We demonstrate the ultra-fast detection of terahertz pulses from a quantum cascade laser (QCL) using superconducting NbN and YBCO detectors. This has enabled both the intrapulse and interpulse dynamics of a THz QCL to be measured directly, including interpulse heating effects on sub-μs timescales

Chowdhury, Siddhant

Davies, A. Giles

Dean, Paul

Il\u27in, Konstantin

Khanna, Suraj P.

Linfield, Edmund H.

Probst, Petra

Salih, Mohammed

Scheuring, Alexander

Siegel, Michael

Stockhausen, Axel

Valavanis, Alex

Wuensch, Stefan

English

Dean, P

Valavanis, A

Scheuring, A

Stockhausen, A

Probst, P

Salih, M

Khanna, SP

Chowdhury, S

Wuensch, S

Il in, K

Linfield, EH

Davies, AG

Siegel, M

White Rose Research Online

This is a repository copy of Ultra-fast sampling of terahertz pulses from a quantum cascade laser using superconducting antenna-coupled NbN and YBCO detectors.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/75246/Article:Dean, P, Valavanis, A, Scheuring, A et al. (10 more authors) (2012) Ultra-fast sampling of terahertz pulses from a quantum cascade laser using superconducting antenna-coupled NbN and YBCO detectors. 2012 37th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz). 1 - 2 . ISSN 2162-2027 https://doi.org/10.1109/IRMMW-THz.2012.6380341eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse See Attached Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. Ultra-fast sampling of terahertz pulses from a quantum cascade laser using superconducting antenna-coupled NbN and YBCO detectorsAbstract—We demonstrate the ultra-fast detection of terahertz pulses from a quantum cascade laser (QCL) using superconducting NbN and YBCO detectors.  This has enabled both the intrapulse and interpulse dynamics of a THz QCL to be measured directly, including interpulse heating effects on sub−s timescales. I. INTRODUCTION AND BACKGROUND HE most commonly employed detectors for terahertz (THz) quantum cascade lasers (QCLs) rely on incoherent thermal detection of radiation. Owing to the slow time-constants of thermal processes (typically ~ms), such schemes are sensitive only to the total pulse energy during typical pulsed QCL operation.  This slow detector response precludes investigation of intrapulse and interpulse dynamics, including interpulse heating effects.  Yet the current requirement for cryogenic cooling of THz QCLs has led to great interest in analyzing their thermal properties.  Such studies generally infer the internal temperature from changes in the emission power [1].  However, heat-extraction occurs on k1׽ たs time-scales [2] that cannot normally be resolved. We report the ultra-fast detection of QCL pulses using superconducting NbN and YBCO detectors with response times of ~165 ps [2] and ~45 ps [3], respectively.  The intrapulse dynamics are interpreted in terms of the time-dependent driving current, enabling the true power-current relationship for the QCL to be determined.  We show that this transfer function differs from that obtained using slow thermal detectors that respond only to the integrated pulse energy.  We also present direct observations of pulse-to-pulse degradation in the emitted power, which enables calculation of time-constants for heat extraction, as well as the thermal esistance and anisotropic thermal conductivity of THz QCLs.  II. INTRAPULSE DYNAMICS The QCL used for this work was based on a three-well resonant-phonon depopulation scheme emitting at ~3.1 THz, processed into a semi-insulating surface plasmon ridge waveguide.  The detector elements consisted of NbN or YBCO micro-bridges fabricated on a sapphire substrate and embedded into a planar log-spiral antenna.  The detectors were operated below the critical temperature and driven onto the superconducting transition by a constant bias voltage.  A 500 MHz real-time oscilloscope was used to monitor the detector output following 40 dB of amplification in the bandwidth 0.1–400 MHz.  Fig. 1. (a) NbN detector responses (solid) to a THz QCL driven by 500-ns current pulses of different amplitude (dashed). The horizontal black line indicates the current value at threshold for this QCL. Fig. 1 shows exemplar NbN detector responses to a 500-ns driving pulse of low duty cycle, for which the recorded current pulses are also shown.  We have confirmed that these dynamic responses can be correlated directly to the time-dependent amplitude of the current pulses.  As can be seen, the shapeof the THz pulse reacts strongly to small changes in the driving pulse, giving rise to drastically different pulse shapes.  Uing the correlation between the instantaneous detector output and driving current measured under a range of driving conditions we have obtained the true power-current relationship for the QCL, as shown in Fig. 2. For comparison the response obtained using a germanium bolometer, which responds only Paul Dean1,*, Alex Valavanis1, Alexander Scheuring2, Axel Stockhausen2, Petra Probst2, Mohammed Salih1, Suraj P. Khanna1, Siddhant Chowdhury1, Stefan Wuensch2, Konstantin Il‘in2, Edmund H. Linfield1, A. Giles Davies1 and Michael Siegel2 1School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, UK 2Institute of Micro- and Nanoelectronic Systems, Karlsruhe Institute of Technology (KIT),  76187 Karlsruhe, Germany Email: *p.dean@leeds.ac.uk T to the pulse energy, is also shown.  This measurement demonstrates the inaccuracy of the power-current relationship measured with the bolometer under pulsed operation.  Fig. 2. Power-current relationships obtained using superconducting NbN (black) and YBCO (red) detectors, and a germanium bolometer (blue). III.  INTRAPULSE DYNAMICS AND HEATING Pulse-to-pulse degradation of the emitted power, arising from Joule heating of the QCL, has also been studied.  Fig.3(a) shows the NbN detector response (average amplitude within each pulse) for a QCL driven by a 1-ms long pulse-train of 500 ns pulses of varying repetition rate, using a fixed heat-sink temperature, TH=15 K.  It can be seen that the THz power decreases in successive pulses, and more severely for higher repetition rates (duty cycles), as a result of the QCL not being able to return to thermal equilibrium with the heat sink. The effect of active-region (AR) temperature, TAR, upon the THz power was measuring with a Ge bolometer; a continuous 1-kHz pulse-train was used, such that TAR≈TH. The AR temperature variation was then inferred for the rapidly-repeating pulse trains in Fig. 3(a) by comparison with the low-duty-cycle data. Fig. 3(b) shows the extracted AR temperatures for each pulse train, along with a regression to the form )/exp()(AR tTTTT H   , where Ĳ is a material-specific thermal time-constant for average heating in the material, and T  is the final, quasi-equilibrium temperature. This functional form represents a general solution of the one-dimensional heat equation for a bulk solidwith temperature-independent material properties. The temperature can be seen to increase over the length of t e pulse train, with a larger final temperature at high repetition rates. Values of TH increase from 25 K at 100-kHz repetition up to 49 K at 500 kHz. Values of Ĳ were found to be between 0.29 and 0.46 ms in each case.   Fig. 3. (a) Detector response to a QCL driven by a pulse train of 500 ns pulses with varying repetition rate.  (b) Time-dependence of the QCL active region temperature deduced from (a). IV. CONCLUSION Direct detection of THz QCL pulses using superconducting detectors enables the observation of intrapulse and interpuls  dynamics.  We have measured the true power-current relationship for a QCL from the correlation between instantaneous driving current and detector response.  Interpulse heating effects have also been studied, from which the thermal properties of QCLs can be [have been?] obtained.  ACKNOWLEDGMENTS We are grateful for support from the EPSRC, the Royal Society, the Wolfson Foundation, and the European Research Council grants ‘TOSCA’ and ‘NOTES’.  This work was partly supported by the German Federal Ministry of Education and Research (Grant Nos. 05K2010 and 13N12025) and by DFG Center for Functional Nanostructures. REFERENCES [1] H. Li, J. C. Cao, Y. J. Han, Z. Y. Tan, and X. C. Guo, “Temperature profile modelling and experimental investigation of thermal resistance of terahertz quantum-cascade lasers,” J. Phys. D, 42, 205102, 2009. [2] M. S. Vitiello, G. Scamarcio, and V. Spagnolo, “Time-resolved measurement of the local lattice temperature in terahertz quantum cascade lasers,” Appl. Phys. Lett., 92, 101116, 2008. [3] A. D. Semenov, H.-W. Hubers, K. S. Il’in, M. Siegel, V. Judin, and A.-S. Muller, “Monitoring coherent THz-synchrotron radiation with superconducting NbN hot-electron detector,” 34th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) 2009.  [4] P. Probst, A. Scheuring, M. Hofherr, D. Rall, S. Wünsch, K. Il’in, M. Siegel, A. Semenov, A. Pohl, H.-W.Hübers, V. Judin, A.-S. Müller, A. Hoehl, R. Müller, and G. Ulm, “YBa2Cu3O7−h quasioptical detectors for fast time-domain analysis of terahertz synchrotron radiation,” Appl. Phys. Lett. 98, 043504, 2011. 

Ultra-fast sampling of terahertz pulses from a quantum cascade laser using superconducting antenna-coupled NbN and YBCO detectors

Ultra-fast sampling of terahertz pulses from a quantum cascade laser using superconducting antenna-coupled NbN and YBCO detectors

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