Martin J Cryan

Silica-on-Silicon Waveguide Quantum Circuits

Quantum technologies based on photons will likely require an integrated optics architecture for improved performance, miniaturization, and scalability. We demonstrate high-fidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits, including two-photon quantum interference with a visibility of 94.8 ± 0.5%; a controlled-NOT gate with an average logical basis fidelity of 94.3 ± 0.2%; and a path-entangled state of two photons with fidelity of >92%. These results show that it is possible to directly “write” sophisticated photonic quantum circuits onto a silicon chip, which will be of benefit to future quantum technologies based on photons, including information processing, communication, metrology, and lithography, as well as the fundamental science of quantum optics.

Broadband Polarization-Independent Perfect Absorber Using a Phase-Change Metamaterial at Visible Frequencies

We report a broadband polarization-independent perfect absorber with wide-angle near unity absorbance in the visible regime. Our structure is composed of an array of thin Au squares separated from a continuous Au film by a phase change material (Ge2Sb2Te5) layer. It shows that the near perfect absorbance is flat and broad over a wide-angle incidence up to 80° for either transverse electric or magnetic polarization due to a high imaginary part of the dielectric permittivity of Ge2Sb2Te5. The electric field, magnetic field and current distributions in the absorber are investigated to explain the physical origin of the absorbance. Moreover, we carried out numerical simulations to investigate the temporal variation of temperature in the Ge2Sb2Te5 layer and to show that the temperature of amorphous Ge2Sb2Te5 can be raised from room temperature to > 433 K (amorphous-to-crystalline phase transition temperature) in just 0.37 ns with a low light intensity of 95 nW/μm2, owing to the enhanced broadband light absorbance through strong plasmonic resonances in the absorber. The proposed phase-change metamaterial provides a simple way to realize a broadband perfect absorber in the visible and near-infrared (NIR) regions and is important for a number of applications including thermally controlled photonic devices, solar energy conversion and optical data storage.

Mid-infrared tunable polarization-independent perfect absorber using a phase-change metamaterial

We present the design of a polarization-independent tunable absorbing metamaterial (MM) in the mid-infrared wavelength regime. Our structure is composed of an array of thin gold (Au) squares separated from a continuous Au film by a phase-change material (PCM) layer. It is shown that a 10% tuning of the absorbance peak can be obtained by switching the PCM between its amorphous and crystalline states. The strong absorbance shows a substantial overlap between TE and TM polarization states over a wide range of incident angles. The electric field, magnetic field, and current distributions in the absorber are investigated to further explain the physical origin of the absorption. The study provides a path toward the realization of tunable absorbers for applications, such as selective thermal emitters, sensors, and bolometers.

Rapid phase transition of a phase-change metamaterial perfect absorber

Phase-change materials (PCMs) have great potential in applications for data storage, optical switching and tunable photonic devices. However, heating the whole of the phase change material at a high speed presents a key challenge. Here, for the first time, we model the incorporation of the phase-change material (Ge2Sb2Te5) within a metamaterial perfect absorber (MMPA) and show that the temperature of amorphous Ge2Sb2Te5 can be raised from room temperature to > 900K (melting point of Ge2Sb2Te5) in just a few nanoseconds with a low light intensity of 150 W/m2, owing to the enhanced light absorption through strong plasmonic resonances in the absorber. Our structure is composed of an array of thin gold (Au) squares separated from a continuous Au film by a Ge2Sb2Te5 layer. A Finite Element Method photothermal model is used to study the temporal variation of temperature in the Ge2Sb2Te5 layer. It is also shown that an absorber with a widely tunable spectrum can be obtained by switching between the amorphous and crystalline states of Ge2Sb2Te5. The study lowers the power requirements for photonic devices based on a thermal phase change and paves the way for the realization of ultrafast photothermally tunable photonic devices.

Fast Tuning of Double Fano Resonance Using A Phase-Change Metamaterial Under Low Power Intensity

In this work, we numerically demonstrate an all-optical tunable Fano resonance in a fishnet metamaterial(MM) based on a metal/phase-change material(PCM)/metal multilayer. We show that the displacement of the elliptical nanoholes from their centers can split the single Fano resonance (FR) into a double FR, exhibiting higher quality factors. The tri-layer fishnet MMs with broken symmetry accomplishes a wide tuning range in the mid-infrared(M-IR) regime by switching between the amorphous and crystalline states of the PCM (Ge2Sb2Te5). A photothermal model is used to study the temporal variation of the temperature of the Ge2Sb2Te5 film to show the potential for switching the phase of Ge2Sb2Te5 by optical heating. Generation of the tunable double FR in this asymmetric structure presents clear advantages as it possesses a fast tuning time of 0.36 ns, a low pump light intensity of 9.6 μW/μm2, and a large tunable wavelength range between 2124 nm and 3028 nm. The optically fast tuning of double FRs using phase change metamaterials(PCMMs) may have potential applications in active multiple-wavelength nanodevices in the M-IR region.

Strongly tunable circular dichroism in gammadion chiral phase-change metamaterials

A metal/phase-change material/metal tri-layer planar chiral metamaterial in the shape of a gammadion is numerically modelled. The chiral metamaterial is integrated with Ge2Sb2Te5 phase-change material (PCM) to accomplish a wide tuning range of the circular dichroism (CD) in the mid-infrared wavelength regime. A photothermal model is used to study the temporal variation of the temperature of the Ge2Sb2Te5 layer and to show the potential for fast switching the phase of Ge2Sb2Te5 under a low incident light intensity of 0.016mW/μm2.

GaN directional couplers for integrated quantum photonics

Large cross-section GaN waveguides are proposed as a suitable architecture to achieve integrated quantum photonic circuits. Directional couplers with this geometry have been designed with aid of the beam propagation method and fabricated using inductively coupled plasma etching. Scanning electron microscopy inspection shows high quality facets for end coupling and a well defined gap between rib pairs in the coupling region. Optical characterization at 800 nm shows single-mode operation and coupling-length-dependent splitting ratios. Two photon interference of degenerate photon pairs has been observed in the directional coupler by measurement of the Hong-Ou-Mandel dip [C. K. Hong, et al., Phys. Rev. Lett. 59, 2044 (1987)] with 96% visibility.

Calculation of losses in 2-D photonic Crystal membrane waveguides using the 3-D FDTD method

The three-dimensional finite-difference time-domain method is used to obtain loss per unit length in a two-dimensional photonic crystal membrane waveguide by simulating three different length guides. Results are shown for propagation both above and below the light line. The results are compared with a Fourier expansion method and good agreement is obtained above and below the light line.

A Fully Bidirectional 2.4-GHz Wireless-Over-Fiber System Using Photonic Active Integrated Antennas (PhAIAs)

This paper describes a low-cost scheme for implementing in-building distributed antenna systems using the photonic-active-integrated-antenna (PhAIA) concept, whereby photonic devices are integrated directly with planar antennas. Deembedded input impedance is measured for an 850-nm vertical-cavity surface-emitting laser and photodiode from 0-10 GHz, and the devices are matched directly to the nonradiating edge of a rectangular-microstrip-patch antenna. Link gain, 1-dB compression point, and spurious-free dynamic range are measured in the links. The fully bidirectional system, which is far from being completely optimized, is then tested over a 300-m laboratory-based multimode fiber link and a 220-m in-building dark-fiber link. Results are shown for throughput and signal-to-noise ratio, and this paper shows that such systems can achieve up to 10-m RF range, at reduced throughput, with no RF amplification.

Investigations of mode control in proton-implanted and oxide-confined VCSELs using PBGs: modelling and experiment

We have investigated theoretically the influence on transverse optical modes of a two-dimensional photonic crystal (PC) defect waveguide embedded into a vertical-cavity surface-emitting laser. In gain-guided structures, where the index profile is weak, the effect of the PC is efficient. In the oxide-confined structures, where the index guidance provided by the oxide is larger, this is not the case, but the efficiency of the PC can be increased by oxide in the node position. It is shown that the thermal lensing effect leads to increased sensitivity of the single-mode conditions on the current injection change.