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Play of Light

Optics research in the Electrical Engineering Department is bringing new insights to data security, medical technology, and many other areas.  

Research involving light—how it moves, propagates, affects, and is affected by objects—impacts a seemingly endless variety of fields. From health care to energy to security, projects involving optics touch on some of the most pressing topics of the modern era. Electrical engineers at UT Arlington are doing their part to advance this important work. Their research includes increasing the amount of data that can securely be transmitted over the Internet, integrating membrane lasers into neural activities to monitor bodily reactions, making large data centers more energy-efficient, and improving biosensors, among others. 

Sending Signals Securely

As savvy Internet users understand, security is a major concern with online transactions and transmissions. That’s why Professor Michael Vasilyev has focused his research on increasing by tenfold both the amount of information that can be securely transmitted on the Web and the distance over which it can be sent. His work is part of an $8 million initiative funded by the Defense Advanced Research Project Agency (DARPA) that enlisted five universities and three companies to study advanced quantum communications.

Professor Michael Vasilyev with research assistant Lu Li

Professor Michael Vasilyev with research assistant Lu Li.

With conventional, or classical, communication, information is transmitted via “bits” that take the value of either one or zero. In contrast, quantum communication uses quantum bits—“qubits”—which can be one, zero, or one and zero simultaneously (something called a “superposition” state). These qubits are represented by quantum-mechanical objects, such as single photons, that can provide a much higher level of protection from eavesdropping than classical communication signals.

“There are all kinds of personal information—both among private citizens and public governments—that require the utmost security,” Dr. Vasilyev says. “Quantum communication offers the most rigorous solution for security because it employs the fundamental laws of quantum mechanics to enforce the exclusive linkage between the sender and the receiver, with no chance of other people intercepting.”

He believes that one of the challenges with current technology is that secure and fast quantum communications can only be done over short distances—about 100 kilometers—before the signal breaks down. The reason is that qubits cannot go through optical amplifiers (commonly used in classical communications) without losing their quantum-mechanical security advantages.

To obviate the issue, Vasilyev’s lab is encoding information in spatial features or pixels of the photons that can be sent through multimode fiber optic lines, thus dramatically increasing the amount of received data without jeopardizing its security. The new technology will be useful for classical communications as well.

“The Internet is facing a capacity crisis,” Vasilyev says. “If the current rates of network traffic growth continue, we could be out of bandwidth by 2020 unless we start harnessing the spatial degrees of freedom of photons in a fiber.”

In collaboration with the University of Vermont, he has also developed regeneration technology that restores the quality of optical signals at multiple wavelengths simultaneously without ever converting them to electrical signals. The project is part of his ongoing research to help dramatically reduce the cost of transporting data over the Internet backbone. 

“The power of optics is in its capability to process many independent, high-speed data streams in parallel,” Vasilyev says. “So far, we have been applying this power to multiple wavelengths. With all possible wavelengths exhausted, we’re now turning to multiple spatial pixels to keep the capacity growing.”

High Capacity, Minimal Energy

Instead of fiber optics and qubits, Weidong Zhou works with silicon chips and lasers. But his work could prove equally important to the tech industry. The electrical engineering professor is trying to reduce energy consumption through thermal-engineered structures by essentially controlling the direction of light. 

Weidong Zhou

Weidong Zhou is using silicon photonics to make large data centers more energy-efficient.

Traditionally, computer and communications devices use low-cost silicon chips to efficiently store integrated electronic circuits for information processing. Lasers, meanwhile, are incorporated into compound semiconductor materials to engineer high-capacity optical networks. Silicon photonics seeks to integrate the two.

“Lasers on silicon remain a major roadblock toward making integrated silicon photonics work,” Dr. Zhou says. “Integrating light or lasers on silicon chips has the potential to increase capacity, increase speed, and lower the energy consumption of what those chips do.”

His technology uses photonic crystals to route laser beams in a way that increases the efficiency of the light on the integrated circuit.
“It’s like building construction vertically in New York City because there’s nowhere to build horizontally,” he explains.

The technology eventually could allow designers to place optical links on silicon chips with much less wasted material, time, and effort. One application is to replace metal wires in data centers with optics. Called optical interconnect, these new wires would allow for faster transfer and less loss. 

“Data centers take a huge amount of power. Google puts theirs near power plants because of the energy draw, for instance,” Zhou says. “Many companies and government agencies, such as DARPA, are working on this problem. The major challenge is to create very high capacity while using minimal energy.” 

Though Zhou’s research in this area began with data centers, it has many other applications, including optical imaging, sensing, bio-integrated electronics, signal processing, and bioimaging. For the latter, the engineer is exploring how a membrane laser can be integrated into neural activity to monitor functions and measure response to stimuli. 

“The computing power of the human brain is similar to a high-performance PC, but the energy consumption is many times lower than anything man-made, around 20 watts,” he says. “The ultimate dream would be to reach that level of power consumption while keeping the same quality.”

Zhou’s work with silicon photonics has helped in this quest. He uses nanoscale-structured photonic crystals as a mirror to form a laser cavity on silicon. Then, using a stamp-assisted printing transfer process, he integrates compound semiconductor structures and silicon cavities. These cavities are very small and use little energy, which also limits temperature rise.

“Sometimes, as long as we know the direction is right, we can see that the applications of our work are unlimited,” Zhou says. “This is engineerable technology for many applications.”

Building for the Future 

Like a certain former United States defense secretary, Robert Magnusson likes to talk about “known unknowns” and “unknown unknowns” when discussing his research process.  

Robert Magnusson in the Nanolithography Laboratory

Robert Magnusson in the Nanolithography Laboratory.

“I encourage my graduate students to avoid the first category and invent fearlessly within the second,” he says. “This philosophy will lead to the greatest discoveries.” 

Dr. Magnusson, the Texas Instruments Distinguished University Chair in Nanoelectronics, works with optics on the nanoscale. His projects have applications in everything from telecommunications to energy to medicine to laser technology. He leads the University’s Nanophotonics Device Group, which models, designs, fabricates, and tests a variety of nanotechnological concepts, including wavelength-selective laser mirrors, efficient broadband reflectors, compact and economic biosensors, wideband absorptive nanostructures, and tunable display pixels. 

Many of these devices use films that are less than one micrometer thick and have been etched with nanopatterns (similar to the anti-reflective coating on some eyeglasses). According to Magnusson, input light resonates within the structure and is generally delayed, which in simple terms means the element temporarily stores the light.  

“When this happens, a great concentration of light occurs inside the device and surrounds it. This then provides attached molecular layers, for example, with intense optical stimulation that can be very useful in the generation of new optical frequencies or in spectroscopy,” he explains. “The combination of periodic layers and homogeneous films creates new effects that surpass ordinary thin-film effects.” 

A nanophotonic chip containing narrowband reflectors for laser applications

A nanophotonic chip containing narrowband reflectors for laser applications.

The class of resonant bio- and chemical sensors invented by Magnusson and his team is of particular importance, as it is a “complete biosensor,” meaning it produces the full parametric quantification of a bioreaction with a single incoherent beam of light. These sensors are now in commercial use.

Additional hot topics under study in the Nanophotonics Device Group include resonant focusing elements, total and wideband absorption in a nanolayer, efficient resonant color-filter displays, coherent perfect absorbers, and nanostructured metasurfaces. 

Source: Engineer Magazine