Luminescent hyperbolic metasurfaces

Abstract

When engineered on scales much smaller than the operating wavelength, metal-semiconductor nanostructures exhibit properties unobtainable in nature. Namely, a uniaxial optical metamaterial described by a hyperbolic dispersion relation can simultaneously behave as a reflective metal and an absorptive or emissive semiconductor for electromagnetic waves with orthogonal linear polarization states. Using an unconventional multilayer architecture, we demonstrate luminescent hyperbolic metasurfaces, wherein distributed semiconducting quantum wells display extreme absorption and emission polarization anisotropy. Through normally incident micro-photoluminescence measurements, we observe absorption anisotropies greater than a factor of 10 and degree-of-linear polarization of emission >0.9. We observe the modification of emission spectra and, by incorporating wavelength-scale gratings, show a controlled reduction of polarization anisotropy. We verify hyperbolic dispersion with numerical simulations that model the metasurface as a composite nanoscale structure and according to the effective medium approximation. Finally, we experimentally demonstrate >350% emission intensity enhancement relative to the bare semiconducting quantum wells.

Figure 1. LuHMS based on nanostructured Ag/InGaAsP MQW.

(a) InGaAsP MQW pillars of 100 nm height and 40 nm width, separated by 40 nm trenches, are defined by EBL and reactive ion etching. (b) Ag is deposited by sputtering, partially filling the trenches to create a multilayer LuHMS with 80 nm period. (c) Optical pumpingof the LuHMS results in collected emission polarized predominantly parallel to the metacrystal Bloch vector, K_B. The wavelength of peak emission and PL intensity depend strongly on pump polarization. Emission from the LuHMS blue shifts as the pump polarization changes from to due to increasing pump absorption. In addition, PL spectra of the LuHMS differ significantly from that of control MQW, regardless of pump polarization, due to a wavelength and pump power dependence of the direction of energy propagation on the surface.

Figure 2. Demonstration of extreme PA in LuHMS.

(a) Dependence of total emission (Total^E) on pump polarization at emission wavelength of 1,350 nm. Maxima and minima are clearly observed for and pumping, respectively (b) Total PL spectra of the LuHMS for parallel and normal polarized pump. The shape of the spectra differ due to band-filling effects. (c) Pump PA of total emission calculated from b. The pump PA increases with frequency due to band-filling associated with the more efficiently absorbed pump polarization. (d) DOLP of emission for parallel-polarized pump. The DOLP is close to unity, indicating nearly linearly polarized light in the direction parallel to K_B. Inset to d shows SEM of nanostructure and definition of polarizations.

Figure 3. Demonstration of modified emission spectra in LuHMS.

(a,c,e) Evolution of total emission (Total^E) with pump power on linear scale and (b,d,f) log–log plot of total emission as a function of pump power for select wavelengths. (a,b) Control InGaAsP MQW. (c,d) LuHMS excited by pump polarized normal and (e,f) parallel to metacrystal Bloch vector, K_B. Significant blue shifting of peak PL occurs in the LuHMS relative to the control MQW, which is attributed to the wavelength dependence of the principal direction of energy propagation in the LuHMS (see Supplementary Fig. 9 and Supplementary Note 4). Additional blue shifting of peak PL occurs in LuHMS under parallel-polarized pumping due to greater pump absorption. The simultaneous dependence of PL on pump polarization and electronic and optical densities of states suggests an avenue for engineering tunable ‘meta-gain' materials (Supplementary Fig. 14 and Supplementary Note 8).

Figure 4. Demonstration of PA reduction via grating coupling.

(a) LuHMS with wavelength-scale grating coupler fabricated with focused ion beam milling. The grating is designed to improve, both in-coupling of the pump and out-coupling of emission polarized normal to the metacrystal Bloch vector, K_B. Comparison of (b) Pump PA and (c) DOLP in the absence and presence of gratings. Improved coupling of normal-polarized light reduces the pump PA and DOLP, consistent with behaviour of simulated LuHMS. The reduction in anisotropy occurs across all measured grating periods and is robust to small changes in geometric parameters of the LuHMS (see Supplementary Fig. 10 and Supplementary Note 6).