Feasibility of spoof surface plasmon waveguide enabled ultrathin room temperature THz GaN quantum cascade laser
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Feasibility of spoof surface plasmon waveguide enabled ultrathin room temperature THz GaN quantum cascade laser Greg Sun1, Jacob B. Khurgin2, and Din Ping Tsai3 1
University of Massachusetts Boston, Boston, Massachusetts 02125, U.S.A.
2
Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.
3
National Taiwan University, Taipei, Taiwan, R.O.C.
Abstract We propose and study the feasibility of a THz GaN/AlGaN quantum cascade laser (QCL) consisting of only five periods with confinement provided by a spoof surface plasmon (SSP) waveguide for room temperature operation. The QCL design takes advantages of the large optical phonon energy and the ultrafast phonon scattering in GaN that allow for engineering favorable laser state lifetimes, and the SSP waveguide provides the optical confinement for the ultrathin QCL. Our analysis has shown that the waveguide loss is sufficiently low for the QCL to reach its threshold at the injection current density around 6 kA/cm2 at room temperature. Introduction The THz spectral range (λ=30-300 μm) is interesting for spectroscopy and imaging with applications in explosive and drug detection, security screening, astronomy, and medical imaging. However, the scope of these applications is limited to a large extent by the availability of THz sources which typically either are very bulky in size or require cryogenic cooling. For THz technology to reach its potential, compact THz lasers operating at room temperature are a necessary component with the performance commensurate of the semiconductor diode lasers. Unfortunately, none of the semiconductors in nature has such a narrow bandgap that gives bandto-band transitions in the THz range, and even if it did, the diode lasers would not have operated at room temperature because of the severe Auger recombination. Quantum cascade lasers (QCLs) relying on intersubband transitions in semiconductor quantum wells (QWs) offer an important alternative where the lasing wavelength can be tuned from mid- to far-IR by engineering subband energy separations. Indeed, QCLs have become the mainstream commercially available sources in mid-IR (2.75µm< λ τ 2 , is easily satisfied throughout the temperature range.
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Fig. 3. (a) Lifetimes of upper ( τ 3 ) and lower laser states ( τ 2 ), as well as scattering between them ( τ 32 ) as a function of temperature, and (b) optical gain vs. injection current density for a range of temperature. With these lifetimes, we can solve the following set of rate equations in the absence of lasing for the subband populations N i ( i = 1, 2,3) , dN 3 J N1 N 2 N 3 J N 3 − N1e − Δ31 / k BT N 3 − N 2 e − Δ32 / k BT = + + − = − − dt e τ 13 τ 23 τ 3 e τ 31 τ 32
(1)
dN 2 N1 N 2 N 3 N 3 − N 2e − Δ32 / k BT N 2 − N1e − Δ21 / k BT = − + = − dt τ 12 τ 2 τ 32 τ 32 τ 21 N = N1 + N 2 + N 3 . Here J is the injection current density, N is the total area doping density per period, and we −Δ / k T have related the scattering rate from subband i to j to that of j to i with τ ij−1 = τ −ji1e ji B where Δ ji = E j − Ei to show explicitly the
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