Performance Of Laser Devices Research Articles

Experimental study of the effect of pump pulse duration on liquid crystal laser performance.

Much work has been done to understand the factors that impact photonic band-edge liquid crystal (LC) laser threshold and slope efficiency, two parameters often stated to quantify performance. Conventionally, LC lasers are optically pumped using Q-switched lasers with a fixed pulse duration, and thus the effect of pump pulse duration on LC laser performance has received little attention. While some studies have been published at different pump pulse durations, these use different laser sources and experimental conditions, making the data incomparable. By exploiting a recent breakthrough in laser diode pumping, our experimental results prove and quantify the detrimental effect of an increase in pump pulse duration on LC laser performance. We also show that the dependency of threshold on pump pulse duration depends on how threshold is defined, owing to an ambiguity in the definition of pulse energy in systems where peak power and pulse duration can be independently controlled. For improved comparison within the literature on LC laser device performance, we thus propose an alternative convention, whereby threshold is stated in units of peak power density.

Temperature dependence of stimulated Raman scattering from a VYO4 crystal.

The microstructural, electrical, and optical properties of crystals are critical to laser device performance, and the temperature-dependence effect of crystals has gained a great deal of attention in research. The linewidth, stimulated Raman scattering (SRS) shift, and intensity of the characteristic peak at 890 cm-1 of YVO4 crystal have been investigated between 148 and 448 K. As the crystal temperature increases, the bond length increases and the crystal force constant decreases, and the phonon softening process arises in the crystal at high temperature. The temperature effect on Raman shift and full width at half maximum is explained in detail in terms of the phonon anharmonic effect. The temperature dependence of SRS can be well demonstrated by an empirical equation. This work can provide new insights into the further understanding of lattice anharmonicity and contribute to the discovery of new optoelectronic materials.

Optical Properties of Crystalline Thin Films of the Organic Laser Gain Material 4,4′-Bis[(N-carbazole)styryl]biphenyl

Enhancing photoluminescence quantum yield and excellent transport associated with molecular order are important for advancing organic laser device performance. However, materials combining excellent quantum yield and transport are rare. 4,4′-Bis[(N-carbazole)styryl]biphenyl (BSB-Cz) has been heavily investigated as a promising material for laser devices. However, research on highly ordered BSB-Cz crystalline films is lacking, despite the much better transport properties to be expected. Here, we show that BSB-Cz thin-film crystals can be prepared using a simple vacuum deposition method, followed by thermal annealing. The resulting crystalline neat BSB-Cz thin films show a very high photoluminescence quantum yield of over 85% and superior optical stability compared to amorphous BSB-Cz films under intense pumping conditions. We believe that our work provides a route for advancing organic laser devices through introducing crystallinity in emitter thin films with a scalable approach.

Room-temperature high-performance CsPbBr3 perovskite tetrahedral microlasers.

With the goals of faster coherent light sources with lower power consumption, laser miniaturization has been intensively pursued in the past four decades. Novel microscale cavities with efficient feedback and distinct emission profiles are essential for excellent performance of laser devices and for exploring their new functionalities. Here, for the first time, well-defined high-quality tetrahedron-shaped CsPbBr3 perovskite microcavities with smooth surfaces were synthesized via the vapor growth method. A room-temperature high-performance tetrahedral microlaser was realized based on CsPbBr3 perovskite single crystals. The three-dimensional total internal reflection mode inside the tetrahedral cavity led to a high-performance microlaser with a linewidth of only ∼0.3 nm at 538 nm emission wavelength and a unique profile with three emission beams into free space with triangular symmetry. In addition, the perovskite tetrahedral microlasers could be pumped by two-photon absorption with threshold only about 2.5 times higher than that of a one-photon laser. The high-performance tetrahedral microlaser with a distinct emission profile enriches the microscale laser family and may find exceptional applications in optical manipulation, communication and on-chip beam steering.

The Anisotropic Thermal Expansion of Non-linear Optical Crystal BaAlBO3F2 Below Room Temperature

Thermal expansion is a crucial factor for the performance of laser devices, since the induced thermal stress by laser irradiation would strongly affect the optical beam quality. For BaAlBO3F2 (BABF), a good non-linear optical (NLO) crystal, due to the highly anisotropic thermal expansion its practical applications are strongly affected by the “tearing” stress with the presence of local overheating area around the laser spot. Recently, the strategy to place the optical crystals in low-temperature environment to alleviate the influence of the thermal effect has been proposed. In order to understand the prospect of BABF for this application, in this work, we investigated its thermal expansion behavior below room temperature. The variable-temperature XRD showed that the ratio of thermal expansion coefficient between along c- and along a(b)- axis is high as 4.5:1 in BABF. The Raman spectrum combined with first-principles phonon analysis revealed that this high thermal expansion anisotropy mainly ascribe to progressive stimulation of the respective vibration phonon modes related with the thermal expansion along a(b)- and c-axis. The good NLO performance in BABF can be kept below room temperature. The work presented in this paper provides an in-depth sight into the thermal expansion behavior in BABF, which, we believe, would has significant implication to the manipulation in atomic scale on the thermal expansion of the materials adopted in strong-field optical facility.

Characteristics of OMVPE grown GaAsBi QW lasers and impact of post-growth thermal annealing

Laser diodes employing a strain-compensated GaAs1−xBix/GaAs1−yPy single quantum well (SQW) active region were grown by organometallic vapor phase epitaxy (OMVPE). High resolution x-ray diffraction, room temperature photoluminescence, and real-time optical reflectance measurements during the OMVPE growth were used to find the optimum process window for the growth of the active region material. Systematic post-growth in situ thermal anneals of various lengths were carried out in order to investigate the impacts of thermal annealing on the laser device performance characteristics. While the lowest threshold current density was achieved after the thermal annealing for 30 min at 630 °C, a gradual decrease in the external differential quantum efficiency was observed as the annealing time increases. It was observed that the temperature sensitivities of the threshold current density increase while those of lasing wavelength and slope efficiency remain nearly constant with increasing annealing time. Z-contrast scanning transmission electron microscopic) analysis revealed inhomogeneous Bi distribution within the QW active region.

In situ annealing enhancement of the optical properties and laser device performance of InAs quantum dots grown on Si substrates.

The addition of elevated temperature steps (annealing) during the growth of InAs/GaAs quantum dot (QD) structures on Si substrates results in significant improvements in their structural and optical properties and laser device performance. This is shown to result from an increased efficacy of the dislocation filter layers (DFLs); reducing the density of dislocations that arise at the Si/III-V interface which reach the active region. The addition of two annealing steps gives a greater than three reduction in the room temperature threshold current of a 1.3 μm emitting QD laser on Si. The active region of structures grown on Si have a room temperature residual tensile strain of 0.17%, consistent with cool down from the growth temperature and the different Si and GaAs thermal expansion coefficients. This strain limits the amount of III-V material that can be grown before relaxation occurs.

Negative differential gain in quantum dot systems: Interplay of structural properties and many-body effects

The saturation behaviour of optical gain with increasing excitation density is an important factor for laser device performance. For active materials based on self-organized InGaAs/GaAs quantum dots, we study the interplay between structural properties of the quantum dots and many-body effects of excited carriers in the optical properties via a combination of tight-binding and quantum-kinetic calculations. We identify regimes where either phase-space filling or excitation-induced dephasing dominates the saturation behavior of the optical gain. The latter can lead to the emergence of a negative differential material gain.

Saturated Absorption Effects in CW-Pumped Solid-State Lasers

Saturation effects in solid-state laser materials are important to accurately model and optimize the performance of laser devices. In this paper, we present analytical and numerical results describing the absorption in continuous wave solid-state lasers with both pump and laser saturation included, for flat-topped and Gaussian spatial distributions. Generalized absorption results are presented as a function of the ratio of pump intensity to pump saturation intensity, laser intensity to laser saturation intensity, optical thickness, and different ratios of laser to pump waists for collimated beams. These results apply to most solid-state laser energy level systems.

Molecular Beam Epitaxial Growth of High Power Quantum Dot Super-Luminecent Diodes

We report optimised epitaxial growth conditions which allow the realization of high power quantum dot super-luminescent diodes. The engineering of the thickness of the low growth temperature GaAs barrier layer results in a significant improvement in laser device performance and a concomitant improvement in super-luminescent diode performance.

Reactive ion etching induced damage evaluation for optoelectronic device fabrication

Reactive ion etching (RIE) processes have been evaluated in terms of material damage introduced and effect on device performance. We have evaluated the damage introduced in the InP∕InGaAsP material system, in terms of surface damage depth, by various RIE-related etching processes (based on CH4∕H2 chemistry) and we have fabricated complete Fabry-Perot laser diode devices to evaluate the effect of RIE-induced damage on device performance. Electrical, optical, and reliability performance results are presented for the laser diode devices. A comparison between the laser device performance and the related surface damage depth is presented and discussed. It has been shown that a correlation exists between RIE-induced damage and optoelectronic device performance. By carefully designing RIE-related processes, which introduce low material damage, good optoelectronic device performance can be realized coupled with high device reliability.

InP based lasers and optical amplifiers with wire-/dot-like active regions

Long wavelength lasers and semiconductor optical amplifiers based on InAs quantum wire-/dot-like active regions were developed on InP substrates dedicated to cover the extended telecommunication wavelength range between 1.4 and 1.65 µm. In a brief overview different technological approaches will be discussed, while in the main part the current status and recent results of quantum-dash lasers are reported. This includes topics like dash formation and material growth, device performance of lasers and optical amplifiers, static and dynamic properties and fundamental material and device modelling.

Band-offset determination for GaInP-AlGaInP structures with compressively strained quantum well active layers

(AlGaIn)P-on-GaAs structures incorporating compressively strained Ga1−xInxP (x≳0.48) quantum well active layers have been studied by low-temperature photoluminescence excitation spectroscopy. The splitting between the lowest energy heavy- and light-hole excitonic transitions is observed to be only weakly dependent on well width over the range 25–300 Å, for sample sets with x=0.56 and x=0.59. Envelope function approximation fitting, based on bulk valence band dispersion calculations which include the strain-induced interaction with the spin split-off band, shows this splitting behavior to be a sensitive function of the heterojunction band offset. Conduction band discontinuities, ΔEc, of 0.67ΔEg (x=0.56) and 0.85ΔEg (x=0.59) provide the best fit to these and all higher lying transitions for the full range of structures examined, indicating that poor hole confinement is a limiting factor in practical compressive strain (AlGaIn)P laser device performance.