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.
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
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.
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.
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 Q's as large as 700.
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.
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.