Research ArticleAPPLIED PHYSICS

An electrically pumped phonon-polariton laser

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Science Advances  12 Jul 2019:
Vol. 5, no. 7, eaau1632
DOI: 10.1126/sciadv.aau1632
  • Fig. 1 Schematics and design of the device structure.

    (A) Scanning electron microscopy picture of the phonon-polariton laser consisting of an active region based on 60 periods of the structure shown in (C) inside a double-metal cavity formed by the top and bottom contacts. (B) Phonon-polariton energy density along the growth direction (z). Most of the energy is concentrated in the barriers where the phonon part of the polariton is confined, while the photon part contributes with an approximately constant energy density. For the computed case with a phonon fraction of 0.5, each constituent makes up about half of the total polariton energy integrated over one period. (C) Band structure diagram of the phonon-polariton laser design (called EV2128), where the gain transition from the upper laser state (uls) to the lower laser state (lls) is indicated. The phonon and photon constituents, traveling in the plane of the laser, are schematically drawn. The color scale shows the electron density computed with a nonequilibrium Green’s function model at 100-K lattice temperature and shows an inverted population at the plotted bias (19 kV/cm).

  • Fig. 2 Device characteristics.

    (A) Measured subthreshold emission spectra (T = 10 K). Two peaks separated by the AlAs reststrahlen band were observed, stemming from the emission from the lower and upper phonon-polariton branches, respectively. The low-temperature laser spectrum at 880 mA is also shown. a.u., arbitrary units. (B) Measured Raman spectrum taken from the active region in the z(x,y)z¯ geometry. (C) Room-temperature direct transmission spectrum using a room-temperature deuterated triglycine sulfate detector. (D) Light output versus current characteristic. A single threshold current (=880 mA) was observed. The inset shows the transport curve of the device. The ridge width of the device is ~30 μm, and the length was 250 μm.

  • Fig. 3 Raman spectroscopy of the phonon-polariton laser and the reference laser with GaAaSb barriers.

    (A) Low-temperature (T = 5.5 K) polarized Raman scattering spectra of the facet of the phonon-polariton laser under operation. The peaks at 28 and 32 meV are attributed to the InAs and GaAs phonons of the active region, respectively. The peak at 35 meV, denoted by “x,” is an artifact attributed to the Raman scattering inside the optics used in the setup. The phonon-polariton peak at 48 meV is seen when the device is driven above the threshold (1060 mA) and disappears when the device is below the threshold (670 mA). (B) Raman spectrum of the reference sample at a current of 835 mA, which is close to the roll-over current of the device. (C) Polarization selection rules for the phonon-polariton peak, which appears only for the in-plane polarization combination [x′(y′, y′)x¯′]. (D) Light and Raman sideband intensity of the phonon polariton and the reference devices as function of the injected current. The inset shows the lasing spectrum of the reference sample (EP1562).

  • Fig. 4 Phonon-polariton laser emission spectra.

    (A) High-resolution emission spectra of the FP devices. The cavity length of all the devices was fixed at 1.0 mm. The longitudinal mode spacing is strongly reduced when the laser emission energy approaches to the AlAs optical phonon energy. The spectral measurements were performed at T = 15 K. (B) Group refractive index (ng) retrieved from the longitudinal mode spacing of the FP devices. The red solid points represent ng of the FP devices containing the AlAs phonon oscillators, while the group index ng of the FP devices without the AlAs phonon oscillators are depicted by the blue solid points (the symbols represent different samples as given in the Supplementary Materials). The red solid line shows ng derived from the computed phonon-polariton dispersion. (C) Computed phonon-polariton dispersion for our laser devices. The red circles represent the emission energies of the measured devices in (B). (D) Computed Hopfield coefficients. In this model, a phonon fraction of 0.65 is obtained at the lowest emission energy (47.2 meV).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/7/eaau1632/DC1

    Supplementary Text

    Fig. S1. Conduction band diagrams.

    Fig. S2. Characteristics of the first order distributed feedback lasers and the FP lasers of the reference InGaAs/GaAsSb structure.

    Fig. S3. Nonequilibrium Green’s function simulation results.

    Fig. S4. Phonon-polariton dispersion (red) and lower polariton Hopfield coefficients of the phonon (hphon.) and photon (hphot.) components for a 4.7-THz GaAs/AlGaAs QCL (53).

    Fig. S5. Raman spectroscopy of the four-well InGaAs/AlInGaAs QCL emitting at 3.6 THz.

    Table S1. Layer structures and properties of all the grown samples.

    Table S2. Selection rules for Raman backscattering from TO phonon modes polarized along the y′ or z directions, as well as LO phonon modes.

    References (4653)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Fig. S1. Conduction band diagrams.
    • Fig. S2. Characteristics of the first order distributed feedback lasers and the FP lasers of the reference InGaAs/GaAsSb structure.
    • Fig. S3. Nonequilibrium Green’s function simulation results.
    • Fig. S4. Phonon-polariton dispersion (red) and lower polariton Hopfield coefficients of the phonon (hphon.) and photon (hphot.) components for a 4.7-THz GaAs/AlGaAs QCL (53).
    • Fig. S5. Raman spectroscopy of the four-well InGaAs/AlInGaAs QCL emitting at 3.6 THz.
    • Table S1. Layer structures and properties of all the grown samples.
    • Table S2. Selection rules for Raman backscattering from TO phonon modes polarized along the y′ or z directions, as well as LO phonon modes.
    • References (4653)

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