Plenary talks at OPTO 2017 explore quantum dots, energy, LiFi

Everything, from your body to the universe itself, emits thermal radiation. "If we can control thermal radiation, we open new possibilities for technological applications," said Shanhui Fan of Stanford University who lead off the OPTO plenary session 30 January in San Francisco, CA (USA).

One example is a passive cooling system. Almost any black material, surrounded by an insulator, radiates heat. By putting such a setup on a rooftop, you can passively cool buildings at night by as much as 15 degrees C below the ambient temperature.

Recently, though, Fan and his colleagues have devised a way to cool a building even during the day. The researchers fabricated a structure made of multiple layers of dielectric materials, which reflect sunlight but still strongly emit infrared radiation in the 8 to 13 micron range.

Using this structure, they built a module that cools running water to below ambient temperature. The water feeds into a condenser, resulting in an air-conditioning unit that doesn't require electricity.

This method uses the ambient environment as a heat sink, so, in principle, if you use the universe itself as a heat sink, you could cool something down to 3 degrees kelvin, the temperature of the cosmos. "If you think about it, the sky is really the limit," Fan said. His group has already reached temperatures below the freezing point of water.

Controlling thermal radiation

These approaches, however, reflect visible light — which isn't always desired. By layering a silica photonic crystal on top of an absorber, the researchers created a material that emits heat and is transparent to the solar spectrum. Such a device could be used to cool photovoltaic cells even while they bake in the sun. For every 10-degree increase in temperature, solar cells drop in efficiency by 1%, so cooling is paramount.

Such passive cooling can also generate electricity. If you place a diode next to a cooler object, photons will flow out from the diode via thermal radiation, generating electricity. Placing a fan-like optical chopper in front of the diode periodically blocks the thermal radiation, allowing the ability to encode signals into the electric current.

Fan also described his work developing textiles for clothes that keep you as cool as if you're wearing nothing. Working with Yi Cui's group at Stanford, the researchers developed a material made of polyethylene, which is typically clear and nearly 100% transparent in infrared.

But by embedding holes ranging in size from 500 nm to 10 microns in the material, they produced a nanoporous polyethylene that's as opaque as cotton, yet transparent to infrared.

Quantum dots for encryption

OPTO plenary speaker Dieter Bimberg of Technische University in Berlin described the benefits and potential of quantum dots in a variety of applications - and how they are vital for quantum cryptography and energy-efficient nanophotonics.

Quantum dots can be fabricated via self-organizing processes. For example, indium arsenide dots are grown on and embedded in gallium arsenide. The dots, appearing as pyramids with a diameter of a few nanometers - 1010 times smaller than those in Egypt - act like individual atoms, with completely quantized energy levels. This allows them to emit light at discrete wavelengths. By embedding quantum dots in a waveguide, for example, you can create a nanophotonic device, like a laser or amplifier.

A single quantum dot, Bimberg explained, can have important uses in quantum cryptography and communication. Within a quantum dot, a hole and an electron, bound together as a quasiparticle called an exciton, can recombine and emit one or at most two polarized photons. One photon can serve as a qubit for sending encrypted signals; two are useful for entanglement.

Unlike in classical encryption, quantum encryption enables the sender and receiver to know immediately if an interloper has broken the coded signal. Quantum dot technology, Bimberg said, is also relatively simple and inexpensive, since it is based on classical semiconductor technology. A single qubit emitter is just a LED with one single quantum dot inside.

A single quantum dot behaves like an individual atom. But in a large assembly of them - say, several million in a semiconductor device - their discrete properties are hidden. But that's what makes them advantageous for creating or transmitting optical signals through communication networks.

Not every quantum dot is identical, so a big collection of them will have a range of sizes and shapes, which means they emit light with a Gaussian distribution of wavelengths. A laser based on quantum dots exploits this broad emission. The broader the emission, the faster the laser can fire pulses - pulses narrower than one picosecond, Bimberg said.

Because quantum dots have both a ground state and an excited state, quantum-dot lasers can generate pulses at different wavelengths. Lastly, such lasers allow for quantum techniques to suppress the slight fluctuations in arrival times of signals called jitter, down to as little as 200 femtoseconds, which would otherwise be very difficult with conventional lasers.

Using quantum-dot technology for other network devices like amplifiers will reduce energy consumption and cost, he said. For instance, devices like an erbium-doped fiber amplifier (EDFA) compensate for the intensity loss of a signal that travels through kilometers of fiber-optic cables. But these amplifiers are complex and expensive and do not operate in the O-band around 1310 nm, which is the range where local and metropolitan area networks operate. Instead, amplifiers based on quantum-dot technology are a cheaper and simpler solution.

Quantum-dot amplifiers have several other advantages. A single device can amplify multiple signals with different wavelengths and does this wavelength division multiplexing without crosstalk between signals. These devices can even change the wavelength of signals, which is sometimes necessary in a network because signals of the same wavelength can interfere with one another.

In general, quantum-dot technology is more energy efficient, which is increasingly important given the rising energy demands of the internet. "We really have to work on energy-efficient devices," Bimberg said. "Quantum-dot-based lasers and amplifiers are absolutely essential."

A quantum-dot laser, for instance, has a threshold current density three to four times lower than that of a conventional quantum well laser, which means it requires much less electricity. Such a device is also more thermally stable, not changing its threshold current density up to 70-80 degrees C, and thus doesn't need an energy-consuming cooling system.

Because a quantum-dot amplifier works for both upstream and downstream signals, a network can cut the number of devices — and thus energy demand — by half. Conventional technology would require a separate amplifier for upstream and downstream signals.

Concluding the plenary session, Harald Haas of the University of Edinburgh discussed the possibilities of LiFi, wireless communication using visible light. The technology could potentially be revolutionary, playing a role in everything from driverless cars and industrial robots to delivering the internet to remote areas and the Internet of Things. 

Haas is chair of mobile communications and director of the LiFi Research and Development Center at University of Edinburgh. He is also the founder of pureLiFi, a startup he created after discovering how to use a simple LED to stream data to a computer.

LiFi installations are proliferating across the globe and are even used for solar panels, he said. "Commercial activity is increasing all the time," Haas said. "The more pilot projects we have, the more we can show that this technology improves our lives."

More than five years ago, Haas demonstrated how he used an LED lamp fitted with a transmitter driver chipset to modulate the light, encode, and then transmit data to a desktop. High-definition video was transmitted through the light beam and LiFi was born.

Since 2011, progress has been as rapid as LiFi's data rates. In the lab, as part of the Ultra-Parallel Visible Light Communications Project (UP-VLC), Haas and colleagues have reached blisteringly fast 10Gbit/s data transmission speeds.

They developed micro-LED arrays to transmit 3.5Gbit/s via each red, green and blue micro-LEDs in parallel. They also applied novel spatial modulation orthogonal frequency divisional multiplexing so the micro-LED elements within the array could beam thousands of streams of light in parallel, multiplying the volume of data transmitted at any one time.

"LEDs have been the bottleneck in data rate. So as part of this project, we wanted to develop a technology that would unlock the vast amount of data rates available in the visible light spectrum," according to Haas. "We've achieved 10Gbit/s with these LEDs, but can reach 100Gbit/s using red, green and blue laser diodes."

Chairs for the OPTO symposium were SPIE Fellow Shibin Jiang of AdValue Photonics (USA) and Jean-Emmanuel Broquin of Institut de Microélectronique Électromagnétisme Photonique/Lab. d'Hyperfréquence et Caractérisation (IMEP-LAHC) in France. Connie J. Chang-Hasnain of UC, Berkeley and SPIE Fellow Graham T. Reed of University of Southampton (UK) were the OPTO cochairs.

Photonics West 2017, 28 January through 2 February at the Moscone Center, encompassed more than 4700 presentations on light-based technologies across more than 95 conferences. It was also the venue for dozens of technical courses for professional development, the Prism Awards for Photonics Innovation, the SPIE Startup Challenge, a two-day job fair, two major exhibitions, and a diverse business program with more than 25 events.

SPIE Photonics West 2018 will run 27 January through 1 February at Moscone Center.