Gain and Threshold Improvements of 1300 nm Lasers based on InGaAs/InAlGaAs Superlattice Active Regions

A detailed experimental analysis of the impact of active region design on the performance of 1300 nm lasers based on InGaAs/InAlGaAs superlattices is presented. Three different types of superlattice active regions and waveguide layer compositions were grown. Using a superlattice allows to downshift the energy position of the miniband, as compared to thin InGaAs quantum wells, having the same composition, being beneficial for high-temperature operation. Very low internal loss (~6$cm^{-1}$), low transparency current density of ~500$A/cm^2$, together with 46$cm^{-1}$ modal gain and 53 % internal efficiency were observed for broad-area lasers with an active region based on a highly strained $In_{0.74}Ga_{0.26}As/In_{0.53}Al_{0.25}Ga_{0.22}As$ superlattice. Characteristic temperatures $T_0$ and $T_1$ were improved up to 76 K and 100 K, respectively. These data suggest that such superlattices have also the potential to much improve VCSEL properties at this wavelength

Polarization Responses of a Solitary and Optically Injected Vertical Cavity Spin Laser

The polarisation properties of a quantum well spin - vertical cavity surface emitting laser (spin - VCSEL), both without injection and with variable polarisation optical injection, are investigated experimentally and compared with the spin flip model (SFM). Without injection, we demonstrate two distinct types of VCSEL-pump response depending on the signs of the linewidth enhancement factor, birefringence and dichroism: firstly where the pump and VCSEL have the same sign of the ellipticity, and secondly where the VCSEL ellipticity, accompanied by the linear polarisation, switches sign. We show that by controlling the injected power, ellipticity or linear angle, near circular polarisation can be obtained. These responses both give insight into the electro-optical injected spin-VCSEL system, and have practical implications for the use of spin VCSELs in unique applications exploiting the ellipticity degree of freedom

Nonlinear Dynamics of solitary and optically-injected spin vertical-cavity lasers

This work investigates the nonlinear dynamics and polarisation properties of Spin-Vertical-(External)-Cavity Surface-Emitting Lasers (V(E)CSELs). The focus is on gaining a broad understanding of the various polarised resolved nonlinear dynamical effects in solitary and injected 1300 nm spin-V(E)CSELs. We report a comprehensive study including theory, based on the Spin Flip Model, and experiments of the stability characteristics of solitary 1300 nm dilute nitride Quantum-Well (QW) spin-VCSELs. Various forms of oscillatory behaviour causing self-sustained oscillations in the polarisation of the spin-VCSEL subject to Continuous-Wave (CW) pumping are found. Additionally, this work is extended to study experimentally and theoretically the evolution of the output polarisation ellipticity, and experimentally the nonlinear dynamics of the light polarisation emitted by the QW spin VCSELs under polarised optical injection. Rich nonlinear dynamics of the optically injected QW spin-VCSEL are reported ranging from polarisation control, polarisation switching and bistability to periodic oscillations and chaos. Good agreement is found between measurements and calculations where theoretical results are available. We also report the first 1300 nm Quantum-Dot (QD) Semiconductor Disk Laser (SDL) using a very simple and compact laser configuration involving a high reflection (HR)-coated fibre as the top mirror. Moreover, by applying spin injection to the 1300 nm SDL via CW polarised optical pumping we also demonstrate the first 1300 nm QD spin Vertical-External-Cavity Surface-Emitting Laser (Spin-VECSEL). This is also accompanied by an investigation of the dynamics of the solitary 1300 nm QD spin-VECSEL. Finally, we present the first experimental study of the evolution of the output polarisation ellipticity and nonlinear dynamics of the 1300 nm QD spin-VECSEL under polarised optical injection. Our findings show nonlinear effects similar to the ones seen in optically injected QW spin-VCSELs

Photonic platform and the impact of optical nonlinearity on communication devices

It is important to understand properties of different materials and the impact they have on devices used in communication networks. This paper is an overview of optical nonlinearities in Silicon and Gallium Nitride and how these nonlinearities can be used in the realization of optical ultra-fast devices targeting the next generation integrated optics. Research results related to optical lasing, optical switching, data modulation, optical signal amplification and photo-detection using Gallium Nitride devices based on waveguides are examined. Attention is also paid to hybrid and monolithic integration approaches towards the development of advanced photonic chips

Polarization Modulation in Quantum-Dot Spin-VCSELs for Ultrafast Data Transmission

Spin-Vertical Cavity Surface Emitting Lasers (spin-VCSELs) are undergoing increasing research effort for new paradigms in high-speed optical communications and photon-enabled computing. To date research in spin-VCSELs has mostly focused on Quantum-Well (QW) devices. However, novel Quantum-Dot (QD) spin-VCSELs, offer enhanced parameter controls permitting the effective, dynamical and ultrafast manipulation of their light emissions polarization. In the present contribution we investigate in detail the operation of QD spin-VCSELs subject to polarization modulation for their use as ultrafast light sources in optical communication systems. We reveal that QD spin-VCSELs outperform their QW counterparts in terms of modulation efficiency, yielding a nearly two-fold improvement. We also analyse the impact of key device parameters in QD spin-VCSELs (e.g. photon decay rate and intra-dot relaxation rate) on the large signal modulation performance with regard to optical modulation amplitude and eye-diagram opening penalty. We show that in addition to exhibiting enhanced polarization modulation performance for data rates up to 250Gbps, QD spin-VCSELs enable operation in dual (ground and excited state) emission thus allowing future exciting routes for multiplexing of information in optical communication links

High-Power Dilute Nitride Lasers Grown by Molecular Beam Epitaxy

Semiconductor lasers are the most widely used type of lasers. This is due to many beneficial properties including compact size, wavelength coverage, and high efficiency. Different semiconductor laser architectures and gain materials can be used to fulfill requirements of different applications. Semiconductor gain materials are easy to tune to emit at desired wavelengths by changing the composition of the material and they can cover a wide range of wavelengths from ultra-violet to mid-infrared. Still, there are some important gaps in the wavelength coverage. Two of these gaps are located at ~600 nm and ~1200 nm, i.e. just below and above the wavelength coverage of traditional GaAs-based semiconductors. Especially the yellow–red (580–620 nm) part of the visible spectrum is important for applications in the fields of medicine, spectroscopy, astronomy and laser projection.This work targeted to cover both of the mentioned wavelength gaps by using dilute nitride GaInNAsSb/GaAs quantum well gain material in novel high-power lasers. This thesis discusses especially the fabrication of the dilute nitride gain materials using plasma-assisted molecular beam epitaxy. Incorporating few percent of nitrogen into InGaAs/GaAs QWs can increase the upper wavelength limit of GaAs-based semiconductors up to 1550 nm by reducing band gap and lattice strain. Using this dilute nitride material system, we fabricated the first multi-watt semiconductor disk lasers (SDLs) emitting at 1180 nm and 1230 nm. The output powers exceeded 10 W at both wavelengths. Although frequency doubling is out of the scope of this thesis, it should be mentioned that these lasers emitted multi-watt powers also at the corresponding frequency doubled wavelengths of 590 nm and 615 nm. In addition, this thesis reports a GaInNAsSb/GaAs SDL emitting at 1550 nm, which is the longest wavelength demonstrated for a monolithic GaAs-based SDL.SDLs, unlike other semiconductor lasers, can emit high-powers (up to 100 W) in nearly diffraction-limited beams and can be efficiently frequency doubled. However, not all applications require multi-watt output powers but would rather benefit from smaller size of the laser source. For this reason we studied also another laser architecture, namely edge-emitting laser diodes. A single-mode laser with record-high output power of 340 mW at 1180 nm, corresponding to yellow (590 nm) frequency-doubled wavelength, was demonstrated. The laser showed also excellent temperature stability, which is important for miniaturization of frequency-doubled lasers.The laser demonstrations could not have been realized without good understanding of the basic properties of the GaInNAs(Sb) gain material and its fabrication. Studies related to these aspects and to calibration of PA-MBE reactors form an important part of this thesis. Especially, effects of growth temperature and As/III beam equivalent pressure ratio on the grown semiconductor structures were studied.In summary, this work is concerned with plasma-assisted molecular beam epitaxy of GaInNAsSb/GaAs gain materials. The fabricated materials were used in novel lasers emitting at wide range of technologically important wavelengths that are difficult to reach otherwise.<br/

Optical characterization of InGaAsN / GaAs quantum wells: Effects of annealing and determination of the band offsets

In the last decade great attention has been given to the characteristics of dilute nitrides. Both their peculiar physical properties and their wide range of possible applications have attracted the interest of many experimental and theoretical groups. In this thesis work some open questions about the fundamental properties of dilute nitrides have been answered. Two important topics have been investigated: the correlation between the optical and morphological properties of InGaAsN / GaAs single quantum well samples (SQWs) and a quantitative, model-independent determination of the band offsets for the same types of structures. In chapter 3, a combined study of photoluminescence (PL) measurements and transmission electron microscopy (TEM) analysis has allowed to find a direct correlation between the degree of carrier localization in the sample and the homogeneity of the material. In particular, the degree of localization increases with increasing the inhomogeneity of the QW layer. On the basis of that, it has been found that the growth temperature (Tg) and the indium content strongly influence the morphology of the InGaAsN QW samples. With increasing Tg or with increasing [In], the inhomogeneity of the sample increases. The growth temperature affects also the optical properties of InGaAsN SQWs. By raising Tg, the PL intensity degrades and the peak emission energy red shifts. On the other hand, the indium content does not remarkably influence the PL efficiency of the QW. The only exception is for very high indium contents ([In] > 34%). In this case, dislocations due to strain relaxation and / or other types of non-radiative recombination centres are created causing a drastic decrease of the PL intensity. After annealing both the morphological and the optical properties are modified. Most notably, by employing samples grown in the range of temperatures between 360 °C and 480 °C annealed in different environments, two important conclusions have been found. First of all, morphology and PL efficiency are not always correlated and secondly, the PL efficiency of a QW directly depends on the density of non-radiative centres. Annealing samples in different atmospheres is a novelty in the literature and it has been the key-point to reach these findings. The first conclusion has been obtained by performing photoluminescence measurements on samples annealed in hydrogen and argon environment, and comparing the results with those of as-grown samples. It has been shown that while the PL intensity of H2-annealed samples is maximum for low values of Tg (400 °C) and minimum for high Tg (450 °C), the PL intensity of the Ar-annealed samples is maximum for high values of Tg (450 °C) and minimum for low Tg (400 °C). In contrast, the degree of localization and the TEM images have shown the same Tg-behaviour, independently of the annealing environment. The second conclusion has been reached by performing time resolved photoluminescence measurements on the same series of samples. It has been shown that whilst the radiative decay time varies with Tg in the same manner for the two annealing atmospheres, i.e. it increases with increasing Tg, the non-radiative decay time varies with the growth temperature in a different way for different annealing environments. In particular, the non-radiative decay time decreases with increasing Tg for H2-annealed samples and increases with increasing Tg for Ar-annealed samples. This behaviour correlates in both cases with the dependence of the PL intensity on the growth temperature. In addition to that, by performing power dependent PL measurements, it has been verified that changes of degree of localization after annealing are only due to morphological modification of the sample. By comparing the results obtained performing PL measurements on GaAsN / GaAs, InGaAs / GaAs, and InGaAsN / GaAs SQW samples, it has been shown that at least two different type of defects are created during the growth of InGaAsN SQWs. One type of defect is related to the presence of nitrogen. The density of these defects increases with Tg and decreases by annealing. Defects of another type are related to the simultaneous presence of indium and nitrogen. They are created at low Tg and tend to agglomerate under annealing. These two types of defects have been employed in a simple model in order to justify the main results obtained in this chapter. In chapter 4, a much debated topic has been analysed: the evolution of the band offsets of InGaAsN / GaAs structures with varying QW parameters. The chapter has been initially focussed on the refinement of the information which can be obtained employing an experimental method developed at Infineon Technologies based on surface photovoltage (SPV) measurements. With this method it is possible to identify optical transitions involving bound states and extended states in a QW sample. In particular, in addition to the bound-to-bound transitions, also the indirect transition from the extended state of the valence band to the first confined state of the conduction band can be identified. This allows the easy determination of the practical band offsets of the QW. These quantities represent the energy values of the conduction (valence) band offset of the heterostructures without the value of the first quantized state of the electrons (holes). For the design of a device, the practical band offsets are fundamental quantities because they quantify the real confinement of the carriers in the well. SPV measurements have been performed on several dedicated series of samples. The results have been compared with those obtained employing other optical techniques and performing theoretical simulations. It has been shown that by using this method, it is possible to gather comprehensive information about a single quantum well which otherwise could be obtained only by combining different experimental techniques and theoretical calculations. With this method transitions related to the ground states of the QW involving both the heavy and light holes states can be detected. Also, the excited states can be identified. As a main condition, it has been shown that only bound-to-bound transitions having the same parity can generate a step in the spectra. This method has been employed to investigate the band states of dilute nitrides SQWs. In particular, the effect of varying nitrogen and indium content on the practical band offsets of InxGa1-xAs1-yNy /GaAs SQW samples has been analysed. As a main result, it has been found that with increasing nitrogen content, the conduction band offset strongly increases (with a rate of about 100 meV / [N]), while the valence band offset is almost unchanged. Moreover, with increasing indium concentration both the conduction and the valence band offsets are modified. In particular, the conduction band offset varies with indium content as in the case of N-free samples. These results represent the first quantitative analysis which directly, i.e. independently of any model, determines the band offsets in dilute nitrides quantum wells. More importantly, it allows to analyse the effect of nitrogen and indium on the conduction and valence band states separately. The practical band offsets are highly important parameters in the design of many devices. Therefore, in the end of this thesis, it has been shown that the SPV method can be employed to determine the practical band offsets of real device structures. In particular, the practical conduction and valence band offsets of lasers emitting at 1.3 µm and 1.5 µm have been determined from the SPV spectra

3D mapping of nanoscale physical properties of VCSEL devices

There is clear lack of methods that allows studies of the nanoscale structure of the VCSEL devices1 that are mainly focused on the roughness of the DBR, or using FIB cross-sectioning and TEM analysis of failed devices to observe the mechanism of the degradation. Here we present a recently developed advanced approach that combines Ar-ion nano-cross-sectioning with material sensitive SPM2 to reveal the internal structure of the VCSEL across the whole stack of top and bottom DBR including active area. We report for the first time the direct observation of local mechanical properties, electric potential and conductance through the 3D VCSEL stack. In order to achieve this, we use beam exit cross-section polishing that creates an oblique section with sub-nm surface roughness through the whole VCSEL structure that is fully suitable for the subsequent cross-sectional SPM (xSPM) studies. We used three different SPM measurement modes – nanomechanical local elastic moduli mapping via Ultrasonic Force Microscopy (UFM) 3, surface potential mapping via Kelvin Probe Force Microscopy (KPFM) and mapping of injected current (local conductivity) via Scanning Spreading Resistance Microscopy (SSRM). xSPM allowed to observe the resulting geometry of the whole device, including active cavity multiple quantum wells (MQW), to obtain profiles of differential doping of the DBR stack, profile of electric potential in the active cavity, and spatial variation of current injection in the individual QW in MQW area. Moreover, by applying forward bias to the VCSEL to initiate laser emission, we were able to observe distribution of the potential in the working regime, paving the way to understanding the 3D current flow in the complete device. Finally, we use finite element modelling (FEM) that confirm the experimental results that of the measurements of the local doping profiles and charge distribution in the active area of the VCSEL around the oxide current confinement aperture. While we show that the new xSPM methodology allowed advanced in-situ studies of VCSELs, it establishes a highly efficient characterisation platform for much broader area of compound semiconductor materials and devices. REFERENCES. 1. D. T. Mathes, R. Hull, K. Choquette, K. Geib, A. Allerman, J. Guenter, B. Hawkins and B. Hawthorne, in Vertical-Cavity Surface-Emitting Lasers Vii, edited by C. Lei and S. P. Kilcoyne (2003), Vol. 4994, pp. 67-82. 2. A. J. Robson, I. Grishin, R. J. Young, A. M. Sanchez, O. V. Kolosov and M. Hayne, Acs Applied Materials & Interfaces 5 (8), 3241-3245 (2013). 3. J. L. Bosse, P. D. Tovee, B. D. Huey and O. V. Kolosov, Journal of Applied Physics 115 (14), 144304 (2014)