Octave-spanning semiconductor laser

Journal name:
Nature Photonics
Year published:
Published online


We present a semiconductor injection laser operating in continuous wave with emission covering more than one octave in frequency and displaying homogeneous power distribution among the lasing modes. The gain medium is based on a heterogeneous quantum cascade structure operating in the terahertz range. Laser emission in continuous wave takes place from 1.64 THz to 3.35 THz with optical powers in the milliwatt range and more than 80 modes above threshold. For narrow waveguides, a collapse of the free-running beatnote to linewidths of 980 Hz, limited by jitter, indicate frequency comb operation on a spectral bandwidth as wide as 624 GHz, making such devices ideal candidates for octave-spanning semiconductor-laser-based terahertz frequency combs.

At a glance


  1. Laser characteristics.
    Figure 1: Laser characteristics.

    a, Calculated gain cross-section gc. Blue curves: individual designs. Green curve: total active region. Inset: arrangement of the different active region designs in the laser. b, SEM image of a processed 50-μm-wide dry-etched laser. Inset: electric field intensity distribution in a metal–metal waveguide. c, LIV characteristics (c.w.) at different temperatures for a 2 mm × 150 µm laser. The first power axis is normalized to a measurement with a broad-area terahertz absolute power meter (TK Instruments, aperture 55 × 40 mm2), and the second axis is from an Ophir THz absolute power meter with a smaller detector surface (aperture diameter 12 mm).

  2. Spectral performance.
    Figure 2: Spectral performance.

    a, Laser spectrum (c.w., 9.5 V, 1.06 A, 350 A cm−2) at 25 K measured with a high-pass filter (black), low-pass filter (red) and without filter (blue) for a 2 mm × 150 µm wet-etched laser ridge. b, Octave-spanning spectrum of a dry-etched 2 mm × 50 µm laser (c.w., 9.7 V, 0.35 A, 350 A cm−2) at 18 K. a.u., arbitrary units.

  3. Beatnote evolution.
    Figure 3: Beatnote evolution.

    a, Evolution of the beatnote spectra as a function of bias current when not facing the FTIR spectrometer. b, Beatnote linewidth as a function of driving current. The resolution bandwidth (RBW) was set to 1 kHz for this measurement (blue line). We therefore have slightly higher linewidths than those shown in Fig. 4 where the RBW is set to 100 Hz. For the multi-beatnote regime the distance between the outermost lines was considered to be the linewidth. All measurements were performed at 25 K.

  4. Beatnote analysis.
    Figure 4: Beatnote analysis.

    Spectral evolution along the LIV for a 3 mm × 50 µm dry-etched laser at 25 K. a, Corresponding LIV range, with the shaded area indicating the comb region. In the light grey area, subcombs are observed. b, Spectra for different currents (for a current of 460 mA we have 105 modes above threshold). a.u., arbitrary units. c, Corresponding electrical beatnote measured with an antenna.

  5. Dispersion analysis.
    Figure 5: Dispersion analysis.

    a, GVD simulations including contributions from the material (GaAs), waveguide and gain. The shaded part indicates the spectral range of the comb regime. b,c, Residuals (green) to a linear fit (blue) of the measured peak positions (red) as a function of peak number for a 3 mm × 50 µm laser at 397 mA (b) and 460 mA (c) in c.w. operation (25 K) compared to the same quantity calculated theoretically from the GVD, showing the effect of dispersion in the lasing spectra. In b, the laser is measured in the comb regime, and in c, the laser is influenced by dispersion, causing the curvature of the green and black curve. The spikes in the green curves seem to be artefacts from the subresolution determination of the peak position.


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  1. ETH Zurich, Institute of Quantum Electronics, Auguste-Piccard-Hof 1, Zurich 8093, Switzerland

    • Markus Rösch,
    • Giacomo Scalari,
    • Mattias Beck &
    • Jérôme Faist


M.R. fabricated the quantum cascade lasers, performed experiments, analysed data, developed the simulations and wrote the paper together with G.S. G.S. designed the quantum cascade lasers, designed and performed experiments, analysed data, developed the simulations and wrote the manuscript together with M.R. M.B. grew the quantum cascade laser material used for this work. J.F. designed the experiments, analysed the data and supervised the work.

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