Center for Energy Efficient Electronics Science

Theme III: Nanophotonics

Faculty

Ming Wu (theme leader), Berkeley
Constance Chang-Hasnain, Berkeley
Eugene Fitzgerald, MIT
Vladimir Stojanović, Berkeley
Eli Yablonovitch, Berkeley

The goal of the Nanophotonics team is to enable optical communications between switches on a chip at unprecedented efficiency levels. In fact, E3S researchers pursue the ultimate goal of experimentally approaching the quantum limit of photons-per-bit in a data-link. This will require reduction of photons per bit from currently used 20,000 to below a hundred photons.

To meet this goal, the Center for E3S strives to improve energy efficiency and sensitivity of both the emitter and the photo-receiver. Central to the E3S nanophotonics research goals has been the demonstration that spontaneous emission from antenna enhanced nano-LEDs can be faster and more energy efficient than the stimulated emission of lasers, the ubiquitous light source in optical communications today. Major advances toward this goal have recently been achieved by demonstrating spontaneous emission enhancements of more 300 times, reaffirming the strategy of the E3S Nanophotonics team of introducing optical antenna enhanced spontaneous light emitters for energy-efficient short distance on-chip optical interconnects.

Challenges

Reaching the quantum limit in terms of photons-per-bit in an optical communications data-link imposes tremendous challenges on choice of materials, nanofabrication of optical components, and their on-chip integration. Ultra-efficient light sources and ultra-sensitive detectors need to be developed and miniaturized to be comparable to the size of transistors. Integration of the optical components in waveguides is also part of the challenges of nanophotonics research.

Current Projects

Antenna-coupled III-V nanoLED

Theme III researchers have recently demonstrated an electrically injected antenna-enhanced III-V nanoLED with a spontaneous emission enhancement close to 200 times. This demonstration was a major breakthrough and places spontaneous emission rate on par with the stimulated emission rate of lasers! The large boost in enhancement factor in the electrically-injected nanoLED was enabled by establishing a fundamental understanding of antenna design, for optical applications and significant fabrication improvements, including a new dry-etching method, an in-situ tilting coating technique, and insertion of a low-index spin-on-glass layer. Further improvements are expected by addressing the pronounced impact of surface recombination on the small nanoLEDs, which have large surface-to-volume ratios. Reaserch to address materials challenges as well as optoelectronic device fabrication are being pursued.

Selected Recent Publications

Antenna-enhanced chalcogenide nanoLED

In parallel to demonstrating the III-V nanoLED, E3S researchers collaborated to develop an optically pumped antenna LED structure with a 2D material as active layer. Using a monolayer of the transition metal dichalcogenide (TMDC) material WSe2 as light emitter, record spontaneous emission rate enhancements of up to 320 times have been achieved. In addition, these results have also shown the benefit of the Center format, as chalcogenide expertise developed in Theme I has been used in this project.

Selected Recent Publications

Ultra-sensitive photo-receivers

A collaborative effort of E3S System Integration and Nanophotonics researchers revealed that a highly sensitive photo-receiver with a low capacitance is the key to reducing the energy consumption of the complete optical link.  The reason is that low detector capacitance  reduces both the photons/bit required from the transmitter side as well as providing larger receiver signal voltage, reducing amplifier energy consumption. Furthermore, in the past, Theme III researchers have identified the device physics necessary for a photo-receiver to be sensitive at ultra-low 20 photons per bit levels. The key requirement is fast transit time of the transistor, and it was found that full performance requires integration of an optical cavity with photo-detector and photo-receiver.

Selected Recent Publications