Surface-texturing represents an effective way for improving efficiency in silicon devices, such as light absorbers, photodetectors, and solar cells. In this paper, we examine the angular property of photocurrent response in surface-textured silicon. We characterize photocurrent spectra of both pyramid-textured silicon and un-etched flat silicon at different incident angles for comparison. Our spectral measurements indicate that pyramid-textured silicon exhibits an overall dramatic decrease in the photocurrent response within the wavelength range of 1–1.18 µm at larger incident angles for both s and p polarizations. This angular property is different for un-etched flat silicon, whose photocurrent decreases in a less angle-sensitive manner for s polarization and increases first and then decreases with the incident angle for p polarization as correlated with light reflectance with a characteristic Brewster’s angle. The absence of Brewster’s angle effect in the photocurrent response of pyramid-textured silicon is in agreement with our reflectance simulations. These results help understand the fundamental optical properties induced by surface-texturing in silicon devices.
Surface-enhanced infrared absorption spectroscopy is attractive for molecular sensing due to its access to chemical bonds with high detection sensitivity. Such a spectroscopic method typically operates on localized resonances in subwavelength structured antennas and metamaterials. In this paper, we demonstrate monolayer octadecanethiol detection by using the leaky guided mode in a metal–insulator–metal waveguide, whose angle-tunable dispersion enables coupling to molecular vibrations with a frequency-variable optical resonance. Our results show that, by changing the incident angle from 15° to 75°, the resonance frequency of the leaky guided mode is scanned around the CH2 vibration modes with frequency detuning from −200 cm−1 to 350 cm−1 in wavenumber. As the frequency detuning increases, the vibration signal of both the CH2 symmetric and asymmetric modes increases first and then decreases. The maximum vibration signal of 1%–1.5% is reached at positive and negative frequency detuning values of ±100 cm−1. These sensing properties are explained with a coupled-oscillator model, which suggests that both enhanced near-field and coupling strength between the optical resonance and molecular vibration play an important role for the optimal sensing performance.
Structure-engineered silicon exhibits a wealth of unique optical properties below its bandgap, which holds promise for mid-infrared and terahertz applications such as photodetection, thermophotovoltaics, radiative cooling, and spectroscopy. In this paper, we investigate enhancement mechanisms of sub-bandgap absorption of black silicon fabricated into periodic pyramids. Our measurements indicate that the pyramid structure leads to an enhanced broadband absorption in the wavelength region from 1.5 to 13.07 μm with an efficiency of over 80%. The broadband absorption enhancement is shown to originate from the Rayleigh–Wood anomaly, localized magnetic plasmonic resonance, and graded-index effect, which together facilitate the interaction between light and free-carriers in silicon. These results are helpful for understanding the interaction between light and black silicon.
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