Andrew D. Hellicar*a, Jia Dub, Stephen M. Hanhama, Li Lia ..... Humphreys R, Hirst P, Heath R, Elliner D, Parker N and Smith M A G 2004 âPassive mm-wave ...
Application of a high temperature superconducting detector to terahertz imaging Andrew D. Hellicar*a, Jia Dub, Stephen M. Hanhama, Li Lia CSIRO ICT Centre, PO Box 76, Epping, NSW 1710, Australia +61 2 9372 4100 b CSIRO MSE, PO Box 218, Lindfield, NSW 2070, Australia +61 2 9413 7641
a
ABSTRACT The performance of a high temperature superconducting junction detector is evaluated. The detector has been built to explore applications of terahertz imaging. The detector device is a high-temperature superconductor (HTS) Josephson junction, which is integrated with a thin-film ring-slot antenna. The ring-slot antenna is patterned on a magnesium oxide (MgO) substrate which is compatible with the detectors YBCO superconducting material lattice. A hyper-hemispherical lens made from high resistivity float zone silicon (HRFZ-Si) is mounted on the rear side of the substrate. The lens couples energy from an imaging system onto the antenna which couples the energy into the device. An existing terahertz imaging system is used in conjunction with the detector to allow for the exploration of relevant applications. The imaging system is based on a conventional quasi-optical design with a backward-wave oscillator as the source and raster scans samples for image acquisition. The imaging capability of the system has been assessed by trialing a range of applications in both transmission and reflection modes. Applications explored include imaging concealed weapons in packaging, non-destructive testing of materials, and imaging devices through plastic structures. The results generated by the imaging system demonstrate its capability for these applications. Keywords: Terahertz Imaging, HTS, Josephson Junctions
1. INTRODUCTION Although imaging at sub-mm wave (terahertz) frequencies has been the subject of exploration over many decades, the last decade has seen an explosion of activity in this area. This is partly due to the advent of new accessible laboratory based imaging technologies [1] and partly due to interest in new terahertz applications in the medical, security and nondestructive testing domains [2, 3]. In the security domain the ability of terahertz waves to penetrate through clothes has led to a number of groups targeting the application of concealed weapon detection. The approaches include both passive and active imaging. Passive imaging (similar to thermal imaging at infrared frequencies) has been achieved with electronic-based systems. As a result the more mature solutions operate at lower frequencies where electronic components are freely available. These systems are now pushing up in frequency from 94 GHz through 250 GHz to 800 GHz. The desire to move to higher frequencies is a result of the benefit of the larger energies radiated by bodies and the higher resolutions achievable for a specified aperture size. Because of the difficulties in building passive systems a number of groups are exploring a second approach: active imaging. Active imaging systems are attractive in the flexibility they present due to the wide range of transmitted waveforms and imaging configurations available, but suffer greatly from the specular reflection problem. Specular reflections at these frequencies often occur, greatly reducing the returned signal from surfaces on objects at angles away from normal incidence resulting in the inability to detect such objects. The detectors presented in this paper are designed for security applications whilst maintaining a flexible approach. The current security market is fluid and application areas in some cases are being solved rapidly or are opening up due to counter measures taken by those posing threats. The detectors presented here keep open the option of passive imaging in the future if detector sensitivity and optical coupling can be optimized. Secondly the frequency response of the detectors is broad-band, being mainly limited to the response of the antenna. Therefore device may be deployed in a narrow-band system at a variety of frequencies, alternative multi-band or broadband solutions are possible, future improvements in device design have the potential to allow coherent operation in the future. A further motivation for the detectors
presented here is one of differentiation. Existing technologies are well developed based on low temperature superconducting (LTS) [4, 5] and room temperature devices [6]. LTS devices need expensive and relatively large coolers to reach 4 K and in some cases sub-Kelvin temperatures. Devices operating at 77 K have the potential to achieve higher sensitivity than the room temperature devices [7, 8] but offer the obvious advantage of higher operating temperature than LTS devices. Such an operating temperature can be conveniently attained by a relatively cheap commercial cryocooler. The presented detector comprises a high temperature superconducting step-edge Josephson junction [9, 10]. The detector is integrated with a ring-slot antenna which couples terahertz from a substrate lens into the device. The detector response is limited by the antenna bandwidth which is approximately 20% centered around 600 GHz. A lens is used to match the antenna beam pattern with an optical imaging system and to eliminate inefficiencies caused by substrate modes. The detector has been tested in a transmission terahertz imaging system that images via raster scanning a sample. Detector parameters such as the responsivity are characterized.
2. HTS TERAHERTZ DETECTOR A HTS detector has been designed with the aim of operation at liquid nitrogen cooling temperatures. An earlier paper by the authors [11] presented a design which unfortunately had to be cooled to well below 77 K before responding to radiation at 600 GHz. Imaging results presented required liquid helium cooling and were very susceptible to vibration due to non-rigid mounting of the cryocooler on the optical table. Subsequently, a process of device parameter optimization has resulted in a device that operates at 77K. A more rigid method of mounting the cryo-cooler was employed and as a result better quality images have been produced.
Fig. 1. Left: Micrograph of a fabricated ring-slot antenna-coupled junction. Right: A close-up view of a YBCO step-edge grain boundary Josephson Junction Fig.1 shows a photograph of a fabricated antenna-coupled step-edge Josephson junction detector; left is gold thin-film ring-slot antenna and right is a scanning electron microscope image of the step-edge Josephson junction with a ~ 2 m 10 m YBCO microstrip across a step-edge on the MgO substrate, forming a grain boundary Josephson junction (the detecting element). The junction is fabricated using a standard photolithography and Ar-ion beam milling (IBE) techniques [9, 10]. The design has similarities with the design used by Qinetiq [12]; however, the operating frequency here is 600 GHz as opposed to 95 GHz. An important advantage of the step-edge junction is that the junction parameters (the junction critical current, Ic, and normal resistance, Rn) can be adjusted to some degree by varying the step-angle and height, ratio of the YBCO film to step-height, and junction width. Therefore, junction characteristic voltage, Vc IcRn and characteristic frequency, fc (2e/h)I cR n (where 2e/h = 0.4836 GHz/ V) can be tailored to the desired operation temperature and frequency.
A limited number of substrates are available for the superconducting device due to a requirement that the superconducting crystalline material (YBCO) have a lattice that matches the substrate. MgO is the lowest permittivity available that matches the YBCO lattice and therefore MgO has been used as the substrate (permittivity of ~ 10). The thickness of the substrate is 0.5mm. Conventional substrate antennas lie on a substrate material that is a fraction of a wavelength. Because of the small wavelengths at terahertz frequencies (in this case 0.5 mm in air) substrate modes will occur. Therefore to eliminate substrate modes a lens has been used to couple energy into the substrate. The lens (Fig. 2) is made from HRFZ-Si. The refractive indices of silicon and MgO are closely matched; therefore reflection losses are limited to the silicon lens-air interface. A future possibility is to use an antireflective coating to minimize reflection loses. A simulation of the currents excited on the antenna structure is shown in Fig. 3. The antenna is a modified version of the antenna presented in [11]. A smaller ground plane is employed which optimizes the antenna gain due to diffraction off the ground plane edge. A co-planar waveguide (CPW) allows the junction to be probed at DC and an RF choke minimises the effect of the CPW on the antennas radiation pattern by ensuring the resonant terahertz field is constrained to the circular slot. This can be seen when looking at the RF currents in Fig. 3 which decay rapidly along the CPW. The junction device is located across the ring-slot at the bottom of the circular slot in Fig. 3. Superconducting lines probe through the inner and top conductor of the CPW (faint double lines in Fig. 1) and allow for low serial resistance DC probing of the junction.
Fig. 2: Photo of the detector device including 3 DC probe lines silicon lens and MgO substrate.
Fig. 3: Excited currents on antenna when struck by incident plane wave.
3. DETECTOR OPERATION The detector DC current voltage response at ~ 80 K is shown in Fig. 4. The blue curve corresponds to the device operation when there is no terahertz incident on the device. The green curve illustrates how the DC-IV curve changes when the terahertz signal is turned on. The device is current biased with a current that maximizes the output voltage variation with incident terahertz power. The horizontal black line represents the current bias at ~500 A which intersects the THz off and on voltage curves at the output voltages of ~0 and 450 V respectively.
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To maximize the responsivity, the device is biased at the peak point of the dynamic or differential resistance, R d = dV/dI, where the resulting output voltage change is the maximum: V(P)= (dV/dI) I(P), where (dV/dI) is the differential resistance of the device (calculated from differentiating the no THz curve of Fig. 4) and I is the change in current occurring on application of a THz field with power P coupled into the junction (calculated from the difference between curves in Fig 4).
4. SYSTEM FOR TESTING DEVICE To test the device the system described in [11] was used. The system employs a backward wave oscillator from Elva-1 to generate a CW 600 GHz beam. A set of optical mirrors focus the beam onto the detector. The beam is chopped at approximately 800 Hz by a chopper wheel and a lock-in amplifier is used for detection. The system can be employed for imaging by raster scanning a sample through a stationary spot at the focus of mirrors M2 and M3 (see Fig 5).
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Fig. 5: Quasi optical configuration.
5. DETECTOR RESULTS The output from the device is passed through an amplifier with gain 100 and then digitized. The digitized signal on application of a chopped terahertz signal to the detector is shown in Fig. 6. It should be noted that a 20 slot chopper wheel has been used to chop the signal at approximately 800 Hz. The device voltage is settling during the chopping period and therefore the time constant of the detector is less than 1 mS. It should be noted the non-instantaneous transitions in the voltage response are not necessarily a result of the device time constant and therefore are not used to predict the time constant. This is because the THz beam covers a significant area as it hits the chopper wheel, and therefore the on/off switching of the THz beam itself is not instantaneous.
Fig. 6: Digitized amplified output of the detector on application of a chopped THz signal.
Figure 6 shows an output voltage level of ~ 45 mV, as expected 100x the signal level of 450uV in Fig. 4. The RMS noise voltage level is ~ 1 V, which gives a signal-to-noise ratio (SNR) of ~ 45,000 or 46 dB in the raw signal. From Fig. 4 (the DC IV curve with/without THz) Ic ~ 500uA. The change in current is I ~ 250uA. Therefore I /Ic = 0.5. We can solve for the power in the junction from the RF voltage in the device. The RF power in the junction is 1.1uW and the responsivity (S) is (dv/di) I /1.1uW: S ~800 V/W. This compares with the liquid helium cooled sensitivity of ~ 12000V/W [11] as the dynamic resistance Rd is much reduced for the I-V curve at 80K (or above 77K). The RMS noise voltage in Fig. 6 is 1uV which is limited by the room temperature electronics such as the lock-in amplifier. However the signal is not integrated, the sample time is ~ 10uS. The post detection bandwidth is therefore ~ 50 kHz. The NEP is then 5x10 -12. It should be noted this is an electrical NEP and does not include the effects of the optics. The detector was used as the detecting element in the system (Fig. 5). The resulting image is shown in Fig. 7.
Fig. 7: Left: Photo of a leaf. Right: Terahertz image of leaf taken with the superconducting device
6. CONCLUSION A HTS superconducting junction detector has been presented and characterized, and the electrical NEP presented. The presented detectors use planar integrated technology which makes it ideal for the next step which involves making closely packed arrays of junction devices with the result of improving the system frame rate and minimizing power wastage in an active system. The ability to make arrays with similar IcRn characteristic voltages to allow for operation at a single temperature will be tested. A further improvement will introduce a scanning element to allow for imaging of stationary targets.
ACKNOWLEDGEMENTS The authors would like to acknowledge Kieran Greene, John Macfarlane and Keith Leslie for advice and technical assistance.
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