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We demonstrate electrical control over coherent optical absorption in a graphene-based Salisbury screen consisting of a single layer of graphene placed in close proximity to a gold back reflector. The screen was designed to enhance light absorption at a target wavelength of 3.2 μm by using a 600 nm-thick, nonabsorbing silica spacer layer. An ionic gel layer placed on top of the screen was used to electrically gate the charge density in the graphene layer. Spectroscopic reflectance measurements were performed in situ as a function of gate bias. The changes in the reflectance spectra were analyzed using a Fresnel based transfer matrix model in which graphene was treated as an infinitesimally thin sheet with a conductivity given by the Kubo formula. The analysis reveals that a careful choice of the ionic gel layer thickness can lead to optical absorption enhancements of up to 5.5 times for the Salisbury screen compared to a suspended sheet of graphene. In addition to these absorption enhancements, we demonstrate very large electrically induced changes in the optical absorption of graphene of ∼3.3% per volt, the highest attained so far in a device that features an atomically thick active layer. This is attributable in part to the more effective gating achieved with the ion gel over the conventional dielectric back gates and partially by achieving a desirable coherent absorption effect linked to the presence of the thin ion gel that boosts the absorption by 40%.

Nanostructured metallic films have the potential to replace metal oxide films as transparent electrodes in optoelectronic devices. An ideal transparent electrode should possess a high, broadband, and polarization-independent transmittance. Conventional metallic gratings and grids with wavelength-scale periodicities, however, do not have all of these qualities. Furthermore, the transmission properties of a nanostructured electrode need to be assessed in the actual dielectric environment provided by a device, where a high-index semiconductor layer can reflect a substantial fraction of the incident light. Here we propose nanostructured aluminum electrodes with space-filling fractal geometries as alternatives to gratings and grids and experimentally demonstrate their superior optoelectronic performance through integration with Si photodetectors. As shown by polarization and spectrally resolved photocurrent measurements, devices with fractal electrodes exhibit both a broadband transmission and a flat polarization response that outperforms both square grids and linear gratings. Finally, we show the benefits of adding a thin silicon nitride film to the nanostructured electrodes to further reduce reflection.

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This paper presents a novel technique for the integration of small CMOS chips into a large area substrate. A key component of the technique is the CMOS chip based self-aligned masking. This allows for the fabrication of sockets in wafers that are at most 5 µm larger than the chip on each side. The chip and the large area substrate are bonded onto a carrier such that the top surfaces of the two components are flush. The unique features of this technique enable the integration of macroscale components, such as leads and microfluidics. Furthermore, the integration process allows for MEMS micromachining after CMOS die-wafer integration. To demonstrate the capabilities of the proposed technology, a low-power integrated potentiostat chip for biosensing implemented in the AMI 0.5 µm CMOS technology is integrated in a silicon substrate. The horizontal gap and the vertical displacement between the chip and the large area substrate measured after the integration were 4 µm and 0.5 µm, respectively. A number of 104 interconnects are patterned with high-precision alignment. Electrical measurements have shown that the functionality of the chip is not affected by the integration process.