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

Flexible metamaterials at visible wavelengths

We report on the fabrication and characterization of plasmonic structures on flexible substrates (Metaflex) and demonstrate the optical properties of a single layer of Metaflex. The layer exhibits a plasmonic resonance in the visible region around 620 nm. We show experimental and numerical results for both nano-antennas and fishnet geometries. We anticipate the use of Metaflex as a building block for flexible metamaterials in the visible range.Publisher PDFPeer reviewe

Integrated Semiconductor Ring Lasers

The concept of a semiconductor laser with a circular resonator, its advantages and particular problems are discussed. The pillbox resonator is introduced and its operation on whispering gallery modes is illustrated using a computer model. The experimental evidence of the guiding mechanism is shown, leading to the first demonstration of continous wave operation in a semiconductor ring laser with a threshold current of 24mA. The parameters of the GaAs/AlGaAs material that are relevant for the low threshold current operation are presented and all aspects of the fabrication procedure are covered, emphasizing the processes that led to smooth sidewalls and the low loss circular cavity. A further reduction of the threshold current to 12.5mA is shown, which is owing to a coating of silicon nitride that supresses the non-radiative recombination current and reduces the scattering loss. The excess bending loss is calculated to be 3dB/360 and found to be independent of the radius between 30mum and 145mum. The influence of the Y-junction on the operation characteristic is studied and shown to cause kinks in the L-I curve; it is also held responsible for the relatively low differential quantum efficiency (0.02-0.04) of the devices. Strip-loaded guiding is demonstrated for radii between 300mum and 600mum and proposed as a solution for the problem of degradation that is caused by etching through the active layer. The integration capability of the structure is demonstrated by the succesful operation of a circuit comprising of a ring laser, a low-loss waveguide and a detector, and an optoelectronic integrated circuit featuring a ring laser and a field-effect transistor. The material parameters that are involved in performing these complicated functions are discussed and modeled numerically

Climate change, resource depletion and population growth: the elephant in the room

Following the COP22 talks in Marrakesh, Alexander Krauss and Thomas Kastning, argue that politicians are ignoring the solution with the largest potential to mitigate climate change: slowing population growth

The Scientific Reach of Multi-Ton Scale Dark Matter Direct Detection Experiments

The next generation of large scale WIMP direct detection experiments have the potential to go beyond the discovery phase and reveal detailed information about both the particle physics and astrophysics of dark matter. We report here on early results arising from the development of a detailed numerical code modeling the proposed DARWIN detector, involving both liquid argon and xenon targets. We incorporate realistic detector physics, particle physics and astrophysical uncertainties and demonstrate to what extent two targets with similar sensitivities can remove various degeneracies and allow a determination of dark matter cross sections and masses while also probing rough aspects of the dark matter phase space distribution. We find that, even assuming dominance of spin-independent scattering, multi-ton scale experiments still have degeneracies that depend sensitively on the dark matter mass, and on the possibility of isospin violation and inelasticity in interactions. We find that these experiments are best able to discriminate dark matter properties for dark matter masses less than around 200 GeV. In addition, and somewhat surprisingly, the use of two targets gives only a small improvement (aside from the advantage of different systematics associated with any claimed signal) in the ability to pin down dark matter parameters when compared with one target of larger exposure.Comment: 23 pages; updated to match PRD versio

Searching for Dark Matter at the LHC with a Mono-Z

We investigate a mono-Z process as a potential dark matter search strategy at the LHC. In this channel a single Z boson recoils against missing transverse momentum, attributed to dark matter particles, $\chi$, which escape the detector. This search strategy is related, and complementary to, monojet and monophoton searches. For illustrative purposes we consider the process $q\bar{q} -> \chi\chi Z$ in a toy dark matter model, where the Z boson is emitted from either the initial state quarks, or from the internal propagator. Among the signatures of this process will be a pair of muons with high pT that reconstruct to the invariant mass of the Z, and large amounts of missing transverse energy. Being a purely electroweak signal, QCD and other Standard Model backgrounds are relatively easily removed with modest selection cuts. We compare the signal to Standard Model backgrounds and demonstrate that, even for conservative cuts, there exist regions of parameter space where the signal may be clearly visible above background in future LHC data, allowing either new discovery potential or the possibility of supplementing information about the dark sector beyond that available from other observable channels.Comment: 11 pages, 13 figure