High-order optical vortex beam generation based on watt-class spatial phase plate-integrated photonic-crystal surface-emitting lasers.

We develop spatial phase plate (SPP)-integrated photonic-crystal surface-emitting lasers featuring double-lattice photonic-crystal structures embedded using a crystal regrowth technique. SPPs possessing eight segmentations per phase rotation number l are fabricated on the top surface to generate optical vortex beams (OVBs) with l = 1-3. The beams exhibit a high output power of ∼5 W and high mode purities of 85%, 78%, and 72% for l = 1-3, respectively. These purity values are comparable with those of a pure Gaussian mode passing through an SPP. The compact, high-power, and high-purity OVB sources can be used in the fields of material processing, optical manipulation, and microscopy.

Generation of optical vortex beam by surface-processed photonic-crystal surface-emitting lasers.

An optical vortex beam possesses a phase singularity that causes a null intensity at the center of the beam, and can be explained as a superposition of a phase distribution along the azimuthal direction and a plane wave. Here, we process the surface of a photonic-crystal surface-emitting laser (PCSEL) to generate an optical vortex beam. By using an eight-segmented phase plate fabricated via three chemical etching steps, a beam having null intensity is obtained. From evaluation of the beam's polarization and interference patterns, we show that the null intensity comes from the phase singularity of the optical vortex.

Polarization control by modulated photonic-crystal lasers.

Modulated photonic-crystal lasers can control the output beam direction two-dimensionally by exciting a two-dimensional cavity mode at the non-diffractive photonic band-edge and diffracting the mode upwards with position modulation of each air hole. In these lasers, the position modulation can be introduced one-directionally, where the modulation is given by the distances between the air holes, or two-directionally, where the modulation is given by the rotation angles of the air holes. For one-directional position modulation, we show that the polarization of output beams is perpendicular to the direction of modulation. For two-directional position modulation, we show that circularly polarized beams are obtained. As such, these lasers can control not only the beam direction but also the polarization.

Investigation of electric field enhancement between metal blocks at the focused field generated by a radially polarized beam.

A radially polarized beam possesses peculiar focusing properties compared with a linearly polarized beam, for example, the generation of a strong longitudinal field and zero intensity of the Poynting vector on the beam axis. In order to exploit these focusing properties, here we consider a system in which gold metal cubes are arranged along the propagation direction of the beam. An electric field enhancement of more than 20-times can be generated between two gold cubes separated by a distance λ/10 on the optical axis. This is because the energy of a radially polarized beam can propagate even if a metal cube is located on the beam axis, and a longitudinal field generated between the cubes can induce a surface plasmon mode. We show that these results are peculiar properties that cannot be produced with an incident linearly polarized beam. We also show that the beam can generate multiple regions of electrical field enhancement in the propagating direction when multiple metal cubes are arranged on the beam axis.

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.

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.

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.

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.

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.