Hybrid Mirrors Research Articles

(Invited) Development of Ultraviolet Vertical-Cavity Surface-Emitting Light Sources

Nitride-based vertical-cavity surface-emitting lasers (VCSELs) at wavelengths below 400 nm have drawn tremendous interest as they are potentially utilized in various applications as highly-efficient and temperature-insensitive optical sources. One example is for a compact, energy efficient, portable ytterbium-ion-based atomic clock that requires a high-stability CW optical pump at near ultraviolet (UV) wavelengths. In this work, we have used metalorganic chemical vapor deposition (MOCVD) to grow optically pumped UV VCSEL epitaxial structures and UV resonant-cavity light-emitting diodes (RCLEDs) to demonstrate several design concepts targeting the ~360-370nm UVA spectral range. The UV VCSELs are designed using a proprietary III-N device simulation tool developed at Georgia Institute of Technology. Previously, we have demonstrated a variety of optically pumped III-N VCSELs lasing at ~375 nm that showed the feasibility of building the near-UV coherent light source on GaN materials platform. The test structures consisted of a five-pair InGaN/Al0.15Ga0.85N multiple-quantum-well (MQW) active region embedded in Al0.1Ga0.9N spacers with the total optical cavity thickness of 3λ. The lowest threshold incident power density for optically pumped VCSELs was estimated to be ~270 kW/cm2 under pulsed conditions at the room temperature. In further work, we implemented a hybrid distributed Bragg reflectors (DBR) combination to demonstrate vertical resonant cavity light emitting diodes (RCLEDs) targeting at 370-nm peak emission wavelengths. The hybrid mirrors use a dielectric DBR with twelve pairs of ¼-wavelength HfO2/SiO2 layers as the topside mirror, and a semiconductor-based mirror composed of five periods of ¾λ air-gap/Al0.05Ga0.95N DBR at the bottom side of the vertical cavity. The RCLED structure comprises three distinct regions: (1) bottom DBR mirror of five-pair 3λ/4-air-gap/ Al0.045Ga0.95fN, (2) a 6λ-cavity LED active region of n-Al0.08Ga0.92N (n~8x1018cm-3) spacer/InGaN-AlGaN MQW/p-AlGaN electron blocking/p-Al0.08Ga0.92N (p~3x1017cm-3) spacer/graded p +-AlGaN, and (3) a 12-pair λ/4 HfO2/SiO2 dielectric DBR as the top-side mirror. The device fabrication employed a nitrogen ion-implantation isolation to form a current confinement aperture. The mesa-type devices have the anode and the cathode electrodes on the topside of the wafer. The air-gap DBRs were formed using optical lithography and subsequent inductively coupled plasma dry etching to create a deep trench, followed by laterally removing sacrificial layers using a conductivity-selective electrochemical etching process. Once formed, this airgap DBR provides a high reflectivity of ~99.9% covering the spectral range from 360 to 400 nm while the dielectric DBR has a reflectivity of >99% covering the range from 350 to 405 nm. The fabricated RCLEDs can be operated at a current density >50 kA/cm2 in the pulsed current mode with a peak emission wavelength of ~375 nm at the room temperature. Details of MOCVD growth, device fabrication, and characterization will be presented.

Femtosecond damage resistance of femtosecond multilayer and hybrid mirrors.

Improving the laser-induced damage threshold of optical components is a basic endeavor in femtosecond technology. By testing more than 30 different femtosecond mirrors with 42 fs laser pulses at 1 kHz repetition rate, we found that a combination of high-bandgap dielectric materials and improved design and coating techniques enable femtosecond multilayer damage thresholds exceeding 2 J/cm2 in some cases. A significant ×2.5 improvement in damage resistance can also be achieved for hybrid Ag-multilayer mirrors exhibiting more than 1 J/cm2 threshold with a clear anticorrelation between damage resistance and peak field strength in the stack. Slight dependence on femtosecond pulse length and substantial decrease for high (megahertz) repetition rates are also observed.

Novel blue continuous-wave current-injected light source

Vertical-cavity surface-emitting lasers (VCSELs) contain two high-reflectivity distributed-Bragg reflectors (DBRs) as well as a short cavity characterized by a small optical-mode volume. They emit a single longitudinal mode in a circularly symmetric beam with little beam divergence. Both characteristics are superior to those achieved with edge-emitting lasers and desirable for many practical applications in, for example, high-density optical storage and laser printing. In addition, the strong field in the microcavity facilitates exploration of cavity quantum-electrodynamics effects, such as single-photon and polariton emission, controlled spontaneous emission, and low threshold (or thresholdless) lasing. Over the past decade, two developments have affected the realization of electrically pumped gallium nitride (GaN)-based VCSELs. First, high-reflectivity, high-quality DBRs composed of AlxGa1−xN and GaN exhibit a large lattice mismatch that forms cracks in the epitaxially grown DBR structure. These cracks could result in a reduction in optical reflectivity and become a leakage-current path. The second is the fabrication of novel, high-transparency, high-conductivity contacts for current injection, which was triggered by the difficulties encountered in constructing low-resistance p-type GaN layers. In 1996, GaN-based edge-emitting laser diodes were first realized at room temperature by Nakamura’s team.1 Arakawa and co-workers subsequently fabricated an In0.1Ga0.9N VCSEL and observed lasing at 77K for the first time in 1998.2 We recently fabricated a crack-free high-reflectivity DBR by incorporating AlN/GaN superlattice layers between the DBR structures to reduce V-shaped defects and act as strain buffers.3 Figure 1 shows the device structure employing hybrid mirrors. It consists of 29 pairs of AlN/GaN DBRs and a 5λ (where λ refers to the operating wavelength) active region with a Figure 1. Schematic diagram of an electrically pumped vertical-cavity surface-emitting laser. DBR: Distributed-Bragg reflector. GaN, AlN: Gallium, aluminum nitride. MQW: Multiquantum well.

Analysis and fabrication of hybrid metal-dielectric omnidirectional Bragg reflectors

We describe chalcogenide glass and polymer based Bragg reflectors with a metallic underlayer and use a transfer matrix model to analyze their performance. The angle-averaged reflectance of a hybrid mirror approaches unity for only a few periods and is much higher than that for a nonmetallized Bragg reflector or for the metallic layer alone. For an angle-averaged reflectance greater than 0.99, the addition of a metallic underlayer enables nearly a tripling of the omnidirectional bandwidth (from approximately 110 to approximately 305 nm) concurrent with a significant reduction in the number of required periods (from 10.5 to 4.5). Hybrid mirrors of 4.5 periods, with a 50 nm Au underlayer and overall thickness of approximately 2 microm, were fabricated atop silicon substrates and characterized. They exhibit an omnidirectional stop band in the 1450-1750 nm wavelength range, in good agreement with theoretical predictions.

Fabrication and performance of blue GaN-based vertical-cavity surface emitting laser employing AlN∕GaN and Ta2O5∕SiO2 distributed Bragg reflector

GaN-based vertical-cavity surface emitting laser with 3 λ cavity and hybrid mirrors, consisting of the 25 pairs AlN∕GaN dielectric Bragg reflector and the 8 pairs Ta2O5∕SiO2, was fabricated. The laser action was achieved under the optical pumping at room temperature with a threshold pumping energy density of about 53mJ∕cm2. The laser emits 448 nm blue wavelength with a linewidth of 0.25 nm and the laser beam has a degree of polarization of about 84%.

Diffraction limited CW optically pumped lasers

The far infrared spectral characteristics of inductive metal mesh dielectric hybrid mirrors were investigated. From experimental and theoretical studies, the mirrors were found to exhibit etalon effects due to the finite Si substrate thickness and to have two separate diffraction conditions. With grid spacing g and substrate index of refraction n , for n^{-1} the mirrors diffract in transmission while for g/\lambda > 1 the mirrors diffract both in transmission and reflection. Using both inductive and capacitive mesh mirrors, EH 11 outputs have been obtained at the milliwatt level in the range 70 μm to 1.2 mm. Powers in excess of 10 mW were obtained from CH 3 OH at 96 and 118 μm, HCOOH at 393 μm, CH 3 I at 447 μm, and CH 3 F at 496μm.