Wave engineering with THz quantum cascade lasers

Journal name:
Nature Photonics
Volume:
7,
Pages:
691–701
Year published:
DOI:
doi:10.1038/nphoton.2013.208
Received
Accepted
Published online

Abstract

Quantum cascade lasers are compact devices based on mature compound semiconductors such as GaAs that take advantage of highly developed optoelectronic fabrication techniques to integrate linear and nonlinear functions. This Review discusses terahertz-wave engineering using quantum cascade lasers with a particular focus on techniques that have been implemented to control their spectral and output beam properties. After briefly introducing the types of active regions and surveying present maximum operating temperatures, we review several photonic structures used for frequency and beam engineering, ranging from distributed feedback lasers to photonic crystals. We then describe techniques that allow the upconversion of terahertz quantum cascade laser radiation in the near-infrared region using nonlinear intracavity mixing. Finally, we review frequency stabilization of terahertz quantum cascade lasers with a special emphasis on phase locking to near-infrared frequency combs.

At a glance

Figures

  1. Maximum operating temperatures of THz QCLs reported to date for pulsed mode operation.
    Figure 1: Maximum operating temperatures of THz QCLs reported to date for pulsed mode operation.

    The straight line represents the equation T = ħω/kB, which was considered to express the empirical limit. It is evident that the reported data do not follow this trend. Inset: timeline indicating the maximum operating temperatures of THz QCLs as a function of time5, 45, 49.

  2. DFB THz QCLs and MEMS-based device for wideband frequency tuning.
    Figure 2: DFB THz QCLs and MEMS-based device for wideband frequency tuning.

    a, Cross section of the intensity profile of the optical mode inside a single-plasmon waveguide. The shaded area indicates the active region. In contrast to metal–metal waveguides, the mode leaks heavily into the substrate (white area). Inset: Scanning electron microscopy image of the top ridge metallization17. b, Single-mode emission spectrum (log scale) of a THz DFB laser with a top metallic grating. The laser was operated in pulsed mode at cryogenic temperatures. Left inset: spectra of three THz DFB lasers with different grating periods17. c, Inset: device with highly subwavelength transverse dimensions. As the 2D colour-scale plot of the mode intensity shows (the black line indicates the mode cross section), a large fraction of the electromagnetic mode resides outside the gain medium. This enables the mode to be manipulated by a movable object that changes the effective index, hence tuning the laser emission frequency. In the main figure, the movable object is a silicon micromachined plunger11. d, Frequency tuning over a range of 330 GHz obtained in a device in which MEMS technology is employed to precisely position the plunger54. Figure reproduced with permission from: a,b ref. 17, © 2005 AIP; d, ref. 54, © 2011 OSA.

  3. THz QCLs based on third-order DFB gratings and photonic crystals.
    Figure 3: THz QCLs based on third-order DFB gratings and photonic crystals.

    a, Schematic of a third-order DFB laser63. b, Light–current–voltage characteristics of a 15-μm-wide third-order DFB laser63. c.w., continuous wave. c, Corresponding measured far-field beam pattern63. d, Schematic cross-section and operating principle of a photonic-crystal THz QCL. The laser active region (blue) is sandwiched between two metallic contact layers. The top metal is patterned with the desired photonic-crystal design72. The photonic crystal acts as a resonator, while simultaneously diffracting light vertically. e, Optical microscopy image of the surface of a photonic-crystal THz QCL73. f, Far-field emission pattern of the photonic-crystal laser designs shown in e, obtained by scanning a Golay cell detector located at a fixed distance from the sample in angular steps of 2° (ref. 73). Figure reproduced with permission from: a–c, ref. 63, © 2010 OSA; e,f ref. 73, © 2010 AIP.

  4. Photonic-crystal structures for high power extraction.
    Figure 4: Photonic-crystal structures for high power extraction.

    a, Transverse magnetic fields (Hy) of the radiative and nonradiative bandedge modes (at the Γ-point of the band structure, kx = 0), which are located above and below the bandgap, respectively9. b, Schematics showing the effect induced by the GPH resonator: the symmetric, radiative modes are confined in the centre, whereas the nonradiative modes are pushed to the highly absorbing device edges. c, Scanning electron microscopy images of GPH lasers9. The red arrow indicates emission from the surface. d, Light–current characteristics of a 214-μm-wide GPH laser for different operating temperatures. Peak output powers of 100 mW and >60 mW are respectively obtained at 20 K and 80 K for pulsed operation9. Inset: typical single-mode emission spectrum. e, The far-field emission pattern of the device in c measured at 78 K (ref. 9).

  5. Nonlinear mixing of THz QCLs with near-infrared radiation.
    Figure 5: Nonlinear mixing of THz QCLs with near-infrared radiation.

    a, Principle of the experiment for intracavity mixing23. A near-infrared beam is coupled to a THz QCL. The nonlinear interaction between the two electric fields generates THz-frequency sidebands on both sides of the optical carrier at ωnear-infrared ± ΩQCL. b, Two types of nonlinear susceptibilities are used for this experiment: (i) bulk GaAs second-order susceptibility (left panel), and (ii) nonlinear susceptibility enhanced by resonant excitation at the bandgap (right panel). c, In GaAs, the nonlinear mixing is phase-matched when a beam in the telecommunication range interacts with a THz wave. As the near-infrared laser operates well within the transparent region of the semiconductor, both sidebands (ωnear-infrared ± ΩQCL) are observed. This is shown by the spectrum (fibre coupled) obtained by pumping at a wavelength of 1.56 μm (ref. 23). d, Enhanced nonlinear susceptibility is generated when ωnear-infrared is tuned in resonance with the interband transition at the energy gap24. In this case, the frequency sum ωnear-infrared + ΩQCL is above the gap and thus completely reabsorbed. Only the frequency difference is observable. This is indicated by the upconverted QCL spectra (left) for different pump excitation energies. Pump beams are normalized to 1. Eg corresponds to the energy above which the near-infrared pump is absorbed; it shows that the generated beam is always below the absorption edge. Inset: high-resolution spectrum of the generated beam, which corresponds to the QCL emission intensity spectral profile.

  6. Generation of a beat-note signal between a THz QCL and a femtosecond-laser comb.
    Figure 6: Generation of a beat-note signal between a THz QCL and a femtosecond-laser comb.

    a, Schematic of the beat-note generation process using electro-optic detection10. Left panel. The femtosecond laser beam is amplitude modulated by the THz field emitted by the QCL. As a result, two amplitude-modulation sideband combs (red lines) are generated on both sides of the femtosecond-laser comb spectrum (blue lines) at νopt ± νQCL, where νopt is the femtosecond-laser carrier frequency. Right panel. Enlargement of the left panel showing the individual comb teeth of the optical carrier and of the upper THz sideband. The blue arrow indicates the optical carrier comb tooth (frequency = νopt + m × frep) that lies closest to the generic comb tooth from the upper sideband at νopt + νQCLn × frep (red arrow). b, Schematic of the beat-note generation using photomixing31, 99. Left panel: femtosecond-laser comb spectrum. Right panel: photocurrent comb spectrum recorded at the output of the photomixer (blue lines). The red line represents the QCL frequency νQCL.

  7. Phase-locking of THz QCLs to electronic multipliers and femtosecond-laser combs.
    Figure 7: Phase-locking of THz QCLs to electronic multipliers and femtosecond-laser combs.

    a, Radiofrequency-beat-note spectrum obtained by phase locking a QCL emitting at 1.5 THz to a solid-state multiplier chain driven by a yttrium iron garnet oscillator96. b, Free-running spectrum of the beat note between a 2.5-THz QCL and n × frep. The resolution bandwidth is 1 MHz. c, Phase-locked spectrum with a resolution bandwidth of 1 Hz (ref. 10). d–f, Coherent detection of an actively mode-locked QCL28. The THz QCL operated at a temperature 20 K and its current was modulated at 13.3 GHz and +10 dBm of radiofrequency power using a synthesizer. d, Downconverted spectrum of the THz QCL obtained using the electro-optic sampling technique illustrated in Fig. 6a. The spectrum was recorded with a resolution bandwidth of 100 kHz; the radiofrequency beat-note linewidths are all below 1 Hz (ref. 28). e, Three periods of the measured (blue) and calculated (red) waveforms obtained assuming that all the modes of d have equal phases28. f, Phases of the Fabry–Pérot modes of d(ref. 101). Modes are numbered from left to right. The phases were obtained by Fourier-transforming 20 individual periods of the sampled waveform of e. Error bars correspond to the obtained standard deviations. Note that the large error bars corresponding to modes 1 and 10 are because those modes have much lower intensities than other modes; consequently, they have negligible contribution to the waveform. Figure reproduced with permission from: a, ref. 96, © 2009 OSA; f, ref. 101, © 2013 IEEE.

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  1. Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot and CNRS, UMR7162, 75205 Paris, France

    • Carlo Sirtori &
    • Stefano Barbieri
  2. Institut d'Electronique Fondamentale, Université Paris Sud and CNRS, UMR8622, 91405 Orsay, France

    • Raffaele Colombelli

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