Sangwoo Ha

Bloch-mode analysis for retrieving effective parameters of metamaterials

We introduce an approach for retrieving effective parameters of metamaterials based on the Bloch-mode analysis of quasiperiodic composite structures. We demonstrate that, in the case of single-mode propagation, a complex effective refractive index can be assigned to the structure, being restored by our method with a high accuracy. We employ both surface and volume averaging of the electromagnetic fields of the dominating (fundamental) Bloch modes to determine the Bloch and wave impedances, respectively. We discuss how this method works for several characteristic examples, and demonstrate that this approach can be useful for retrieval of both material and wave effective parameters of a broad range of metamaterials.

Polychromatic nonlinear surface modes generated by supercontinuum light

We study propagation of polychromatic light near the edge of a nonlinear waveguide array. We describe simultaneous spatial and spectral beam reshaping associated with power and wavelength-dependent tunneling between the waveguides. We present experimental verifications of the effects predicted theoretically including the first observation of supercontinuum nonlinear surface modes.

Dispersionless tunneling of slow light in antisymmetric photonic crystal couplers

We predict that robust routing of slow-light pulses is possible between antisymmetrically coupled photonic-crystal waveguides. We demonstrate that for all pulses with the group velocities varying by several orders of magnitude, the complete switching occurs at the fixed coupling length of just several unit cells of the photonic crystal.

Experimental observation of evanescent modes at the interface to slow-light photonic crystal waveguides

We experimentally study the fields close to an interface between two photonic crystal waveguides that have different dispersion properties. After the transition from a waveguide in which the group velocity of light is v(g) ~ c/10 to a waveguide in which it is v(g) ~ c/100, we observe a gradual increase in the field intensity and the lateral spreading of the mode. We attribute this evolution to the existence of a weakly evanescent mode that exponentially decays away from the interface. We compare this to the situation where the transition between the waveguides only leads to a minor change in group velocity and show that, in that case, the evolution is absent. Furthermore, we apply novel numerical mode extraction techniques to confirm experimental results.

Bloch-mode extraction from near-field data in periodic waveguides

We demonstrate that the spatial profiles of both propagating and evanescent Bloch modes in a periodic structure can be extracted from a single measurement of an electric field at the specified optical wavelength. We develop a systematic extraction procedure by extending the concepts of high-resolution spectral methods previously developed for temporal data series to take into account the symmetry properties of Bloch modes simultaneously at all spatial locations. We illustrate the application of our method to a photonic crystal waveguide interface and confirm its robustness in the presence of noise.

Slow-light switching in nonlinear Bragg-grating couplers

We study propagation and switching of slow-light pulses in nonlinear couplers with phase-shifted Bragg gratings. We demonstrate that power-controlled nonlinear self-action of light can be used to compensate for dispersion-induced broadening of pulses through the formation of gap solitons, to control pulse switching in the coupler, and to tune the propagation velocity.

Slow-light vortices in periodic waveguides

We reveal that the reduction of the group velocity of light in periodic waveguides is generically associated with the presence of circulating energy flows or optical vortices. We show that the energy flows are gradually frozen for slow-light at the Brillouin zone edge, whereas vortices persist for slow-light states having non-vanishing phase velocity inside the Brillouin zone. We also demonstrate that the presence of vortices can be linked to the absence of slow-light at the zone edge, and the presented calculations illustrate these general results.

Nonlinear switching and reshaping of slow-light pulses in Bragg-grating couplers

We study the propagation and switching of slow-light pulses in nonlinear directional couplers composed of two parallel waveguides, where each waveguide contains a Bragg grating. We show that by optimizing the phase shift between the Bragg gratings, one can obtain specific dispersion characteristics enabling all-optical pulse manipulation in space and in time. We demonstrate that the power-controlled nonlinear self-action of light can be used to compensate dispersion-induced broadening of pulses through the formation of gap solitons, to control pulse switching in the coupler, and to tune the propagation velocity. We also confirm that the switching is tolerant to deviations of the phase shift from the optimal value, which can occur in the fabrication process.

Dispersion extraction with near-field measurements in periodic waveguides

We formulate and demonstrate experimentally the high-resolution spectral method based on Bloch-wave symmetry properties for extracting mode dispersion in periodic waveguides from measurements of near-field profiles. We characterize both the propagating and evanescent modes, and also determine the amplitudes of forward and backward waves in different waveguide configurations, with the estimated accuracy of several percent or less. Whereas the commonly employed spatial Fourier-transform (SFT) analysis provides the wavenumber resolution which is limited by the inverse length of the waveguide, we achieve precise dispersion extraction even for compact photonic structures.

Trapped supercontinuum and multi-color gap solitons

tonic structures in the form of waveguide arrays. The arrays feature the refractive index modulation in the transverse spatial dimension [see (a)] with the characteristic period of several wavelengths, resembling the periodic cladding of PCFs. In such structures, back-scattering is absent and transmission coefficients can approach unity simultaneously for all spectral components. In addition, the spatial beam propagation in waveguide arrays tends to change smoothly as the optical wavelength is varied by hundreds of nanometers, in contrast to the sharp spectral sensitivity in photonic crystals, where the refractive index is modulated in the propagation direction on wavelength scale. Following the theoretical analysis, 2 we demonstrated spatio-spectral reshaping of supercontinuum light achieved through nonlinear interaction of spectral components in an array of optical waveguides fabricated in a LiNbO 3 crystal. 3,4 At