Four-wave mixing in slow light photonic crystal waveguides with very high group index

This work was supported by the EPSRC - UK Silicon Photonics consortium.We report efficient four-wave mixing in dispersion engineered slow light silicon photonic crystal waveguides with a flat band group index of n(g) = 60. Using only 15 mW continuous wave coupled input power, we observe a conversion efficiency of -28 dB. This efficiency represents a 30 dB enhancement compared to a silicon nanowire of the same length. At higher powers, thermal redshifting due to linear absorption was found to detune the slow light regime preventing the expected improvement in efficiency. We then overcome this thermal limitation by using oxide-clad waveguides, which we demonstrate for group indices of n(g) = 30. Higher group indices may be achieved with oxide clad-waveguides, and we predict conversion efficiencies approaching -10 dB, which is equivalent to that already achieved in silicon nanowires but for a 50x shorter length.Publisher PDFPeer reviewe

Ultracompact and low-power optical switch based on silicon photonic crystals

Switching light is one of the most fundamental functions of an optical circuit. As such, optical switches are a major research topic in photonics, and many types of switches have been realized. Most optical switches operate by imposing a phase shift between two sections of the device to direct light from one port to another, or to switch it on and off, the major constraint being that typical refractive index changes are very small. Conventional solutions address this issue by making long devices, thus increasing the footprint, or by using resonant enhancement, thus reducing the bandwidth. We present a slow-light-enhanced optical switch that is 36 times shorter than a conventional device for the same refractive index change and has a switching length of 5.2 m.The work was funded through the EU FP6-FET “Splash” project and we acknowledge the Nanostructuring Platform of EU FP6-NoE “epixnet” for technical support. T. P. White is supported by an 1851 Royal Commission Research Fellowship

Electro-optic modulation in bulk silicon using surface plasmon resonance

The authors acknowledge funding from the EPSRC in the UK under the UK Silicon Photonics project.We propose and present simulated results for a new design of an optical modulator based on Surface Plasmon Polariton (SPP) resonance. The modulator is realized on a bulk silicon substrate, thus offering an opportunity for front-end integration with electronic circuits. The device consists of a dielectric waveguide evanescently coupled to a SPP mode at the interface between bulk silicon and metal. By using SPP resonance we achieved an ultra-high spectral sensitivity (∼5000 nm/refractive index unit) with large modulation bandwidth (90 nm). For a refractive index change of 0.02, we achieved 100 nm shift in resonance wavelength and a modulation depth of ∼10 dB.PostprintPeer reviewe

Control of Q-factor in nanobeam cavities on substrate

In this paper, we demonstrate how to efficiently control the quality factor of silicon nitride nanobeam cavities, grown on a silica substrate and embedded in an upper cladding, by engineering the nanobeam cross-section and the shape of the periodic holes. We propose optimized configurations that are able to overcome the decreasing of the Q-factor when the nanobeam is embedded in an asymmetric medium. More precisely, we show that the maximum achievable quality factor can be designed and tuned in asymmetric configurations where the upper cladding is particularly different from the substrate one. These optimized configurations exhibit high-Q factor and small mode volume over a wide range of the upper cladding refractive index paving the way for the realization of innovative optical sensors and for the compensation of fabrication tolerances in embedded optical nanobeam cavities.Postprin

Optimizing band-edge slow light in silicon-on-insulator waveguide gratings

A systematic analysis of photonic bands and group index in silicon grating waveguides is performed, in order to optimize band-edge slow-light behavior in integrated structures with low losses. A combination of numerical methods and perturbation theory is adopted. It is shown that a substantial increase of slow light bandwidth is achieved when decreasing the internal width of the waveguide and the silicon thickness in the cladding region. It is also observed that a reduction of the internal width does not undermine the performance of an adiabatic taper

Toolkit for photonic integrated circuits based on inverted rib waveguides

This work was supported by an EPSRC Doctoral Prize and a European Research Council Starting under Grant 337508.We have performed an exploration of inverted rib waveguide platform for use in optical backplanes. This entailed the design, optimization, and characterization of a variety of passive optical components that may serve as a basis for the functions required of an on-chip optical networks. The presented design introduces an inverted-rib template, which consists of a polymer waveguide layer. We have successfully fabricated and demonstrated low-loss waveguides, and also functional passive devices such as directional couplers, multimode interferometers, waveguide bends and crossings, and distributed Bragg reflectors. We also demonstrate a way of coupling active components (e.g., in silicon) to such a photonic integrated circuit.PostprintPeer reviewe

Slow light with interleaved p-n junction to enhance performance of integrated Mach-Zehnder silicon modulators

AbstractSlow light is a very important concept in nanophotonics, especially in the context of photonic crystals. In this work, we apply our previous design of band-edge slow light in silicon waveguide gratings [M. Passoni et al, Opt. Express 26, 8470 (2018)] to Mach-Zehnder modulators based on the plasma dispersion effect. The key idea is to employ an interleaved p-n junction with the same periodicity as the grating, in order to achieve optimal matching between the electromagnetic field profile and the depletion regions of the p-n junction. The resulting modulation efficiency is strongly improved as compared to common modulators based on normal rib waveguides, even in a bandwidth of 20–30 nm near the band edge, while the total insertion loss due to free carriers is not increased. The present concept is promising in view of realizing slow-light modulators for silicon photonics with reduced energy dissipation

Hybrid external cavity laser with an amorphous silicon-based photonic crystal cavity mirror

Funding: EU ERC-SG 337508 DANCER, EU H2020 Marie Skłodowska-Curie 713654 and SFI 18/TIDA/6128.The authors present results on the performance of a hybrid external cavity photonic crystal laser-comprising semiconductor optical amplifier, and a 2D photonic crystal cavity fabricated in low-temperature amorphous silicon. The authors demonstrate that lithographic control over amorphous silicon photonic crystal cavity-resonant wavelengths is possible, and that single-mode lasing at optical telecommunications wavelengths is possible on an amorphous silicon platform.Publisher PDFPeer reviewe

Tight focusing of azimuthally polarized optical vortex produced by subwavelength grating

Focusing of an optical vortex with sectorial azimuthal polarization is numerically investigated. An effect of sector numbers on the results of focusing is estimated. It is shown that the focal spot produced by a beam with six sectors does not differ from the ideally azimuthally polarized optical vortex; a difference in the focal spot diameter does not exceed 0.001 of the wavelength. For a four-sectoral beam, the difference does not exceed 0.028 of the wavelength. We have investigated a four-Sector transmission Polarization Converter for a wavelength of 633 nm, that enables the conversion of a linearly polarized incident beam into a mixture of linearly and azimuthally polarized beams. It was experimentally shown that light propagated through the four-Sector transmission Polarization Converter and focused by Fresnel zone plate with a focal length of 532 nm produces focal spot with diameters 0.46 and 0.57 of wavelength.Publisher PD

Highly efficient coupling between a monolithically integrated photonic crystal cavity and a bus waveguide

We experimentally demonstrate a new optical filter design comprising of a photonic crystal cavity and a low index bus waveguide which are monolithically integrated on a silicon-on-insulator (SOI) platform. We have fabricated oxide clad PhC cavities with a silicon nitride waveguide positioned directly above, such that there is an overlap between the evanescent tails of the two modes. We have realised an extinction ratio of 7.5dB for cavities with total Q of 50,000.Postprin