Progress and challenges in electrically pumped GaN-based VCSELs

The Vertical-Cavity Surface-Emitting Laser (VCSEL) is an established optical source in short-distance optical communication links, computer mice and tailored infrared power heating systems. Its low power consumption, easy integration into two-dimensional arrays, and low-cost manufacturing also make this type of semiconductor laser suitable for application in areas such as high-resolution printing, medical applications, and general lighting. However, these applications require emission wavelengths in the blue-UV instead of the established infrared regime, which can be achieved by using GaN-based instead of GaAs-based materials. The development of GaN-based VCSELs is challenging, but during recent years several groups have managed to demonstrate electrically pumped GaN-based VCSELs with close to 1 mW of optical output power and threshold current densities between 3-16 kA/cm2. The performance is limited by challenges such as achieving high-reflectivity mirrors, vertical and lateral carrier confinement, efficient lateral current spreading, accurate cavity length control and lateral optical mode confinement. This paper summarizes different strategies to solve these issues in electrically pumped GaN-VCSELs together with state-of-the-art results. We will highlight our work on combined transverse current and optical mode confinement, where we show that many structures used for current confinement result in unintentionally optically anti-guided resonators. Such resonators can have a very high optical loss, which easily doubles the threshold gain for lasing. We will also present an alternative to the use of distributed Bragg reflectors as high-reflectivity mirrors, namely TiO2/air high contrast gratings (HCGs). Fabricated HCGs of this type show a high reflectivity (>95%) over a 25 nm wavelength span

Optical microprism cavities based on dislocation-free GaN

Three-dimensional growth of nanostructures can be used to reduce the threading dislocation density that degrades III-nitride laser performance. Here, nanowire-based hexagonal GaN microprisms with flat top and bottom c-facets are embedded between two dielectric distributed Bragg reflectors to create dislocation-free vertical optical cavities. The cavities are electron beam pumped, and the quality (Q) factor is deduced from the cavity-filtered yellow luminescence. The Q factor is similar to 500 for a 1000nm wide prism cavity and only similar to 60 for a 600nm wide cavity, showing the strong decrease in Q factor when diffraction losses become dominant. Measured Q factors are in good agreement with those obtained from quasi-3D finite element frequency-domain method and 3D beam propagation method simulations. Simulations further predict that a prism cavity with a 1000nm width will have a Q factor of around 2000 in the blue spectral regime, which would be the target regime for real devices. These results demonstrate the potential of GaN prisms as a scalable platform for realizing small footprint lasers with low threshold currents

High-resolution macromolecular crystallography at the FemtoMAX beamline with time-over-threshold photon detection

Protein dynamics contribute to protein function on different time scales. Ultrafast X-ray diffraction snapshots can visualize the location and amplitude of atom displacements after perturbation. Since amplitudes of ultrafast motions are small, high-quality X-ray diffraction data is necessary for detection. Diffraction from bovine trypsin crystals using single femtosecond X-ray pulses was recorded at FemtoMAX, which is a versatile beamline of the MAX IV synchrotron. The time-over-threshold detection made it possible that single photons are distinguishable even under short-pulse low-repetition-rate conditions. The diffraction data quality from FemtoMAX beamline enables atomic resolution investigation of protein structures. This evaluation is based on the shape of the Wilson plot, cumulative intensity distribution compared with theoretical distribution, I/σ, Rmerge /Rmeas and CC1/2 statistics versus resolution. The FemtoMAX beamline provides an interesting alternative to X-ray free-electron lasers when studying reversible processes in protein crystals

Diffractive Optics Design

Diffractive optical elements (kinoforms) change the way light propagates, and can perform very complex tasks. They can split an incident beam into any number of outgoing, and possibly focused, beams (fan-out). Other kinoforms shape the cross sectional intensity distribution of the beam, which is often Gaussian, into a rectangle with constant intensity, for instance. What function the kinoform implements depends on the surface relief etched on the kinoform. This work considers important aspects of the design of diffractive optical elements, within the scalar optics approximation, such as - efficient optimization of the kinoform relief with the optimal-rotation-angle method. For example, shallow, phase-swing restricted kinoforms are designed. - non-diffraction-limited (beam shaping) design. One experimental example is a semiconductor laser beam shaping system consisting only of a multiple-function kinoform. - finding a model for the effects of fabrication on the relief (the proximity effect) and trying to compensate for this effect already in the design, which can yield very uniform fan-out patterns even when the proximity effect is considerable. - integrating diffractive optics with semiconductor optics. Examples are kinoforms illuminated by VCSELs and dislocated binary gratings that outcouple a guided wave and also impose a continuous phase modulation on the outcoupled wave. - using the exact (no Fresnel approximation, for instance) scalar theory, based on the scalar wave (Helmholtz) equation, in an efficient formulation that enables the design of kinoforms producing virtually any desired, three-dimensional, fan-out light distribution

Diffractive Optics Design

Diffractive optical elements (kinoforms) change the way light propagates, and can perform very complex tasks. They can split an incident beam into any number of outgoing, and possibly focused, beams (fan-out). Other kinoforms shape the cross sectional intensity distribution of the beam, which is often Gaussian, into a rectangle with constant intensity, for instance. What function the kinoform implements depends on the surface relief etched on the kinoform. This work considers important aspects of the design of diffractive optical elements, within the scalar optics approximation, such as - efficient optimization of the kinoform relief with the optimal-rotation-angle method. For example, shallow, phase-swing restricted kinoforms are designed. - non-diffraction-limited (beam shaping) design. One experimental example is a semiconductor laser beam shaping system consisting only of a multiple-function kinoform. - finding a model for the effects of fabrication on the relief (the proximity effect) and trying to compensate for this effect already in the design, which can yield very uniform fan-out patterns even when the proximity effect is considerable. - integrating diffractive optics with semiconductor optics. Examples are kinoforms illuminated by VCSELs and dislocated binary gratings that outcouple a guided wave and also impose a continuous phase modulation on the outcoupled wave. - using the exact (no Fresnel approximation, for instance) scalar theory, based on the scalar wave (Helmholtz) equation, in an efficient formulation that enables the design of kinoforms producing virtually any desired, three-dimensional, fan-out light distribution