Ivonne Escorcia Carranza, James Grant, and David R.S. Cumming. Microsystems Technology Group, School of Engineering, University of Glasgow, G12 8LT, ...
Terahertz Imaging using a Monolithic Metamaterial Based Detector Ivonne Escorcia Carranza, James Grant, and David R.S. Cumming Microsystems Technology Group, School of Engineering, University of Glasgow, G12 8LT, UK Abstract—A terahertz detector composed of a metamaterial absorber and micro-bolometer sensor integrated in a standard CMOS process is presented. The prototype demonstrates an innovative, uncooled, low cost, compact terahertz detector that is readily scaleable to high resolution focal plane array formats. The detector imaging capability is demonstrated in a transmission mode experiment.
I. INTRODUCTION
T
he unique characteristics of terahertz (THz) technology have led to the development of advanced imaging systems. The ability of THz waves to penetrate nonconductive materials coupled with its non-ionizing characteristics make it a promising candidate for a variety of applications ranging from airport security, non-invasive package inspection and medical imaging [1], [2]. Current THz imaging technologies present major limitations such as bulky equipment, expensive components, incompatibility with CMOS process and/or necessity for cryogenic cooling of THz sources and detectors [3], [4]. THz imaging has also been hindered by the lack of available materials able to absorb THz radiation. Therefore, we have introduced the use of metamaterials (MMs) in our prototype due to their ability to absorb THz radiation and their easy integration in a CMOS process. The integration of MMs with a sensor demonstrates a new technique towards THz imaging that focuses on the development of a monolithic THz detector integrated in a standard CMOS process targeting single THz frequencies. II.
DESIGN AND CHARACTERIZATION
A 3D schematic diagram of one pixel of our MM based THz detector is shown in Fig. 1a. It is composed of a metamaterial absorber coupled with a vanadium oxide (VOx) microbolometer sensor positioned directly above. To benefit from the commercial availability of the CMOS technology, the already available metallic and insulating layers were used to design the MM absorber. Three electric ring resonators (ERRs), formed in M4, M5 and M6, a continuous ground
Fig. 1. a) THz pixel detector 3D schematic diagram showing the VOx microbolometer, MM structure and ground plane. The Ti/Al tracks connect the micro-bolometer sensor to the electronics. b) Scanning electron micrograph of a section of the post-processed array showing the VOx sensors placed above the MM periodic structure.
plane (M3), and the inter-metal dielectric layers comprise a
broadband metamaterial absorber. MM based absorption is inherently narrowband (FWHM typically less than 20% the centre frequency) and presents a significant challenge when designing a MM device to operate at a specific frequency. In our work we use a CO2 laser pumped methanol vapor laser operating at 2.52 THz. We therefore require the MM device to strongly absorb the same frequency. To maximize the likelihood of attaining strong absorption in the region of 2.5 THz, three stacked ERRs are employed to broaden the absorption spectrum. Owing to the periodic structure of the pixel, it is readily scaled to large format high resolution focal plane arrays as shown in Fig. 1b. III. RESULTS Fig. 2a shows the simulated spectral characteristics for the MM absorber. There is a broad absorption peak of magnitude 88% at 2.65 THz while at 2.5 THz, our frequency of interest, the absorption magnitude is 50%. Complete post-processed devices were experimentally characterized under vacuum in a Bruker IFS 66v/S Fourier Transform Infrared Spectrometer. The experimental spectrum has two resonance peaks at 2.5 THz and 2.81 THz of magnitude 57% and 70% respectively. Overall there is reasonable agreement between the experimental and simulation spectra. The deviation between the curves can be attributed to several factors; the most significant two are the possibility of a discrepancy in the real and assumed value of the inter-metal dielectric refractive index and potential thickness non-uniformity of the three inter-metal dielectric layers. Figure 2b depicts the angular dependence of the metamaterial based THz detector for polar incident angles ranging from 0 to 60º. There is little change in the absorption characteristics for incident angles up to 50º, with a maximum absorption magnitude of 88% occurring at 2.65 THz. As a practical demonstration of the capability of the monolithic resonant detector we employed it in a Nipkow based 2.5 THz transmission imaging experiment. The Nipkow disk has been used for imaging systems since the first television was invented in 1929 [5]. The experimental set-up is shown in Fig. 3a and is based on a similar approach described by Ma et al [6]. However, in the present work the apertures in the disk were replaced by Fresnel lenses fabricated in a 100 mm silicon wafer. Each lens is a 16-level binary phase shifting diffractive optic with a diameter of 10 mm and a focal length of 5.5 cm. The lenses in the disk are positioned in the form of a single-turn spiral starting from an external radial point of the disk and proceeding to the center. The Fresnel lens is scanned across the incident 15 mm diameter Gaussian laser beam, sampling a small portion as it moves. The sampled portion is focused into a diameter of 0.7 mm, onto the object. As the disk rotates, the same Fresnel lens travels in a spiral pattern and interrogates a different portion of the object, giving rise to a line scan.
Fig. 3. a) Experimental set up for the imaging system based on a Nipkow disk. b) Optical image of the cut out “T” shape in aluminum used in the THz imaging experiment. c) THz image taken using the monolithic metamaterial based THz detector.
IV. CONCLUSION
Fig. 2. a) Simulated and experimental absorption spectra of the monolithic metamaterial based THz detector. b) Simulated spectral absorption response as a function of incident angle.
Each lens in turn contributes to a new line-scan hence a 2D image is created. When an object is positioned at the image plane, each focused beam behaves as a probe and scans the object sequentially in both the x (radial) and y (rotational) directions. Only 1 lens samples the incident 15 mm beam at any time. In our experiment the object is a 10 mm x 10 mm T shape cut out of a sheet of aluminum (see Fig. 3b). The transmitted beam is reflected and re-focused by a 90º off axis parabolic mirror onto the monolithic resonant THz detector. A mechanical chopper is placed in front of the detector to modulate the THz beam. The same current bias and lock-in detection method, as before, is used. Both the lock-in signal and the timing signal are monitored on a digital oscilloscope. A Labview (National Instruments) program is used to acquire data from the oscilloscope, process the raw data and consequently build a 2D image using the processed data. Fig. 3c shows the captured image using the monolithic resonant THz detector for a bias current of 450 nA, modulation frequency of 1 Hz, lock-in amplifier time constant of 300 ms and a disk rotation speed of 0.2 rpm.
We have designed, simulated and characterized a monolithic resonant terahertz detector composed of a metamaterial absorber and a micro-bolometer sensor element. The broadband metamaterials absorber is realized directly in the layers of a standard 0.18 µm CMOS process and the micro-bolometer sensor element is defined by post-processing procedures. We have demonstrated the capability of the monolithic resonant THz detector in a practical 2.5 THz imaging experiment based on a Nipkow disk. Furthermore, MM absorbers are extremely flexible in that their operating frequency can be scaled simply by modifying the unit cell size, ERR dimensions and insulating layer thickness between the ERR and the ground plane. Our new technique to detect THz radiation and the MM natural scalability to large array formats presents a major advance towards the production of compact, cost-effective and real-time THz imaging systems. REFERENCES [1]. J. F. Federici et al., "THz imaging and sensing for security applicationsexplosives, weapons and drugs," Semiconductor Science and Technology, vol. 20, pp. S266–S280, 2005. [2]. K. Humphreys et al., "Medical applications of terahertz imaging: a review of current technology and potential applications in biomedical engineering.," Conference Proceedings: IEEE Engineering in Medicine and Biology Society, vol.2, pp. 1302–1305, 2004. [3]. A.G. Davies et al., “The development of terahertz sources and their applications,” Physics in Medicine and Biology, vol. 47, pp. 3679-3689, 2002. [4]. X.-C. Zhang, "Terahertz wave imaging: horizons and hurdles," Physics in Medicine and Biology, vol. 47, pp. 3667–3677, 2002. [5]. J. L. Baird, “Apparatus for transmitting views or images to a distance.” U.S. Patent 1699270 A, Jan 15, 1929. [6]. Y. Ma et al., “Terahertz single pixel imaging based on a Nipkow disk,” Optics Letters, vol. 37, pp. 1484-1486, 2012.
