Search

We investigated cross nonlinear absorption in silicon waveguides using the probe-pump scheme. An analytic solution of the probe is derived including XTPA. We experimentally extracted the effect of each loss on powers using a simple method.

We investigated non-degenerate two-photon absorption in silicon waveguides using a probe-pump scheme. An analytic solution of the probe is derived and a simple approach is provided to experimentally extract the effect of each loss on the pump and probe powers.

Reliable sources of photons are imperative to the development of photonic quantum technologies. I will review the use of ultra-compact slow light photonic crystal structures to create heralded single photon sources using nonlinear processes.

In recent years integrated waveguide devices have emerged as an attractive platform for scalable quantum tech- nologies. In contrast to earlier free-space investigations, one must consider additional effects induced by the media. In amorphous materials, spontaneous Raman scattered photons act as a noise source. In crystalline materials two-photon absorption (TPA) and free carrier absorption (FCA) are present at large intensities. While initial observations noted TPA affected experiments in integrated semiconductor devices, at present the nuanced roles of these processes in the quantum regime is unclear. Here, using single photons generated via spontaneous four-wave mixing (SFWM) in silicon, we experimentally demonstrate that cross-TPA (XTPA) between a classical pump beam and generated single photons imposes an intrinsic limit on heralded single photon generation, even in the single pair regime. Our newly developed model is in excellent agreement with experimental results.

This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (FOM), which is financially supported by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). The work is also supported by the research programs NanonextNL and MEMPHIS, funded by the Dutch ministry of economic affairs. We also acknowledge financial support by the EPSRC through the “UK Silicon Photonics” grant. Wavelength-scale optical modulators are essential building blocks for future on-chip optical interconnects. Any modulator design is a trade-off between bandwidth, size and fabrication complexity, size being particularly important as it determines capacitance and actuation energy. Here, we demonstrate an interesting alternative that is only 3 mm long, only uses silicon on insulator (SOI) material and accommodates several nanometres of optical bandwidth at 1550 nm. The device is based on a photonic crystal waveguide: by combining the refractive index shift with slow-light enhanced absorption induced by free-carrier injection, we achieve an operation bandwidth that significantly exceeds the shift of the bandedge. We compare a 3 mm and an 80 mm long modulator and surprisingly, the shorter device outperforms the longer one. Despite its small size, the device achieves an optical bandwidth as broad as 7 nm for an extinction ratio of 10 dB, and modulation times ranging between 500 ps and 100 ps. Publisher PDF

Single photons are of paramount importance to future quantum technologies, including quantum communication and computation. Nonlinear photonic devices using parametric processes offer a straightforward route to generating photons, however additional nonlinear processes may come into play and interfere with these sources. Here we analyse these sources in the presence of multi-photon processes for the first time. We conduct experiments in silicon and gallium indium phosphide photonic crystal waveguides which display inherently different nonlinear absorption processes, namely two-photon (TPA) and three-photon absorption (ThPA), respectively. We develop a novel model capturing these diverse effects which is in excellent quantitative agreement with measurements of brightness, coincidence-to-accidental ratio (CAR) and second-order correlation function g(2)(0), showing that TPA imposes an intrinsic limit on heralded single photon sources. We devise a new figure of merit, the quantum utility (QMU), enabling direct comparison and optimisation of single photon sources., 6 figures, 12 pages

We demonstrate phase sensitive amplification (PSA) in a 196μm silicon slow-light photonic crystal with an extinction ratio of 11dB. This record smallest phase sensitive amplifier is also the first demonstration of PSA in a photonic crystal.

Using a silicon slow-light photonic crystal waveguide, we demonstrate the operation of a dynamic delay line based on an ultrafast indirect photonic transition to alter the group velocity of a pulse. We show continuously tuneable delays of up to 20 ps, using a waveguide just 300 μm in length. Our approach is scalable, in that both the maximum delay and capacity of the buffer are proportional to the length of the waveguide, and flexible, in that individual pulses in a pulse stream can be controlled independently, which we demonstrate by operating on pulses separated by just 30 ps.

The non-deterministic nature of photon sources is a key limitation for single-photon quantum processors. Spatial multiplexing overcomes this by enhancing the heralded single-photon yield without enhancing the output noise. Here the intrinsic statistical limit of an individual source is surpassed by spatially multiplexing two monolithic silicon-based correlated photon pair sources in the telecommunications band, demonstrating a 62.4% increase in the heralded single-photon output without an increase in unwanted multipair generation. We further demonstrate the scalability of this scheme by multiplexing photons generated in two waveguides pumped via an integrated coupler with a 63.1% increase in the heralded photon rate. This demonstration paves the way for a scalable architecture for multiplexing many photon sources in a compact integrated platform and achieving efficient two-photon interference, required at the core of optical quantum computing and quantum communication protocols., Photonic quantum technologies will require efficient single-photon sources and spatial multiplexing has been explored as a route to achieve this. Here, the authors present a scheme to integrate several single-photon sources using spatial multiplexing for on-chip applications at telecommunications wavelengths.

This paper reports on experimental investigations of the evolution of the spectral density of an ultrashort laser pulse as it propagates through a composite sample consisting of a slow-light photonic crystal (PhC) waveguide and polymer access waveguides. A novel method of near-field scanning optical microscopy was used to measure the spectrum at different locations, and for different powers, only inside the PhC waveguide. This is in contrast to traditional transmission measurements where the evolution of the spectrum due to the entire system is observed.

Here we present the first experimental demonstration of spatial multiplexing of two integrated heralded single photon sources so as to enhance the heralded photon rate for a given single waveguide output power.

In this work we use ultrafast pump-probe measurements to demonstrate a slow light photonic crystal (PhC) optical modulator that is based on free-carrier dispersion, and which operates in transmission.

We devise a new figure of merit, the quantum utility (QMU), enabling direct comparison of single photon sources. Measuring counts, coincidence-to-accidental ratios and second-order correlation functions, we compute the QMU for sources in two materials.

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals(1-4) and the enhancement of both linear(5-7) and nonlinear devices.(8-11) Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically(12) and electrically(13) pumped ultra-low threshold lasers and the enhancement of nonlinear effects.(14-16) Furthermore, passive filters(17) and modulators(18-19) have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption. To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues. The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum(20-21) and interferometric techniques.(22-25) Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.(26) Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans. When characterising photonic crystal cavities, techniques involving internal sources(21) or external waveguides directly coupled to the cavity(27) impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.(28) and further developed by Galli et al.(29).

We introduce the concept of an indirect photonic transition and demonstrate its use in a dynamic delay line to alter the group velocity of an optical pulse. Operating on an ultrafast time scale, we show continuously tuneable delays of up to 20 ps, using a slow light photonic crystal waveguide only 300 $\mu$m in length. Our approach is flexible, in that individual pulses in a pulse stream can be controlled independently, which we demonstrate by operating on pulses separated by just 30 ps. The two-step indirect transition is demonstrated here with a 30% conversion efficiency., Comment: "Copyright (2012) by the American Physical Society."

We report our progress on quantum information processing on an ultra-compact silicon photonic crystal platform. We achieved correlated photon-pair generation with a coincidence to accidental ratio as high as 130 in a 196 μm-long waveguide.

We use an indirect photonic transition to alter the group velocity of a pulse in a silicon photonic crystal waveguide. Delays of up to 20ps, with efficiencies of 30%, are achievable in a 300μm waveguide.

We propose a new design for compact tunable optical delay lines, based on adiabatic wavelength conversion and group velocity dispersion, in engineered silicon slow light photonic crystal waveguides. Actuation is performed over a small fraction of the device area, allowing for efficient operation in terms of switching power. We report on the experimental demonstration of variable delays up to 15 ps in a 300 µm long waveguide using this scheme, thus realising an ultrafast tunable delay line in an integrated format.

The prospect for low pump-power Raman amplification in silicon waveguides has recently been boosted by theoretical studies discussing the enhancement of nonlinear phenomena in slow-light structures. In principle, the slowing down of either the pump or the signal beam is equivalent in terms of Raman gain, but in the presence of losses, we show that they play different roles in determining the net signal gain. We also investigate the impact of the mode profile in realistic slow-light waveguides on the total gain, an effect that is usually neglected in the context of stimulated Raman scattering. By taking representative losses and mode shapes into account, we provide a realistic estimation of the achievable performance of slow-light photonic crystal waveguides. Publisher PDF

By performing ultrafast pump-probe experiments, we show the 0.3THz adiabatic frequency conversion of pulses in a slow-light photonic crystal waveguide with 80% efficiency. We demonstrate the use of this conversion scheme in a delay line.

Engineering dispersion and loss in photonic crystal waveguides allows us to control propagation up to moderate group indices. Novel results on 'overengineered' waveguides give insights into loss vs. both propagation constant and group index.

We analytically and experimentally investigate cross-phase modulation (XPM) in silicon waveguides. In contrast to the well known result in pure Kerr media, the spectral broadening ratio of XPM to self-phase modulation is not two in the presence of either two-photon absorption (TPA) or free carriers. The physical origin of this change is different for each effect. In the case of TPA, this nonlinear absorption attenuates and slightly modifies the pulse shape due to differential absorption in the pulse peak and wings. When free carriers are present two different mechanisms modify the dynamics. First, free-carrier absorption performs a similar role to TPA, but is additionally asymmetric due to the delayed free-carrier response. Second, free-carrier dispersion induces an asymmetric blue phase shift which competes directly with the symmetric Kerr-induced XPM red shift. We confirm this analysis with pump-probe experiments in a silicon photonic crystal waveguide.

We theoretically and experimentally investigate the nonlinear evolution of two optical pulses in a silicon waveguide. We provide an analytic solution for the weak probe wave undergoing non-degenerate two-photon absorption (TPA) from the strong pump. At larger pump intensities, we employ a numerical solution to study the interplay between TPA and photo-generated free carriers. We develop a simple and powerful approach to extract and separate out the distinct loss contributions of TPA and free-carrier absorption from readily available experimental data. Our analysis accounts accurately for experimental results in silicon photonic crystal waveguides.

We demonstrate continuous wave four-wave mixing in silicon photonic crystal waveguides of 396 mu m length with a group index of n(g) = 30. The highest observed conversion efficiency is -24 dB for 90 mW coupled input pump power. The key question we address is whether the predicted fourth power dependence of the conversion efficiency on the slowdown factor (eta approximate to S-4) can indeed be observed in this system, and how the conversion efficiency depends on device length in the presence of propagation losses. We find that the expected dependencies hold as long as both realistic losses and the variation of mode shape with slowdown factor are taken into account. Having achieved a good agreement between a simple analytical model and the experiment, we also predict structures that can achieve the same conversion efficiency as already observed in nanowires for the same input power, yet for a device length that is 50 times shorter. (C) 2011 Optical Society of America Publisher PDF