Direct-focusing surface-emitting laser

Abstract

Focusing is a fundamental optical technique that has been widely implemented via lenses. Here, we demonstrate direct focusing from a band-edge surface-emitting laser, whose emission area is 200 µm × 200 µm, without any lenses. To achieve this, a phase-modulating layer is incorporated into the laser cavity. This layer acts simultaneously as a lasing cavity similar to that of a photonic crystal laser and as a holographic spatial-phase modulator, which transforms the output beam into a focusing beam by slightly shifting the positions of holes from a periodic square lattice. Beam profiles along the surface normal clearly show that direct focusing occurs with a focal length and focal spot size of 310 µm and 6.1 µm, respectively. The focal length agrees well with the theoretical value, and the focal spot size is 2.0 times the diffraction-limited size, which indicates that the higher transverse modes are sufficiently suppressed. In addition, the power density at the focus is 540 times higher than that at the near-field plane. Interestingly, a focus pattern is also observed in the opposite direction at the near-field plane, which indicates that a converging beam and a diverging beam are simultaneously emitted because of the nature of the in-plane band-edge laser. The conventional beam patterns of semiconductor laser cavities are limited to the regime of two-dimensional projection based on a Fourier hologram. In contrast, we demonstrate the simplest form of a three-dimensional point cloud based on a Fresnel hologram, which is quite useful for micro-sensing applications such as microfluidics, flow cytometry, blood sensors, and endoscopy.