graphene layers grown by chemical vapor deposition and transferred on THz transparent and flexible substrates. Room temperature ionic liquid, inserted between the graphene, provides mutual gating between the graphene layers. We used passive matrix addressing to control local charge density thus the THz transmittance. With this device configuration, we were able to obtain 5×5 arrays of graphene modulator with 65% modulation between 0.1 to 1.5 THz.

340 GHz imaging system for detection of concealed threat objects at 5 meters stand-off distance (Conference Presentation)

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.

Transmission and detection of terahertz radiation in a weakly ionized plasma (Conference Presentation)

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.