Dielectric metasurfaces that exploit the different Mie resonances of nanoscale dielectric resonators are a powerful platform for manipulating electromagnetic fields and can provide novel optical behavior. In this work, we experimentally demonstrate independent tuning of the magnetic dipole resonances relative to the electric dipole resonances of split dielectric resonators (SDRs). By increasing the split dimension, we observe a blue shift of the magnetic dipole resonance toward the electric dipole resonance. Therefore, SDRs provide the ability to directly control the interaction between the two dipole resonances within the same resonator. For example, we achieve the first Kerker condition by spectrally overlapping the electric and magnetic dipole resonances and observe significantly suppressed backward scattering. Moreover, we show that a single SDR can be used as an optical nanoantenna that provides strong unidirectional emission from an electric dipole source.
We have performed two-dimensional Fourier transform spectroscopy on intrinsic and modulation doped quantum wells in external magnetic fields up to 10 T. In the undoped sample, the strong Coulomb interactions and the increasing separations of the electron and hole charge distributions with increasing magnetic fields lead to a nontrivial in-plane dispersion of the magneto-excitons. Thus, the discrete and degenerate Landau levels are coupled to a continuum. The signature of this continuum is the emergence of elongated spectral line shapes at the Landau level energies, which are exposed by the multidimensional nature of our technique. Surprisingly, the elongation of the peaks is completely absent in the lowest Landau level spectra obtained from the modulation doped quantum well at high fields.
Korkusinski, Marek K.; Bogan, Alex B.; Studenikin, Sergei S.; Aers, Geoff A.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Sachrajda, Andrew S.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
Sachrajda, Andrew S.; Bogan, Alex B.; Studenikin, Sergei S.; Korkusinski, Marek K.; Aers, Geoff A.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
Xu, Luyao; Chen, Daguan; Curwen, Christopher A.; Memarian, Mohammad; Reno, J.L.; Itoh, Tatsuo; Williams, Benjamin S.
Dynamic control of a laser’s output polarization state is desirable for applications in polarization sensitive imaging, spectroscopy, and ellipsometry. Using external elements to control the polarization state is a common approach. Less common and more challenging is directly switching the polarization state of a laser, which, however, has the potential to provide high switching speeds, compactness, and power efficiency. Here, we demonstrate a new approach to achieve direct and electrically controlled polarization switching of a semiconductor laser. This is enabled by integrating a polarization-sensitive metasurface with a semiconductor gain medium to selectively amplify a cavity mode with the designed polarization state, therefore leading to an output in the designed polarization. Here, the demonstration is for a terahertz quantum-cascade laser, which exhibits electrically controlled switching between two linear polarizations separated by 80°, while maintaining an excellent beam with a narrow divergence of ∼3° × 3° and a singlemode operation fixed at ∼3.4 THz, combined with a peak power as high as 93 mW at a temperature of 77 K. The polarization-sensitive metasurface is composed of two interleaved arrays of surface-emitting antennas, all of which are loaded with quantum-cascade gain materials. Each array is designed to resonantly interact with one specific polarization; when electrical bias is selectively applied to the gain material in one array, selective amplification of one polarization occurs. The amplifying metasurface is used along with an output coupler reflector to build a vertical-external-cavity surface-emitting laser whose output polarization state can be switched solely electrically. This work demonstrates the potential of exploiting amplifying polarization-sensitive metasurfaces to create lasers with desirable polarization states—a concept which is applicable beyond the terahertz and can potentially be applied to shorter wavelengths.
Korkusinski, Marek K.; Bogan, Alex B.; Studeinkin, Sergei S.; Aers, Geoff A.; Gaudreau, Louis G.; Zawadski, Piotr Z.; Sachrajda, Andrew S.; Tracy, Lisa A.; Reno, J.L.; Hargett, Terry H.
Terahertz (THz) double-metal plasmonic resonators enable enhanced light-matter coupling by exploiting strong field confinement. The double-metal design however restricts access to the internal fields. We propose and demonstrate a method for spatial mapping and spectroscopic analysis of the internal electromagnetic fields in double-metal plasmonic resonators. We use the concept of image charges and aperture-type scanning near-field THz time-domain microscopy to probe the fields confined within the closed resonator. The experimental method opens doors to studies of light-matter coupling in deeply sub-wavelength volumes at THz frequencies.
A new tuning mechanism is demonstrated for single-mode metal-clad plasmonic lasers, in which the refractive-index of the laser's surrounding medium affects the resonant-cavity mode in the same vein as the refractive-index of gain medium inside the cavity. Reversible, continuous, and mode-hop-free tuning of ∼57 GHz is realized for single-mode narrow-beam terahertz plasmonic quantum-cascade lasers (QCLs), which is demonstrated at a much more practical temperature of 78 K. The tuning is based on post-process deposition/etching of a dielectric (silicon-dioxide) on a QCL chip that has already been soldered and wire-bonded onto a copper mount. This is a considerably larger tuning range compared to previously reported results for terahertz QCLs with directional far-field radiation patterns. The key enabling mechanism for tuning is a recently developed antenna-feedback scheme for plasmonic lasers, which leads to the generation of hybrid surface-plasmon-polaritons propagating outside the cavity of the laser with a large spatial extent. The effect of dielectric deposition on QCL's characteristics is investigated in detail including that on maximum operating temperature, peak output power, and far-field radiation patterns. Single-lobed beam with low divergence (<7°) is maintained through the tuning range. The antenna-feedback scheme is ideally suited for modulation of plasmonic lasers and their sensing applications due to the sensitive dependence of spectral and radiative properties of the laser on its surrounding medium.
The efficiency of photoconductive (PC) devices, including terahertz detectors, is constrained by the bulk optical constants of PC materials. Here, we show that optical absorption in a PC layer can be modified substantially within a hybrid cavity containing nanoantennas and a Distributed Bragg Reflector. We find that a hybrid cavity, consisting of a GaAs PC layer of just 50 nm, can be used to absorb >75% of incident photons by trapping the light within the cavity. We provide an intuitive model, which describes the dependence of the optimum operation wavelength on the cavity thickness. We also find that the nanoantenna size is a critical parameter, small variations of which lead to both wavelength shifting and reduced absorption in the cavity, suggesting that impedance matching is key for achieving efficient absorption in the optically thin hybrid cavities.
We demonstrate the ultrafast formation of terahertz (THz) metasurfaces through all-optical creation of spatially modulated carrier density profiles in a deep-subwavelength GaAs film. The switch-on of the transient plasmon mode, governed by the GaAs effective electron mass and electron− phonon interactions, is revealed by structured-optical pump THz probe spectroscopy, on a time scale of 500 fs. By modulating the carrier density using different pump fluences, we observe a wide tuning of the electric dipole resonance of the transient GaAs metasurface from 0.5 THz to 1.7 THz. Furthermore, we numerically demonstrate that the metasurface presented here can be generalized to more complex architectures for realizing functionalities such as perfect absorption, leading to a 30 dB modulation depth. The platform also provides a pathway to achieve ultrafast manipulation of infrared beams in the linear and, potentially, nonlinear regime.
International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz
Li, Xinwei; Zhang, Qi; Lou, Minhan; Reno, J.L.; Pan, Wei P.; Watson, John D.; Manfra, Michael J.; Kono, Junichiro
We have integrated an ultrahigh mobility twodimensional electron gas with a high-quality-factor terahertz photonic cavity. With a quantizing magnetic field and at low temperatures, we demonstrated collective nonperturbative coupling of the electron cyclotron resonance with terahertz cavity photons with a high cooperativity. Due to the suppression of superradiance-induced broadening of cyclotron resonance by the high-quality-factor cavity, our hybrid quantum system exhibited unprecedentedly sharp polariton lines and a large vacuum Rabi splitting simultaneously.
Plasmonic lasers generate coherent long-range or localized surface-plasmon-polaritons (SPPs), where the SPP mode exists at the interface of the metal (or a metallic nanoparticle) and a dielectric. Metallic-cavities sup- porting SPP modes are also utilized for terahertz quantum-cascade lasers (QCLs). Due to subwavelength apertures, plasmonic lasers have highly divergent radiation patterns. Recently, we theoretically and experimentally demonstrated a new technique for implementing distributed-feedback (DFB), which is termed as an antenna- feedback scheme, to establish a hybrid SPP mode in the surrounding medium of a plasmonic laser's cavity with a large wavefront. This technique allows such lasers to radiate in narrow beams without requirement of any specific design considerations for phase-matching. Experimental demonstration is done for terahertz QCLs that show beam-divergence as small as 4-degrees. The antenna-feedback scheme has a characteristic feature in that refractive-index of the laser's surrounding medium affects its radiative frequency in the same vein as refractive- index of the cavity. Hence, any perturbations in the refractive-index of the surrounding medium could lead to large modulation in the laser's emission frequency. Along this line, we report ∼57 GHz reversible, continuous, and mode-hop-free tuning of such QCLs operating at 78 K based on post-process deposition/etching of a dielectric on an already mounted QCL chip. This is the largest tuning range achieved for terahertz QCLs when operating much above the temperature of liquid-Helium. We review the aforementioned experimental results and discuss methods to increase optical power output from terahertz QCLs with antenna-feedback. Peak power output of ∼13 mW is realized for a 3.3 THz QCL operating in a Stirling cooler at 54 K. A new dual-slit photonic structure based on antenna-feedback scheme is proposed to further improve output power as well as provide enhanced tunability.
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 metasurface. 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.
A key parameter underlying the efficacy of any nonlinear optical process is group velocity dispersion. In quantum cascade lasers (QCLs), there have been several recent demonstrations of devices exploiting nonlinearities in both the mid-infrared and the terahertz. Though the gain of QCLs has been well studied, the dispersion has been much less investigated, and several questions remain about its dynamics and precise origin. In this work, we use time-domain spectroscopy to investigate the dispersion of broadband terahertz QCLs, and demonstrate that contributions from both the material and the intersubband transitions are relevant. We show that in contrast to the laser gain—which is clamped to a fixed value above lasing threshold—the dispersion changes with bias even above threshold, which is a consequence of shifting intersubband populations. We also examine the role of higher-order dispersion in QCLs and discuss the ramifications of our result for devices utilizing nonlinear effects, such as frequency combs.
We present all-dielectric 2D and 3D metamaterials that are monolithically fabricated from III-V semiconductor nanostructures. The active/gain and high optical nonlinearity properties of the metamaterials can lead to new classes of active devices.
We experimentally observe large enhancement of second-harmonic generation (SHG) from GaAs metasurfaces. The SHG polarization when excited at the electric and magnetic dipole resonances is orthogonal and can be attributed to different nonlinear generation mechanisms.
Contemporary research into diagonal active region terahertz quantum cascade lasers for high temperature operation has yielded little success. We present evidence that the failure of high diagonality alone as a design strategy is due to a fundamental trade-off between large optical oscillator strength and long upper-level lifetime. We hypothesize that diagonality needs to be paired with increased doping in order to succeed, and present evidence that highly diagonal designs can benefit from much higher doping than normally found in terahertz quantum cascade lasers. In assuming the benefits of high diagonality paired with high doping, we also highlight important challenges that need to be overcome, specifically the increased importance of carrier induced band-bending and impurity scattering.
Xu, Luyao; Chen, Daguan; Itoh, Tatsuo; Reno, J.L.; Williams, Benjamin S.
A terahertz vertical-external-cavity surface-emitting-laser (VECSEL) is demonstrated using an active focusing reflectarray metasurface based on quantum-cascade gain material. The focusing effect enables a hemispherical cavity with flat optics, which exhibits higher geometric stability than a plano-plano cavity and a directive and circular neardiffraction limited Gaussian beam with M2 beam parameter as low as 1.3 and brightness of 1.86 � 106 Wsr-1m-2. This work initiates the potential of leveraging inhomogeneous metasurface and reflectarray designs to achieve high-power and high-brightness terahertz quantum-cascade VECSELs.