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

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

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

Near-Field Spectroscopy of Individual Asymmetric Split-Ring Terahertz Resonators.

Metamaterial resonators have become an efficient and versatile platform in the terahertz frequency range, finding applications in integrated optical devices, such as active modulators and detectors, and in fundamental research, e.g., ultrastrong light-matter investigations. Despite their growing use, characterization of modes supported by these subwavelength elements has proven to be challenging and it still relies on indirect observation of the collective far-field transmission/reflection properties of resonator arrays. Here, we present a broadband time-domain spectroscopic investigation of individual metamaterial resonators via a THz aperture scanning near-field microscope (a-SNOM). The time-domain a-SNOM allows the mapping and quantitative analysis of strongly confined modes supported by the resonators. In particular, a cross-polarized configuration presented here allows an investigation of weakly radiative modes. These results hold great potential to advance future metamaterial-based optoelectronic platforms for fundamental research in THz photonics

Perfectly absorbing dielectric metasurfaces for photodetection

Perfect absorption of light by an optically thin metasurface is among several remarkable optical functionalities enabled by nanophotonics. This functionality can be introduced into optoelectronic devices by structuring an active semiconductor-based element as a perfectly absorbing all-dielectric metasurface, leading to improved optical properties while simultaneously providing electrical conductivity. However, a delicate combination of geometrical and material parameters is required for perfect absorption, and currently, no general all-dielectric metasurface design fulfills these conditions for a desired semiconductor and operation wavelength. Here, using numerical simulations, we demonstrate that Mie resonators with subwavelength-size interconnecting channels allow this combination of perfect absorption requirements to be satisfied for different wavelengths of operation and different levels of intrinsic material absorption. We reveal the underlying physics and show that interconnecting channels play a critical role in achieving perfect absorption through their effects on the resonant wavelengths and losses for the electric dipole and magnetic dipole modes in Mie resonators. By adjusting only the channel widths, perfect absorption can be achieved for an optically thin GaAs-based metasurface at a desired wavelength of operation in a range from 715 nm to 840 nm, where the intrinsic absorption level in GaAs varies by more than a factor of 2. Optical transmission experiments confirm that these metasurfaces resonantly enhance optical absorption. This work lays out the foundation and guidelines for replacing bulk semiconductors with electrically connected, optically thin, perfectly absorbing metasurfaces in optical detectors

THz Detectors with Photoconductive Metasurfaces Operating at Microwatt Gate Power Levels

We demonstrate terahertz (THz) detectors enabling high signal-to-noise ratio photoconductive detection at gate powers as low as 5μW. This is achieved by integrating a photoconductive metasurface, which enhances the detector efficiency and sensitivity

Thin THz QCL active regions for improved continuous-wave operating temperature

We compare the performance of 10 and 5 μm thick metal-metal waveguide terahertz quantum-cascade laser ridges operating around 2.7 THz and based on a 4-well phonon depopulation active region design. Thanks to reduced heat dissipation and lower thermal resistance, the 5 μm thick material shows an 18 K increase in continuous wave operating temperature compared to the 10 μm material, despite a lower maximum pulsed-mode operating temperature and a larger input power density. A maximum continuous wave operating temperature of 129 K is achieved using the 5 μm thick material and a 15 μm wide ridge waveguide, which lased up to 155 K in the pulsed mode. The use of thin active regions is likely to become increasingly important to address the increasing input power density of emerging 2- and 3-well active region designs that show the highest pulsed operating temperatures