We realize a femtosecond high-power spontaneous mode-locked operation with gigahertz oscillation in a vertical-external cavity surface-emitting laser under the condition of eliminating the internal and external unwanted reflection. We find that the reflectivity of the output coupler has a significant influence not only on the output power but also on the output pulse duration. With an incident pump power of 20 W, we have achieved 2.35 W of average output power with 778 fs pulse duration at a repetition rate of 2.17 GHz. The shortest pulse duration was 654 fs at an average output power of 0.45 W.

The criterion for achieving the self-mode-locking (SML) in an optically pumped semiconductor laser (OPSL) with a linear cavity is systematically explored. Experimental results reveal that the occurrence of SML can be assisted by the existence of high-order transverse modes. Numerical analysis is performed to confirm that the critical pump power for obtaining the SML operation agrees very well with the pump threshold for exciting TEM 1,0 mode. The present finding offers an important insight into laser physics and a useful indication for obtaining the SML operation in OPSLs.

Nitride-based UV, vertical-injection light-emitting diodes ͑VLEDs͒ with GaN-free and surface roughness structures operating at 365 nm were proposed and demonstrated by a combination of wafer bonding and laser lift-off processes. The GaN-free structure offers a promising potential for enhancing the light output of UV-VLEDs. The 3.2ϫ light output enhancement was performed by removing the GaN. With the help of adopting a roughened surface, the light-output power of the UV-VLEDs could be further enhanced by a factor of 2.3 as compared with that of UV-VLED without a roughened surface. The total enhancement of surface-roughened GaN-free UV-VLEDs was increased by a factor of 7.8 compared to that of conventional UV-LEDs at a driving current injection of 250 mA.High-brightness GaN-based blue and green light emitting diodes ͑LEDs͒ have already been extensively used in full-color displays, traffic signals, and exterior automotive lighting, etc., and show a greater potential to replace incandescent bulbs and fluorescent lamps. 1 UV-LEDs are of interest as a light source for exciting phosphors, medical equipment, air cleaners, and environmental sensors. In particular, most of the phosphors for white fluorescent lamps have a high conversion efficiency of less than 370 nm in the UV spectral region. 2 Therefore, it is important to develop high-efficiency UVLEDs that emit less than 370 nm light in order to fabricate high luminous white LEDs by coupling UV-LEDs and phosphors for solid-state lighting applications. Although the blue/green LEDs are commercially available, it is still difficult to manufacture highbrightness UV-LEDs due to poor quantum efficiency. Several works focusing on efficient current injection by introducing current blocking layers, 3 highly efficient radiative carrier recombination by introducing a high-quality bulk GaN substrate, 4 the emission enhancement of LEDs by the introduction of an AlInGaN quaternary active layer, 5 and the improvement in quality of LEDs by utilizing the AlGaN epitaxial lateral overgrowth technique 6 have attempted to improve device performance. However, light extraction efficiency is also limited due to the self-absorption effect of the GaN layer. The self-absorption of UV lights in the bulk GaN substrate, thick GaN buffer/contact layer under the active layer, or the p-type GaN contact layer result in the lower external quantum efficiency. Therefore, GaN-free structure design is a key issue for enhancing the light extraction of UV-LEDs. 7 Recently, state-of-the art vertical injection GaN-based LEDs were demonstrated to be high-potential lightemitting devices capable of achieving high brightness operation due to their excellent thermal dissipation. 8,9 In addition, the surface roughness technique seems to have great potential to provide large enhancement due to random scattering from the roughened surface. 10,11 Zhou et al. 12 reported Ͼ2.5 times light extraction gain for deep UV-LEDs by utilizing laser lift-off ͑LLO͒ and surface roughening to enhance the extraction efficiency of near-...

Flexibility is one of remarkable characteristics of flexible electronics. Therefore, the analysis of flexible electronics deformation caused by external torques will help its design. In this work, analytical derivation is conducted for flexible electronics treated as multilayer structures. In experiments, an electric motor under computer control exerts a torque to generate torsion, whose twist angles are obtained by using both optical encoder measurement and theoretical calculation. Comparisons are made between experimental and theoretical results for bare PEN (polyethylene naphthalate) substrates of different sizes, multilayer PET (polyethylene terephthalate) substrates, and ITO (indium tin oxide)‐coated PET substrates. Finally, a laser displacement sensor is used to measure the out‐of‐plane deformation for validating the proposed theoretical model.

In contrast to liquid-crystal displays (LCDs) and light-emitting-diode (LED) displays, flexible displays allow deformation. Therefore, it is worth studying the deformation of substrates in flexible displays. In this work, an analytical derivation is carried out for the torsion behavior of flexible substrates. Comparisons are made between deflection results of linear and non-linear elasticity analyses for substrates subject to four corner forces and torque, respectively. They show that when deflection becomes large, torques or forces needed for unit deformation also increase. Simulation results also show that when the substrate aspect ratio of length/width in display sizes increases, displays are easier to deform under torsion. In addition, polyethylene terephthalate (PET) substrates subject to torque are compared with polyethylene naphthalate (PFN) substrates. Finally, in experiments a fringe-reflection method and a four-step phase-shifting technique were carried out to validate the proposed non-linear model.