Terahertz nano-spectroscopy with resonant scattering probes

We propose and demonstrate tunable resonant scattering probes for terahertz (THz) near-field microscopy, using sharp indium tips fabricated to the tine of a quartz tuning fork. We find the antenna resonance of the indium tips can be tuned by altering the tip length, which we support with numerical models. We also demonstrate the indium tips can provide nanoscale field confinement at the tip apex, with spatial resolution better than 100 nm

THz Aperture Near-Field Spectroscopy of Dirac Plasmons in Topological Insulator Bi2Se3

We map the dispersion relation in ribbon gratings of topological insulator Bi2Se3 using both far-field and aperture-type near-field THz spectroscopy methods. The results are consistent with theoretical predictions of coupling between the optical phonon and massless Dirac plasmons

Tunable Fully Absorbing Metasurfaces for Efficient THz Detection

Terahertz photoconductive antennas with a nanostructured active region have been actively investigated recently with a goal to achieve high efficiency THz detectors and emitters. Here we provide a novel design of perfectly-absorbing photoconductive region without plasmonic elements using a metasurface, and provide a systematic method by which the metasurface can be designed to work optimally for varying optical gate frequencies across the GaAs band-gap. This paves the way to using metasurface devices for THz detection and other applications in a wide range of laser systems operating at different wavelengths or with different photoconductive materials

Perfectly-absorbing photoconductive metasurfaces for THz applications

Ultrafast switching of photoconductivity is essential for many terahertz (THz) technologies, however this process is inefficient. Recently developed concepts of all-dielectric metasurfaces can improve efficiency of ultrafast switches, overcoming material limitations, reducing the thickness of the photoconductive region and lowering optical power requirements for THz devices. We will consider two types of perfectly absorbing metasurfaces compatible with the photoconductive switch architecture and discuss performance of THz detectors with integrated metasurfaces. We will show that optical power level required for optimum operation for these THz detectors is more than one order of magnitude lower in comparison to devices without metasurfaces

Perfect absorption in GaAs metasurfaces near the bandgap edge

Perfect optical absorption occurs in a metasurface that supports two degenerate and critically-coupled modes of opposite symmetry. The challenge in designing a perfectly absorbing metasurface for a desired wavelength and material stems from the fact that satisfying these conditions requires multi-dimensional optimization often with parameters affecting optical resonances in non-trivial ways. This problem comes to the fore in semiconductor metasurfaces operating near the bandgap wavelength, where intrinsic material absorption varies significantly. Here we devise and demonstrate a systematic process by which one can achieve perfect absorption in GaAs metasurfaces for a desired wavelength at different levels of intrinsic material absorption, eliminating the need for trial and error in the design process. Using this method, we show that perfect absorption can be achieved not only at wavelengths where GaAs exhibits high absorption, but also at wavelengths near the bandgap edge. In this region, absorption is enhanced by over one order of magnitude compared a layer of unstructured GaAs of the same thickness

Sensitivity and Noise in THz Photoconductive Metasurface Detectors

Photoconductive antenna THz detectors based on highly absorbing LT-GaAs metasurfaces enable high sensitivity and high signal-to-noise ratio (> 106) at optical gate powers as low as 5 μW. By investigating the dependence of detector performance on optical gate power, we compare several metasurface detectors with standard PCAs and develop a general model for quantifying the sensitivity and optimal gate power for detector operation. We also show that the LT-GaAs metasurface can even enhance sub bandgap absorption, enabling the use of these detectors in telecom wavelength systems

Nonlinear Terahertz Generation in Semiconductor Metasurfaces

We demonstrate ultra-thin semiconductor metasurfaces for generation of THz pulses. By investigating the dependence of the THz amplitude and phase on excitation field polarization and crystal orientation, we deduce that the underlying THz emission mechanism in metasurfaces differs from bulk semiconductor wafers with second order nonlinearity playing a dominant role. The metasurface enables control of the THz phase and can therefore be used to spatially structure the THz emitted field. We use this effect to design and demonstrate a metasurface which simultaneously emits and focusses THz pulses

Terahertz Generation from GaAs Metasurfaces: Role of Surface Nonlinearity

We show that a GaAs metasurface can generate THz radiation with comparable efficiency to a bulk GaAs crystal. We attribute the enhanced generation to second order nonlinearity with the surface making a strong contribution