Tomas Sarmiento

Ultrafast direct modulation of a single-mode photonic crystal nanocavity light-emitting diode

We demonstrate an electrically driven single mode photonic crystal cavity LED with record speed of operation (10 GHz) and 0.25 fJ/bit energy consumption, the lowest of any optical transmitter to date.

Nanobeam photonic crystal cavity quantum dot laser

The lasing behavior of one dimensional GaAs nanobeam cavities with embedded InAs quantum dots is studied at room temperature. Lasing is observed throughout the quantum dot PL spectrum, and the wavelength dependence of the threshold is calculated. We study the cavity lasers under both 780 nm and 980 nm pump, finding thresholds as low as 0.3 microW and 19 microW for the two pump wavelengths, respectively. Finally, the nanobeam cavity laser wavelengths are tuned by up to 7 nm by employing a fiber taper in near proximity to the cavities. The fiber taper is used both to efficiently pump the cavity and collect the cavity emission.

Photon-enhanced thermionic emission from heterostructures with low interface recombination

Photon-enhanced thermionic emission is a method of solar-energy conversion that promises to combine photon and thermal processes into a single mechanism, overcoming fundamental limits on the efficiency of photovoltaic cells. Photon-enhanced thermionic emission relies on vacuum emission of photoexcited electrons that are in thermal equilibrium with a semiconductor lattice, avoiding challenging non-equilibrium requirements and exotic material properties. However, although previous work demonstrated the photon-enhanced thermionic emission effect, efficiency has until now remained very low. Here we describe electron-emission measurements on a GaAs/AlGaAs heterostructure that introduces an internal interface, decoupling the basic physics of photon-enhanced thermionic emission from the vacuum emission process. Quantum efficiencies are dramatically higher than in previous experiments because of low interface recombination and are projected to increase another order of magnitude with more stable, low work-function coatings. The results highlight the effectiveness of the photon-enhanced thermionic emission process and demonstrate that efficient photon-enhanced thermionic emission is achievable, a key step towards realistic photon-enhanced thermionic emission based energy conversion.

Single-cell photonic nanocavity probes

We demonstrate for the first time high Q photonic nanocavities operating inside single biological cells. We show in vitro protein detection with our tool as a route towards real-time label-free sensing in an intracellular environment.

Electrically pumped photonic crystal nanocavity light sources using a laterally doped p-i-n junction

A technique to electrically pump photonic crystal nanocavities using a lateral p-i-n junction is described. Ion implantation doping is used to form the junction, which under forward bias pumps a gallium arsenide photonic crystal nanocavity with indium arsenide quantum dots. Efficient cavity-coupled electroluminescence is demonstrated and the electrical characteristics of the diode are presented. The fabrication improvements necessary for making an electrically pumped nanocavity laser using a lateral junction are discussed.

Optical fiber tips functionalized with semiconductor photonic crystal cavities

We develop a new method to transfer photonic crystal resonators to the tips of optical fibers. High Q (2000-4000) cavities are coupled via transmission or PL emission to the fibers in both Si and GaAs.

Design and growth of III–V nanowire solar cell arrays on low cost substrates

State-of-the-art III–V multijunction cells have achieved a record efficiency of 42.8%, which has fueled great interest in the utility sector for large-scale deployment. However, III–V solar cells have thus far proven too expensive for widespread terrestrial applications due to the combined cost of substrates, growth processes, and materials. Here, we propose a novel III–V solar cell based on the epitaxial growth of AlGaAs/GaAs on Ge nanowires, pre-patterned on low cost substrates to achieve cost-effective, large-scale deployment. This approach is based on our recent discovery that the surface kinetics and epitaxial growth by MBE and MOCVD are dramatically altered when growing on nanostructures instead of planar surfaces. These growth kinetics enable uniform, single crystal growth of low-defect, lattice mismatched materials on nanostructures with high aspect ratios. We present the device design, TCAD simulation results, and experimental growth results for GaAs/Ge core-shell nanowires on silicon substrates. Finite-difference time-domain (FDTD) simulation results show that this GaAs/Ge nanowire array has reduced reflection and wider incident angle acceptance than its planar counterpart, and outperforms planar anti-reflective coatings under some conditions. GaAs is epitaxially grown on Ge nanowires via MBE and MOCVD. TEM measurements on the wires confirm that the GaAs/Ge core-shell structure is single crystal. Based on these results, we are in the process of fabricating GaAs/Ge nanowire solar cell arrays. We will present further characterization of these core-shell arrays as well as electrical measurements of solar cell devices.

Coherent Generation of Nonclassical Light on Chip via Detuned Photon Blockade.

The on-chip generation of nonclassical states of light is a key requirement for future optical quantum hardware. In solid-state cavity quantum electrodynamics, such nonclassical light can be generated from self-assembled quantum dots strongly coupled to photonic crystal cavities. Their anharmonic strong light-matter interaction results in large optical nonlinearities at the single photon level, where the admission of a single photon into the cavity may enhance (photon tunneling) or diminish (photon blockade) the probability for a second photon to enter the cavity. Here, we demonstrate that detuning the cavity and quantum-dot resonances enables the generation of high-purity nonclassical light from strongly coupled systems. For specific detunings we show that not only the purity but also the efficiency of single-photon generation increases significantly, making high-quality single-photon generation by photon blockade possible with current state-of-the-art samples.

Antenna electrodes for controlling electroluminescence.

Optical antennas can control the emission from quantum emitters by modifying the local density of optical states via the Purcell effect. A variety of nanometallic antennas have been implemented to enhance and control key photoluminescence properties, such as the decay rate, directionality and polarization. However, their implementation in active devices has been hampered by the need to precisely place emitters near an antenna and to efficiently excite them electrically. Here we illustrate a design methodology for antenna electrodes that for the first time facilitates simultaneous operation as electrodes for current injection and as antennas capable of optically manipulating the electroluminescence. We show that by confining the electrically excited carriers to the vicinity of antenna electrodes and maximizing the optical coupling of the emission to a single, well-defined antenna mode, their electroluminescence can be effectively controlled. This work spurs the development of densely integrated, electrically driven light sources with tailored emission properties.

Nanobeam photonic crystal cavity light-emitting diodes

We present results on electrically driven nanobeam photonic crystal cavities formed out of a lateral p-i-n junction in gallium arsenide. Despite their small conducting dimensions, nanobeams have robust electrical properties with high current densities possible at low drive powers. Much like their two-dimensional counterparts, the nanobeam cavities exhibit bright electroluminescence at room temperature from embedded 1250 nm InAs quantum dots. A small room temperature differential gain is observed in the cavities with minor beam self-heating suggesting that lasing is possible. These results open the door for efficient electrical control of active nanobeam cavities for diverse nanophotonic applications.