Spotlight on Photonics at STI (continued)

Solar cells, fiber optics and cancer detection

Tomorrow’s solar cells
The sun is the ultimate renewable energy source, and work is constantly being done on ways to use the sun to produce energy. Various teams of scientists are attempting to determine the best way to collect sunlight and convert it into electricity, and results have been promising thus far. Christophe Ballif and his Photovoltaics and Thin Film Electronics Lab at EPFL have designed hybrid solar cells with a yield of 21.4% by finely layering amorphous silicon on crystalline silicon. The technique could pave the way for a new generation of solar cells.
 
Another approach has proven to be extremely efficient: using nanowires. As the name suggests, nanowires are tiny threads which, when set up vertically, function as a kind of light funnel, absorbing large amounts of light. Anna Fontcuberta of the EPFL’s Semiconductor Materials Lab has developed gallium arsenide nanowires that can absorb 12 times more light than traditional photovoltaic cells.    

And here is another promising idea being worked on at STI: why not use sunlight to generate renewable energy via chemical reactions? This is the approach taken by Sophia Haussener, in the Renewable Energy Science and Engineering Lab (LRESE). She uses multi-scale solar simulators to test various ways of producing fuel from water and sunlight, varying the designs of electrochemical and thermo-chemical reactors and comparing the results obtained. 
  
Fiber optics as guides and sensors
When it is in the form of a laser, light works differently. In telecommunications, it is used to send data along fiber optic cables, which are in fact tiny transparent tubes. The electrical signals in the copper cables used before the advent of fiber optics were affected by electromagnetic disturbances.  Fiber optic cables, on the contrary, aren’t affected by such disturbances, and can guide electromagnetic waves filled with data over very long distances with few signal losses. The technology is still fairly recent and isn’t yet totally understood or optimized. Scientists all over the world are currently working to perfect fiber optics.

One such scientist is EPFL’s Camille Brès. In her Photonics Systems Lab, Professor Brès is working on how to employ and optimize non-linear effects in fiber optics. She is attempting to design the best possible fibers using non-linear materials that are adapted to various frequencies. Non-linear materials can change the frequency of an electromagnetic wave —or light wave— that goes through them, as long as the wave is sufficiently intense and the dispersion of the waveguide isn’t too great. This property makes it possible to use non-linear materials to process, copy or multiply waves, simply by adjusting the light waves that travel along the fiber. The technique is a simple, surprising method for improving data transmission. 

In addition to being a perfect telecommunications tool, fiber optics can function as “artificial nerves” that can detect changes in temperature and structural changes in buildings along the entire length of the fiber. Luc Thévenaz and his GFO (or Group for Fiber Optics) are working on inserting fibers in bridges, along pipes and in other constructions in order to detect changes that might weaken the structure. One example where the GFO’s technique could be implemented is the ITER fusion reactor, where a fiber-optics “nervous system” is planned as a way of detecting any leaks of ultra-cooled helium.

The approach works like this: a light wave is beamed to each end of the fiber optic cable. By analyzing the tone of the vibrations the waves emit when they collide, it is possible to determine the state of the structure –and potentially prevent structural failures and other accidents. The same principle also makes it possible to accelerate, slow down or even briefly stop a light signal, which represents the first step towards optical memory.     

Detecting the biomarkers of certain types of cancer
Photonics has been incredibly useful in medicine. It enables laser surgery and laser treatments, needless to say, but it is also used in diagnostics and screening. In collaboration with the Lausanne University Hospital (CHUV) EPFL’s Edurado Charbon of the Quantum Architecture Group (AQUA) has developed a single-photon detector that can detect any remaining cancer cells when a tumor has been removed.

EPFL’s Olivier Martin works on nanophotonic methods at the Nanophotonics and Metrology Laboratory. As part of the SPEDOC project, Professor Martin uses light to detect molecules in a drop of blood. But not just any molecules: his technique can detect the protein biomarkers of several types of cancer. The blood is first put into a chip that contains microfluidic channels with mettalic nanostructures that can trap the biomarker proteins in question. Then the chip is illuminated, and changes in the wavelength of the refracted light are detected using a spectrometer. The underlying phenomenon is known as surface plasmon resonance (SPR), and it makes it possible to determine the number of cancer biomarkers in the blood sample.

Biomedical imaging and building new devices

What is happening at the cellular and molecular level?
The ingenious use of light can make it possible to observe the unobservable. Several EPFL labs are working on such approaches, in particular at very small scales.

Aleksandra Radenovic’s team in the Nanoscale Biology Lab recently presented a technique based on PALM florescence microscopy that makes it possible to observe substance and nutrient exchanges taking place at cell-membrane level. The method can precisely track the number of proteins at the site observed. Since cell membranes function as intermediaries between cells and their environments, the approach can provide insight into how cells react to medications, to take one example, or pollutants, to take another. The technique is based on the ability of certain nanometric objects to “capture” light, meaning that when the targeted molecules are illuminated by successive flashes of light an extremely high-resolution assemblage of images can be made, showing where the proteins are present at very small scales. 

A live 3D image of a cell
The world of the infinitely small is also the object of study for Christian Depeursinge’s Microvision and Microdiagnostics Group. They have designed a device for observing cells in real time and in 3D, without using any contrast agents or floroscopy.

The device combines holographic microscopy and computational image processing. The object is swept by a laser beam and a digital camera takes holographic images that are assembled by a computer and “deconvoluted” in order to eliminate noise. Finally, the assembled three-dimensional image of the cell can be virtually “sliced” to expose its internal elements, such as the nucleus or organelles.

Can the mysteries of water be solved with optical methods?
Sylvie Roke analyzes water molecules. Of course, the basic molecular structure of water is relatively simple, with one oxygen atom bound to two hydrogen atoms (whence the symbol H2O). However, liquid water molecules’ behavior is often unpredictable and cannot be explained scientifically. In her Fundamental Biophotonics Lab Professor Roke is using novel optical approaches to try to unravel the mysteries of what occurs on the surface of tiny water droplets, a little-understood zone that can provide a wealth of information. She combines two light-based techniques, non-linear optics and light scattering, in her approach. Non-linear optics makes it possible to home in on the interface zone. Light scattering provides information about the surface molecules once they have been identified. This includes characteristics like their size and mass. The combined use of these two techniques may make it possible to verify or contradict several major scientific hypotheses and thereby clarify what occurs during some processes that take place in the human body, which is, after all, composed mostly of water.

Live on-screen viewing of blood circulation

Théo Lasser of the Biomedical Optics Lab uses light to conduct real-time observations of blood-flow in micro-vessels just under the skin. Aïmago, a startup that spun off from the lab, recently demonstrated the technique.

A screen with a camera shows a real-time image of a sort of “topographical map” of the blood vessel structure of any part of the body. This can be used to assess the seriousness of a burn, to take one example. The device is based on Doppler Laser technology. It relies on the Doppler Effect (the wave frequency lag between emitted and reflected light beams) and uses a laser to measure the light reflected by red blood cells as opposed to static tissues. The Doppler effect is illustrated by different colors on the screen, thanks to the camera. In this way, all one has to do is hold a hand under the camera to see a color image of blood circulation on the screen.

Glasses from the future
 
Photonics also makes it possible to invent radical, fun new devices. For example, the “augmented reality glasses” made by the Laboratory of Applied Photonics Devices (LAPD), a group led by Professor Christophe Moser. The HD glasses, which were recently presented in prototype stage, give whoever is wearing them various information, such as the time and date, the weather, or a route the wearer would like to follow – it’s all displayed right on the lens! This is made possible by a mini-projector affixed to the arms of the glasses and by a laminated holographic film on the lenses. However, the remaining technical challenges are considerable: how can users take in both the information on the glasses and the surrounding world? The current solution is a special contact lens that is worn along with the glasses. Work continues to make the device more user-friendly.

Quantum aspects of light and optofuidics

Designing new, more energy-efficient lasers
On the level of fundamental research, the quantum aspects of light are of great interest to scientists. Nicolas Grandjean, who is on the fence between the STI and SB faculties in his Laboratory of Advanced Semiconductors for Photonics and Electronics, studies a generation of blue-light mini-lasers. These devices use very little energy. They generate laser beams using polaritons, which are fascinating quantum particles that are at once photons and matter, and are created using semiconductors and mirrors. One possible application for these energy-efficient devices would be in medicine, where there is a desire to develop systems that can generate lasers within the human body, for uses like detecting sick cells.

Opto-mechanics: turning light into mechanical oscillation
The quantum effects of light can be harnessed in a wide variety of fields, including opto-mechanics. Tobias Kippenberg, who is also a member of both STI and the School of Basic Sciences and heads the Laboratory of Photonics and Quantum Measurements, has succeeded in transforming light into mechanical oscillations with his team. They then reconverted it back into light – all on an object that was visible to the naked eye! Generally quantum effects are not observable on large objects, because larger objects interact too much with their environment, leading to a phenomenon known as “quantum decoherence.” However, by cooling the oscillator until its temperature was close to absolute zero, the team was able to control its movements on a level where quantum effects take place. This represents the first time that it was possible to observe the strange effects of quantum mechanics on man-made objects. (Up until now, they have only been observed at atomic or molecular level.)

New fields like optofluidics    
STI is also home to research in new, cutting-edge fields relating to photonics. For example, STI is the world leader in optofluidics, which combines optics and microfluidics. STI’s Dean, Demetri Psaltis, who directs the Optics Lab, is this field’s pioneer.      

Concretely, optofluidics brings together a source of light and microfluidic devices. These devices have tiny channels that transport extremely small quantities of liquid. Optofluidics makes it possible to guide and concentrate light using the liquids. A recent development consists in applying this technology to the energy sector. For example, Professor Psaltis is working on equipping the channels with surface catalyzers, which would make them into micro-reactors that could convert water and sunlight into hydrogen. The idea is to collect the sunlight and direct it to the microchannels in order to set off a chemical reaction.

The advantage compared with the current generation of larger reactors lies in the tiny dimensions of the channels: this makes for a greater surface area, and thus for more reactions. The approach therefore can lead to a far greater number of reactions in a much smaller volume. The challenge remaining is to succeed in grouping a large number of the micro-reactors: this entails quite a few different parameters that must all be managed.

A promising future
Clearly, photonics has a bright future ahead of it at STI Faculty. While many technologies are still being perfected, some photonics-based approaches have already been spun off as startups. “STI has recently hired several young professors working on photonics-related projects,” notes Professor Martin. “These activities have become greatly diversified at EPFL.” For example, although it was not covered in this article, the photonics-related research in the School of Basic Sciences is also extremely rich.

Two new photonics professors at EPFL

Fabien Sorin joined STI as a new tenure track assistant professor in March. His focus is on developing fibers and processes capable of creating nanostructured optical and optoelectrical devices that can be deployed at macroscopic scales. His work has included photovoltaic systems and he has shown that crystalline semiconductors can be made from fibers that were drawn from an amorphous state, thus enabling dimensional control and high electronic performance.

Professor Hatice Altug will join STI in June. Her work focuses on ultra-sensitive bio-nano sensors and photonic chip devices. By combining nanofluidics and plasmonics, she has developed a system of portable sensors that can detect living viruses in less than 30 minutes, as well as biomolecules and chemical substances. 

Although not covered in this article, the photonics research currently underway at the Basic Sciences Faculty is also extremely varied and rich.

Other STI labs using photonics include:
– The Applied Optics Lab (Hans Peter Herzig)
– The Biomedical Imaging Lab (Michäel Unser)
– The Microelectronic Systems Lab (Yusuf Leblebic)
– The Electromagnetics and Acoustics Lab (Juan Mosig)
– The Laboratory for Photonic Materials and Characterization (Gian-Luca Bona)
– The Microsystems for Space Technologies Laboratory (Herbert Shea)

For an overview of all photonics-based research
go to the photonics PhD sites at http://phd.epfl.ch/edpo and http://photonics.epfl.ch/.

A few key dates:
1947: Appearance of the transistor: the electronics era is born    
1960: The first lasers: photonics takes off as a field of study.    
1962: The first LED display   
1968: The first CDD image sensor, one of the basic components of digital cameras   
1970s: development of fiber optics   
2000s: creation and development of optofluidics