Achieving high-power single-mode operation in Vertical-Cavity Surface-Emitting Lasers (VCSELs) has received renewed interest due to performance needs driven by facial recognition and 3D imaging in mobile telephones. Two distinct mode control methods that rely on exploiting the spatial field distribution of optical transverse modes to achieve high-power single-mode operation in VCSELs will be discussed. The first method uses a surface-deposited optical coating of multilayer SiO2/TiO2 or single-layer silicon to achieve single-mode emission. The capability to pattern these layers in a wafer-scale process makes this method attractive for high-volume manufacturing. The second method utilizes impurity-induced layer disordering (IID) to selectively intermix the top distributed Bragg reflector (DBR) in a VCSEL, thereby creating a mirror whose reflectivity spatially varies across the aperture. Using these techniques, single-mode output power in excess of 10 mW has been demonstrated with side-mode suppression ratios in excess of 35 db.
The impact of diffusion profile shaping through the use of tensilely and compressively strained diffusion masks on the modulation response of single-mode vertical-cavity surface-emitting lasers (VCSELs) using disorder-defined apertures is investigated. VCSELs designed for high-power single-fundamental-mode emission through the use apertures defined via impurity-induced disordering (IID) in conjunction with a standard oxide-confinement process are characterized to extract high-frequency optical cavity parameters across oxide aperture and diffusion mask strain variations. The 3-dB small-signal bandwidth is maintained for a 7.68 mW single-mode 850 nm VCSEL with an oxide aperture of 13 μm using a tensilely strained diffusion mask relative to a non-disordered multimode device of the same oxide aperture. A large K-factor reduction is also observed, from 0.248 ns to 0.045 ns, indicating that damping and photon lifetimes within the cavity of VCSELs employing disorder-defined apertures are substantially reduced. Performance implications to data communication applications are discussed.
IEEE Photonics Technology Letters Article Link
The strain of diffusion masks utilized for impurity-induced disordering is demonstrated to control the curvature of the diffusion front, and therefore disordering front, of the disordered distributed Bragg reflector (DBR) aperture. As a result, the disordered apertures formed under strain conditions varying from compressive to tensile are shown to significantly impact the electro-optical performance and spectral characteristics of impurity-induced disordered (IID) VCSELs designed for single-fundamental-mode operation. An investigation and analysis of the electro-optical performance and spectral characteristics of IID-VCSELs as a result of varying diffusion mask strain is presented.
A wafer-scale CMOS-compatible process for heterogeneous integration of III-V epitaxial material onto silicon for photonic device fabrication is presented. Transfer of AlGaAs-GaAs Vertical-Cavity Surface-Emitting Laser (VCSEL) epitaxial material onto silicon using a carrier wafer process and metallic bonding is used to form III-V islands which are subsequently processed into VCSELs. The transfer process begins with the bonding of III-V wafer pieces epitaxy-down on a carrier wafer using a temporary bonding material. Following substrate removal, precisely-located islands of material are formed using photolithography and dry etching. These islands are bonded onto a silicon host wafer using a thin-film non-gold metal bonding process and the transfer wafer is removed. Following the bonding of the epitaxial islands onto the silicon wafer, standard processing methods are used to form VCSELs with non-gold contacts. The removal of the GaAs substrate prior to bonding provides an improved thermal pathway which leads to a reduction in wavelength shift with output power under continuous-wave (CW) excitation. Unlike prior work in which fullyfabricated VCSELs are flip-chip bonded to silicon, all photonic device processing takes place after the epitaxial transfer process. The electrical and optical performance of heterogeneously integrated 850nm GaAs VCSELs on silicon is compared to their as-grown counterparts. The demonstrated method creates the potential for the integration of III-V photonic devices with silicon CMOS, including CMOS imaging arrays. Such devices could have use in applications ranging from 3D imaging to LiDAR.
Patrick Su demonstrates record high-power single-mode performance in VCSELs using strain-controlled impurity-induced disordering apertures. Oxide-VCSELs of aperture sizes from 9-13 um are shown to achieve record single-mode output powers of 8.52 mW, 9.57 mW, 10.20 mW, 10.57 mW, and 10.95 mW respectively.
Applied Physics Letter Article Link
A method of preparing passive materials suitable for magnetooptic interactions is shown using manganese implantation into gallium nitride (GaN) epitaxial layers, which establishes both dilute ferromagnetism and a three-level optical system with persistence to over 300 K. A sweep of thermal anneal parameters for a high implant dose into Mg-doped p-type GaN films is presented, and the materials are tested for both their magnetization and photoluminescence (PL). The optimal anneal process at 825 °C for 5 min maintains ferromagnetism with TC>305 K, confirming magnetic alignment at room temperature with a coercivity of ~100 Oe. PL and spectrophotometry of the optimally prepared materials show the effects of the mid-gap defect state on the material’s optical characteristics. The anneal process returns the real part index to its baseline dispersion while retaining an onset of absorption starting at the defect level EA=1.8 eV, signifying a stable mid-gap energy transition with a measured state lifetime of τPL=2.7 ns. The scalability of this process for producing three-level transition magnetic materials suggests passive optical or magnetooptic devices can be constructed that interconnect photonic and spintronic effects for emerging system designs and potential applications in quantum information.
IEEE Transactions on Electron Device Article Link
This work reports comprehensive modeling of photocurrent spectra generated by an InGaN/InGaN disk-in-wire photodiode based on the effective bond-orbital model. The photocurrent spectra contributed by both single-photon absorption and two-photon absorption are calculated. The physical mechanisms for the observed prominent peaks are identified and investigated.
Single transverse mode operation in vertical-cavity surface-emitting lasers (VCSELs) is desired over multimode operation in a number of applications. This work presents a method of achieving single-mode operation by depositing an annulus-shaped semiconductor coating atop an 850 nm oxide-confined VCSEL to modify the standing wave of the optical field in the laser cavity. The devices are benchmarked for singlemode performance as well as optical output power via optical spectra and light-current-voltage (LIV) measurements respectively.
The promise of silicon integrated circuits with their electronic or photonic functionality enhanced via heterogeneous integration has motivated significant work in understanding and overcoming the barriers to realizing such an IC. Additional hurdles must be overcome when integrating devices that are highly sensitive to temperature variation such as semiconductor lasers. Challenges in the heterogeneous integration process will be reviewed, and approaches for heterogeneous integration that have the potential to enable silicon ICs with enhanced functionality will be discussed in the context of integrated photonic systems.
The transistor-injected quantum cascade laser (TI-QCL) is a novel design for a mid-wave infrared (MWIR) laser that seeks to overcome some of the primary limitations of standard quantum cascade lasers (QCLs). By growing the active cascade region within the base-collector junction of an npn heterojunction bipolar transistor (HBT), independent control of the injection current and active region bias is achievable through the emitter current and base-collector reverse bias respectively. The active region bias is important to properly align the lasing states and to control the lasing wavelength. Physical design limitations of the TI-QCL and their effects on the fabrication process of samples is presented. In order to characterize device performance and validate fabrication improvements, InP-based device samples designed for λ= 7.3 µm emission are fabricated. Preliminary characterization results are shown in the form of diode measurements to validate the HBT electrical operation of the TI-QCL which is necessary to realize the optical benefits of the device.
