Optomechanical detection of light with orbital angular momentum.

We present the design of an optomechanical device that allows sensitive transduction of the orbital angular momentum of light. An optically induced twist imparted on the device is detected using a photonic crystal cavity optomechanical system. This device allows the measurement of the orbital angular momentum of light when photons are absorbed by the mechanical element or the detection of the presence of photons when they are scattered into new orbital angular momentum states by a sub-wavelength grating patterned on the device. Such a system allows the detection of optical pulses with an l = 1 orbital angular momentum field that have an average photon number of 3.9 × 103 at a 5 MHz repetition rate, assuming that detector noise is not limiting measurement sensitivity. This scheme can be extended to higher order orbital angular momentum states.

Large color gamut displays with diffraction gratings.

The ability to display a broad variety of colors has great benefits not only in the context of entertainment but also as a means to streamline design in prototyping and manufacturing processes. Displays that use RGB filters or backlights cannot span all colors that occur in nature. To improve the accuracy of color reproduction, there have been attempts to include additional color primaries in displays. Existing solutions, however, have an impact on cost, scalability, and spatial resolution and are predominantly applicable to projection systems. We propose an approach based on combining diffraction grating extractors and the HANS imaging pipeline initially developed for printing. This combination offers unprecedented potential to attain large color gamuts with the same backlights commercially used today.

Sensitivity analysis and optimization of sub-wavelength optical gratings using adjoints.

Numerical optimization of photonic devices is often limited by a large design space the finite-differences gradient method requires as many electric field computations as there are design parameters. Adjoint-based optimization can deliver the same gradients with only two electric field computations. Here, we derive the relevant adjoint formalism and illustrate its application for a waveguide slab, and for the design of optical sub-wavelength gratings.

Picosecond all-optical switching in hydrogenated amorphous silicon microring resonators.

We utilize cross-phase modulation to observe all-optical switching in microring resonators fabricated with hydrogenated amorphous silicon (a-Si:H). Using 2.7-ps pulses from a mode-locked fiber laser in the telecom C-band, we observe optical switching of a cw telecom-band probe with full-width at half-maximum switching times of 14.8 ps, using approximately 720 fJ of energy deposited in the microring. In comparison with telecom-band optical switching in undoped crystalline silicon microrings, a-Si:H exhibits substantially higher switching speeds due to reduced impact of free-carrier processes.

A multi-directional backlight for a wide-angle, glasses-free three-dimensional display.

Multiview three-dimensional (3D) displays can project the correct perspectives of a 3D image in many spatial directions simultaneously. They provide a 3D stereoscopic experience to many viewers at the same time with full motion parallax and do not require special glasses or eye tracking. None of the leading multiview 3D solutions is particularly well suited to mobile devices (watches, mobile phones or tablets), which require the combination of a thin, portable form factor, a high spatial resolution and a wide full-parallax view zone (for short viewing distance from potentially steep angles). Here we introduce a multi-directional diffractive backlight technology that permits the rendering of high-resolution, full-parallax 3D images in a very wide view zone (up to 180 degrees in principle) at an observation distance of up to a metre. The key to our design is a guided-wave illumination technique based on light-emitting diodes that produces wide-angle multiview images in colour from a thin planar transparent lightguide. Pixels associated with different views or colours are spatially multiplexed and can be independently addressed and modulated at video rate using an external shutter plane. To illustrate the capabilities of this technology, we use simple ink masks or a high-resolution commercial liquid-crystal display unit to demonstrate passive and active (30 frames per second) modulation of a 64-view backlight, producing 3D images with a spatial resolution of 88 pixels per inch and full-motion parallax in an unprecedented view zone of 90 degrees. We also present several transparent hand-held prototypes showing animated sequences of up to six different 200-view images at a resolution of 127 pixels per inch.

Strong optical confinement between nonperiodic flat dielectric gratings.

We present a novel design of optical microcavity where the optical energy resides primarily in free space and therefore is readily accessible to foreign objects such as atoms, molecules, mechanical resonators, etc. We describe the physics of these resonators and propose a design method based on stochastic optimization. Cavity designs with diffraction-limited mode volumes and quality factors in the range of 10(4)-10(6) are presented. With a purely planar geometry, the cavity can be easily integrated on-chip by using conventional micro- and nanofabrication processes.

Reflective silicon binary diffraction grating for visible wavelengths.

We introduce a device based on subwavelength resonant grating technology. Using a single lithography step we built a reflective binary grating that mimics the functionality of a blazed diffraction grating in a flat geometry. We have also demonstrated that efficient subwavelength resonant devices for visible wavelengths can be built using silicon.

Nanoengineered optical resonance sensor for composite material refractive-index measurements.

We present an optical resonance sensor capable of measurement of refractive index in highly nonhomogeneous materials. Traditional optical resonance sensors fail when the size of particles is comparable with the wavelength (100 nm and larger). Our new nanoengineered design allows incorporation of a highly delocalized mode into a resonance structure. The sensing depth of the device was measured to be 1 mum, the largest reported in the literature as far as we know, with a quality factor of 500. We demonstrate two applications.

Crosstalk-free design for the intersection of two dielectric waveguides.

We propose an efficient method to reduce the crosstalk, reflection and radiation at the crossing of two dielectric waveguides in a on-chip optical interconnect network. By increasing the vertical thickness of the guides locally in the crossing region, we create better mode-matching interfaces that dramatically reduce losses. The idea is demonstrated using numerical simulations. More than 95% crosstalk power reduction and 90% reflection power reduction are observed, while the radiation power can be reduced by 40%. The method is compatible with the planar integrated circuit technique.

Silicon microring resonators with 1.5-microm radius.

We demonstrate a junction between a silicon strip waveguide and an ultra-compact silicon microring resonator that minimizes spurious light scattering and increases the critical dimensions of the geometry. We show cascaded silicon microring resonators with radii around 1.5 microm and effective mode volumes around 1.0 microm(3) that are critically coupled to a waveguide with coupled Q's up to 9,000. The radius of 1.5 microm is smaller than the operational wavelength, and is close to the theoretical size limit of the silicon microring ring resonator for the same Q. The device is fabricated with a widely-available SEM-based lithography system using a stitch-free design based on a U-shaped waveguide.

Coherent population trapping of single spins in diamond under optical excitation.

Coherent population trapping is demonstrated in single nitrogen-vacancy centers in diamond under optical excitation. For sufficient excitation power, the fluorescence intensity drops almost to the background level when the laser modulation frequency matches the 2.88 GHz splitting of the ground states. The results are well described theoretically by a four-level model, allowing the relative transition strengths to be determined for individual centers. The results show that all-optical control of single spins is possible in diamond.

Coherent population trapping in diamond N-V centers at zero magnetic field.

Coherent population trapping at zero magnetic field was observed for nitrogen-vacancy centers in diamond under optical excitation. This was measured as a reduction in photoluminescence when the detuning between two excitation lasers matched the 2.88 GHz crystal-field splitting of the color center ground states. This behavior is highly sensitive to strain, which modifies the excited states, and was unexpected following recent experiments demonstrating optical readout of single nitrogen-vacancy electron spins based on cycling transitions. These results demonstrate for the first time that three-level Lambda configurations suitable for proposed quantum information applications can be realized simultaneously for all four orientations of nitrogen-vacancy centers at zero magnetic field.

Controlling the spontaneous emission rate of single quantum dots in a two-dimensional photonic crystal.

We observe large spontaneous emission rate modification of individual InAs quantum dots (QDs) in a 2D photonic crystal with a modified, high-Q single-defect cavity. Compared to QDs in a bulk semiconductor, QDs that are resonant with the cavity show an emission rate increase of up to a factor of 8. In contrast, off-resonant QDs indicate up to fivefold rate quenching as the local density of optical states is diminished in the photonic crystal. In both cases, we demonstrate photon antibunching, showing that the structure represents an on-demand single photon source with a pulse duration from 210 ps to 8 ns. We explain the suppression of QD emission rate using finite difference time domain simulations and find good agreement with experiment.

Entanglement formation and violation of Bell's inequality with a semiconductor single photon source.

We report the generation of polarization-entangled photons, using a quantum dot single photon source, linear optics, and photodetectors. Two photons created independently are observed to violate Bell's inequality. The density matrix describing the polarization state of the postselected photon pairs is reconstructed and agrees well with a simple model predicting the quality of entanglement from the known parameters of the single photon source. Our scheme provides a method to create no more than one entangled photon pair per cycle after postselection, a feature useful to enhance quantum cryptography protocols based on shared entanglement.

Indistinguishable photons from a single-photon device.

Single-photon sources have recently been demonstrated using a variety of devices, including molecules, mesoscopic quantum wells, colour centres, trapped ions and semiconductor quantum dots. Compared with a Poisson-distributed source of the same intensity, these sources rarely emit two or more photons in the same pulse. Numerous applications for single-photon sources have been proposed in the field of quantum information, but most--including linear-optical quantum computation--also require consecutive photons to have identical wave packets. For a source based on a single quantum emitter, the emitter must therefore be excited in a rapid or deterministic way, and interact little with its surrounding environment. Here we test the indistinguishability of photons emitted by a semiconductor quantum dot in a microcavity through a Hong-Ou-Mandel-type two-photon interference experiment. We find that consecutive photons are largely indistinguishable, with a mean wave-packet overlap as large as 0.81, making this source useful in a variety of experiments in quantum optics and quantum information.