Compact Mid-Infrared Gas Sensing Enabled by an All-Metamaterial Design.

The miniaturization of mid-infrared optical gas sensors has great potential to make the "fingerprint region" between 2 and 10 µm accessible to a variety of cost-sensitive applications ranging from medical technology to atmospheric sensing. Here we demonstrate a gas sensor concept that achieves a 30-fold reduction in absorption volume compared to conventional gas sensors by using plasmonic metamaterials as on-chip optical filters. Integrating metamaterials into both the emitter and the detector cascades their individual filter functions, yielding a narrowband spectral response tailored to the absorption band of interest, here CO2. Simultaneously, the metamaterials' angle-independence is maintained, enabling an optically efficient, millimeter-scale cavity. With a CO2 sensitivity of 22.4 ± 0.5 ppm·Hz-0.5, the electrically driven prototype already performs at par with much larger commercial devices while consuming 80% less energy per measurement. The all-metamaterial sensing concept offers a path toward more compact and energy-efficient mid-infrared gas sensors without trade-offs in sensitivity or robustness.

Engineering Graphene Phototransistors for High Dynamic Range Applications.

Phototransistors are light-sensitive devices featuring a high dynamic range, low-light detection, and mechanisms to adapt to different ambient light conditions. These features are of interest for bioinspired applications such as artificial and restored vision. In this work, we report on a graphene-based phototransistor exploiting the photogating effect that features picowatt- to microwatt-level photodetection, a dynamic range covering six orders of magnitude from 7 to 107 lux, and a responsivity of up to 4.7 × 103 A/W. The proposed device offers the highest dynamic range and lowest optical power detected compared to the state of the art in interfacial photogating and further operates air stably. These results have been achieved by a combination of multiple developments. For example, by optimizing the geometry of our devices with respect to the graphene channel aspect ratio and by introducing a semitransparent top-gate electrode, we report a factor 20-30 improvement in responsivity over unoptimized reference devices. Furthermore, we use a built-in dynamic range compression based on a partial logarithmic optical power dependence in combination with control of responsivity. These features enable adaptation to changing lighting conditions and support high dynamic range operation, similar to what is known in human visual perception. The enhanced performance of our devices therefore holds potential for bioinspired applications, such as retinal implants.

Bulk-suppressed and surface-sensitive Raman scattering by transferable plasmonic membranes with irregular slot-shaped nanopores.

Raman spectroscopy enables the non-destructive characterization of chemical composition, crystallinity, defects, or strain in countless materials. However, the Raman response of surfaces or thin films is often weak and obscured by dominant bulk signals. Here we overcome this limitation by placing a transferable porous gold membrane, (PAuM) on the surface of interest. Slot-shaped nanopores in the membrane act as plasmonic antennas and enhance the Raman response of the surface or thin film underneath. Simultaneously, the PAuM suppresses the penetration of the excitation laser into the bulk, efficiently blocking its Raman signal. Using graphene as a model surface, we show that this method increases the surface-to-bulk Raman signal ratio by three orders of magnitude. We find that 90% of the Raman enhancement occurs within the top 2.5 nm of the material, demonstrating truly surface-sensitive Raman scattering. To validate our approach, we quantify the strain in a 12.5 nm thin Silicon film and analyze the surface of a LaNiO3 thin film. We observe a Raman mode splitting for the LaNiO3 surface-layer, which is spectroscopic evidence that the surface structure differs from the bulk. These results validate that PAuM gives direct access to Raman signatures of thin films and surfaces.

Highly Responsive Mid-Infrared Metamaterial Enhanced Heterostructure Photodetector Formed out of Sintered PbSe/PbS Colloidal Quantum Dots.

Efficient and simple-to-fabricate light detectors in the mid infrared (MIR) spectral range are of great importance for various applications in existing and emerging technologies. Here, we demonstrate compact and efficient photodetectors operating at room temperature in a wavelength range of 2710-4250 nm with responsivities as high as 375 and 4 A/W. Key to the high performance is the combination of a sintered colloidal quantum dot (CQD) lead selenide (PbSe) and lead sulfide (PbS) heterojunction photoconductor with a metallic metasurface perfect absorber. The combination of this photoconductor stack with the metallic metasurface perfect absorber provides an overall ∼20-fold increase of the responsivity compared against reference sintered PbSe photoconductors. More precisely, the introduction of a PbSe/PbS heterojunction increases the responsivity by a factor of ∼2 and the metallic metasurface enhances the responsivity by an order of magnitude. The metasurface not only enhances the light-matter interaction but also acts as an electrode to the detector. Furthermore, fabrication of our devices relies on simple and inexpensive methods. This is in contrast to most of the currently available (state-of-the-art) MIR photodetectors that rely on rather expensive as well as nontrivial fabrication technologies that often require cooling for efficient operation.

Metamaterial graphene photodetector with bandwidth exceeding 500 gigahertz.

Although graphene has met many of its initially predicted optoelectronic, thermal, and mechanical properties, photodetectors with large spectral bandwidths and extremely high frequency responses remain outstanding. In this work, we demonstrate a >500 gigahertz, flat-frequency response, graphene-based photodetector that operates under ambient conditions across a 200-nanometer-wide spectral band with center wavelengths adaptable from <1400 to >4200 nanometers. Our detector combines graphene with metamaterial perfect absorbers with direct illumination from a single-mode fiber, which breaks with the conventional miniaturization of photodetectors on an integrated photonic platform. This design allows for much higher optical powers while still allowing record-high bandwidths and data rates. Our results demonstrate that graphene photodetectors can outperform conventional technologies in terms of speed, bandwidth, and operation across a large spectral range.

Modeling Hydrodynamic Charge Transport in Graphene.

Graphene has exceptional electronic properties, such as zero band gap, massless carriers, and high mobility. These exotic carrier properties enable the design and development of unique graphene devices. However, traditional semiconductor solvers based on drift-diffusion equations are not capable of modeling and simulating the charge distribution and transport in graphene, accurately, to its full extent. The effects of charge inertia, viscosity, collective charge movement, contact doping, etc., cannot be accounted for by the conventional Poisson-drift-diffusion models, due to the underlying assumptions and simplifications. Therefore, this article proposes two mathematical models to analyze and simulate graphene-based devices. The first model is based on a modified nonlinear Poisson's equation, which solves for the Fermi level and charge distribution electrostatically on graphene, by considering gating and contact doping. The second proposed solver focuses on the transport of the carriers by solving a hydrodynamic model. Furthermore, this model is applied to a Tesla-valve structure, where the viscosity and collective motion of the carriers play an important role, giving rise to rectification. These two models allow us to model unique electronic properties of graphene that could be paramount for the design of future graphene devices.

Photonic response and temperature evolution of SiO2/TiO2 multilayers.

The microstructural and optical reflectivity response of photonic SiO2/TiO2 nanomultilayers have been investigated as a function of temperature and up to the material system's melting point. The nanomultilayers exhibit high, broadband reflectivities up to 1350 °C with values that exceed 75% for a 1 µm broad wavelength range (600-1600 nm). The optimized nanometer sized, dielectric multilayers undergo phase transformations from anatase TiO2 and amorphous SiO2 to the thermodynamically stable phases, rutile and cristobalite, respectively, that alter their structural morphology from the initial multilayers to that of a scatterer. Nonetheless, they retain their photonic characteristics, when characterized on top of selected substrate foils. The thermal behavior of the nanometer sized multilayers has been investigated by differential thermal analysis (DTA) and compared to that of commercially available, mm-sized, annealed powders. The same melting reactions were observed, but the temperatures were lower for the nm-sized samples. The samples were characterized using X-ray powder diffraction before DTA and after annealing at temperatures of 1350 and 1700 °C. The microstructural evolution and phase compositions were investigated by scanning electron microscopy and energy-dispersive X-ray spectroscopy measurements. The limited mutual solubility of one material to another, in combination with the preservation of their optical reflectivity response even after annealing, makes them an interesting material system for high-temperature, photonic coatings, such as photovoltaics, aerospace re-entry and gas turbines, where ultra-high temperatures and intense thermal radiation are present. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10853-021-06557-y.

Design of CMOS-compatible metal-insulator-metal metasurfaces via extended equivalent-circuit analysis.

Photonic metasurfaces compatible with large-scale production such as CMOS are of importance because they promise cointegration of electronics with photonics for detection, communication and sensing. The main challenges on the way of designing such metasurfaces are: (1) large variety of possible geometrical shapes of metasurface elements that makes finding the most appropriate shape difficult; (2) poor compatibility of available electronic layer stacks with photonics. In this paper we show how to address both of these challenges utilizing extended equivalent-circuit analysis. In a first step we classify the behavior of different metasurfaces using the equivalent circuit. We discover that metasurfaces that use inverted-dipole resonator type exhibit higher tolerance to dielectric spacer thickness, higher angular stability and have similar resonance quality-factor as other types. In the second step we utilize the equivalent-circuit scheme to efficiently optimize the parameters of inverted-dipole based metasurfaces for a layer stack such as given in a CMOS process. Finally, as an example we demonstrate how an inverted-cross structure can be adapted to a commercial 110 nm CMOS process with Al metal layers. We measured peak absorption above 90% at center wavelength around 4 µm with quality factor of approximately 5 and angular stability larger than 60°.

Template Stripping of Perovskite Thin Films for Dry Interfacing and Surface Structuring.

Combining excellent optoelectronic properties with the benefits of solution processability, metal-halide perovskites are promising materials for photovoltaic and light-emitting technologies. To facilitate the integration of perovskite thin films into optoelectronic devices, a need exists for simple and efficient fabrication methods. Here, we present a template-stripping technique to produce ultraflat and flexible perovskite thin films. We apply a one-step spin-coating procedure to produce high-quality CH3NH3PbBr3 perovskite thin films on top of ultraflat silicon templates. These films can be mechanically cleaved from the template using a polymer adhesive to reveal the ultraflat perovskite surfaces. We demonstrate how the flatness and flexibility of the template-stripped films enable new processing strategies based on dry interfacing, by interfacing them with plasmonic hole arrays. Moreover, we show that by using prepatterned silicon templates, structured perovskite surfaces can be produced with feature sizes down to a micrometer.

Broadband, High-Temperature Stable Reflector for Aerospace Thermal Radiation Protection.

A simple and thermally stable photonic heterostructure exhibiting high average reflectivity (⟨R⟩ ≈ 88.8%) across a broad wavelength range (920-1450 nm) is presented. The design combines a thin, highly reflective and broadband metallic substrate (Ta) with an optimized dielectric coating (10 layers) to create an enhanced reflector with improved optical and thermal properties compared to its constituents. The heterostructure exhibits temperature-reversible reflective properties up to 1000 °C. In order to take advantage of the high reflectivity and temperature stable properties of this coating, in a wide range of non-photonic composite materials, we have fabricated heterostructure platelets as additives. By impregnating these additives into other types of materials, their response can be photonically enhanced.  Platelets of such a heterostructure have been introduced inside an organic matrix to increase its broadband reflection performance. The platelet-impregnated matrix displays an average reflectivity improvement from 5% to an average of 55% over a 1000 nm range, making it a suitable additive for next generation thermal protection systems (TPS).