Analysis of optically pumped compact laterally coupled distributed feedback lasers with three symmetric defect regions

This article analyzes compact laterally coupled distributed feedback DFB lasers with three defect regions. These devices are more flexible and smaller than conventional DFB lasers, having typical lengths between 20 and 50 m and a width less than 1 m lateral gratings are inserted in a single-mode waveguide . We optimize the defect regions to achieve an improved performance. In other words, an adequate choice of phase shifts may lead to single-mode operation, lower threshold optical power, higher quantum differential efficiency, and more uniform field distribution. This device is designed to operate under optical pumping

Coupling of light from microdisk lasers into plasmonic nano-antennas

An optical dipole nano-antenna can be constructed by placing a sub-wavelength dielectric (e.g., air) gap between two metallic regions. For typical applications using light in the infrared region, the gap width is generally in the range between 50 and 100 nm. Owing to the close proximity of the electrodes, these antennas can generate very intense electric fields that can be used to excite nonlinear effects. For example, it is possible to trigger surface Raman scattering on molecules placed in the vicinity of the nano-antenna, allowing the fabrication of biological sensors and imaging systems in the nanometric scale. However, since nano-antennas are passive devices, they need to receive light from external sources that are generally much larger than the antennas. In this article, we numerically study the coupling of light from microdisk lasers into plasmonic nanoantennas. We show that, by using micro-cavities, we can further enhance the electric fields inside the nano-antennas

Steady-State Analysis of a Directional Square Lattice Band-Edge Photonic Crystal Lasers

The original square-lattice band-edge structure emitted light in different directions. In a recent paper, we have improved the directivity of these structures by using a honeycomb photonic crystal to shield the original structure. This has improved the performance of such devices, in such a way that light can be coupled directionally to a large waveguide. In this paper, we present a steady-state analysis of the performance of this directive device and study the amount of power which can be coupled into the waveguide, analyzing its performance as an in-plane emitter and the limitations caused by vertical losses

Increasing the coupling efficiency of a microdisk laser to waveguides by using well designed spiral structures

In this article, we optimize the coupling efficiency from a GaAs microdisk resonator into a single mode spiral waveguide. A classical microdisk resonator coupling light into a nonevanescent straight waveguide reaches a typical coupling efficiency of 67%. We show that the introduction of a spiral waveguide that works both as a waveguide and circular Bragg reflector can improve such efficiency to almost 90%. The same structure with the addition of a taper can couple up to 80% of the generated power into a slot waveguide.The authors would like to acknowledge the funding from the Australian Research Council for this project

Terahertz focusing of multiple wavelengths by graphene metasurfaces

Metasurfaces can achieve nearly arbitrary wavefront control based on manipulation of the wave phase profile. We propose a metasurface based on double graphene cut-wire resonators which can cover the complete 2p phase region with high reflection efficiency. This full phase coverage is essential for efficient wavefront manipulation, without reflecting energy into unwanted channels. A mirror capable of focusing multiple wavelengths is demonstrated numerically based on the proposed structure. The mirror can effectively focus terahertz (THz) waves from 1.2 to 1.9 THz to the same focal point by changing the Fermi level of each graphene resonator separately. The presented metasurface could provide a powerful platform for controlling THz waves, including focusing, beam steering, beam shaping, and holography

Post-processing approach for tuning multi-layered metamaterials

We propose a post-processing approach to efficiently tune the resonance frequency in double-layered terahertz metamaterials separated by a bonding agent. By heating the bonding agent, it is possible to move one metamaterial layer laterally with respect to the other. This changes the coupling between adjacent layers, thereby shifting the resonance frequency. The resonance frequency of the stacked layers continuously shifts as a function of the lateral displacement, reaching a maximum shift of 92 GHz (31% of the center frequency). We discuss the effects of vertical separation on the tunability of the two-layered structure. The post-processing approach is rather general and can be applied to different paired metamaterials in various wavelength ranges, paving the way to efficiently assemble and fine tune metamaterial sensors and filters.The authors would like to acknowledge the financial support provided by the Australian Research Council and the Asian Office of Aerospace Research and Development–U.S. Air Force

A Tale of Two Tantalum Borides as Potential Saturable Absorbers for Q-Switched Fiber Lasers

In this paper, we analyze the performance of two tantalum-based boride (TaB and TaB 2 ) microparticles as potential saturable absorbers for high-power fiber lasers. Both materials are ultrahigh temperature ceramics with melting points above 3000 °C, but with different crystalline structures: TaB has an orthorhombic structure (nearly isotropic), whereas TaB 2 has a hexagonal structure (uniaxial, anisotropic). Despite their different crystalline structures, the microparticles have a similar low fluence attenuation (between 2.3 and 2.60 dB/μm) and modulation depths (around 2.0 dB/μm), but remarkable different saturation fluences: TaB has a saturation fluence of 160 μJ/cm 2 , whereas TaB 2 has a saturation fluence of 110 μJ/cm 2 . The measured damage thresholds are 112 and 10 6 mJ/cm 2 /pulse for TaB and TaB 2 , respectively. When incorporated to a fiber laser, the materials produce pulses with durations of 345 ns, lower than those reported by our group in previous papers. The results show that the materials can find potential applications in high-power Q-switched lasers

High Fluence Chromium and Tungsten Bowtie Nano-antennas

Nano-antennas are replicas of antennas that operate at radio-frequencies, but with considerably smaller dimensions when compared with their radio frequency counterparts. Noble metals based nano-antennas have the ability to enhance photoinduced phenomena such as localized electric fields, therefore-they have been used in various applications ranging from optical sensing and imaging to performance improvement of solar cells. However, such nano-structures can be damaged in high power applications such as heat resisted magnetic recording, solar thermo-photovoltaics and nano-scale heat transfer systems. Having a small footprint, nano-antennas cannot handle high fluences (energy density per unit area) and are subject to being damaged at adequately high power (some antennas can handle just a few milliwatts). In addition, given that nano-antennas are passive devices driven by external light sources, the potential damage of the antennas limits their use with high power lasers: this liability can be overcome by employing materials with high melting points such as chromium (Cr) and tungsten (W). In this article, we fabricate chromium and tungsten nano-antennas and demonstrate that they can handle 110 and 300 times higher fluence than that of gold (Au) counterpart, while the electric field enhancement is not significantly reduced.Te authors gratefully acknowledge the fabrication facilities provided by Australian National Fabrication Facility (ANFF ACT node, Australia). We would acknowledge the fnancial support from UNSW Canberra, Australia. We also would like to thank Te Asian Ofce of Aerospace Research and Development (AOARD US Air Force FA2386-15-1-4084), Australian Research Council (ARC LP160100253, DP170103778 and DE190100413) to provide the funding