Hyperbolic metamaterials (HMMs) have attracted much attention because they allow for broadband enhancement of spontaneous emission and imaging below the diffraction limit. However, HMMs with traditional metals as metallic component are not suitable for applications in the infrared spectral range. Using Ga-doped ZnO, we demonstrate monolithic HMMs operating at infrared wavelengths. We identify the material's hyperbolic character by various optical measurements in combination with theoretical calculations. In particular, negative refraction of the extraordinary wave and propagation of light with wave vector values exceeding that of free-space are demonstrated in the entire telecommunication window. These findings reveal a considerable potential for creating novel functional elements at telecommunication wavelengths.
Demonstration of hyperbolic metamaterials at telecommunication wavelength using Ga-doped ZnO, Optics Express, 23,32555-32560 (2015)
We demonstrate negative refraction at telecommunication wavelengths through plasmon-photon hybridization on a simple microcavity with metallic mirrors. Instead of using conventional metals, the plasmonic excitations are provided by a heavily doped semiconductor which enables us to tune them into resonance with the infrared photon modes of the cavity. In this way, the dispersion of the resultant hybrid cavity modes can be widely adjusted. In particular, negative dispersion and negative refraction at telecommunication wavelengths on an all-ZnO monolithical cavity are demonstrated.
Negative refraction at telecommunication wavelengths through plasmon-photon hybridization, Opt. Express , 23, 30079-30087 (2015), Special Issue Surface Plasmon Photonics 2015
Sn-doped In2O3 films with free-electron concentration varied up to 1.7 x 1021 cm-3 are prepared by molecular beam epitaxy. In this way, a metallic Drude-type dielectric function with a negative real part extending beyond λ = 1050 nm is created. Despite essentially polycrystalline structure of the layers, the plasmonic damping Γ is found not to exceed 70 meV in the entire doping range making excitation of low-loss surface plasmon polaritons at frequencies fully covering the telecommunication band feasible. A monotonically increasing discrepancy between the carrier concentration obtained from Hall-effect and the concentration extracted from fitting the optical spectra hints at a change of the band-structure related parameters of In2O3 with increasing Sn-doping.
Surface plasmon polaritons with plasma frequencies above 1 eV in Sn-doped In2O3, Phys. Status Solidi B, (2015)
Longitudinal bulk plasmons in an n-doped ZnO layer system are studied by two-color femtosecond pump-probe spectroscopy in the midinfrared. The optical bulk plasmon resonance identified in linear reflectivity spectra undergoes a strong redshift and a limited broadening upon intraband excitation of electrons. The nonlinear changes of plasmon absorption decay on a time scale of 2 ps and originate from the intraband redistribution of electrons. Theoretical calculations explain the plasmon redshift by the transient increase of the ensemble-averaged electron mass and the concomitantly reduced plasma frequency in the hot electron plasma. The observed bulk plasmon nonlinearity holds strong potential for applications in plasmonics.
Ultrafast Nonlinear Response of Bulk Plasmons in Highly Doped ZnO Layers, Phys.Rev. Lett., 115, 147401 (2015)
Single crystalline thin films of Er2O3, demonstrating efficient 1.5 lm luminescence of Er3+ at room temperature were grown on Al2O3 substrate by molecular beam epitaxy. The absorption coefficient at 1.536 lm was found to reach 270 cm-1 translating in a maximal possible gain of 1390 dBcm-1. In conjunction with the 10% higher refractive index as compared to Al2O3, this opens the possibility to use Er2O3:sapphire films as short-length waveguide amplifiers in telecommunication.
Single crystalline Er2O3:sapphire films as potentially high-gain amplifiers at telecommunication wavelength, Appl. Phys. Lett., 105, 191111 (2014)
Adjusting the free-electron concentration, the surface plasmon frequency of the semiconductor ZnOGa is tuned into resonance with the molecular vibrations of the n-alkane tetracontane. Closed molecular films deposited on the semiconductor's surface in the monolayer regime generate distinct signatures in total-attenuated-reflection spectra at the frequencies of the symmetric and asymmetric stretching vibrations of the CH2 group. Their line shape undergoes profound changes from absorptive to dispersive and even antiresonance behavior when moving along the surface-plasmon dispersion by the angle of incidence. We demonstrate that this line-shape diversity results from a phase-sensitive perturbation of the surface-plasmon-polariton generation at the molecule-metal interface.
Resonant interaction of molecular vibrations and surface plasmon polaritons: The weak coupling regime, Phys. Rev. B, 90, 125423 (2014)
The use of the free-electron gas in heavily doped ZnO:Ga enables the realization of almost arbitrarily shaped surface-plasmon-polariton dispersion curves in planar geometries. In particular, by preparing metal-metal-type interfaces, we demonstrate surface-plasmon polaritons exhibiting finite frequencies in the long-wavelength limit. Moreover, coupling of surface plasmon polaritons at adjacent interfaces allows for the controlled formation of frequency gaps or, alternatively, the opening of otherwise forbidden regions by an appropriate layer design. Our findings reveal a considerable plasmonic potential of this semiconductor-based approach, e.g., for achieving propagation control or phase matching for nonlinear optical processes as well as novel many-particle phenomena. Given the advanced possibilities to structure semiconductors down to the nanometer-length scale, we anticipate novel plasmonic functional elements and devices even beyond the planar setting of our study.
ZnO as a Tunable Metal: New Types of Surface Plasmon Polaritons, Phys.Rev. Lett., 112, 137401 (2014)
Zn(Mg)O:Ga films of high crystalline perfection can be grown by molecular beam epitaxy in a two-dimensional mode up to Ga mole fractions of about 6 %. The doping efficiency close to 100 % results in free-carrier concentrations of almost 1021 cm-3. The structural quality of the films deposited on sapphire shows only slight degradation with increasing the doping level and can be further significantly improved by using ZnO wafers. The Hall mobilities of the samples with Ga concentrations n = 1019-1021 cm-3 lay in the range of 45-55 cm2/Vs and 25-39 cm2/Vs for ZnO:Ga and ZnMgO:Ga, correspondingly. Both, the high electron concentrations and the carrier mobilities result in low resistivities of the layers comparable with ITO films. The Ga doping is associated with the emergence of a distinct reflectivity electron-plasma edge in the optical spectra shifting towards higher energies with increasing n. Fitting the optical reflectivity, absorption and transmission spectra by the Drude model for a free electron gas provides zero-crossover wavelength of the real part of the dielectric function as well as plasmonic damping. The crossover wavelenght can be tuned by selecting the doping level in the range of 8-1.36 µm, thus reaching telecommunication wavelengths, while the damping does not exceed 50 meV.
The capability of 2D growth together with similar findings for ZnMgO:Ga enable the fabrication of well-defined multilayer heterostructures with tailored electrical properties and dispersion relations, e.g. for use as transparent highly conductive layers or as an active part of plasmonic metamaterials.
letzte Änderung: 21.09.2015 id