Laser Active Region Research Articles

An IH-QCL based gas sensor for simultaneous detection of methane and acetylene

Extended wavelength tuning of an IH-QCL (integrated heater quantum cascade laser) is exploited for simultaneous detection of methane and acetylene using direct absorption spectroscopy. The integrated heater, placed within few microns of the laser active region, enables wider wavelength tuning than would be possible with a conventional DFB (distributed feedback) QCL. In this work, the laser current and heater resistor current are modulated simultaneously at 25 kHz to tune the laser over 1279.6–1280.1 cm−1, covering absorption transitions of methane and acetylene. The laser is characterized extensively to understand the dependence of wavelength tuning on modulation frequency, modulation amplitude and phase difference between laser/heater modulation. Thereafter, the designed sensor is validated in both room-temperature static cell experiments and non-reactive high-temperature-measurements in methane-acetylene-argon gas mixtures in the shock tube. Finally, the sensor is applied for simultaneous detection of methane and acetylene during the high-temperature pyrolysis of iso-octane behind reflected shock waves.

The effect of p-doping on multi-state lasing in InAs/InGaAs quantum dot lasers for different cavity lengths

The effect of modulation p-doping on multi-state lasing in InAs/InGaAs quantum dot (QD) lasers is studied for different levels of acceptor concentration. It is shown that in case of the short laser cavities, p-doping results in higher output power of the ground-state optical transitions of InAs/InGaAs QDs whereas in longer samples p-doping may result in the decrease of this power component. On the basis of this observation, the optimal design of laser active region and optimal doping level are discussed in details.

Laser initiation of mixtures of PETN and aluminum by a deposit

The formation of a deposit from mixtures of PETN with fine aluminum under the action of neodymium laser radiation in a free generation mode is studied. This paper also describes the efficiency of a deposit as a means to reduce the energy of laser initiation of mixtures as a function of fineness of PETN, aluminum content in initiated mixtures and deposit, mixture density, and diameter of the laser action region on the explosive. The compositions of mixtures for preparing deposits optimal in composition, which reduce the initiation energy of mixtures of PETN and aluminum down to 3.75 times, are determined. The functioning mechanism of a deposit during laser initiation of mixtures of PETN and aluminum is under discussion.

Optical gain analysis of GaAs-based InGaAs/GaAsSbBi type-II quantum wells lasers.

A novel active region design based on a type-II InGaAs/GaAsSbBi quantum wells on GaAs substrate is proposed and studied in this work. The band structures of the InGaAs/GaAsSbBi type-II quantum wells are studied based on a self-consistent 14-band k·p model. The electronic and optical properties of dilute-bismide InGaAs/GaAsSbBi type-II quantum well structures are investigated theoretically. Moreover, the room temperature gain characteristics of the laser active region are studied with different Bi composition. The theoretical results indicate that adding Bi into InGaAs/GaAsSb type-II active regions on GaAs substrate extends the laser emission wavelength beyond 1550nm without sacrificing the peak gain value. It is shown that these type-II quantum well structures are suitable for 1550nm wavelength region operation at room temperature.

Active Region Design and Gain Characteristics of InP-Based Dilute Bismide Type-II Quantum Wells for Mid-IR Lasers

The electronic and optical properties of InP-based InGaAs/GaAsSbBi type-II quantum well (QW) structures are investigated theoretically. The detailed analyses of the InGaAs/GaAsSbBi type-II QW laser structures were based on a 14-band $\text {k}\cdot \text {p}$ model. The theoretical results indicate that adding Bi into InGaAs/GaAsSb type-II active regions on InP allows wavelength extension, but with minimal impact to transition matrix element. Moreover, the gain characteristics of the active region of laser were studied as a function of the Bi composition. It is shown that these type-II QW structures are suitable for mid-infrared (2– $4~{\mu }\text{m}$ ) operation at room temperature.

Optical properties of InGaAs/InGaAlAs quantum wells for the 1520–1580 nm spectral range

The optical properties of elastically strained semiconductor heterostructures with InGaAs/InGaAlAs quantum wells (QWs), intended for use in the formation of the active region of lasers emitting in the spectral range 1520–1580 nm, are studied. Active regions with varied lattice mismatch between the InGaAs QWs and the InP substrate are fabricated by molecular beam epitaxy. The maximum lattice mismatch for the InGaAs QWs is +2%. The optical properties of the elastically strained InGaAlAs/InGaAs/InP heterostructures are studied by the photoluminescence method in the temperature range from 20 to 140°C at various power densities of the excitation laser. Investigation of the optical properties of InGaAlAs/InGaAs/InP experimental samples confirms the feasibility of using the developed elastically strained heterostructures for the fabrication of active regions for laser diodes with high temperature stability.

Mechanism of the polarization control in intracavity- contacted VCSEL with rhomboidal oxide current aperture

The possible mechanisms of the polarization control in single-mode intracavity- contacted vertical-cavity surface-emitting lasers (IC-VCSELs) with the rhomboidal selectively- oxidized current aperture were investigated. It was found that the lasing emission polarization of all single-mode VCSELs is fixed along the minor diagonal of the rhomboidal-shape aperture (the [110] direction). Numerical modelling of carrier transport did not reveal any sufficient injection anisotropy in the laser active region, while the transverse optical confinement factors calculated for the fundamental mode with two orthogonal polarizations are identical. Optical loss anisotropy and/or gain anisotropy are the most likely mechanisms of inducing the polarization fixation.

Sub-nanometrically resolved chemical mappings of quantum-cascade laser active regions

A procedure that produces sub-nanometrically resolved chemical mappings of MOCVD-grown InGaAs/InAlAs/InP quantum cascade lasers is presented. The chemical mappings reveal that, although the structure is lattice-matched to InP, the InAlAs barriers do not attain the nominal aluminum content—48%—and are, in fact, InGaAlAs quaternaries. This information is used to adjust the aluminum precursor flow and fine-tune the composition of the barriers, resulting in a significant improvement of the fabricated lasers.

Optically pumped 1.3 μm room-temperature InAs quantum-dot micro-disk lasers directly grown on (001) silicon.

Direct integration of high-performance laser diodes on silicon will dramatically transform the world of photonics, expediting the progress toward low-cost and compact photonic integrated circuits (PICs) on the mainstream silicon platform. Here, we report, to the best of our knowledge, the first 1.3 μm room-temperature continuous-wave InAs quantum-dot micro-disk lasers epitaxially grown on industrial-compatible Si (001) substrates without offcut. The lasing threshold is as low as hundreds of microwatts, similar to the thresholds of identical lasers grown on a GaAs substrate. The heteroepitaxial structure employed here does not require the use of an absorptive germanium buffer and/or dislocation filter layers, both of which impede the efficient coupling of light from the laser active regions to silicon waveguides. This allows for full compatibility with the extensive silicon-on-insulator (SOI) technology. The large-area virtual GaAs (on Si) substrates can be directly adopted in various mature in-plane laser configurations, both optically and electrically. Thus, this demonstration represents a major advancement toward the commercial success of fully integrated silicon photonics.

Variation of the emission frequency of a terahertz quantum cascade laser

The variation of the emission frequency of a quantum cascade laser of the 3.2 THz range within a single radiation pulse was studied using high-resolution Fourier spectroscopy. A significant (108 MHz) increase in frequency in the first 4 μs of a pulse was found. This increase is attributed to a change in the electron part of permittivity of the laser active region coinciding with a rise in the emission intensity. The frequency dropped by 81 MHz in the following 40 μs of a radiation pulse due to a change in the lattice part of permittivity occurring as the laser active region was heated during a radiation pulse.

Spectroscopic Study of Terahertz Generation in Mid-Infrared Quantum Cascade Lasers.

Terahertz quantum cascade laser sources based on intra-cavity difference-frequency generation are currently the only room-temperature mass-producible diode-laser-like emitters of coherent 1–6 THz radiation. Device performance has improved dramatically over the past few years to reach milliwatt-level power output and broad tuning from 1.2 to 5.9 THz, all at room-temperature. Terahertz output in these sources originates from intersubband optical nonlinearity in the laser active region. Here we report the first comprehensive spectroscopic study of the optical nonlinearity and investigate its dependence on the mid-infrared pump frequencies. Our work shows that the terahertz generation efficiency can vary by a factor of 2 or greater depending on the spectral position of the mid-infrared pumps for a fixed THz difference-frequency. We have also measured for the first time the linewidth for transitions between the lower quantum cascade laser states, which is critical for determining terahertz nonlinearity and predicting optical loss in quantum cascade laser waveguides.

Regrowth of quantum cascade laser active regions on metamorphic buffer layers

Metamorphic buffer layers (MBLs) were used as substrates with lattice constants selected for designing and fabricating intersubband transition sources involving strained superlattices (SLs) such as Quantum Cascade Lasers (QCLs). Chemical mechanical planarization (CMP) was used to prepare the InGaAs-based MBLs for epitaxial growth. Indium enrichment of the InGaAs layer on the MBL surfaces was observed when annealed at the regrowth temperatures. This post-anneal enhancement was eliminated by including a wet-etch treatment after CMP, which results in an epi-ready surface for regrowth. Ten stages of a QCL core region structure, designed for emission at a 3.4μm wavelength are regrown on a surface-optimized MBL. Such structures exhibit well defined X-ray diffraction pendellösung fringes, and transmission electron microscopy confirms planar superlattice interfaces with layer thicknesses that are in good agreement with the design target.

Distinct morphology of keyhole wall during high power fibre laser deep penetration welding

The keyhole wall is the interaction interface between the laser and the material during the laser deep penetration welding. Measuring the morphology of the keyhole wall is thus of significance for understanding the high power fibre laser deep penetration welding process. In this paper, the clear keyhole wall was reserved by suddenly closing laser during high power fibre laser welding of copper alloy. The results indicate that the keyhole can be divided into laser action region and metallic vapour pressure maintenance region. The laser action region is on the front wall of keyhole, and a series of concentric elliptical rings are observed in this region. In another region, its diameter is significantly larger than the spot diameter and the keyhole wall is basically smooth. The results are different from those generally accepted viewpoints, which clarify the flowing law of molten fluid in the keyhole and are thus of great guiding significance for optimising the welding technology.

A novel method for observing the micro-morphology of keyhole wall during high-power fiber laser welding

Most of the laser welding defects are likely to be related to the welding process transport phenomena induced by the energy conversion process in the keyhole. The keyhole wall is the interaction interface between the laser and welded material, and studying its micro-morphology is significant for understanding the energy conversion process. Because the current measuring methods are limited, observation of keyhole wall micro-morphology is still a challenge. In this paper, a novel method was designed. The keyhole wall was preserved as the high-power fiber laser was suddenly shut off during welding process, and then it was measured by using a scanning electron microscopy (SEM). The results show that the laser beam directly acted on the keyhole front wall during welding. The concentric elliptical rings, wrinkles, and ripples were found on the laser-action region. These micro-morphologies indicate that the energy coupling is mainly the absorption of the keyhole front wall, and the keyhole formation is similar to a laser drilling on the keyhole front wall during welding. The novel method can be used to investigate on the formation of the welding defects during high-power fiber laser welding in the future study.

Spatial inhomogeneities in AlxGa1−xN quantum wells induced by the surface morphology of AlN/sapphire templates

The effects of the template on the optical and structural properties of Al0.75Ga0.25N/Al0.8Ga0.2N multiple quantum well (MQWs) laser active regions have been investigated. The laser structures for optical pumping were grown on planar c-plane AlN/sapphire as well as on thick epitaxially laterally overgrown (ELO) AlN layers on patterned AlN/sapphire. Two ELO AlN/sapphire templates were investigated, one with a miscut of the sapphire surface to the m-direction with an angle of 0.25°, the other with a miscut angle of 0.25° to the sapphire a-direction. The MQWs are studied by atomic force microscopy, plan-view cathodoluminescence (CL) at room temperature and 83 K as well as transmission electron microscopy using high-angle annular dark-field imaging and energy-dispersive x-ray spectroscopy. The results are compared to optical pumping measurements. It was found that the surface morphology of the templates determines the lateral wavelength distribution in the MQWs observed by spectral CL mappings. The lateral wavelength spread is largest for the laser structures grown on ELO AlN with miscut to sapphire a-direction caused by the local variation of the MQW thicknesses and the Ga incorporation at macrosteps on the ELO-AlN. A CL peak wavelength spread of up to 7 nm has been found. The MQWs grown on planar AlN/sapphire templates show a homogeneous wavelength distribution. However, due to the high threading dislocation density and the resulting strong nonradiative recombination, laser operation could not be achieved. The laser structures grown on ELO AlN/sapphire show optically pumped lasing with a record short wavelength of 237 nm.

Power increase in Q-switched two-sectional quantum well lasers due to Stark effect

Two-sectional laser diodes emitting light pulses in Q-switched (QS) mode are investigated. The active region of lasers contains three quantum wells. The shift of absorption edge due to Stark effect leads to red shift of lasing wavelength and pulse power doubling in QS mode.

One-Dimensional Nature of InAs/InP Quantum Dashes Revealed by Scanning Tunneling Spectroscopy.

We report on low-temperature cross-sectional scanning tunneling microscopy and spectroscopy on InAs(P)/InGaAsP/InP(001) quantum dashes, embedded in a diode-laser structure. The laser active region consists of nine InAs(P) quantum dash layers separated by the InGaAsP quaternary alloy barriers. The effect of the p-i-n junction built-in potential on the band structure has been evidenced and quantified on large-scale tunneling spectroscopic measurements across the whole active region. By comparing the tunneling current onset channels, a consistent energy shift has been measured in successive quantum dash or barrier layers, either for the ground state energy of similar-sized quantum dashes or for the conduction band edge of the barriers, corresponding to the band-bending slope. The extracted values are in good quantitative agreement with the theoretical band structure calculations, demonstrating the high sensitivity of this spectroscopic measurement to probe the electronic structure of individual nanostructures, relative to local potential variations. Furthermore, by taking advantage of the potential gradient, we compared the local density of states over successive quantum dash layers. We observed that it does not vanish while increasing energy, for any of the investigated quantum dashes, in contrast to what would be expected for discrete level zero-dimensional (0D) structures. In order to acquire further proof and fully address the open question concerning the quantum dash dimensionality nature, we focused on individual quantum dashes obtaining high-energy-resolution measurements. The study of the local density of states clearly indicates a 1D quantum-wirelike nature for these nanostructures whose electronic squared wave functions were subsequently imaged by differential conductivity mapping.

Temperature-dependent investigation of carrier transport, injection, and densities in AlGaAs-based multi-quantum-well active layers for vertical-cavity surface-emitting lasers

The electro-optical efficiency of vertical-cavity surface-emitting lasers (VCSELs) strongly depends on the efficient carrier injection into the quantum wells (QWs) in the laser active region. Carrier injection degrades with increasing temperature, which limits VCSEL performance in high-power applications where self-heating imposes high-operating temperatures. In a numerical model, we investigate the transport of charge carriers in an 808-nm AlGaAs multi-quantum-well structure with special attention to the temperature dependence of car- rier injection into the QWs. Experimental reference data were extracted from oxide-confined, top-emitting VCSELs. The transport simulations follow a drift-diffusion-model complemented by an energy-resolved car- rier-capture model. The QW gain was calculated in the screened Hartree-Fock approximation. With the combi- nation of the gain and transport model, we explain experimental reference data for the injection efficiency and threshold current. The degradation of the injection efficiency with increasing temperature is not only due to increased thermionic escape of carriers from the QWs, but also to state filling in the QWs initiated from higher threshold carrier densities. With a full opto-electro-thermal VCSEL model, we demonstrate how changes in VCSEL properties affecting the threshold carrier density, like mirror design or optical confinement, have con- sequences on the thermal behavior of the injection and the VCSEL performance. © 2015 Society of Photo-Optical

The analytical approach to optimization of active region structure of quantum dot laser

Using the analytical approach introduced in our previous papers we analyse the possibilities of optimization of size and structure of active region of semiconductor quantum dot lasers emitting via ground-state optical transitions. It is shown that there are optimal length' dispersion and number of QD layers in laser active region which allow one to obtain lasing spectrum of a given width at minimum injection current. Laser efficiency corresponding to the injection current optimized by the cavity length is practically equal to its maximum value.

Regrowth-free single-mode quantum cascade lasers with power consumption below 1 W

We report on single-mode distributed-feedback quantum cascade lasers emitting at 4.8 μm with continuous-wave threshold power consumption as low as 0.76 W at 20 °C and 0.98 W at 50 °C. Following growth of the laser active region and semiconductor cladding layers by a single molecular beam epitaxy process, devices with 4-μm-wide ridges and vertical sidewall gratings were fabricated using plasma etching and standard dielectric and metal deposition processes. In terms of mode stability, output power, and efficiency, we show that lasers with 1-mm cavity length and high-reflectivity back-facet coatings can match the performance of buried heterostructure devices, but with the advantage of requiring only a single epitaxial growth step.