In optical communications, laser pulses are used to transmit information along fiber-optic cables for telephone service, the Internet and cable television.
Researchers at Purdue University have shown how an optical material made of aluminum-doped zinc oxide (AZO) is able to modulate – or change – how much light is 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.”
Being able to modulate the amount of light reflected is necessary for potential industrial applications such as data transmission.
“We can engineer the film to provide either a decrease or an increase in reflection, whatever is needed for the particular application,” said Kinsey, working with a team of researchers led by Alexandra Boltasseva, an associate professor of electrical and computer engineering, and Vladimir M. Shalaev, scientific director of nanophotonics at Purdue’s Birck Nanotechnology Center and a distinguished professor of electrical and computer engineering. “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.”
Findings were detailed in a research paper appearing in July in the journal Optica, published by the Optical Society of America.
The material has been shown to work in the near-infrared range of the spectrum, which is used in optical communications, and it is compatible with the complementary metal–oxide–semiconductor (CMOS) manufacturing process used to construct integrated circuits. Such a technology could bring devices that process high-speed optical communications.
The researchers have proposed creating an “all optical plasmonic modulator using CMOS-compatible materials,” or an optical transistor.
In electronics, silicon-based transistors are critical building blocks that switch power and amplify signals. An optical transistor could perform a similar role for light instead of electricity, bringing far faster systems than now possible.
The Optica paper, featured on the cover of the journal, was authored by Kinsey, graduate students Clayton DeVault and Jongbum Kim; visiting scholar Marcello Ferrera from Heriot-Watt University in Edinburgh, Scotland; Shalaev and Boltasseva.
Exposing the material to a pulsing laser light causes electrons to move from one energy level called the valence band to a higher energy level called the conduction band. As the electrons move to the conduction band they leave behind “holes” in the valance band, and eventually the electrons recombine with these holes.
The switching speed of transistors is limited by how fast it takes conventional semiconductors such as silicon to complete this cycle of light to be absorbed, excite electrons, produce holes and then recombine.
“So what we would like to do is drastically speed this up,” Kinsey said.
This cycle takes about 350 femtoseconds to complete in the new AZO films, which is 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.
“We were surprised that it was this fast,” Kinsey said.
The increase in speed could translate into devices at least 10 times faster than conventional silicon-based electronics.
The AZO films are said to be “Epsilon-near-zero,” meaning the refractive index is near zero, a quality found normally in metals and new “metamaterials,” which contain features, patterns or elements that enable unprecedented control of light by harnessing clouds of electrons called surface plasmons. Unlike natural materials, metamaterials are able to reduce the index of refraction to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material.
The pulsing laser light changes the AZO’s index of refraction, which, in turn, modulates the amount of reflection and could make higher performance possible.
“If you are operating in the range where your refractive index is low then you can have an enhanced effect, so enhanced reflection change and enhanced transmission change,” he said.
The researchers “doped” zinc oxide with aluminum, meaning the zinc oxide is impregnated with aluminum atoms to alter the material’s optical properties. Doping the zinc oxide causes it to behave like a metal at certain wavelengths and like a dielectric at other wavelengths.
A new low-temperature fabrication process is critical to the material’s properties and for its CMOS compatibility.
“For industrial applications you can’t go to really high fabrication temperatures because that damages underlying material on the chip or device,” Kinsey said. “An interesting thing about these materials is that by changing factors like the processing temperature you can drastically change the properties of the films. They can be metallic or they can be very much dielectric.”
The AZO also makes it possible to “tune” the optical properties of metamaterials, an advance that could hasten their commercialization, Boltasseva said.
Purdue University researchers have demonstrated a CMOS-compatible all-optical transistor capable of 4THz speeds, potentially over a 1000 times faster than silicon transistors.
Nano-photonic transistors processed at low-temperatures can be fabricated atop complementary metal oxide semiconductors (CMOS) to boost switching time by ~5,000-times less than 300 femtoseconds (fs) or almost 4 terahertz (THz), according to researchers at Purdue University. The aluminum-doped zinc oxide (AZO) material from which these optical transistors are fabricated has a tunable dielectric permittivity compatible with all telecommunications infrared (IR) standard.
The AZO plasmonic oxide material is predicted by the researchers to be capable of 10-times faster communications speeds at all popular telecommunications wavelengths. The all-optical technology uses light for both the data stream and the control signals that modulate the data, rather than use electrical signals to control the modulation like today. The AZO films can be engineered to either increase or decrease the reflection index to encode the 1s and 0s during data transmissions. Their next step is to fabricate a working device in a simple application
Photonics test bench at Purdue University for testing its unique aluminum-doped zinc oxide (AZO) material.
(Source: Purdue University)
(a) Schematic of the pump-probe setup. Filter F1 transmits 1.3 μm and reflects 650 nm light. F3 removes residual 650 nm. 𝑅 provides a delay line. Lenses L1 and L2 focus the light onto BBO to generate 325 nm light. Filter F2 removes residual 650 nm. Lenses L3 and L5 focus light onto the sample. F4 filters any stray light for detector D1. Normalized change in the (b) reflected power and (c) transmitted power as a function of the delay time between the pump and probe pulses. The absolute reflection (transmission) of the AZO sample without pumping is 25% (40%) at 1.3 μm.
Transparent conducting oxides have recently gained great attention as CMOS-compatible materials for applications in nanophotonics due to their low optical loss, metal-like behavior, versatile/tailorable optical properties, and established fabrication procedures. In particular, aluminum-doped zinc oxide (AZO) is very attractive because its dielectric permittivity can be engineered over a broad range in the near-IR and IR. However, despite all these beneficial features, the slow (>100 ps) electron-hole recombination time typical of these compounds still represents a fundamental limitation impeding ultrafast optical modulation. Here we report the first epsilon-near-zero AZO thin films that simultaneously exhibit ultrafast carrier dynamics (excitation and recombination time below 1 ps) and an outstanding reflectance modulation up to 40% for very low pump fluence levels (less than 4 mJ/cm2) at a telecom wavelength of 1.3 μm. The unique properties of the demonstrated AZO thin films are the result of a low-temperature fabrication procedure promoting deep-level defects within the film and an ultrahigh carrier concentration.
SOURCES – Purdue University, Optical Society, EETimes
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