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Abstract:Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers. For example, the ability to control optical field confinement, enhancement, and scattering almost independently, allows nonlinear frequency conversion efficiencies to be enhanced by many orders of magnitude compared to bulk materials. Also, the subwavelength length scale renders phase matching issues irrelevant. Compared with plasmonic nanostructures, dielectric resonator metamaterials show great promise for enhanced nonlinear optical processes due to their larger mode volumes. Here, we present, for the first time, resonantly enhanced second-harmonic generation (SHG) using Gallium Arsenide (GaAs) based dielectric metasurfaces. Using arrays of cylindrical resonators we observe SHG enhancement factors as large as 10 4 relative to 2 unpatterned GaAs. At the magnetic dipole resonance we measure an absolute nonlinear conversion efficiency of ~2 × 10 −5 with ~3.4 GW/cm 2 pump intensity. The polarization properties of the SHG reveal that both bulk and surface nonlinearities play important roles in the observed nonlinear process.

Resonant-phonon terahertz quantum-cascade lasers operating up to a heat-sink temperature of 186 K are demonstrated. This record temperature performance is achieved based on a diagonal design, with the objective to increase the upper-state lifetime and therefore the gain at elevated temperatures. The increased diagonality also lowers the operating current densities by limiting the flow of parasitic leakage current. Quantitatively, the diagonality is characterized by a radiative oscillator strength that is smaller by a factor of two from the least of any previously published designs. At the lasing frequency of 3.9 THz, 63 mW of peak optical power was measured at 5 K, and approximately 5 mW could still be detected at 180 K.

We report the development of a quantum cascade laser, at ϭ87.2 m, corresponding to 3.44 THz or 14.2 meV photon energy. The GaAs/Al 0.15 Ga 0.85 As laser structure utilizes longitudinal-optical ͑LO͒ phonon scattering for electron depopulation. Laser action is obtained in pulsed mode at temperatures up to 65 K, and at 50% duty cycle up to 29 K. Operating at 5 K in pulsed mode, the threshold current density is 840 A/cm 2 , and the peak power is approximately 2.5 mW. Based on the relatively high operating temperatures and duty cycles, we propose that direct LO-phonon-based depopulation is a robust method for achieving quantum cascade lasers at long-wavelength THz frequencies.

Magnetotransport in a laterally confined two-dimensional electron gas (2DEG) can exhibit modified scattering channels owing to a tilted Hall potential. Transitions of electrons between Landau levels with shifted guiding centers can be accomplished through a Zener tunneling mechanism, and make a significant contribution to the magnetoresistance. A remarkable oscillation effect in weak field magnetoresistance has been observed in high-mobility 2DEGs in GaAs-AlGa0.3As0.7 heterostructures, and can be well explained by the Zener mechanism.PACS numbers: 71.70. Di, 73.43.Qt, 73.43.Jn Scattering and dissipation are central issues in quantum transport in electronic systems. Of particular interest are the peculiar phenomena in a quantum Hall (QH) system, realized when a two-dimensional electron gas (2DEG) is subject to a strong magnetic field, B. In the integer quantum Hall effect (IQHE) regime the Hall plateau is formed and the longitudinal transport is dissipationless [1]. It has been known for quite sometime that, when a substantial current is passed through the 2DEG, the IQHE tends to break down. Above a certain critical current density the quantized Hall plateau disappears and the transport in this regime becomes dissipative. Such phenomena have been observed in confined QH systems, such as Hall bar [2] or Corbino geometries [3].The mechanism leading to the breakdown effect is still under debate. Among the possible explanations, the Zener tunneling mechanism was originally proposed by Tsui et al [4]. In essence, if a sufficient Hall field, E y , is established in a laterally confined QH system, the degeneracy of the Landau orbits is lifted. Zener tunneling can occur between the occupied Landau orbits below the Fermi level, E F , and the empty orbits above the E F , separated at a distance equivalent to the cyclotron diameter, 2R c . The condition for energy conservation is satisfied whenever 2R c eE y = l ω c , where l = 1, 2, 3... integers, ω c = eB/m * the cyclotron frequency, and m * the electron effective mass. By this mechanism, a critical current density of j ∼ 60 A/m is needed to initiate the IQHE breakdown in GaAs, at a typical magnetic field of B ∼ 10 T [4]. In the breakdown effect of IQHE, such tunneling events are likely to take place near the edges of the QH system where a high field can build up.Quite surprisingly, we have observed the Zener tunneling effect in a different regime. Specifically, our effect takes place in the bulk of a 2DEG subjected to a weak magnetic field. Remarkable magnetoresistance oscillations are observed in high-mobility Hall bar specimens.The period of oscillation, ∆(1/B), is tunable by a dc bias current, J dc : ∆(1/B) ∝ n 1/2 e /J dc , where n e is the electron sheet density. This observation confirms a general selection rule for electron transport in a weak magnetic field, namely, ∆k x ≈ 2k F for momentum transfer between the initial and final Landau levels, where x labels the direction of the current and 2k F is the Fermi wave vector of the 2DEG at zero magnetic field. This...

We combine photonic crystal and quantum cascade band engineering to create an in-plane laser at terahertz frequency. We demonstrate that such photonic crystal lasers strongly improve the performances of terahertz quantum cascade material in terms of threshold current, waveguide losses, emission mode selection, tunability and maximum operation temperature. The laser operates in a slow-light regime between the M saddle point and K band-edge in reciprocal lattice. Coarse frequency control of half of a terahertz is achieved by lithographically tuning the photonic crystal period. Thanks to field assisted gain shift and cavity pulling, the single mode emission is continuously tuned over 30 GHz.

We present a new approach to dielectric metasurface design that relies on a single resonator per unit cell and produces robust, high quality-factor Fano resonances. Our approach utilizes symmetry breaking of highly symmetric resonator geometries, such as cubes, to induce couplings between the otherwise orthogonal resonator modes. In particular, we design perturbations that couple "bright" dipole modes to "dark" dipole modes whose radiative decay is suppressed by local field effects in the array. Our approach is widely scalable from the near-infrared to radio frequencies. We first unravel the Fano resonance behavior through numerical simulations of a germanium resonator-based metasurface that achieves a quality-factor of ~1300 at ~10.8 µm. Then, we present two experimental demonstrations operating in the near-infrared (~1 µm): a silicon-based implementation that achieves a quality-factor of ~350; and a gallium arsenide-based structure that achieves a quality-factor of ~600 -the highest near-infrared quality-factor experimentally demonstrated to date with this kind of metasurfaces. Importantly, large electromagnetic field enhancements appear within the resonators at the Fano resonant frequencies. We envision that combining high quality-factor, high field enhancement resonances with nonlinear and active/gain materials such as gallium arsenide will lead to new classes of active optical devices.Metasurfaces are currently the subject of intensive research worldwide since they can be tailored to produce a wide range of optical behaviors. However, metasurfaces generally exhibit broad spectral resonances, and it is difficult to obtain narrow (i.e. high quality-factor, Q) spectral features. Attaining such high-Q features from metasurfaces would greatly expand their application space, particularly in the areas of sensing, spectral filtering, and optical modulation. Early metasurfaces were fabricated from metals and exhibited particularly broad resonances at infrared and optical frequencies as a result of Ohmic losses. Dielectric resonator-based metasurfaces were introduced to overcome these losses and have enabled, among others, wave-front manipulation and cloaking devices, perfect reflectors, and ultrathin lenses [1-10] but, although absorptive losses were reduced, the metasurface resonances remained broad due to strong coupling with the external field (i.e. large radiation losses).Recently, new strategies based on "electromagnetically induced transparency" or "Fano resonances" have been developed that show great promise for achieving high-Q resonances [11][12][13][14][15]. In this approach, the resonator system is designed to support both "bright" and "dark" resonances. The incident optical field readily couples to the bright resonance, but cannot couple directly to the dark resonance. Through proper design, a weak coupling between the two resonances can be introduced, allowing energy from the incident wave to be indirectly coupled to the dark resonance. The metasurface transmission and reflection spectra resulting from ...