Power/Performance Bits: Aug. 4

Plasmonics in progress: superfast fluorescence; a compact optical converter; low power quick switching material.

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Superfast fluorescence

Duke University researchers developed an ultrafast light-emitting device, pushing semiconductor quantum dots to emit light at more than 90 gigahertz. This device could one day be used in optical computing chips or for optical communication between traditional electronic microchips.

The new speed record was set using plasmonics. When a laser shines on the surface of a silver cube just 75 nanometers wide, the free electrons on its surface begin to oscillate together in a wave. These oscillations create their own light, which reacts again with the free electrons. Energy trapped on the surface of the nanocube in this fashion is called a plasmon.

The plasmon creates an intense electromagnetic field between the silver nanocube and a thin sheet of gold placed a mere 20 atoms away. This field interacts with quantum dots – spheres of semiconducting material just six nanometers wide – that are sandwiched in between the nanocube and the gold. The quantum dots, in turn, produce a directional, efficient emission of photons that can be turned on and off at more than 90 gigahertz.

A nanoscale view of the new superfast fluorescent system using a transmission electron microscope. The silver cube is just 75-nanometers wide. The quantum dots (red) are sandwiched between the silver cube and a thin gold foil. (Source: Maiken Mikkelsen/Duke University)

A nanoscale view of the new superfast fluorescent system using a transmission electron microscope. The silver cube is just 75-nanometers wide. The quantum dots (red) are sandwiched between the silver cube and a thin gold foil. (Source: Maiken Mikkelsen/Duke University)

“There is great interest in replacing lasers with LEDs for short-distance optical communication, but these ideas have always been limited by the slow emission rate of fluorescent materials, lack of efficiency and inability to direct the photons,” said Gleb Akselrod, a postdoctoral researcher at Duke. “Now we have made an important step towards solving these problems.”

The group is now working to use the plasmonic structure to create a single photon source, which is a necessity for extremely secure quantum communications, by sandwiching a single quantum dot in the gap between the silver nanocube and gold foil. They are also trying to precisely place and orient the quantum dots to create the fastest fluorescence rates possible.

Compact optical converter

Compact optical transmission presents possibilities for faster and more energy-efficient data exchange between electronic chips. Scientists at the Karlsruhe Institute of Technology (KIT) and ETH Zurich developed a plasmonic Mach-Zehnder modulator (MZM) capable of converting digital electrical signals into optical signals at a rate of up to 108 gigabit per second.

The MZM developed by the team is only 12.5 micrometers long, roughly one tenth the thickness of a hair. It consists of two arms, each of which contains one electro-optical modulator. Each modulator is made up of a metal-insulator-metal waveguide with a gap approximately 80 nanometers wide and filled with an electro-optical polymer, and sidewalls made of gold which, at the same time, act as electrodes. The electrodes carry a voltage which is modulated in line with the digital data. The electro-optical polymer changes its index of refraction as a function of the voltage. The waveguide and the coupler made of silicon route the two parts of a split light beam to the gaps or from the gaps.

In the respective gap, the light beams of the waveguides initiate electromagnetic surface waves, the so-called surface plasmons. The voltage applied to the polymer modulates the surface waves. Modulation is different in both gaps but coherent, as the same voltage is applied with different polarities. After passing through the gaps, the surface waves initially enter the output optical waveguides as modulated light beams and are then superimposed. The result is a light beam in whose intensity (amplitude), the digital information was encoded.

In the experiment, the MZM works reliably over the entire spectral range of the broad-band optical fiber networks of 1500 – 1600 nanometers at an electric bandwidth of 70 gigahertz with data flows of up to 108 gigabit per second. The large depth of modulation is a consequence of the high manufacturing accuracy in silicon technology. The MZM can also be made by means of the widespread CMOS-processes in microelectronics, and thus can easily be integrated into current chip architectures.

Low power, quick switching material

Researchers at Purdue University showed the potential of an optical material made of aluminum-doped zinc oxide (AZO) to modulate the amount of light reflected by 40 percent, while requiring less power than other “all-optical” semiconductor devices.

“Low power is important because if you want to operate very fast – and we show the potential for up to a terahertz or more – then you need low energy dissipation,” said doctoral student Nathaniel Kinsey. “Otherwise, your material would heat up and melt when you start pushing it really fast. All-optical means that unlike conventional technologies we don’t use any electrical signals to control the system. Both the data stream and the control signals are optical pulses.”

“We can engineer the film to provide either a decrease or an increase in reflection, whatever is needed for the particular application,” said Kinsey. “You can use either an increase or a decrease in the reflection to encode data. It just depends on what you are trying to do. This change in the reflection also results in a change in the transmission.”

The material has been shown to work in the near-infrared range of the spectrum and it is compatible with the CMOS manufacturing process used to construct integrated circuits. Such a technology could bring devices that process high-speed optical communications.

The switching speed of the new AZO films is about 350 femtoseconds, roughly 5,000 times faster than crystalline silicon and so fleeting that light travels only about 100 microns, or roughly the thickness of a sheet of paper, in that time.

The increase in speed could translate into devices at least 10 times faster than conventional silicon-based electronics.