Computational Optical Sensing and Processing
Laboratory

TeraHertz focal-plane detection and imaging group

Staff

Leader: Péter Földesy, PhD
Researchers: Ákos Zarándy, PhD
PhD students: Domonkos Gergely, Zoltán Kárász, László Kozák, Fuzy Csaba, and several BSc and MSc students

Research topic and mission

The THz and mmWave electronics and imaging is a relatively novel field of the electronics. The classic CMOS technology is capable of detecting sub-THz radiation well above its cut-off frequency. The phenomena behind is the so-called electron plasmawave detection. Our mission is to combine our laboratory's and partner's focal-plane processor design and application background with this unique area.

To utilize its potential in material inspection, skin diagnostics, resolution enhancement, CT, we have built a laboratory environment and developed several characterization and imaging ASICs. We have created several variants in test chips, of which characterization serves for generalized model and optimization in collaboration Computational Fluid Dynamics and Material Scientists groups.

Current enhanced detection

A recent direction is to investigate the current bias effect on FET detection and sensitivity. An Applied Physics Letters article is accepted and under final review process and a simple consequence of the model is presented in an Optics Letters article already (see below the publication list). It seems that the current flow does not affect the sensitivity at all. Rather, the biasing sets a different inversion charge density on the drain and source terminals, which leads to the same situation that was found at non biased cases on the two terminals with similar source-gate or gate-drain voltage.

A 360 GHz polarization image taken by a single MOSFET detector. Two polarization sensitive antennas are connected to the source and the drain. By driving the FET into saturation, and swapping the direction of current, the two antenna signal is separately received by the single FET.

THz imager

As a step further in CW sub-THz imaging, we have designed a complex 90 nm TSMC technology SoC chip including per pixel signal conversion and processing cores. The sensor-processor array contains four by three sensors with various antenna shapes, direction, and polarization properties. Each sensor pixel embeds an ADC, lock-in amplification, and finally a strandard SPI protocol output. The combined sensitivity of the detectors covers 250-700 GHz with average 5 kV/W and 180 kV/W peak response at 360 GHz with NEP=40 pW/Hz-1. The highest response of the H shaped dipole is more than 3000 kV/W and 200 pW/Hz-1 NEP with around 50-80 nAmp source-drain current and 0.2V gate-source voltage at 2KHz modulation frequency.

Microphoto of two sensors, the bowtie has 550 GHz peak and the H shape dipole at 360 GHz with MV/W sensitivity

Another array of detector and antenna pairs, resonating near 500 GHz with 50 kV/W responsivity. The array is used now for compressed sensing imaging and waveform detection.

Homodyne interferometry

A single-shot quadrature phase-shifting interferometry architecture and optical arrangement have been developed. It is applicable to any antenna coupled detector technologies. The method is based on orthogonally polarized object and reference beams and on linear and circular polarization sensitive antennas in space-division multiplexing. The solution is patented as well. The technique can be adapted to two, three, four-step and Gabor holography recordings. It is also demonstrated that the space-division multiplexing does not necessary causes sparse sampling.

Here a two-step phase shifting reconstruction results are given at 360GHz.

Photograph of the PMMA equiconvex lens illustrating the raster scanned area of 30 by 30 mm (a), in phase interferogram captured by the bow-tie antenna detector (b), quadrature interferogram captured by the spiral antenna detector (c), reconstructed phase image (d), unwrapped phase image (e), pseudo 3D view of the estimated OPD (f).

Skin cancer trials

Water has very high absorption coefficient in the covered frequency range. Hence, in reflective setup water concentration changes can be registered confidently. Cancerous skin areas often show increased water concentration (e.g. in the case of superficial basal cell carcinoma, squamous cell carcinoma). In vivo skin water concentration changes person by person: according to skin type, sustained stress and actual status (daily biorhythm). Moreover skin structure (thickness of different layers) changes according to skin area (placement on the body) as well. For that reason local contrast of absorption intensity can be used to distinguish between healthy and diseased skin areas. In vivo measurements will be carried out involving both diseased tissue and its broader surroundings.

THz laboratory

The quasi optical setup below shows focusing case with fiber coupled laser positioning aid. The mmWave and the visible light is mixed with a proper dichronic mirror. The CW source's capabilities is illustrated as well.

The main sub-THz source is a VDI product includes a WR9.0 amplifier multiplier chain (AMC) with a YIG oscillator to drive it and several multipliers to extend into the WR4.3, WR2.8, and WR2.2, WR1.2 waveguide bands (80...750 GHz). The source is CW, has electronic modulation and attenuation.

The setup is mounted on an optical table with several positioners, off-axis mirrors, X-Z scanning table, connected to PCs by data acquision cards, etc.

A typical quasi-optical setup with fiber coupled laser positioning aid during characterization of a detector array.

THz space light modulation

Digital microfluidics chip as THz space light modulation (SLM). A digital microfluidics chip has been designed and manufactured in the MTA-MFA (it is covered by passivation and teflon hidrofobic layers). The motivation for this chip is that the image formation compressed sensing method requires masked light projection to the sensor(s). In exotic wavelength, like THz range, there is no real solution for this – apart from the trivial scanned mirrors. Using the digital microfluidics, droplets of absorbing and/or reflecting fluids can be circulated performing as radiation masking.

Visual and raster scan of two droplets in the microfluidic spatial light modulator array at 0.48 THz (lambda=620um) by focused irradiation.

Prototype of the THz space light modulator

Other designs

Several 180 nm UMC CMOS triple-well technology prototype of several test structures are designed and manufactured. One published in ISCAS 2012 has a low responsibility the range of 20-30 V/W @ 400 GHz for CW radiation, and about 20 mA/W for fsec pulsed radiation, but its distinguishing feature is the variety of formerly unpresented detector arrangements of three serial gates. Another array of serially connected 16 sensors is under manufacturing to fill up the focal plane spot size with high sensitity detectors.

As a VLSI design team we run several other projects as well. A few from our latest designs:

  • MITLL 3D process with 3 silicon layer, a true 3D technology. Complex navigation aid vision system on a chip.
  • Vision chips with various capabilities based on Silicon and InGaAs detectors.
  • Time-to-digital converter (student project) with about 30 psec resolution, based on two propagating binary wave collosion.

Publications

  • P. Foldesy, "Current steering detection scheme of three terminal antenna-coupled terahertz field effect transistor detectors" Optics Letters, Vol. 38, Issue 15, pp. 2804-2806 (2013) paper [This paper was published in Optics Letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: [http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-38-15-2804]. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.]
  • P. Foldesy, "Terahertz single-shot quadrature phase-shifting interferometry" Optics Letters, Vol. 37, Issue 19, pp. 4044-4046 (2012) paper [This paper was published in Optics Letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: [http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-37-19-4044]. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.]
  • P. Foldesy, Z. Fekete, T. Pardy, D. Gergelyi, "Terahertz Spatial Light Modulator with Digital Microfluidic Array", 26th Eurosensors Conference, September 9-12, 2012, Kraków, appears in Elsevier Journal of Procedia Engineering, DOI: 10.1016-j.proeng.2012.09.307 paper
  • P. Foldesy, D. Gergelyi, C. Fuzy, and G. Karolyi. "Test and configuration architecture of a sub-thz cmos detector array", 2012 IEEE 15th Symposium on Design and Diagnostics of Electronic Circuits and Systems (DDECS ) pages 101-104, April 18-20, 2012, Tallinn, Estonia. paper
  • P. Foldesy, "Array of serially connected silicon CMOS sub-terahertz detectors per pixel architecture", 3rd EOS Topical Meeting on Terahertz Science and Technology (TST 2012), 17-20 June 2012, Prague abstract, poster
  • P. Foldesy, "Characterization of silicon field effect transistor sub-THz detectors for imaging systems", IEEE International Symposium on Circuits and Systems (ISCAS 2012), May 20-23 2012, Seoul, Korea paper
  • P. Foldesy, A. Zarandy, "Integrated CMOS sub-THz imager array", 13th International Workshop on Cellular Nanoscale Networks and their Applications (CNNA 2012 ), August 29-31 Turin, Italy (accepted)

Infrastructure

  • VDI CW sub-THz source + Ericsson Power meter
  • Quasi optical setup – Thorlabs, Edmund optics, etc.
  • DLP® DiscoveryTM 4000 Starter Kit Board .7 XGA (VIS), 1024 x 768 micro mirrors, 13.6 µm pitch
  • Cadence, Mentor Graphics, Xilinx complete CAD tool flows
  • Application server with project centric version control system

Partners

Supporting grants

The Hungarian RD grant: KTIA-OTKA-77564 of 600 k$. We lead the consortium in which several Hungarian institutions are involved (PTE, MTA-MFA, BMGE). The goal is to develop a THz range imager including sensors, processing elements and imaging setup.

MTA-SzTAKI internal grant lead by Ákos Zarándy, goal: multispectral imaging system