Tsuyoshi Yamashita

Polarization beam splitter based on a photonic crystal heterostructure.

The design and characterization of a photonic crystal (PC) polarization beam splitter (PBS) that operates with an extinction ratio of greater than 15 dB for both polarizations are presented. The PBS is fabricated on a silicon-on-insulator (SOI) wafer where the input and output ports consist of 5 mum wide ridge waveguides. A large spectral shift is observed in the dispersion plots of the lowest-order even (TE-like) and odd (TM-like) modes due to the SOI confinement. Because of this shift, the TE-like mode is close to a directional gap at the top of the band, and the TM-like mode is in a low-frequency regime where the dispersion surface is almost isotropic. We show that the TE-like mode has very high reflection at the interface between the two PCs, whereas the TM-like mode exhibits a very high transmission.

Photonic band tuning in two-dimensional photonic crystal slab waveguides by atomic layer deposition

The photonic bands of two-dimensional (2D) triangular lattice photonic crystal Si slab waveguides were statically tuned using low temperature atomic layer deposition (ALD) of TiO2. Angular dependent reflectance measurements of bare and coated devices were well fitted by three-dimensional finite-difference time-domain calculations. The technique not only allows the physics of photonic band effects in 2D photonic crystals to be systematically studied but also demonstrates large static tuning and precise fine-scale control over band frequency and dispersion, with a frequency tuning range of 12% and precision of 0.005% per ALD cycle. Band tuning to achieve zero group velocity is demonstrated.

Evaluation of self-collimated beams in photonic crystals for optical interconnect

Self-collimated beams and photonic bandgap mirrors in photonic crystals are evaluated for applicability in an on-chip interconnect system. Simulations using the plane-wave expansion and finite-difference time-domain methods are utilized to design and evaluate the theoretical performance of these systems, called a virtual waveguide due to borderless confinement of the signal. The effect of systematic and random fabrication errors on the performance is characterized. Coupling efficiency is virtually unaffected by misalignment, but is found to be a strong function of the length of the waveguide and the frequency of light. Additional routing capabilities of sharp 90/spl deg/ turns and signal crossings with no crosstalk are demonstrated. Photonic crystal virtual waveguides are ideal structures for on-chip optical signal routing.

Wave front evolution of negatively refracted waves in a photonic crystal

Using a heterodyne near-field scanning microscope, the authors analyze the phase and amplitude of the electric field of an optical wave across the boundary of positive to negative refraction media. The photonic crystal acts as an extremely anisotropic material with a negative curvature of its dispersion surface whose shape is resolved experimentally. This extreme anisotropy results in the beam having a peculiar phase evolution through propagation that does not occur in isotropic media. A focusing wave is produced by negative refraction, which has diverging wave fronts before the internal focus and converging wave fronts after the focus.

Observation of Brillouin zone folding in photonic crystal slab waveguides possessing a superlattice pattern

The optical properties of superlattice photonic crystal (PC) patterns in two-dimensional slab waveguides were investigated using coupled-resonance angular dependent reflectivity measurements. Additional features were found in the superlattice PC spectra in comparison to that of the triangular lattice, indicating Brillouin zone folding (BZF) due to the larger unit cell and reduced symmetry of the superlattice. Calculated band structures corroborate these measurements and confirm the BZF effect which was shown to extract portions of the triangular lattice guided bands into the waveguide radiating regime, making the dielectric bands excitable by an out-of-plane source.

TUNING OF PHOTONIC CRYSTAL BAND PROPERTIES BY ATOMIC LAYER DEPOSITION

We report the application of atomic layer deposition to manipulate the dielectric architecture of conventional and superlattice two-dimensional photonic crystal waveguides fabricated in silicon. Conformal deposition of a second dielectric layer is shown to have a dramatic influence on the photonic band structure and produces unique effects that cannot be emulated in a single dielectric slab photonic crystal material. With additional dielectric coatings, a strong decrease in photonic band frequencies and change in band slope are observed, which for the lowest photonic states produces strong degeneracies. The capability, in principle, to tune the position of bands to within 0.005% accuracy, is demonstrated. Additionally, new features are observed when differential band shifts result in band-crossing and for which like polarizations activate perturbation mechanisms that result in local and strong band curvatures. The extremely strong band bending resulting from band-band interactions could have applications, in slow light devices, and provide a way to introduce non-linear effects into tunable photonic crystal structures.

Manipulation of Dispersion Properties of Two-dimensional Photonic Crystal Slab Waveguides by Atomic Layer Deposition

In addition to the growing interest within the semiconductor industry, atomic layer deposition (ALD) has also become an attractive tool to engineer and manipulate with extreme precision the complex dielectric architecture of three-dimensional photonic crystals (PCs). Very recently, we proposed to apply this technique to 2D PC slab waveguides.

Photonic Crystal Reflection Prisms

We present a photonic crystal cube-like polarization beam splitter and total internal reflection prism. By controlling diffraction, we experimentally show that free space optical prisms can now be implemented onto a planar silicon platform.

Phase and group index of the second photonic crystal band

Light propagation in slab photonic crystals can have a negative phase velocity and accumulate a negative optical path length. We present experimental results on guiding and dispersion properties when the phase index is approximately -1.

Photonic band tuning in 2D photonic crystals by atomic layer deposition

In this paper, we have demonstrated a technique in which the photonic bands of two-dimensional triangular lattice photonic crystal slab waveguides can be statically tuned by nanoscale modifications, thereby enabling unprecedented adjustment to the dispersion properties of any 2D photonic crystal. Additionally, the technique facilitates the formation of layered and composite 2D PC waveguides thus opening new fabrication routes to control dispersion, propagation and dielectric contrast