The objective of this research is the development of low-threshold, high-switching speed, programmable/reconfigurable, single-mode nanolasers. The proposed transformative program will address important problems that pertain to nanolasers including electrical injection and tunability. The intellectual merits of the proposed program are in the fundamental understanding of optical processes in sub-wavelength scale nanocavities, development of material growth techniques, and demonstration of nanolasers with following features: i) physical footprint 20-30 times smaller than state-of-the-art, ii) electrical injection, iii) wide tunability (tens of nanometers). The photonic crystal nanobeam cavities, based on an optical waveguide perforated with 1-D lattice of holes, enable efficient control of spontaneous emission at sub-wavelength-scale. Furthermore, their physical footprint is exactly the same as that of an optical waveguide, and therefore, they represent the smallest possible high-Q cavity geometry. Finally, by taking advantage of their flexibility, dynamical wavelength tunability over a wide wavelength range, with negligible steady-state power consumption, can be achieved. The broader impact of the proposed program is the development of techniques and devices for efficient light generation, collection, and localization, with applications ranging from optical and quantum-optical communication to life sciences and bio-chemical sensing. The program represents a unique interdisciplinary research and educational opportunity for students that encompass laser theory and design, epitaxial material growth, nanofabrication, and device characterization. An essential component of the program is outreach: the team members will continue their collaboration with Museum of Science in Boston, their involvement in the development of high-school physics curricula, and participation in Research Experience for Undergraduates program.
Project Description and Motivation: Developing an electrically driven ultra-small nanolaser is critical to making nanolaser a competitive technology, towards applications including photonic interconnect and index sensing. However, most of the nanolasers previously reported were optically pumped with another laser diode, with few exceptions. The big obstacle lies in configuring the electrical contacts close to the optical cavity in order to effectively inject carriers into the cavity mode, but the large absorption of conventional metal contacts will degrade the cavity’s Quality factor dramatically, and prevents lasing behavior. Project Results: We have developed nanobeam lasers operating at room temperature and composed of InAlGaAs/InP laser structures. Harvard University focused on the design and processing of the nanobeam laser structures while Georgia Tech’s contributions focused on the development of epitaxial materials structures of photonic crystal nanobeam lasers and photonic crystal disk lasers with compressive-strained In0.58Ga0.42As QWs (for TE gain) that are optically pumped. The Inx(AlyGa1-y)1-xAs heterostructures grown on InP substrates by MOCVD and have tight control of p- and n-type doped cladding layers . Especially, zinc (Zn) for p-type doping is known to be diffusive and could significantly degrade the internal quantum efficiency of the laser when it is diffused in the InGaAs quantum wells. In order to control the diffusion of Zn, growth condition and Zn dopant profile were optimized. Employing optimized epitaxial growth conditions, several injection nanobeam laser structures were delivered to Harvard team. In addition, Georgia Tech team delivered InAlGaAs heterostructures with embedded tensile-strained In0.48Ga0.52As quantum wells (QWs) for TM-gain nanobeam lasers. We also studied the ohmic contacts to these ultra-small lasers. Graphene, the one-atomic-thick layer of carbon atoms that are aligned in a honeycomb crystal lattice, on the other hand has both high electrical conductivity and high optical transparency, and thus is an ideal material candidate for transparent conducting electrodes. Graphene has already been applied in devices, such as light emitting diodes, photovoltaic cells, and liquid crystal displays. In addition, graphene has extremely high mechanical flexibility, and can conform to a patterned surface over large areas. Therefore, we believe, graphene could provide a promising solution to forming the electrical contacts to electrically driven nanolasers. Intellectual merit: The results of this program are among the lowest threshold nanolasers reported as of our publication dates and establish that such nanolasers are potentially useful for many applications. We have published our results in major refereed journals and at major conferences. Broader Impact: Ultra-small-scale nanolasers are required in photonic circuits for many critical applications, e.g., low-power, high-speed optical interconnects in Si-based electronic integrated circuits and InP-based photonic integrated circuits. We believe our results establish a path to apply nanolasers to these important applications worth billions of dollars world-wide
