Silicon Nanophotonics
After dominating the electronics industry for decades, silicon is on the verge of entering the field of integrated photonics - the traditional stronghold of III–V semiconductors. The development of silicon based photonics lags far behind, despite silicon nano-fabrication methods being extremely well developed. This is primarily due to lack of practical silicon light sources, i.e., efficient silicon light-emitting diodes and injection lasers, since other integrated optical components (e.g. modulators, switches and detectors all exist with excellent performance.
Silicon is an indirect-band-gap semiconductor material for which light emission is a phonon mediated process with a low probability. This gives rise to spontaneous e-h recombination lifetimes in the millisecond range, much longer than the typical timescale for non-radiative recombination. As a result, the internal quantum efficiency for intrinsic silicon is very low with typical values being around ~10−6. With regard to lasing in silicon, fast and efficient non-radiative processes such as Auger or free-carrier absorption prevent population inversion at the high pumping rates needed to achieve optical amplification.
The overall aim of the silicon photonics project is to develop efficient monolithic light sources on a silicon chip. We investigate two different approaches employing nanophotonic effects from passive silicon photonic crystal nanocavities (click for further details) and germanium-islands as active emitter in a silicon photonic crystal nanocavities (click for further details).
Novel photonic bio-sensing based on Silicon nanostructures
Over recent years Silicon photonic crystals have received great attention due to their strong potential applications in the field of integrated optics and sensing. Optical resonators formed by introducing defects into two-dimensional photonic crystal slabs are capable of localizing light and can exhibit high-Q optical modes. The cavity mode frequency and Q of such structures can be tailored by careful design of their structural parameters and is, furthermore highly sensitive to the refractive index of the environment, like the surface of the device. Unlike alternative sensing platforms that utilize the interaction between the small evanescent tail of the electromagnetic field and the analyte, PhCs can be designed to localize the electric field in the low refractive index region, which makes the sensors extremely sensitive to a small refractive index change. When compared to conventional sensors with typical sensitive areas of ≈1mm2, the area is dramatically decreased to ≈1 μm2 and analyte volumes in the order of fl are sufficient.
We take advantage of these properties to investigate label-free bio-sensor applications based on the linear optical response of the system. The detection of specific bio-molecules without the use of radioactive or fluorescent labels reduces the complexity in the screening process and allows time resolved measurements without affecting the intrinsic properties of the target molecules. Silicon technology allows us to integrate these structures into lab-on-chip devices leading to highly sensitive bio-sensors based on the optical response of semiconductor nanostructures.
News
We present a temperature-dependent photoluminescence study of silicon optical nanocavities formed by introducing point defects into two-dimensional photonic crystals. In addition to the prominent TO-phonon-assisted transition from crystalline silicon at ~1.10 eV, we observe a broad defect band luminescence from ~1.05 to ~1.09 eV. Detectable emission from the cavity mode persists up to room temperature; in strong contrast, the background emission vanishes for T≥150 K.
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Funding
We gratefully acknowledge funding from: