Wah Tung Lau

Tuning coherent radiative thermal conductance in multilayer photonic crystals

Photonic crystals can be used to drastically influence coherent radiative thermal conductance. In a multilayer crystal, radiative thermal conductance can transit from above to below vacuum value when temperature increases, due to photonic band effects.

Anomalous modal structure in a waveguide with a photonic crystal core

We analyze a dielectric waveguide with a photonic crystal core. Using constant frequency contour analysis, we show that the modal behavior of this structure is drastically different from that of a conventional slab waveguide. In particular, at a given frequency the lowest-order guided mode can have an odd symmetry or can have more than one nodal plane in its field distribution. Also, there exist several single-mode regions with a different modal profile in each region. Finally, a single-mode waveguide for the fundamental mode with a large core and strong confinement can be realized. All these behaviors are confirmed by our three-dimensional finite-difference time-domain simulations.

Ultra-small coherent thermal conductance using multi-layer photonic crystal

A multi-layer photonic crystal can be used to suppress coherent thermal conductance below the vacuum conductance value, over the entire high-temperature range. With interlacing layers of silicon and vacuum, heat can only be carried by photons. The thermal conductance of the crystal would then be determined by the photonic band structure. Partial photonic band gaps that present over most of the thermal spectrum, as well as the suppression of evanescent coupling of photons across the vacuum layers at high frequencies, would reduce the amount heat conducting photon channels below that of the vacuum. Thus such multi-layer structures can be very efficient thermal insulators. Besides, the thermal conductance of such structures can exhibit substantial tunability, by merely changing the size of the vacuum spacing.

Exponential Suppression of Thermal Conductance using Coherent Transport and Heterostructures

We consider coherent thermal conductance through multilayer photonic crystal heterostructures, consisting of a series of cascaded nonidentical photonic crystals. We show that thermal conductance can be suppressed exponentially with the number of cascaded crystals due to the mismatch between photonic bands of all crystals in the heterostructure.

Spatial coherence of the thermal electromagnetic field in the vicinity of a dielectric slab

We analyze the spatial coherence of thermal field emitted from a lossy dielectric slab using fluctuation-dissipation theorem.1 For a given wavelength λ, the coherence property varies drastically with the distance from the slab surface. The coherence length is roughly λ / 2 in the far-field zone, but in the extreme near-field zone, it is many orders of magnitude smaller than λ, due to spatially fluctuating surface charges at the air-dielectric interface. On the other hand, in the intermediate near-field zone, the coherence length can be much longer than λ / 2 if the loss is small, because of the presence of waveguide modes of the slab. Such long-ranged coherence falls off approximately as 1/√x , in contrast to 1/x for a blackbody radiator, where x refers to displacement parallel to the slab surface. Furthermore, at a point of fixed distance from the slab surface, the frequency spectrum of the local energy density exhibits distinct fluctuation pattern, which is shown to be closely related to the waveguide dispersion relation.

In-plane photonic crystal and nanophotonic devices

We report our recent works on magneto-optical photonic crystals, anomalous modal behaviors in waveguides with photonic crystal core, and all-angle negative refraction in metal-dielectric-metal structures.

Controlling diffraction and waveguide modes by exploiting spatial dispersions in photonic crystals

We present a theoretical condition for achieving three-dimensional self-collimation of light in a photonic crystal. Such effects provide a very interesting mechanism for developing integrated circuits in 3D crystals that can be synthesized in a large scale. We also show that in a dielectric waveguide with a photonic crystal core, the modal properties are very unusual. In particular, a single-mode waveguide for the fundamental mode with a large core and a strong confinement can be realized. This is potentially important for suppressing modal competition in laser structures.

Fabrication and characterization of tightly confining AlGaAs waveguides and microcavities for nonlinear optical applications

For applications such as fiber optic networks, wavelength conversion, or extracting information from a predetermined channel, are required operations. All-optical systems, based on non-linear optical frequency conversion, offer advantages compared to present systems based on optical-electronic-optical (OEO) conversion. Thanks to the large nonlinear susceptibility of AlGaAs (d14 = 90pm/V) and mature device fabrication technologies, quasi-phasematched non-linear interactions in orientation-patterned AlGaAs waveguides for optical wavelength conversion have already been demonstrated. However, they require long interaction length (~ centimeters) and a complex fabrication process. Moreover, the conversion efficiency remains relatively low, due to losses and poor confinement. We present here the design and fabrication of a very compact (~ tens of microns long) device based on tightly confining waveguides and photonic crystal microcavities. Our device is inherently phase-matched due to the short length and should significantly increase the conversion efficiency due to tight confinement and high cavity-Q value. We characterized the waveguides, measuring the propagation loss by the Fabry-Perot method and by a variant of the cutback method, and both give a consistent loss value (~5 dB/mm for single-mode waveguides and ~3 dB/mm for multimode waveguide). We also characterized the microcavities measuring the transmission spectrum and the cavity-Q value, obtaining Qs as large as 700.

Tightly confining AlGaAs waveguides and microcavities for optical frequency conversion

We present the design and fabrication process for an AlGaAs optical frequency conversion device based on tightly confining waveguides and a Photonic Bandgap Crystal Microcavity. We first theoretically analyze the improvement in non-linear conversion efficiency due to a high confinement cavity, compared to traditional QPM waveguides. The theoretical analysis is supported by finite difference frequency and time domain simulations. The theoretical conversion efficiency estimated with these tools is ~4%/mW for a device ~10 μm long. Influence of sidewall roughness on the Q of the cavity is also analyzed. Then, we describe the fabrication process of our device, which involves molecular beam epitaxy, electron beam lithography and plasma etching.

Creating large-bandwidth line defects by embedding dielectric waveguides into photonic crystal slabs

We introduce a general design procedure of creating a linear defect in any photonic crystal slabs that functioned as a waveguide. This involves embedding a dielectric block waveguide into a photonic crysatl slab such that the group velocity dispersion relation is phase matched with the bandgap of the crystal. A specific scheme of integration is required to remove any edge states within the guiding bandwidth. Such methodology allows a systematic tuning of waveguide properties such that it can be non-dispersive, and possesses wide bandwidth that contains only a single mode band. As a support of our argument, we demonstrate a particualr waveguide design that is single mode with essentially no group velocity dispersion, and possesses a bandwidth of 13% of the center band frequency.