Larry A. Coldren - US grants

WPC> 2 B V J Z Courier T ? x x x x 6 X @ K X @ QMS JetScript QMSJETSC.PRS x @ h h h h }X @ #| x 2 < 9222440 Coldren Recently, several novel schemes to provide tunable lasers with relative wavelength tuning ranges much larger than the achievable index change have been proposed. Such lasers are desired in high capacity wavelength division networks for applications in tele and data communications, and they may find use in optical sensors and memories as well as in spectroscopies applications. Two of the most promising structures have been studied at UC Santa Barbara. Wavelength tuning ranges> 30 nm have been demonstrated. Unfortunately, current funding for this and related programs has ended without any renewal prospect due to the sponsoring company's policy. One of the problems with such lasers is that they are difficult to directly modulate without simultaneously shifting the wavelength. Thus, it is desirable to integrate them with separately controlled modulator external to the laser cavity. At UCSB work on a simple and robust guide/antiguide modulator has indicated that is an excellent candidate for this function. Again, funding for this work has also ended. One goal of this proposed program would involve integrating this modulator with one of the tunable laser structures. It is shown that this can be accomplished with no extra processing steps.

9313529 Hu The University of California at Santa Barbara will use this award to modernize a research facility in order to appropriately house a custom, high-resolution electron beam writing machine. The machine (VS1) was made available to this university by a donation from IBM. It will provide a unique sub-micron lithography tool capable of registration and alignment over large areas, commensurate with a "manufacturing" quality machine. The proper operation of the machine requires precise control of the local environment: temperature and humidity of the ambient must be kept within a limited range. Appropriate vibration isolation of this machine is also essential. The situation of the VS1 will be adjacent to the clean room fabrication area which is an optimal location from the point of view of easy access to and integration with the total device/structure fabrication process. This award will allow the university to ensure that this facility gets optimum use by modernizing the structural, electrical and air handling capabilities required for the operation of the VS1. This facility will form a critical capability of a research effort that already encompasses over 100 graduate students, postdoctoral fellows and faculty in the Departments of Electrical and Computer Engineering, Materials, Physics and Chemistry. This "in house" capability allows the formation of test structures that facilitate rapid prove-in of novel device concepts, but also allows rapid turn-around and adjustment of technology and processes. ***

Instrumentation will be acquired for the metallo-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) synthesis of III-V nitrides using funds from the Academic Research Infrastructure Program. The major items include a RF heated III-V MOCVD nitride system and a nitrogen plasma source for MBE growth. The aluminum, gallium and indium nitride materials are the most promising materials for optoelectronic devices to operate in the ultraviolet to blue wavelength regime. The research activities will focus on: 1) development of novel epitaxial growth technologies for III-V nitride semiconductors, 2) fundamental studies of the epitaxial growth mechanism, and 3) the fabrication of UV/blue laser diodes, high temperature electronics, and vacuum electronics devices. This research involves substantial support and interest from industrial collaborators. Studies of the epitaxial growth of III-V nitride semiconductors will be conducted with the ultimate objective of producing high quality laser diodes for use in the ultraviolet and blue wavelength regimes. The III-V nitride semiconductors are useful materials for high temperature electronics and also for vacumn electronics devices. The III-V nitrides are technologically important materials for devices, particularly at shorter wavelengths and at higher temperatures.

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Coldren

The intent of this program is to demonstrate the highest performance 1550 nm VCSELs ever, using a very robust, manufacturable, InP-based technology. It is also intended to demonstrate WDM array technology using this approach as well as operability in the range between 1300 and 1550 nm. This technology is based on a recently patented VCSEL design philosophy[1] that has already demonstrated record-level performance using high-index-contrast As-Sb DBR mirrors together with InP conductive layers for low resistance electrical contacts and heat extraction, all lattice-matched and grown in a single epitaxial step on an InP substrate[2-4]. However, in this program it is proposed to incorporate several new and novel concepts that promise significant performance breakthroughs on top of the prior milestones. These concepts include the incorporation of novel 1) dielectric apertures for low-loss lateral optical and current confinement, 2) electron barriers for reduced vertical carrier leakage, and 3) quantum-well intermixing around the circumference of the device for lateral carrier confinement. These elements will lead to significant improvements in output power, maximum operating temperature, wall-plug efficiency, and available wavelength range, as desired for future low-cost optical networks.

Prior work funded by NSF and DARPA has been successful in demonstrating the huge promise of the general approach. Most recently, experiments on this InP platform demonstrated the best overall 1550nm VCSEL results[2-4] as compared to all of the various monolithic approaches. These results illustrate the first ever 1550 nm VCSEL to have a sub-milliamp threshold current with over a milliwatt of light out at room temperature. And even with a non-optimal active region design and some excess optical loss in this case, it provided 0.2 mW of output at 70C. Moreover, because several limitations that were present in these early devices are now understood, the proposed improvements have been identified, and preliminary experiments and modeling suggest the 'significant performance breakthroughs' indicated above.

The intellectual merit of this work derives from its originality, contribution to knowledge, and experience and infrastructure of the PI. Eleven inventions related to the proposed technology have been filed as patents. The proposed novel VCSEL designs include the use of new dielectric-aperturing techniques for both lateral current and optical confinement, an electron barrier layer for improved high-temperature performance, and optionally, a novel implant and anneal procedure to selectively intermix quantum wells on the periphery for lateral carrier confinement. Previously, new approaches for creating WDM arrays of such VCSELs were proposed[5]; a reproducible MBE growth procedure was developed to create low optical loss, high-index contrast DBR mirrors using compounds of AlGaAsSb lattice-matched to InP; high-conductivity InP layers for low thermal and electrical impedance were included; and very low-voltage-drop tunnel junctions (TJs) incorporating InP and AlGaInAs were developed to enable VCSELs with only n-doped contact layers for low optical loss and electrical resistance[6].

The broader impacts of this activity include its potential for having a major impact on reducing the cost of sources for the optical communications industry. Current long wavelength VCSELs do not provide the required output power, and their temperature range of operation is limited. Higher-power, higher-efficiency, and higher operating temperatures in a low-cost technology for the 1300 - 1550 nm range will be offered by the results of this research, and this will be very enabling to the optical communications industry in its attempts to recover from its current slump. The project will provide an excellent teaching vehicle for the graduate student involved, who will need to learn various aspects of OE device physics, materials growth and processing, and device characterization.

Photons have proven to be most useful for encoding special quantum states and for transmitting them through free space or optical fibers. For local quantum-state operations photons are less favorable and well-localized quantum systems are desirable. In this respect quantum dots, often referred to as artificial atoms, are particularly attractive. This research aims at combining the advantages of photons with those of artificial atoms.

The main objective is to transfer the polarization quantum state of a single photon onto excitons in quantum dots and visa versa. The anticipated results are: a novel positioning technique for a quantum dot in the center of an optical waveguide, the demonstration of a single-photon absorption and reemission by a single quantum dot inside a micro-pillar with intrinsic lensing, the demonstration of the polarization quantum-state transfer between single photons and single quantum dots, and creating entanglement between a quantum dot and a photon and between two quantum dots. The first requirement to achieve the objectives is that the coupling between photons and quantum dots has to be resonant in order to preserve the quantum-phase coherences. For this optical-cavities resonant both with the incoming photon and the quantum dot inside the cavity will be used. Two novel ways of achieving a strong optical mode overlap with the quantum dots will be explored. The first is to use quantum dots inside micro pillars that containing optical lensing through the use of tapered oxidation layer. The second is to develop a technique to position a single quantum dot in the center of an optical micro cavity. The second requirement is that the quantum dots have to be effectively symmetric in order to obtain exciton spin degeneracy. For this magnetic fields and/or strain-induced effects on the micro-pillars will be explored. The third requirement is that the reemitted photon from the quantum dot should be distinguishable from photons reflected from the sample surface. For this a Michelson interferometer will be used where the two end mirrors are replaced by one micro-cavity containing a quantum dot on resonance and one micro-pillar containing no quantum dots on resonance.

Reaching the objectives will be a major step forwards in quantum-state control and harnessing and understanding quantum decoherence in nano-structures. The research is based on a close collaboration between the Materials, Engineering and Physics Departments at the University of California Santa Barbara. This collaboration provides an excellent opportunity for young researchers to perform interdisciplinary research on important topics in quantum (and classical) communication and information processing and in nano-structure fabrication. Reaching the objectives will initiate future research in storage of quantum information and in implementing the quantum repeater scheme (enabling long-distance quantum cryptography), quantum error correction and quantum networks.

Objective:
An entirely new type of semiconductor laser modulation process will be explored in this industry-university collaborative project. A novel Field-Induced Charge Separation Laser (FICSL) structure will provide direct modulation of the gain to enable much higher modulation speeds relative to conventional diode lasers in which current modulation is employed. Preliminary modeling suggests modulation speeds in the 100 GHz range. Collaborations with Ziva Corp., our GOALI partner, will help guide these activities.

Intellectual Merit:
The FICSL involves new physics, that of separating holes and electrons with an applied field via a gate structure placed above the active gain region in order to directly modulate the gain. An enhanced understanding of laser dynamics in multi-terminal configurations should result. The team brings high levels of expertise in MBE growth (Palmstrøm), high-speed transistors (Rodwell) and efficient, high-speed vertical-cavity lasers (Coldren) to uniquely address this new problem area. The UCSB labs in MBE growth, III-V nanofabrication, and materials and device characterization, are second to none for the VCSEL-like laser studies to be doneS. Complementary skills exist at Ziva.

Broader impact:
Such more-efficient, higher-speed devices may revolutionize optical interconnect approaches and enable more efficient computers and data centers. Interaction with Ziva greatly benefits the students involved. New processes developed within NSF-NNIN facility, will be available to future generations of students. Physics will be integrated into the ECE227 series, and it will be disseminated in journal and conference publications. The co-PIs and graduate students will continue to participate in one or more of the ten internship programs for under-represented minorities, high school, and undergrad students that are sponsored by NSF-MRL, NNIN, & COE.

Title: Integrated Synthesized Optical Sources


The successful pursuit of this research could lead to the first widely-tunable, integrated synthesized optical sources, which would be much more stable, and have smaller size, weight and power requirements than components currently available. The combination of photonic and electronic integration will produce a chip-scale tunable sources that do not require accurate temperature control or high input power to operate. The project's GOALI partner, Freedom Photonics LLC., intends to commercialize some version of the proposed device that may impact the optical components marketplace. The new technology will be developed within the NSF-NNIN facility and the new fabrication processes will be made to students and outside users. The PIs have a strong commitment to involving high school, and undergraduate students in research activities, including summer internships, and students of under-represented groups in STEM learning programs.

The proposed research aims to develop a highly-integrated widely-tunable optical frequency synthesizer (OFS) with sub-Hz-level accuracy. The integrated OFS is based on a monolithic, 4-section, widely-tunable laser offset-locked to any one of the lines of a frequency comb. By employing a combination of photonic and electronic integration, sub-Hz-level accuracy can be obtained with an integrated optical phase-locked loop (OPLL) technology. All of the photonics, except for the external reference laser, the photodiodes, and the comb generator, will be integrated on a single InP photonic IC . The tunable laser's output can be tuned across the entire comb by tuning the laser between the comb lines with the RF offset source, and alternately selecting different comb lines. With this approach sub-Hz-level relative frequency accuracy, as well as sub-kHz linewidth, should be possible. If successful, the proposed work would have a transformative impact on the design of optical systems for a series of new optical technologies.