Terahertz laser frequency combs

Journal name:
Nature Photonics
Volume:
8,
Pages:
462–467
Year published:
DOI:
doi:10.1038/nphoton.2014.85
Received
Accepted
Published online

Abstract

Terahertz light can be used to identify numerous complex molecules, but has traditionally remained unexploited due to the lack of powerful broadband sources. Pulsed lasers can be used to generate broadband radiation, but such sources are bulky and produce only microwatts of average power. Conversely, although terahertz quantum cascade lasers are compact semiconductor sources of high-power terahertz radiation, their narrowband emission makes them unsuitable for complex spectroscopy. In this work, we demonstrate frequency combs based on terahertz quantum cascade lasers, which combine the high power of lasers with the broadband capabilities of pulsed sources. By fully exploiting the quantum-mechanically broadened gain spectrum available to these lasers, we can generate 5 mW of terahertz power spread across 70 laser lines. This radiation is sufficiently powerful to be detected by Schottky-diode mixers, and will lead to compact terahertz spectrometers.

At a glance

Figures

  1. GVD of laser gain medium.
    Figure 1: GVD of laser gain medium.

    a, Calculated GVD of GaAs at 40 K, defined here as . The marked frequencies are 3.5 THz and 42 THz. Inset: phase acquired by a time-domain terahertz pulse after travelling through a 775 µm QCL twice, with a parabolic fit. b, Schematic of the integrated dispersion compensation scheme used. The longer-wavelength wave travels further before it reflects, while the shorter-wavelength wave reflects earlier, resulting in a shorter effective cavity.

  2. Continuous-wave spectrum and beat notes.
    Figure 2: Continuous-wave spectrum and beat notes.

    a, Spectrum of a THz QCL comb biased to 0.9 A at a temperature of 50 K. The device is 20 µm wide, 5 mm long and emits 4.7 mW at 45 K. Atmospheric absorption is shown in yellow. b, QCL-generated beat notes offset relative to 6.82 GHz, shown with the environmental fluctuations stabilized and unstabilized. (The spectra are discontinuous at 40 MHz because the signal below 40 MHz was measured by downconverting and sampling, while the signal above was measured with a spectrum analyser.) c, HEB-detected beat-note offset relative to 6.80 GHz, stabilized. The linewidth of 1.53 kHz is limited by the instrument resolution. d, Bias dependence of the beat-note frequency measured by the QCL and by the HEB. A repetition rate tuning of 12 MHz is possible, giving a total frequency shift of 6.80 GHz at 3.8 THz, approximately the mode spacing.

  3. Results of SWIFT measurement.
    Figure 3: Results of SWIFT measurement.

    a, Normal interferogram and homodyne interferograms, as measured by the HEB. The normal interferogram represents a d.c. measurement, while the homodyne interferogram represents an RF measurement. The QCL is biased to 0.9 A and is at 50 K. Inset: zoomed-in view near the zero-path delay. b, Blue line: SWIFT correlation spectrum (calculated from homodyne interferograms). Red line: spectrum product (calculated from normal interferogram). Even though they are fundamentally different measurements, their excellent agreement indicates that most of the spectral power is in the frequency comb.

  4. Bias dependence of beat-note and SWIFT spectra.
    Figure 4: Bias dependence of beat-note and SWIFT spectra.

    a, RF power measured from a bias-tee (amplified by 66 dB, with wiring losses of 20 dB) and terahertz power (calibrated) emitted by the QCL as a function of bias at 50 K, over the dynamic range of lasing operation. b, Linewidth of the RF signal emitted from the QCL as a function of bias, as measured with a spectrum analyser. (The linewidth metric used here is standard deviation.) The regions of stable comb formation are shaded and denoted I, II and III. c, SWIFT correlation spectra (measured with the HEB) and gain spectra (measured with THz TDS) corresponding to each of the three regions.

  5. Heterodyne beat note between single-mode laser and comb.
    Figure 5: Heterodyne beat note between single-mode laser and comb.

    a, Correlation spectrum of comb at 0.9 A (green) and spectrum of DFB laser (red). b, Heterodyne beating of free-running comb laser at 0.885 A with free-running DFB laser at various biases. The two lines correspond to beating between the two nearest comb lines and sum to the repetition rate of 6.8 GHz. The tuning coefficient of the DFB is 28 MHz mA−1. c, Zoomed-in view of one of the lines, showing a convolved FWHM of 2.5 MHz. From this, we estimate that our comb lines are 1.8 MHz wide.

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Author information

Affiliations

  1. Department of Electrical Engineering and Computer Science, Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    • David Burghoff,
    • Tsung-Yu Kao,
    • Ningren Han,
    • Chun Wang Ivan Chan,
    • Xiaowei Cai,
    • Yang Yang &
    • Qing Hu
  2. SRON Netherlands Institute for Space Research, 9747 AD, Groningen, The Netherlands

    • Darren J. Hayton &
    • Jian-Rong Gao
  3. Kavli Institute of NanoScience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands

    • Jian-Rong Gao
  4. Center for Integrated Nanotechnology, Sandia National Laboratories, Albuquerque, New Mexico 87123, USA

    • John L. Reno

Contributions

D.B. conceived the strategy, designed the devices, performed the measurements and completed the analysis. D.B. and N.H. performed electromagnetic simulations. T.-Y.K., N.H. and C.W.I.C. fabricated the devices. D.B., X.C. and Y.Y. performed the heterodyne beat-note measurements. D.J.H. and J.-R.G. provided the hot electron bolometer mixer. J.L.R. provided the material growth. All work was performed under the supervision of Q.H.

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The authors declare no competing financial interests.

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