Frequency tripled semiconductor disk laser with over 0.5 W ultraviolet output power.

Semiconductor disk lasers can produce high output power and good beam quality simultaneously. The high intracavity circulating power of about hundreds of watts, along with the flexibility of tailorable emitting wavelengths, make it an attractive light source for obtaining ultraviolet (UV) radiation from near-infrared lasers through nonlinear frequency conversion. This work reports a frequency tripled 327 nm semiconductor disk laser with record output power and wavelength tuning range by using a type-I phase-matched LiB3O5 (LBO) crystal and a type-I phase-matched ß-BaB2O4 (BBO) crystal as the frequency-doubling and -tripling crystals respectively. Thanks to the obviously larger nonlinear coefficient of the type-I phase-matched BBO compared to the commonly used type-II phase-matched LBO, as well as the small spot size specifically designed at the crystal location, the maximum output power of UV lasers reaches 538 mW, corresponding to an optical-to-optical conversion efficiency from pump to UV laser of about 1.14%. A wavelength tuning range of about 8.6 nm and good power stability with a standard deviation of about 0.94 are also achieved.

Boosting Perovskite Photodetector Performance in NIR Using Plasmonic Bowtie Nanoantenna Arrays.

Triple-cation mixed metal halide perovskites are important optoelectronic materials due to their high photon to electron conversion efficiency, low exciton binding energy, and good thermal stability. However, the perovskites have low photon to electron conversion efficiency in near-infrared (NIR) due to their weak intrinsic absorption at longer wavelength, especially near the band edge and over the bandgap wavelength. A plasmonic functionalized perovskite photodetector (PD) is designed and fabricated in this study, in which the perovskite ((Cs0.06 FA0.79 MA0.15 )Pb(I0.85 Br0.15 )3 ) active materials are spin-coated on the surface of Au bowtie nanoantenna (BNA) arrays substrate. Under 785 nm laser illumination, near the bandedge of perovskite, the fabricated BNA-based plasmonic PD exhibits ≈2962% enhancement in the photoresponse over the Si/SiO2 -based normal PD. Moreover, the detectivity of the plasmonic PD has a value of 1.5 × 1012 with external quantum efficiency as high as 188.8%, more than 30 times over the normal PD. The strong boosting in the plasmonic PD performance is attributed to the enhanced electric field around BNA arrays through the coupling of localized surface plasmon resonance. The demonstrated BNA-perovskite design can also be used to enhance performance of other optoelectronic devices, and the concept can be extended to other spectral regions with different active materials.

High power femtosecond semiconductor lasers based on saw-toothed master-oscillator power-amplifier system with compressed ASE.

High power femtosecond semiconductor laser based on saw-toothed taper mode-locked laser and amplifier was demonstrated with compressed amplified spontaneous emission (ASE). The external-cavity mode-locked taper laser generated the clean optical pulses without any sub-pulse components. A semiconductor optical amplifier (SOA) with tilted taper waveguide and saw-toothed edge reduced evidently the ASE background. The saw-tooth microstructures were optimized and it was found that the saw-tooth of right-right angled triangle showed the best effect. The ratio of the maximum intensity to background radiation was increased by 21.9% and the power was increased by 30.5% due to the saw-tooth microstructure in the SOA. The pulse duration of 495 fs and a peak power over 1.5 kW with repetition rate of 579 MHz were realized after a double-pass grating compressor.

Beam waist shrinkage of high-power broad-area diode lasers by mode tailoring.

A new approach was proposed and its role in improvement of the beam quality of high-power broad-area diode lasers was demonstrated, in which a composite arrow array and trench microstructure was used to suppress the beam waist and tailor the high order lateral modes. The beam waist shows a special shrinkage with increasing injection current resulting from the combined effect of mode tailoring and the thermal lens effect. A 58% improvement in lateral beam parameter product was realized compared with conventional broad-area diode lasers.

Loss tailoring of high-power broad-area diode lasers.

Broad-area diode lasers (BALs) with high power are highly desirable for a variety of applications. However, such lasers suffer from strongly deteriorated beam quality due to multimode behavior in the lateral direction. In this Letter, we present an approach to flexibly tailor the optical loss of different-order lateral modes by etching micro-holes on the laser mesa with controlled position and numbers. Through arranging the micro-holes at the peak positions of high-order lateral modes with an increasing number from the mesa center to both edges, high-order modes are suppressed due to a larger propagation loss than the fundamental mode. As a result of enhanced mode discrimination, we demonstrate that this technique provides a greatly improved beam quality and about two times higher brightness for 100 µm wide BAL, while maintaining high power and slope efficiency output.

Dynamically tunable multi-lobe laser generation via multifocal curved beam.

Beams with curved properties, represented by Airy beam, have already shown potential applications in various fields. Here we propose a simple method to achieve a multifocal curved beam (MCB). The scheme is based on the ability of microspheres to control the distribution of the light field. Combined with the caustic effect, the dynamic control of the beam curvature and the foci can be realized. The simulation results confirm the mechanism behind this phenomenon. Furthermore, MCB is applied experimentally into the end-pumped microchip laser. This work has verified the theory of MCB and achieved a dynamically tunable multi-lobe laser, which has a wide application prospect.

High-brightness diode lasers obtained via off-axis spectral beam combining with selective feedback.

We proposed a modified off-axis spectral beam combining method, based on the concept of selective feedback. A high reflectivity mirror with a fixed width was used to select and couple back the optical modes in the external cavity. The emission power exceeding 20 W with M2 factors of 2.7 × 4.4 in the fast and slow axes was demonstrated. The beam quality of the system was improved by a factor of three to four compared with that of a single emitter, and a high brightness of 190 MW cm-2 str-1 was achieved.

Going beyond the beam quality limit of spectral beam combining of diode lasers in a V-shaped external cavity.

A novel spectral beam combining (SBC) approach based on off-axis feedback in a V-shaped external cavity (VSBC) was proposed and demonstrated. A highly reflecting mirror was used to supply the optical feedback by partial overlapping the beam. The advantages of simple setup, output coupler free, tunable beam quality and emission power over traditional SBC were presented. The beam quality exceeds single emitter with the similar energy conversion efficiency to the traditional SBC. The M2 factors of 2.31 × 3.76 in fast and slow axes and a brightness of 122 MWcm-2sr-1 were realized at 30 A based on a commercial broad-area diode laser array.

Asymmetric light diffraction of two-dimensional electromagnetically induced grating with PT symmetry in asymmetric double quantum wells.

An asymmetric double semiconductor quantum well is proposed to realize two-dimensional parity-time (PT) symmetry and an electromagnetically induced grating. In such a nontrivial grating with PT symmetry, the incident probe photons can be diffracted to selected angles depending on the spatial relationship of the real and imaginary parts of the refractive index. Such results are due to the interference mechanism between the amplitude and phase of the grating and can be manipulated by the probe detuning, modulation amplitudes of the standing wave fields, and interaction length of the medium. Such a system may lead to new approaches of observing PT-symmetry-related phenomena and has potential applications in photoelectric devices requiring asymmetric light transport using semiconductor quantum wells.

Investigation of regime switching from mode locking to Q-switching in a 2 µm InGaSb/AlGaAsSb quantum well laser.

A two-section InGaSb/AlGaAsSb single quantum well (SQW) laser emitting at 2 µm is presented. By varying the absorber bias voltage with a fixed gain current at 130 mA, passive mode locking at ~18.40 GHz, Q-switched mode locking, and passive Q-switching are observed in this laser. In the Q-switched mode locking regimes, the Q-switched RF signal and mode locked RF signal coexist, and the Q-switched lasing and mode-locked lasing happen at different wavelengths. This is the first observation of these three pulsed working regimes in a GaSb-based diode laser. An analysis of the regime switching mechanism is given based on the interplay between the gain saturation and the saturable absorption.

Near-diffraction-limited Bragg reflection waveguide lasers.

We report a near-diffraction-limited tapered Bragg reflection waveguide laser (BRL) with a 10 µm ridge width, which is significantly larger than the conventional design. The large mode expansion in the vertical waveguide enables a scalable increase in the ridge width for single lateral mode operation. The role of the taper angle in the performance of tapered BRLs with the intrinsic characteristics of a thick vertical waveguide was investigated. The results indicate that the BRL with a taper angle of 3° shows the best far-field performance. An extremely low vertical divergence angle of 14.5° and a lateral divergence as low as 2.8° for 95% power inclusion were realized. A continuous-wave power exceeding 1 W was demonstrated. Over the entire operating current range, the vertical beam is almost unchanged with an excellent beam quality (M2) of about 1.3. Lateral beam width increases slightly at higher currents due to the increasing contribution of side lobes, but it still remains nearly diffraction-limited with a lateral M2 of less than 2. Narrow beam divergence and high beam quality of the lasers allow simple and inexpensive collimation and coupling.

High-power dual-wavelength lasing in bimodal-sized InGaAs/GaAs quantum dots.

In this paper, we demonstrate high power, dual-wavelength (dual-λ) lasing stemming from bimodal-sized InGaAs/GaAs quantum dots (QDs). The device exhibits simultaneous dual-λ lasing at 1015.2 nm and 1023.0 nm with total power of 165.6 mW at 700 mA under room temperature continuous wave (CW) mode. Gaussian fitting analyses of the electroluminescence (EL) spectrum attribute the excellent performance to independent carrier transitions from the first excited states of large dot ensemble (LD ES1) and small dot ensemble (SD ES1), respectively. This formation provides a new possibility to achieve high power dual-λ operation only using Fabry-Pérot (FP) cavity, which is significant for compact size and low fabrication cost.

Low lateral divergence 2 µm InGaSb/ AlGaAsSb broad-area quantum well lasers.

High power and high brightness mid-infrared GaSb based lasers are desired for many applications, however, the high lateral divergence is still the influence factor for practical application. In this paper, a simple and effective approach based on the fishbone-shape microstructure was proposed, the effective improvement on both the lateral divergence and output power of 2 µm GaSb based broad-area lasers was demonstrated. The lateral divergence is reduced averagely by 55% and 15.8° for 95% power content is realized. The continuous-wave emission power is increased about 19% with the decreased threshold current. The other merits for this microstructure are the unchanged intrinsic characteristic of broad-area lasers and the low cost fabrication.

Design considerations of high-performance InGaAs/InP single-photon avalanche diodes for quantum key distribution.

InGaAs/InP single-photon avalanche diodes (SPADs) are widely used in practical applications requiring near-infrared photon counting such as quantum key distribution (QKD). Photon detection efficiency and dark count rate are the intrinsic parameters of InGaAs/InP SPADs, due to the fact that their performances cannot be improved using different quenching electronics given the same operation conditions. After modeling these parameters and developing a simulation platform for InGaAs/InP SPADs, we investigate the semiconductor structure design and optimization. The parameters of photon detection efficiency and dark count rate highly depend on the variables of absorption layer thickness, multiplication layer thickness, excess bias voltage, and temperature. By evaluating the decoy-state QKD performance, the variables for SPAD design and operation can be globally optimized. Such optimization from the perspective of specific applications can provide an effective approach to design high-performance InGaAs/InP SPADs.

Continuous-wave operation of 1550 nm low-threshold triple-lattice photonic-crystal surface-emitting lasers.

Benefitting from narrow beam divergence, photonic crystal surface-emitting lasers are expected to play an essential role in the ever-growing fields of optical communication and light detection and ranging. Lasers operating with 1.55 µm wavelengths have attracted particular attention due to their minimum fiber loss and high eye-safe threshold. However, high interband absorption significantly decreases their performance at this 1.55 µm wavelength. Therefore, stronger optical feedback is needed to reduce their threshold and thus improve the output power. Toward this goal, photonic-crystal resonators with deep holes and high dielectric contrast are often used. Nevertheless, the relevant techniques for high-contrast photonic crystals inevitably complicate fabrication and reduce the final yield. In this paper, we demonstrate the first continuous-wave operation of 1.55 µm photonic-crystal surface-emitting lasers by using a 'triple-lattice photonic-crystal resonator', which superimposes three lattice point groups to increase the strength of in-plane optical feedback. Using this geometry, the in-plane 180° coupling can be enhanced threefold compared to the normal single-lattice structure. Detailed theoretical and experimental investigations demonstrate the much lower threshold current density of this structure compared to 'single-lattice' and 'double-lattice' photonic-crystal resonators, verifying our design principles. Our findings provide a new strategy for photonic crystal laser miniaturization, which is crucial for realizing their use in future high-speed applications.

Observation of the fluorescence spectrum for a driven cascade model system in atomic beam.

We experimentally study the resonance fluorescence from an excited two-level atom when the atomic upper level is coupled by a nonresonant field to a higher-lying state in a rubidium atomic beam. The heights, widths and positions of the fluorescence peaks can be controlled by modifying the detuning of the auxiliary field. We explain the observed spectrum with the transition properties of the dressed states generated by the coupling of the two laser fields. We also attribute the line narrowing to the effects of Spontaneously Generated Coherence between the close-lying levels in the dressed state picture generated by the auxiliary field. And the corresponding spectrum can be viewed as the evidence of Spontaneously Generated Coherence. The experimental results agree well with calculations based on the density-matrix equations.

Parity-time symmetry in coherent asymmetric double quantum wells.

A coherently prepared asymmetric double semiconductor quantum well (QW) is proposed to realize parity-time (PT) symmetry. By appropriately tuning the laser fields and the pertinent QW parameters, PT-symmetric optical potentials are obtained by three different methods. Such a coherent QW system is reconfigurable and controllable, and it can generate new approaches of theoretically and experimentally studying PT-symmetric phenomena.

Enhancing Third- and Fifth-Order Nonlinearity via Tunneling in Multiple Quantum Dots.

The nonlinearity of semiconductor quantum dots under the condition of low light levels has many important applications. In this study, linear absorption, self-Kerr nonlinearity, fifth-order nonlinearity and cross-Kerr nonlinearity of multiple quantum dots, which are coupled by multiple tunneling, are investigated by using the probability amplitude method. It is found that the linear and nonlinear properties of multiple quantum dots can be modified by the tunneling intensity and energy splitting of the system. Most importantly, it is possible to realize enhanced self-Kerr nonlinearity, fifth-order nonlinearity and cross-Kerr nonlinearity with low linear absorption by choosing suitable parameters for the multiple quantum dots. These results have many potential applications in nonlinear optics and quantum information devices using semiconductor quantum dots.

Extracting more light for vertical emission: high power continuous wave operation of 1.3-µm quantum-dot photonic-crystal surface-emitting laser based on a flat band.

For long distance optical interconnects, 1.3-µm surface-emitting lasers are key devices. However, the low output power of several milliwatts limits their application. In this study, by introducing a two-dimensional photonic-crystal and using InAs quantum dots as active materials, a continuous-wave, 13.3-mW output power, 1.3-µm wavelength, room-temperature surface-emitting laser is achieved. In addition, such a device can be operated at high temperatures of up to 90 °C. The enhanced output power results from the flat band structure of the photonic crystal and an extra feedback mechanism. Surface emission is realized by photonic crystal diffraction and thus the distributed Bragg reflector is eliminated. The proposed device provides a means to overcome the limitations of low-power 1.3-µm surface-emitting lasers and increase the number of applications thereof.

Fabrication of TiCx-TiB2/Al Composites for Application as a Heat Sink.

Metal matrix composites reinforced with ceramic particles have become the most attractive material in the research and development of new materials for thermal management applications. In this work, 40-60 vol. % TiCx-TiB2/Al composites were successfully fabricated by the method of combustion synthesis and hot press consolidation in an Al-Ti-B4C system. The effect of the TiCx-TiB2 content on the microstructure and compression properties of the composites was investigated. Moreover, the abrasive wear behavior and thermo-physics properties of the TiCx-TiB2/Al composite were studied and compared with the TiCx/Al composite. The compression properties, abrasive wear behavior and thermo-physics properties of the TiCx-TiB2/Al composite are all better than those of the TiCx/Al composite, which confirms that the TiCx-TiB2/Al composite is more appropriate for application as a heat sink.