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ISSN 1063 7834, Physics of the Solid State, 2010, Vol. 52, No. 11, pp. 2241­2245. © Pleiades Publishing, Ltd., 2010. Original Russian Text © V.L. Vaks, V.Yu. Balakirev, A.N. Panin, S.I. Pripolzin, V.P. Koshelets, O.S. Kiselev, 2010, published in Fizika Tverdogo Tela, 2010, Vol. 52, No. 11, pp. 2100­2103.

PROCEEDINGS OF THE XIV INTERNATIONAL SYMPOSIUM "NANOPHYSICS AND NANOELECTRONICS 2010"
(Nizhni Novgorod, Russia, March 15­19, 2010)

METALS AND SUPERCONDUCTORS

Development of the Physical Principles of the Design and Implementation of a 500­700 GHz Spectrometer with a Superconducting Integrated Receiver
V. L. Vaksa, V. Yu. Balakireva, A. N. Panina, S. I. Pripolzina, V. P. Kosheletsb, and O. S. Kiselevb
b

Institute for Physics of Microstructures, Russian Academy of Sciences, ul. Ul'yanova 46, Nizhni Novgorod, 603950 Russia Kotel'nikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, ul. Mokhovaya 11, building 7, Moscow, 125009 Russia e mail: elena@ipm.spi nnov.ru

a

Abstract--A spectrometer based on the effect of freely decaying polarization in the frequency range 500­ 700 GHz has been designed. Radiation sources are harmonics from a quantum semiconductor superlattice frequency multiplier. The receiving system of this spectrometer is constructed using a superconducting inte grated receiver based on a superconductor­insulator­superconductor mixer and a flux­flow oscillator oper ating as a heterodyne oscillator. The spectrometer has been used to measure absorption lines of NH3 in a sam ple of expired air (572 GHz). DOI: 10.1134/S1063783410110041

The terahertz (THz) and sub terahertz (sub THz) frequency ranges are very attractive for the use in many spectroscopic and, primarily, precise analytical inves tigations, because it is in these frequency ranges that the highest intensity absorption lines of many impor tant materials have been observed. In this respect, the implementation of a high sensitivity spectrometer that is suitable for the control over high tech pro cesses, the use in medical diagnostics, and the design of safety related systems seems to be a very important and topical problem. Nowadays, there exists only one commercial spec trometer of this class. The Microtech Instruments Inc. (the United States) has developed a spectrometer operating in the sub THz frequency range and employing a frequency multiplied backward wave oscillator, which covers the range from 100 to 1500 GHz. In this instrument, a standard pyroelectric detector is used as the receiving system. The spec trometer operates with a spectral resolution of 1­ 10 MHz and a sensitivity of the order of 5 â 10­7. These characteristics are sufficient for solving a num ber of spectroscopic problems; however, there exist applications that require a higher frequency resolution and a higher sensitivity of the analysis. First and fore most, the case in point is the analysis of multicompo nent gas mixtures in which the concentration of indi vidual components can be at the level of ppb or even

ppt. As an example of such problems, we should note the determination of impurities in high purity materi als, detection of toxic gases in the ambient air, moni toring of the processes occurring in chemical reactors, etc. Among the existing methods of gas analysis, the best approximation to the theoretical threshold of sen sitivity and the good frequency resolution limited only by the Doppler effect are provided by nonstationary microwave spectroscopy based on the effect of coher ent spontaneous radiation [1­3]. The applicability of these methods in the sub THz frequency range has become possible owing to the use of quantum semi conductor superlattice (QSSL) mixers and multipliers [4]. It has been demonstrated that these structures are more effective for the frequency conversion [4], because, in this case, as compared to Schottk diodes, the inertia of an electron transit through the active region and the parasitic capacitance become smaller, which makes it possible to increase the boundary oper ating frequency of the diode. Moreover, the quantum semiconductor superlattice has a current­voltage characteristic with a negative differential conductivity, which is retained up to frequencies above 1 THz. The QSSL mixers were used in the design of a new family of frequency synthesizers operating in the ranges 667­ 857, 789­968, and 882­1100 GHz [5], as well as a solid state harmonic generator based on a Gunn gen

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VAKS et al.

Solid state synthesizer (phase stable generator 89 ­117 GHz)

THz QSSL multi plier Cell Integrated receiver

Fig. 1. Schematic diagram of the terahertz frequency spec trometer with a superconducting integrated receiver.

erator operating at frequencies up to 8.1 THz. Com pared to the existing sub THz sources produced by Microtech Instruments Inc., the harmonic generators are compact in form and simple in operation and have a longer service life. Among the existing receiving systems, which oper ate in the sub THz range, the most suitable version for the use in the design of a high sensitivity spectrometer is the receiver based on a superconductor­insulator­ superconductor (SIS) mixer and a flux­flow oscillator (FFO) employed as a heterodyne oscillator [6­9]. The sensitivity of this receiver, which is close to the quan tum limit, is several orders of magnitude higher than the sensitivity of the existing receivers (piezoelectric sensors, thermocouples, Schottky diode detectors). The superconducting integrated receiver (SIR) also has a number of advantages, such as the compact form, the wide range of FFO frequency tuning, and the low energy consumption [10, 11]. At present, the frequency range of the majority of heterodyne receivers is limited by the heterodyne fre quency tuning with typical values of 10­15% for a cir
Fine frequency step control

cuit of solid state multipliers [12]. The bandwidth in the SIR is determined by a tunable structure of the SIS mixer and the matching circuit between the SIS and the FFO. A bandwidth up to 30­40% can be achieved by the design integration of a pair of SIS mixers. Another potential advantage is the use of a lattice of SIR channels inside one cryostat, which can operate at the same frequency or at different frequencies of the heterodyne oscillator. In this paper, we present a spectrometer with phase manipulation of the exposing radiation and a super conducting integrated receiver operating in the fre quency range 500­700 GHz. The physical principle of the spectrometer operation is as follows: the interac tion of frequency modulated radiation with reso nantly absorbing molecules results in a periodic pro cess of induction and decay of macroscopic polariza tion of the molecules [1]. A signal reemitted by the molecules lags behind the emission signal in phase, and this effect is used to receive a desired signal. The simplified schematic diagram of the spectrom eter is presented in Fig. 1. A signal from a reference generator is multiplied with the use of solid state devices, i.e., quantum semiconductor superlattices. In the proposed scheme, gas molecules interact with a resonant THz harmonic. The high Q cell can be used to increase the power of the resonant mode and to sup press the other modes. Radiation sources in the THz range are the har monics (up to 54th harmonic in the vicinity of 8100 GHz) obtained by multiplying the frequency of a synthesizer based on a Gunn generator operating in the frequency modulation mode in the frequency range 100­120 GHz with the use of a QSSL multi plier.

Coarse frequency step control

Reference generator 400 ­ 440 MHz

Reference synthesizer 9­10.5 GHz

FM modulator Synchronous to receiver

FPD

IF amplifier

Harmonic mixer

Gunn generator with varicap

Frequency multiplier Output

PLL filter

Frequency and amplitude modulation control

Fig. 2. Schematic diagram of the solid state frequency synthesizer. PHYSICS OF THE SOLID STATE Vol. 52 No. 11 2010


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The schematic diagram of the solid state frequency synthesizer with radiation frequency modulation is presented in Fig. 2. The harmonic of a reference coarse frequency step synthesizer operating in the frequency range 9.0­10.5 is mixed on a harmonic mixer with a part of the separated power of the output signal from the Gunn generator. The intermediate frequency (IF) signal in the range 400­440 MHz is amplified and fed to a frequency phase detector (FPD) in the phase locked loop (PLL) system. The reference synthesizer for FPD is the fine frequency step synthesizer operating in the range 400­440 MHz with a minimum frequency step of 10 kHz. The signal from the FPD output passes through a PLL filter and is fed to the frequency control input of the Gunn gen erator, thus closing the PLL system. The frequency modulator sets the frequency and deviation of the reference IF generator, which then are transferred to the output signal of the Gunn generator. The PLL band is chosen so as to transfer the frequency modulation from the IF channel to the output signal without a distortion. All parameters of the solid state synthesizer, namely, the radiation frequency and mod ulation parameters, are set from a computer through a microcontroller. The receiving system of the spectrometer is designed using a superconducting integrated receiver based on a SIS mixer and an FFO operating as a het erodyne oscillator. The schematic diagram of the SIR microchip (4.0 â 4.0 â 0.5 mm in size) designed at the Kotel'nikov Institute of Radio Engineering and Elec tronics of the Russian Academy of Sciences (Moscow, Russia) is presented in Fig. 3. The frequency resolu tion of the receiver (along with the noise temperature and directional pattern) is one of the main parameters of the spectrometer. In order to obtained the required frequency resolution, the superconducting hetero dyne oscillator of the integrated receiver should be synchronized to the reference synthesizer. For these requirements to be satisfied, we developed the concept of an integrated receiver with the cryogenic PLL sys tem of the heterodyne oscillator [13]. According to this concept, a signal from the superconducting het erodyne oscillator is distributed between two SIS mix ers, one of which is used as a receiving quasiparticle element and the other operates as a harmonic mixer in the PLL system. The integrated receiver operates under the follow ing conditions: the frequency range is 500­700 GHz, the noise temperature is lower than 200 K, the IF band is 4­8 GHz, the directional pattern with side lobes is at the level of less than ­17 dB, and the spectral reso lution is 1 MHz. Compared to the existing systems with close parameters, the proposed spectrometer is characterized by a wider range of input frequencies, smaller dimensions, and a lower energy consumption. Figure 4 shows the schematic diagram of the IF processor and the data collection system with a mixer
PHYSICS OF THE SOLID STATE Vol. 52 No. 11

4 K Dewar 5 6 7 4­8 GHz 8 5 0.1­1 GHz PLL 400 MHz

2 SIR chip 1 3 4

20 GHz 9 Reference sinthesizer

Fig. 3. Schematic diagram of the superconducting inte grated receiver (SIR) chip in a Dewar vessel (indicated by the dotted line) at a temperature of 4 K: (1) optical input, 500­700 GHz; (2) superconductor­insulator­supercon ductor (SIS) mixer; (3) harmonic mixer (HM); (4) flux­ flow oscillator (FFO) operating as a heterodyne oscillator, 500­700 GHz; (5) high electron mobility transistor (HEMT) amplifiers; (6) amplifier; (7) intermediate fre quency (IF) processor and digital to analog converter (DAC); (8) control and data processing system; and (9) FFO, SIS, and HM control system.

in which the output signal in the range 4­8 GHz is mixed with a reference signal at 3.6 GHz. The IF sig nal passes through a 0.4­0.8 GHz bandpass filter and is fed to a Schottky barrier diode detector. The separated modulation signal is amplified by a video frequency amplifier and fed to the input of the phase locked detector. For this detector, the reference signal is the phase controlled signal from the fre quency modulator. The phase control makes it possible to separate the largest amplitude of the desired signal and to reduce interfering signals due to both the interference in the THz channel and irregularities in the IF channels. After passing the low pass filter, the desired signal is digitized using a 16 bit digital to analog converter (DAC), where the preliminary storage of signals takes place. The further storage is performed with software in the computer. The performance of the spectrometer was tested in measurements of absorption spectra of a number of molecules. In particular, measurements were per formed in samples of expired air at a frequency of the absorption line of ammonia (572 GHz), because, spectroscopically, the expired air represents a multi component gas mixture. The cell had the form of a glass tube 10 cm in diameter and 60 cm long with opti cally transparent flat windows at the ends. The large
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2244 4 ­ 8 GHz Mixer IF filter 0.4 ­ 0.8 GHz

VAKS et al. Detector

Reference generator 3.6 GHz

Video amplifier

Controllable phaser Synchronous input from FM modulator

Synchronous phase detector

1/100 proportion. As a result, the gas concentration in the calibrated sample was equal to 10­4 mole fractions. For a more accurate calibration, it is necessary to pre pare precision calibration mixtures and to check them using other methods. Figure 5 shows the writing of the absorption line of NH3 in the sample of expired air. According to the above calibration, the NH3 concen tration in the expired air can be measured to a certain accuracy with the aim of subsequent medical diagnos tics. Thus, the performed test measurements of rota tional spectra of a number of basic molecules have confirmed the high sensitivity of the instrument (no worse than 1 ppb) with a spectral resolution limited only by the Doppler effect. In conclusion, we note that, in this paper, the pos sibility of designing a laboratory purpose sub THz spectrometer based on a harmonic generator and a superconducting integrated receiver with a quantum sensitivity has been demonstrated for the first time. The characteristics of this spectrometer satisfy the requirements of the precision gas analysis. ACKNOWLEDGMENTS

Low pass filter

16 bit DAC Record and storage of signals
Fig. 4. Schematic diagram of the IF processor and data collection system.

550 540 530 mV 520 510 500 490 572487.0 572491.5 572496.0 572500.5 572505.0 MHz
Fig. 5. Writing of the absorption line of NH3 at a frequency of 572 GHz in the sample of expired air.

This study was supported by the Russian Founda tion for Basic Research (project nos. 09 02 97039 r_povolzh'e_a, 09 02 00246, and 09 02 12172 ofi m) and the Presidium of the Russian Academy of Sci ences within the framework of the program "Princi ples of Basic Research in Nanotechnologies and Nanomaterials," the project "Quantum Coherent Nanostructures for Detection and Generation of Electromagnetic Radiation in the Terahertz Range." REFERENCES
1. V. L. Vaks, V. V. Khodos, and E. V. Spivak, Rev. Sci. Instrum. 70 (8), 3447 (1999). 2. V. L. Vaks, A. B. Brailovsky, and V. V. Khodos, Infrared Millimeter Waves 20 (5), 883 (1999). 3. V. V. Khodos, D. A. Ryndyk, and V. L. Vaks, Eur. Phys.: J. Appl. Phys. 25, 203 (2004). 4. V. L. Vaks, Yu. I. Koshurinov, D. G. Pavel'ev, and A. N. Panin, Izv. Vyssh. Uchebn. Zaved., Radiofiz. 48 (10­11), 933 (2005). 5. V. L. Vaks, A. Illiyuk, A. Panin, S. Pripolsin, S. Basov, and D. Paveliev, in Proceedings of the International Con ference "European Microwave Week," Munich, Ger many, 2007 (Munich, 2007). 6. V. P. Koshelets, S. V. Shitov, L. V. Filippenko, A. M. Ba ryshev, H. Golstein, T. de Graauw, W. Luinge, H. Scha effer, and H. van de Stadt, Appl. Phys. Lett. 68, 1273 (1996). 7. V. P. Koshelets and S. V. Shitov, Supercond. Sci. Tech nol. 13, R53 (2000).
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diameter of the cell made it possible to considerably decrease the influence exerted on the overall result of the measurements by gas molecules deposited onto the walls of the cell. The diameter of the radiation beam passing through the central axis of the cell did not exceed 3 cm. The products of gas exchange with the cell walls did not penetrate into this beam. The mea surements were carried out in the regime of continu ous circulation of the gas under investigation at a con stant pressure of ~3 â 10­2 Torr attained in the cell. The calibration was performed using a 1% ammonia solution, which was additionally diluted in a

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DEVELOPMENT OF THE PHYSICAL PRINCIPLES 8. P. Yagoubov, R. Hoogeveen, M. Torgashin, A. Khud chenko, V. Koshelets, N. Suttiwong, G. Wagner, and M. Birk, in Proceedings of the 17th International Sympo sium on Space Terahertz Technology (ISSTT), Paris, France, 2006 (Paris, 2006), p. 338. 9. V. P. Koshelets, A. B. Ermakov, L. V. Filippenko, A. Khudchenko, O. S. Kiselev, A. S. Sobolev, M. Yu. Torgashin, P. A. Yagoubov, R. W. M. Hoogev een, and W. Wild, IEEE Trans. Appl. Supercond. 17, 336 (2007). 10. T. Nagatsuma, K. Enpuku, K. Sueoka, K. Yoshida, and F. Irie, J. Appl. Phys. 58, 441 (1985).

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11. V. P. Koshelets, P. N. Dmitriev, A. B. Ermakov, A. S. Sobolev, M. Yu. Torgashin, V. V. Kurin, A. L. Pan kratov, and J. Mygind, IEEE Trans. Appl. Supercond. 15, 964 (2005). 12. I. Mehdi, Proc. SPIE 5498, 103 (2004). 13. S. V. Shitov, V. P. Koshelets, A. B. Ermakov, P. N. Dmi triev, L. V. Filippenko, V. V. Khodos, V. L. Vaks, P. A. Yagoubov, W. J. Vreeling, and P. R. Wesselius, IEEE Trans. Appl. Supercond. 13, 684 (2003).

Translated by O. Borovik Romanova

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