Theory and simulation of subwavelength high contrast gratings and their applications in vertical-cavity surface-emitting laser devices

This work intends to fully explore the qualities and applications of subwavelength gratings. Subwavelength gratings are diffraction gratings with physical dimensions less than the wavelength of incident light. It has been found that by tailoring specific dimension parameters, a number of different reflection profiles can be attained by these structures including high reflectivity or low reflectivity with broad and narrow spectral responses. In the course of this thesis the physical basis for this phenomenon will be presented as well as a mathematical derivation. After discussion of the mechanics of the reflection behavior, the methods used in modeling subwavelength gratings and designing them for specific functions will be explored. Following this, the fundamentals of vertical-cavity surface-emitting lasers (VCSELs) will be discussed, and the applications of subwavelength gratings when used with these lasers will follow. Several devices, both theoretical proposals and fabricated examples, will be presented in addition to the available performance measurements. Finally, the fabrication challenges that restrict subwavelength gratings from adoption as standard components in VCSEL design will be considered with regard to ongoing fabrication research

Tunable MEMS VCSEL on Silicon substrate

We present design, fabrication and characterization of a MEMS VCSEL which utilizes a silicon-on-insulator wafer for the microelectromechanical system and encapsulates the MEMS by direct InP wafer bonding, which improves the protection and control of the tuning element. This procedure enables a more robust fabrication, a larger free spectral range and facilitates bidirectional tuning of the MEMS element. The MEMS VCSEL device uses a high contrast grating mirror on a MEMS stage as the bottom mirror, a wafer bonded InP with quantum wells for amplification and a deposited dielectric DBR as the top mirror. A 40 nm tuning range and a mechanical resonance frequency in excess of 2 MHz are demonstrated

Integration of GaAs-based VCSEL array on SiN platform with HCG reflectors for WDM applications

We present a GaAs-based VCSEL structure, BCB bonded to a Si3N4 waveguide circuit, where one DBR is substituted by a free-standing Si3N4 high-contrast-grating (HCG) reflector realized in the Si3N4 waveguide layer. This design enables solutions for on-chip spectroscopic sensing, and the dense integration of 850-nm WDM data communication transmitters where individual channel wavelengths are set by varying the HCG parameters. RCWA shows that a 300nm-thick Si3N4 HCG with 800nm period and 40% duty cycle reflects strongly (>99%) over a 75nm wavelength range around 850nm. A design with a standing-optical-field minimum at the III-V/airgap interface maximizes the HCG's influence on the VCSEL wavelength, allowing for a 15-nm-wide wavelength setting range with low threshold gain (<1000 cm(-1))

20-Gb/s Modulation of Silicon-Integrated Short-Wavelength Hybrid-Cavity VCSELs

We investigate the dynamics of silicon-integrated 850-nm-wavelength hybrid-cavity vertical-cavity surface-emitting lasers (VCSELs). The VCSELs consist of a GaAs-based half-VCSEL attached to a dielectric distributed Bragg reflector on a silicon substrate using ultra-thin divinylsiloxane-bis-benzocyclobutene adhesive bonding. A 5-µm oxide aperture diameter VCSEL, with a small signal modulation bandwidth of 11 GHz, supports data transmission at bit rates up to 20 Gb/s. The modulation bandwidth and the large signal modulation characteristics are found to be impaired by the high thermal impedance

Progress and challenges in electrically pumped GaN-based VCSELs

The Vertical-Cavity Surface-Emitting Laser (VCSEL) is an established optical source in short-distance optical communication links, computer mice and tailored infrared power heating systems. Its low power consumption, easy integration into two-dimensional arrays, and low-cost manufacturing also make this type of semiconductor laser suitable for application in areas such as high-resolution printing, medical applications, and general lighting. However, these applications require emission wavelengths in the blue-UV instead of the established infrared regime, which can be achieved by using GaN-based instead of GaAs-based materials. The development of GaN-based VCSELs is challenging, but during recent years several groups have managed to demonstrate electrically pumped GaN-based VCSELs with close to 1 mW of optical output power and threshold current densities between 3-16 kA/cm2. The performance is limited by challenges such as achieving high-reflectivity mirrors, vertical and lateral carrier confinement, efficient lateral current spreading, accurate cavity length control and lateral optical mode confinement. This paper summarizes different strategies to solve these issues in electrically pumped GaN-VCSELs together with state-of-the-art results. We will highlight our work on combined transverse current and optical mode confinement, where we show that many structures used for current confinement result in unintentionally optically anti-guided resonators. Such resonators can have a very high optical loss, which easily doubles the threshold gain for lasing. We will also present an alternative to the use of distributed Bragg reflectors as high-reflectivity mirrors, namely TiO2/air high contrast gratings (HCGs). Fabricated HCGs of this type show a high reflectivity (>95%) over a 25 nm wavelength span