Coherent superposition of light from subwavelength sources is an attractive prospect for the manipulation of the direction, shape and polarization of optical beams. This phenomenon constitutes the basis of phased arrays, commonly used at microwave and radio frequencies. Here we propose a new concept for phased-array sources at infrared frequencies based on metamaterial nanocavities coupled to a highly nonlinear semiconductor heterostructure. Optical pumping of the nanocavity induces a localized, phase-locked, nonlinear resonant polarization that acts as a source feed for a higher-order resonance of the nanocavity. Varying the nanocavity design enables the production of beams with arbitrary shape and polarization. As an example, we demonstrate two second harmonic phased-array sources that perform two optical functions at the second harmonic wavelength (∼5μm): a beam splitter and a polarizing beam splitter. Proper design of the nanocavity and nonlinear heterostructure will enable such phased arrays to span most of the infrared spectrum.
We report continuous, dynamic, reversible, and widely tunable lasing from 367 to 337 nm from single GaN nanowires (NWs) by applying hydrostatic pressure up to ∼7 GPa. The GaN NW lasers, with heights of 4-5 μm and diameters ∼140 nm, are fabricated using a lithographically defined two-step top-down technique. The wavelength tuning is caused by an increasing Γ direct bandgap of GaN with increasing pressure and is precisely controllable to subnanometer resolution. The observed pressure coefficients of the NWs are ∼40% larger compared with GaN microstructures fabricated from the same material or from reported bulk GaN values, revealing a nanoscale-related effect that significantly enhances the tuning range using this approach. This approach can be generally applied to other semiconductor NW lasers to potentially achieve full spectral coverage from the UV to IR.
We experimentally demonstrate efficient third harmonic generation from an indium tin oxide nanofilm (λ/42 thick) on a glass substrate for a pump wavelength of 1.4 μm. A conversion efficiency of 3.3 × 10-6 is achieved by exploiting the field enhancement properties of the epsilon-near-zero mode with an enhancement factor of 200. This nanoscale frequency conversion method is applicable to other plasmonic materials and reststrahlen materials in proximity of the longitudinal optical phonon frequencies.
We employ both the effective medium approximation (EMA) and Bloch theory to compare the dispersion properties of semiconductor hyperbolic metamaterials (SHMs) at mid-infrared frequencies and metallic hyperbolic metamaterials (MHMs) at visible frequencies. This analysis reveals the conditions under which the EMA can be safely applied for both MHMs and SHMs. We find that the combination of precise nanoscale layering and the longer infrared operating wavelengths puts the SHMs well within the effective medium limit and, in contrast to MHMs, allows for the attainment of very high photon momentum states. In addition, SHMs allow for new phenomena such as ultrafast creation of the hyperbolic manifold through optical pumping. In particular, we examine the possibility of achieving ultrafast topological transitions through optical pumping which can photo-dope appropriately designed quantum wells on the femtosecond time scale.
Lasing is demonstrated from gallium nitride nanotubes fabricated using a two-step top-down technique. By optically pumping, we observed characteristics of lasing: a clear threshold, a narrow spectral, and guided emission from the nanotubes. In addition, annular lasing emission from the GaN nanotube is also observed, indicating that cross-sectional shape control can be employed to manipulate the properties of nanolasers. The nanotube lasers could be of interest for optical nanofluidic applications or application benefitting from a hollow beam shape.
Ultrafast optical excitation of photocarriers has the potential to transform undoped semiconductor superlattices into semiconductor hyperbolic metamaterials (SHMs). In this paper, we investigate the optical properties associated with such ultrafast topological transitions. We first show reflectance, transmittance, and absorption under TE and TM plane wave incidence. In the unpumped state, the superlattice exhibits a frequency region with high reflectance (>80%) and a region with low reflectance (<1%) for both TE and TM polarizations over a wide range of incidence angles. In contrast, in the photopumped state, the reflectance for both frequencies and polarizations is very low (<1%) for a similar range of angles. Interestingly, this system can function as an all-optical reflection switch on ultrafast timescales. Furthermore, for TM incidence and close to the epsilon-near-zero point of the longitudinal permittivity, directional perfect absorption on ultrafast timescales may also be achieved. Finally, we discuss the onset of negative refraction in the photopumped state.