Marco Liscidini

Photonic light-trapping versus Lambertian limits in thin film silicon solar cells with 1D and 2D periodic patterns.

We theoretically investigate the light-trapping properties of one- and two-dimensional periodic patterns etched on the front surface of c-Si and a-Si thin film solar cells with a silver back reflector and an anti-reflection coating. For each active material and configuration, absorbance A and short-circuit current density Jsc are calculated by means of rigorous coupled wave analysis (RCWA), for different active materials thicknesses in the range of interest of thin film solar cells and in a wide range of geometrical parameters. The results are then compared with Lambertian limits to light-trapping for the case of zero absorption and for the general case of finite absorption in the active material. With a proper optimization, patterns can give substantial absorption enhancement, especially for 2D patterns and for thinner cells. The effects of the photonic patterns on light harvesting are investigated from the optical spectra of the optimized configurations. We focus on the main physical effects of patterning, namely a reduction of reflection losses (better impedance matching conditions), diffraction of light in air or inside the cell, and coupling of incident radiation into quasi-guided optical modes of the structure, which is characteristic of photonic light-trapping.

Ultra-low power generation of twin photons in a compact silicon ring resonator

We demonstrate efficient generation of correlated photon pairs by spontaneous four wave mixing in a 5 μm radius silicon ring resonator in the telecom band around 1550 nm. By optically pumping our device with a 200 μW continuous wave laser, we obtain a pair generation rate of 0.2 MHz and demonstrate photon time correlations with a coincidence-to-accidental ratio as high as 250. The results are in good agreement with theoretical predictions and show the potential of silicon micro-ring resonators as room temperature sources for integrated quantum optics applications.

Micrometer-scale integrated silicon source of time-energy entangled photons

Entanglement is a fundamental resource in quantum information processing. Several studies have explored the integration of sources of entangled states on a silicon chip, but the devices demonstrated so far require millimeter lengths and pump powers of the order of hundreds of milliwatts to produce an appreciable photon flux, hindering their scalability and dense integration. Microring resonators have been shown to be efficient sources of photon pairs, but entangled state emission has never been proven in these devices. Here we report the first demonstration, to the best of our knowledge, of a microring resonator capable of emitting time-energy entangled photons. We use a Franson experiment to show a violation of Bell’s inequality by more than seven standard deviations with an internal pair generation exceeding 107  Hz. The source is integrated on a silicon chip, operates at milliwatt and submilliwatt pump power, emits in the telecom band, and outputs into a photonic waveguide. These are all essential features of an entangled state emitter for a quantum photonic network.

Light trapping regimes in thin-film silicon solar cells with a photonic pattern

By patterning thin-film silicon solar cells with a periodic etching in addition to an antireflection coating, we increase the short-circuit current up to 36.5%. The pattern and the coating are investigated to recognise different coupling regimes.

Analysis of Bloch-surface-wave assisted diffraction-based biosensors

A systematic study of Bloch surface wave (BSW) properties and applications in diffraction-based biosensors is presented. The design of such devices starts with the calculation of the BSW dispersion relation for a semi-infinite one-dimensional photonic crystal. We propose an approach in which polarization and 1DPC termination effects are simply described. Since in a realistic device the number of periods is limited, we investigate the issues arising from finite size effects and the choice of a structure substrate. Diffraction efficiency is studied as a function index contrast, multilayer termination, grating thickness, and number of periods. Numerical examples for Si/SiO2 and a-Si1−xNx:H periodic dielectric stacks are presented, showing that BSW can be exploited for the realization of efficient diffraction-based biosensors from the infrared to the visible range.

Spontaneous four-wave mixing in microring resonators

We consider spontaneous four-wave mixing in a microring resonator, presenting photon-pair generation rates and biphoton wave functions. We show how generation rates can be simply predicted from the performance of the device in the classical regime and that a wide variety of biphoton wave functions can be achieved by varying the pump pulse duration.

Direct evidence of light confinement and emission enhancement in active silicon-on-insulator slot waveguides

The authors experimentally demonstrate strong light confinement and enhancement of emission at 1.54μm in planar silicon-on-insulator waveguides containing a thin layer (slot) of SiO2 with Er3+ doped Si nanoclusters. Angle-resolved attenuated total reflectance is used to excite the slab guided modes, giving a direct evidence of the strong confinement of the electric field in the low-index active material for the fundamental transverse-magnetic mode. By measuring the guided photoluminescence from the cleaved-edge of the sample, the authors observe a more than fivefold enhancement of emission for the transverse-magnetic mode over the transverse-electric one. These results show that Si-based slot waveguides could be important as starting templates for the realization of Si-compatible active optical devices.

Porous silicon structures for low-cost diffraction-based biosensing

We present a strategy for label-free biosensing using porous silicon diffraction gratings. The gratings are fabricated using a cost-effective, high-throughput stamping technique. Unlike traditional diffraction-based biosensors that rely on microcontact printing or lithography to create gratings for the localization of analytes on the top surface of the grating, in our structure analytes are free to infiltrate the porous network and increase the effective refractive index of the grating. The large surface area of porous silicon available for molecular binding offers the potential for enhanced diffraction response compared to nonporous gratings with limited surface area. Small molecule detection of 3-aminopropyltriethoxysilane is demonstrated.

Engineering Gaussian disorder at rough interfaces for light trapping in thin-film solar cells

A theoretical study of randomly rough interfaces to obtain light trapping in thin-film silicon solar cells is presented. Roughness is modeled as a surface with Gaussian disorder, described using the root mean square of height and the lateral correlation length as statistical parameters. The model is shown to describe commonly used rough substrates. Rigorous calculations, with short-circuit current density as a figure of merit, lead to an optimization of disorder parameters and to a significant absorption enhancement. The understanding and optimization of disorder is believed to be of general interest for various realizations of thin-film solar cells.

Enhancement of diffraction for biosensing applications via Bloch surface waves

We propose a biosensor based on the diffraction of Bloch surface waves (BSWs) in periodic dielectric stacks. Significant enhancement of diffraction efficiency by a biomolecule grating placed on the multilayer is predicted when a BSW is excited through a prism in the Kretschmann configuration. Numerical calculations for BSW in a Si∕SiO2 dielectric stack show an increase of diffraction intensity up to three orders of magnitude with respect to the case of surface plasmon wave enhancement. The mechanism that leads to large field confinement and the absence of absorption losses in the dielectric system make this solution flexible and suitable to different grating periods.