Gary Shambat

Room temperature 1.6 µm electroluminescence from Ge light emitting diode on Si substrate

We report the room temperature electroluminescence (EL) at 1.6 microm of a Ge n+/p light emitting diode on a Si substrate. Unlike normal electrically pumped devices, this device shows a super linear luminescence enhancement at high current. By comparing different n type doping concentrations, we observe that a higher concentration is required to achieve better efficiency of the device. Thermal enhancement effects observed in temperature dependent EL spectra show the capability of this device to operate at room temperature or above. These detailed studies show that Ge can be a good candidate for a Si compatible light emitting device.

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.

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.

Direct band Ge photoluminescence near 1.6 μm coupled to Ge-on-Si microdisk resonators

We fabricate and optically characterize germanium microdisks formed out of epitaxial germanium grown on silicon. Resonators coupled to fiber tapers display clear whispering gallery modes in transmission and photoluminescence with quality factors limited by germanium’s material absorption. Continuous wave pumping of the cavities resulted in a dominant heating effect for the cavity modes in both transmission and photoluminescence. Pulsed optical pumping proved to be more effective in minimizing heating, but was not sufficient to observe material gain or lasing. We believe that significantly higher doping levels are critical in order to achieve lasing at reasonable pump conditions.

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.

Ultra-low power fiber-coupled gallium arsenide photonic crystal cavity electro-optic modulator.

We demonstrate electro-optic modulation in a GaAs laterally doped photonic crystal cavity diode with ultra-low switching energy of several fJ/bit. A short non-radiative carrier lifetime allows fast switching with an upper threshold of 100 GHz.

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.

Cavity-enhanced direct band electroluminescence near 1550 nm from germanium microdisk resonator diode on silicon

We electrically and optically characterize a germanium resonator diode on silicon fabricated by integrating a germanium light emitting diode with a microdisk cavity. Diode current-voltage characteristics show a low ideality factor and a high on/off ratio. The optical transmission of the resonator features whispering gallery modes with quality factors of a few hundred. Direct band gap electroluminescence under continuous current injection shows a clear enhancement of emission by the cavity. At this stage, the pumping level is not high enough to cause linewidth narrowing and invert the material. A higher n-type activated doping of germanium is necessary to achieve lasing.

Photonic crystal filters for multi-band optical filtering on a monolithic substrate

Many applications require the ability to image a scene in several different narrow spectral bands simultaneously. Conventional multi-layer dielectric filters require control of film thickness to change the resonant wavelength. This makes it difficult to fabricate a mosaic of multiple narrow spectral band transmission filters monolithically. We adjusted the spectral transmission of a multi-layer dielectric filter by drilling a periodic array of subwavelength holes through the stack. Multi-band photonic crystal filters were modeled and optimized for a specific case of filtering six optical bands on a single substrate. Numerical simulations showed that there exists a particular air hole periodicity which maximizes the minimum hole diameter. Specifically for a stack of SiO2 and Si3N4 with the set of filtered wavelengths (nm): 560, 576, 600, 630, 650, and 660, the optimal hole periodicity was 282 nm. This resulted in a minimum hole diameter of 90 nm and a maximum diameter of 226 nm. Realistic fabrication tolerances were considered such as dielectric layer thickness and refractive index fluctuations, as well as vertical air hole taper. Our results provide a reproducible methodology for similar multi-band monolithic filters in either the optical or infrared regimes.