Kyoko Kitamura

Sub-wavelength focal spot with long depth of focus generated by radially polarized, narrow-width annular beam

We demonstrate the formation of a sub-wavelength focal spot with a long depth of focus using a radially polarized, narrow-width annular beam. Theoretical analysis predicts that a tighter focal spot (approximately 0.4 lambda) and longer depth of focus (more than 4 lambda) can be formed by a longitudinal electric field when the width of the annular part of the beam is decreased. Experimental measurements using a radially polarized beam from a photonic crystal laser agree well with these predictions. Tight focal spots with long depths of focus have great potential for use in high-tolerance, high-resolution applications in optical systems.

Higher-order vector beams produced by photonic-crystal lasers

We have successfully generated vector beams with higher-order polarization states using photonic-crystal lasers. We have analyzed and designed lattice structures that provide cavity modes with different symmetries. Fabricated devices based on these lattice structures produced doughnut-shaped vector beams, with symmetries corresponding to the cavity modes. Our study enables the systematic analysis of vector beams, which we expect will lead to applications such as high-resolution microscopy, laser processing, and optical trapping.

Focusing properties of vector vortex beams emitted by photonic-crystal lasers

We experimentally investigate the focusing properties of first- and second-order vector beams and vector vortex beams generated by photonic-crystal lasers. When the azimuthal indices of the vector beam (l) and the phase dependence (n) match, strong intensity appears at the center of focus. Our theoretical analyses agree well with the experimental results and predict that the central intensity has circular polarization.

Three-dimensional coupled-wave analysis for triangular-lattice photonic-crystal surface-emitting lasers with transverse-electric polarization

Three-dimensional coupled-wave theory is extended to model triangular-lattice photonic-crystal surface-emitting lasers with transverse-electric polarization. A generalized coupled-wave equation is derived to describe the sixfold symmetry of the eigenmodes in a triangular lattice. The extended theory includes the effects of both surface radiation and in-plane losses in a finite-size laser structure. Modal properties of interest including the band structure, radiation constant, threshold gain, field intensity profile, and far-field pattern (FFP) are calculated. The calculated band structure and FFP, as well as the predicted lasing mode, agree well with experimental observations. The effect of air-hole size on mode selection is also studied and confirmed by experiment.

Needle-like focus generation by radially polarized halo beams emitted by photonic-crystal ring-cavity laser

Focused fields that possess a small spot size and long depth of focus (DOF) are expected to lead to the further development of optical applications. Here, we develop a photonic-crystal ring-cavity laser that emits a beam with a radially polarized halo shape (rinner/router > 0.9). This beam has a needle-like focus with a spot size of less than 0.4λ and a depth of focus longer than 10 λ for a 0.9 numerical aperture objective lens. We evaluate the focusing properties of the emitted beam and demonstrate that it has a longer depth of focus than conventional beams.

Mode stability in photonic-crystal surface-emitting lasers with large κ1DL

We study mode stability in photonic-crystal surface-emitting lasers (PCSELs) with large coupling-coefficient-length product κ1DL(>6). We observe that mode competition occurs at high current levels above threshold. Our combined experimental and theoretical study provides the first evidence of the mode competition originating from the high-order band-edge modes. The decreased threshold margin between these competing high-order modes and the main lasing mode with increasing cavity length as well as the spatial hole burning effect may deteriorate the single-mode stability. Our finding is essential for designing single-mode high-power PCSELs for which the strategy to suppress the high-order modes must be considered.

Air-Hole Retained Growth by Molecular Beam Epitaxy for Fabricating GaAs-Based Photonic-Crystal Lasers

We report a method of embedding air/semiconductor two-dimensional photonic-crystal structures into semiconductor crystals using molecular beam epitaxy. We show that we can form air/GaAs photonic crystals composed of retained air holes only by growing an AlGaAs layer onto a GaAs substrate with photonic-crystal patterns. A variety of air/GaAs photonic-crystal structures can also be obtained by adjusting the relative directions between the molecular-beam irradiation and the crystalline orientation of GaAs. Finally, a flat surface, where the root-mean-square value is less than 1 nm, are obtained when AlGaAs layers with sufficient thickness are grown even after the air-hole retained growth.

Finite-difference time-domain (FDTD) analysis on the interaction between a metal block and a radially polarized focused beam.

Radially polarized focused beams have attracted a great deal of attention because of their unique properties characterized by the longitudinal field. Although this longitudinal field is strongly confined to the beam axis, the energy flow, i.e., the Poynting vector, has null intensity on the axis. Hence, the interaction of the focused beam and matter has thus far been unclear. We analyzed the interactions between the focused beam and a subwavelength metal block placed at the center of the focus using three-dimensional finite-difference time-domain (FDTD) calculation. We found that most of the Poynting energy propagates through to the far-field, and that a strong enhancement of the electric field appeared on the metal surface. This enhancement is attributed to the constructive interference of the symmetric electric field and the coupling to the surface plasmon mode.

Photonic-Crystal Surface-Emitting Lasers: Review and Introduction of Modulated-Photonic Crystals

Photonic-crystal surface-emitting lasers (PCSELs) have attracted much attention for their unrivaled capabilities, such as broad area, coherent resonance, tailored beam patterns, and beam steering. In this paper, we first review the progress of PCSELs, then introduce a novel concept of modulated photonic-crystal surface-emitting lasers (M-PCSELs) for realizing both lasing oscillation and on demand, beam diffraction for any two-dimensional direction in free space without the need for external elements. This unique concept paves the way toward the development of semiconductor lasers with completely controllable beams.

Position-modulated photonic-crystal lasers and control of beam direction and polarization

We propose and demonstrate position-modulated photonic-crystal lasers which can control the output-beam directions two-dimensionally and have various polarizations. The results promise to greatly enhance the range of possible laser-scanning applications.