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We report on uncooled mid-infrared photovoltaic responses at 300 K arising in heterojunctions of reduced graphene oxide with p-Si. Two major photoresponse spectral peaks are observed, one in the near infrared starting at 1.1 μm corresponding to electron-hole pair generation in the Si substrate, and another at wavelengths below 2.5 μm, arising from properties of the reduced graphene oxide-Si heterojunction. Our analysis of the current-voltage characteristics at various temperatures suggests that the two materials form a type-II (broken-gap) heterojunction, with a characteristic transition between direct tunneling to field emission, to over-the-barrier excitation with increasing reverse voltage. Illumination was found to affect the onset of the transition between direct tunneling and field-emission, suggesting that the mid infrared response results from the excitation of minority carriers (electrons) from the Si and their collection in the reduced graphene oxide contact. The photoresponse near 1.1 μm showed a time constant at least five times faster than the one at 2.5 μm, which points to surface defects as well as high series resistance and capacitance as potentially limiting factors in this mode of operation. With proper device engineering considerations, these devices could be promising as a graphene-based platform for infrared sensing.

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Physical coloration without chemicals offers a pathway to develop pollution-free coloration technology, and can be applied to colorimetric sensing of gases, toxic and chemical agents. In this paper, we report on realization of a high-purity red color using mechanism of mode-selective absorption in a thin-film optical cavity. By placing an ultra-thin absorber layer at the antinode of a targeted spectral band in a Fabry-Perot cavity, its otherwise conventional dichroic reflection spectrum is shaped into a broad rectangular flat-bottom one that gives the desired vivid red. The purity of our demonstrated red color reaches 76%, which is increased by 16% compared with those reported in prior thin-film structures. Our method of mode-selective absorption is adaptable to more general-purpose spectral shaping, and could be applied in producing other target colors as well as broadband light absorption for energy harvesting and infrared detection.

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Structural coloration is a quickly growing field, encompassing physical and photonic processes such as interference, diffraction, and scattering. In this study, we investigated the optical effects in the visible wavelength range, and in particular, the colour gamuts achievable with absorber–dielectric–metal sandwich structures. These chemical‐free layered structures are highly tunable, easily scaled, optical cavities that are capable of generating remarkable colours whose properties are determined completely by material and structural parameters. We employed experimental and numerical strategies to demonstrate that each absorber spans a unique colour gamut, i.e. a subset of the full chromaticity space. While gamut overlap is observed between different absorber types, the gamut areas unique to each absorber occur at different hues of high excitation purity. A comprehensive understanding of how these colour gamuts develop and how different materials may be combined to expand larger subsets of the chromaticity space is required in order to maximize the variety of colours achievable with this system and elevate it into a ‘structural coloration technology’.

We demonstrate optical rectification in a reconfigurable and relatively simple nanoscopic tunneling junction formed via resistive switching. In optical rectification, electrons must keep up with the rapid oscillations of an illuminating optical field and harness the nonlinearities of a tunneling contact to produce the desired DC field. Among the intrinsic requirements for such devices are tunneling junctions with an exceedingly small capacitance and surface area. In contrast to tunneling junctions formed by different methods, the resistive switching approach explored here allows the system to be tuned, set, and reset via the application of DC electric fields. This makes it ideally suitable for exploring optical rectification phenomena under different tunneling conditions and for dynamically tuning the device's responsivity. This “on-the-go” tunability opens the possibility for adaptive devices, such as ultrafast photon detectors, wireless power transmitters, and energy harvesting systems.

Existing structural coloration methods using thin films, commonly implemented in high-purity aluminium, produce colours which are highly dependent on the viewing angle because of the inherent angular dependence of thin film interference. Adapting the thin film coloration mechanism to anodisation of industrial-quality aluminium alloys, which scatter light more efficiently than their high-purity counterparts, reduces angle dependence in the colour produced. This reduction of angle dependence, as well as the wide use of anodised aluminium in consumer products, suggests that structural colour based on anodised aluminium could potentially be scaled up for commercial scale production.