Efficient on-chip molecule and bio-agent detection can be achieved by accessing strong molecular absorption lines in the mid-infrared, but it requires high output power broadband mid-IR sources. Here, we report supercontinuum generation in an air-clad Si 0.6 Ge 0.4 ∕Si waveguide that emits a broad spectrum spanning from 3.0 μm to 8.5 μm. These waveguides have anomalous dispersion and low propagation loss (<0.4 dB∕cm) in the mid-IR, which leads to a supercontinuum output with a high average power of more than 10 mW on-chip. The realization of broadband mid-IR sources with high spectral brightness makes the SiGe-on-Si platform promising for a wide range of applications.

Abstract:We characterize the nonlinear optical response of low loss Si 0.6 Ge 0.4 / Si waveguides in the mid-infrared between 3.3 μm and 4 μm using femtosecond optical pulses. We estimate the three and four-photon absorption coefficients as well as the Kerr nonlinear refractive index from the experimental measurements. The effect of multiphoton absorption on the optical nonlinear Kerr response is evaluated and the nonlinear figure of merit estimated providing some guidelines for designing nonlinear optical devices in the mid-IR. Finally, we compare the impact of free-carrier absorption at mid-infrared wavelengths versus near-infrared wavelengths for these ultra-short pulses. References and links1. R. Soref, "Mid-infrared photonics in silicon and germanium," Nat. Photonics 4(8), 495-497 (2010). 2. B. Jalali, "Nonlinear optics in the mid-infrared," Nat. Photonics 4(8), 506-508 (2010). Brun, S. Ortiz, P. Labeye, S. Nicoletti, and C. Grillet, "Nonlinear optical response of low loss silicon germanium waveguides in the mid-infrared," Opt. Express 23(7), 8261-8271 (2015).

Dark-field microscopy is a widely used tool for measuring the optical resonance of plasmonic nanostructures. However, current numerical methods for simulating the dark-field scattering spectra were carried out with plane wave illumination either at normal incidence or at an oblique angle from one direction. In actual experiments, light is focused onto the sample through an annular ring within a range of glancing angles. In this paper, we present a theoretical model capable of accurately simulating the dark-field light source with an annular ring. Simulations correctly reproduce a counterintuitive blue shift in the scattering spectra from gold nanodisks with a diameter beyond 140 nm. We believe that our proposed simulation method can be potentially applied as a general tool capable of simulating the dark-field scattering spectra of plasmonic nanostructures as well as other dielectric nanostructures with sizes beyond the quasi-static limit.

Localized surface plasmon resonance (LSPR) on gold nanoparticles and nanostructures shows its capability to modulate the spontaneous emission rate for photoluminescence. However, beyond the quasi-static limit with oblique incidence conditions, probing a retardation-based plasmonic resonant state excited by the horizontal incident wave-vector component has not been reported yet in the plasmon-mediated photoluminescence of gold nanostructures. In this paper, we investigated the photoluminescence of individual and coupled gold nanodisks with increasing size, and the peaks in the size-dependent photoluminescence spectra are shown to originate from the preferential excitation of vertical (LSPR) and horizontal (retardationbased) plasmonic resonant states. We have also proposed a three-step model to well describe the contribution of vertical and horizontal plasmonic resonant states in the photoluminescence of gold nanodisks. Our study makes a new contribution to the understanding of photoluminescence from gold nanostructures, and it paves the way toward the applications of plasmonmediated luminescence and biological studies.

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