Scaling the Power and Tailoring the Wavelength of Semiconductor Disk Lasers

Optically pumped semiconductor disk lasers (SDLs) provide a unique combination of high output power, high beam quality and possible emission wavelengths spanning from the ultraviolet to the mid-infrared spectral range. In essence, SDLs combine the wavelength versatility of semiconductor gain media with the power scaling principles of optically pumped solid state disk lasers. The emission wavelength of SDLs can be tailored to match the desired application simply by altering the composition of the gain material. High power operation, however, requires efficient thermal management, which can be realized using thin structures that are integrated with industrial diamond heat spreaders. The main objective of this thesis was to develop methods for increasing the output power of optically pumped SDLs, especially in challenging wavelength regions. The work included integrating SDL gain elements onto diamond heat spreaders using thin intermediate gold layers. This configuration enabled 45–50 % higher output powers than conventional bonding with indium solder. In addition, the reflectivity of the SDL gain mirror was enhanced using a semiconductor-dielectric-metal compound mirror. This procedure enabled 30 % thinner mirror structures when compared with the conventional design, where the reflectivity of the semiconductor mirror is enhanced with a metal layer. Finally, thin GaAs-based semiconductor mirrors were integrated with InP-based active regions. Such integration is necessary for high power operation in the spectral range 1.3–1.6 µm, because InP-based compounds for a highly reflective thin mirror section are not available. The configuration enabled record-high output powers of 6.6 W and 4.6 W at the wavelengths of 1.3 µm and 1.58 µm, respectively. The second objective of this thesis was to generate high output powers in single-frequency operation and via intracavity frequency-doubling. In single-frequency operation, record-high output powers of 4.6 W and 1 W were demonstrated at the wavelengths of 1.05 µm and 1.56 µm, respectively. Such light sources are required for numerous applications including free-space communications and high resolution spectroscopy. In addition, second-harmonic generation was demonstrated with SDLs emitting at 1.3 µm and 1.57 µm. The output powers reached 3 W at 650 nm and 1 W at 785 nm, which represent record-high output powers from SDLs in this wavelength range. These types of lasers could be especially useful in biophotonics and medical applications

High-performance III-V quantum structures and devices grown on Si substrates

III-V material laser monolithically grown on silicon (Si) substrate is urgently required to achieve low-cost and high-yield Si photonics. Due to the material dissimilarity between III-V component and Si, however, several challenges, such as dislocations and antiphase domains, remain to be solved during the epitaxial growth. In this regard, quantum dot (QD) laser diodes have been demonstrated with impressive characteristics of temperature insensitive, low power consumption and defects tolerance, and thus QD material is regards as an ideal material for laser directly grown on Si substrate. In this thesis, both QD laser diodes with 1.3 µm wavelength and quantum dot cascade laser with mid-infrared wavelength have been investigated. To understand the unique advantages of QD material, the comparison of QD and quantum well (QW) materials and devices grown on Si substrate is carried out in chapter 3. Based on identical fabrication and growth conditions, Si-based QW devices are unable to operate at room temperature, while the room-temperature Si-based QD is obtained with threshold current density of 160 A/cm2 and single-facet output power of >100 mW under continuous wave (c.w.) injection current driving. Besides, Si-based QD laser also shows remarkable temperature stability which the c.w. operation temperature reaches 66 ℃. The results point out that QD material has great potential in monolithic growth of III-V on Si for silicon photonics. Then, a novel approach of all-MBE grown QD laser on Si substrate is reported in chapter 4, with the optimization of buffer layer. The all-MBE grown QD laser on on-axis Si substrate with maximum operation temperature of 130 oC is achieved by utilizing thin Germanium (Ge) buffer. The mid-infrared silicon photonics has wide applications and market, but the lack of Si-based mid-infrared laser is a subsistent problem. Because the bandgap of conventional QW and QD materials is impossible to match the wavelength in mid-infrared range (3 µm to 20 µm), the Si-based quantum cascade laser (QCL) devices is regarded as an effective method to meet the requirement. Therefore, the high-performance QCL is firstly explored in chapter 5, and then, several methods in fabrication process are researched to enhance the performance for QCL devices. After the optimization of structure design and development of fabrication process, the InP-based QCL shows impressive properties with 600 mW emission power and over 100℃ operation temperature under c.w. mode. Following the previous work on Si-based QD laser, the quantum dot cascade laser (QDCL) is expected as a suitable solution for Si-based QCL devices. With the continuous improvement in structure design, the QDCL with multilayer QDs shows comparable performance, compared with conventional QCL devices. It is noted that the QDCL generates both TE and TM modes output, which is a breakthrough towards surface emitting QCL because the common QW-based QCL has only-TM emission in principle. Finally, the Si-based QCL is attempted with different structure design based on the pervious results

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)

Integrated butt-coupled membrane laser for Indium Phosphide on Silicon platform

In this work we present the design and technology development for an integrated butt-coupled membrane laser in the IMOS (Indium Phosphide Membrane On Silicon) platform . Laser is expected to have a small footprint (less than 50 µm 2 ), 1 mA threshold current and a direct modulation frequency of 10 GHz

Recent Advances and Future Trends in Nanophotonics

Nanophotonics has emerged as a multidisciplinary frontier of science and engineering. Due to its high potential to contribute to breakthroughs in many areas of technology, nanophotonics is capturing the interest of many researchers from different fields. This Special Issue of Applied Sciences on “Recent advances and future trends in nanophotonics” aims to give an overview on the latest developments in nanophotonics and its roles in different application domains. Topics of discussion include, but are not limited to, the exploration of new directions of nanophotonic science and technology that enable technological breakthroughs in high-impact areas mainly regarding diffraction elements, detection, imaging, spectroscopy, optical communications, and computing