Bending the Light with a Tiny Chip

A silicon chip developed by Caltech researchers acts as a lens-free projector—and could one day end up in your cell phone.

Imagine that you are in a meeting with coworkers or at a gathering of friends. You pull out your cell phone to show a presentation or a video on YouTube. But you don't use the tiny screen; your phone projects a bright, clear image onto a wall or a big screen. Such a technology may be on its way, thanks to a new light-bending silicon chip developed by researchers at Caltech.

The chip was developed by Ali Hajimiri, Thomas G. Myers Professor of Electrical Engineering, and researchers in his laboratory. The results were presented at the Optical Fiber Communication (OFC) conference in San Francisco on March 10.

Traditional projectors—like those used to project a film or classroom lecture notes—pass a beam of light through a tiny image, using lenses to map each point of the small picture to corresponding, yet expanded, points on a large screen. The Caltech chip eliminates the need for bulky and expensive lenses and bulbs and instead uses a so-called integrated optical phased array (OPA) to project the image electronically with only a single laser diode as light source and no mechanically moving parts.

Hajimiri and his colleagues were able to bypass traditional optics by manipulating the coherence of light—a property that allows the researchers to "bend" the light waves on the surface of the chip without lenses or the use of any mechanical movement. If two waves are coherent in the direction of propagation—meaning that the peaks and troughs of one wave are exactly aligned with those of the second wave—the waves combine, resulting in one wave, a beam with twice the amplitude and four times the energy as the initial wave, moving in the direction of the coherent waves.

"By changing the relative timing of the waves, you can change the direction of the light beam," says Hajimiri. For example, if 10 people kneeling in line by a swimming pool slap the water at the exact same instant, they will make one big wave that travels directly away from them. But if the 10 separate slaps are staggered—each person hitting the water a half a second after the last—there will still be one big, combined wave, but with the wave bending to travel at an angle, he says.

Using a series of pipes for the light—called phase shifters—the OPA chip similarly slows down or speeds up the timing of the waves, thus controlling the direction of the light beam. To form an image, electronic data from a computer are converted into multiple electrical currents; by applying stronger or weaker currents to the light within the phase shifter, the number of electrons within each light path changes—which, in turn, changes the timing of the light wave in that path. The timed light waves are then delivered to tiny array elements within a grid on the chip. The light is then projected from each array in the grid, the individual array beams combining coherently in the air to form a single light beam and a spot on the screen.

As the electronic signal rapidly steers the beam left, right, up, and down, the light acts as a very fast pen, drawing an image made of light on the projection surface. Because the direction of the light beam is controlled electronically—not mechanically—it can create a sort of line very quickly. Since the light draws many times per second, the eye sees the process as a single image instead of a moving light beam, says Hajimiri.

"The new thing about our work is really that we can do this on a tiny, one-millimeter-square silicon chip, and the fact that we can do it very rapidly—rapidly enough to form images, since we phase-shift electronically in two dimensions," says Behrooz Abiri, a graduate student in Hajimiri's group and a coauthor on the paper. So far, the images Hajimiri and his team can project with the current version of the chip are somewhat simple—a triangle, a smiley face, or single letters, for example. However, the researchers are currently experimenting with larger chips that include more light-delivering array elements that—like using a larger lens on a camera—can improve the resolution and increase the complexity of the projected images.

In their recent experiments, Hajimiri and his colleagues have used the silicon chip to project images in infrared light, but additional work with different types of semiconductors will also allow the researchers to expand the tiny projector's capabilities into the visible spectrum. "Right now we are using silicon technology, which works better with infrared light. If you want to project visible light, you can take the exact same architecture and do it in what's called compound semiconductor III-V technology," says Firooz Aflatouni, another coauthor on the paper, who in January finished his two-year postdoctoral appointment at Caltech and joined the University of Pennsylvania as an assistant professor. "Silicon is good because it can be easily integrated into electronics, but these other compound semiconductors could be used to do the same thing."

"In the future, this can be incorporated into a phone, and since there is no need for a lens, you can have a phone that acts as a projector all by itself," Hajimiri says. However, although the chip could easily be incorporated into a cell phone, he points out that a tiny projection device can have many applications—including light-based radar systems (called "LIDAR"), which are used in positioning, robotics, geographical measurements, and mapmaking. Such equipment already exists, but current LIDAR technology requires complex, bulky, and expensive equipment—equipment that could be streamlined and simplified to a single chip at a much lower cost.

"But I don't want to limit the device to just a few purposes. The beauty of this thing is that these chips are small and can be made at a very low cost—and this opens up lots of interesting possibilities," he says.

These results were described in a presentation titled "Electronic Two-Dimensional Beam Steering for Integrated Optical Phased Arrays." Along with Hajimiri, Abiri, and Aflatouni, Caltech senior Angad Rekhi also contributed to the work. The study was funded by grants from the Caltech Innovation Initiative, and the Information Science and Technology initiative at Caltech.

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Monday, March 17, 2014
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Caltech New Media Art Exhibition

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Unleashing Collaborative Learning through Technology: A Study of Tablet-Mediated Student Learning

Building Artificial Cells Will Be a Noisy Business

Engineers like to make things that work. And if one wants to make something work using nanoscale components—the size of proteins, antibodies, and viruses—mimicking the behavior of cells is a good place to start since cells carry an enormous amount of information in a very tiny packet. As Erik Winfree, professor of computer science, computation and neutral systems, and bioengineering, explains, "I tend to think of cells as really small robots. Biology has programmed natural cells, but now engineers are starting to think about how we can program artificial cells. We want to program something about a micron in size, finer than the dimension of a human hair, that can interact with its chemical environment and carry out the spectrum of tasks that biological things do, but according to our instructions."

Getting tiny things to behave is, however, a daunting task. A central problem bioengineers face when working at this scale is that when biochemical circuits, such as the one Winfree has designed, are restricted to an extremely small volume, they may cease to function as expected, even though the circuit works well in a regular test tube. Smaller populations of molecules simply do not behave the same as larger populations of the same molecules, as a recent paper in Nature Chemistry demonstrates.

Winfree and his coauthors began their investigation of the effect of small sample size on biochemical processes with a biochemical oscillator designed in Winfree's lab at Caltech. This oscillator is a solution composed of small synthetic DNA molecules that are activated by RNA transcripts and enzymes. When the DNA molecules are activated by the other components in the solution, a biological circuit is created. This circuit fluoresces in a rhythmic pulse for approximately 15 hours until its chemical reactions slow and eventually stop.

The researchers then "compartmentalized" the oscillator by reducing it from one large system in a test tube to many tiny oscillators. Using an approach developed by Maximilian Weitz and colleagues at the Technical University of Munich and former Caltech graduate student Elisa Franco, currently an assistant professor of mechanical engineering at UC Riverside, an aqueous solution of the DNA, RNA, and enzymes that make up the biochemical oscillator was mixed with oil and shaken until small portions of the solution, each containing a tiny oscillator, were isolated within droplets surrounded by oil.

"After the oil is added and shaken, the mixture turns into a cream, called an emulsion, that looks somewhat like a light mayonnaise," says Winfree. "We then take this cream, pour it on a glass slide and spread it out, and observe the patterns of pulsing fluorescence in each droplet under a microscope." 

When a large sample of the solution is active, it fluoresces in regular pulses. The largest droplets behave as the entire solution does: fluorescing mostly in phase with one another, as though separate but still acting in concert. But the behavior of the smaller droplets was found to be much less consistent, and their pulses of fluorescence quickly moved out of phase with the larger droplets.

Researchers had expected that the various droplets, especially the smaller ones, would behave differently from one another due to an effect known as stochastic reaction dynamics. The specific reactions that make up a biochemical circuit may happen at slightly different times in different parts of a solution. If the solution sample is large enough, this effect is averaged out, but if the sample is very small, these slight differences in the timing of reactions will be amplified. The sensitivity to droplet size can be even more significant depending on the nature of the reactions. As Winfree explains, "If you have two competing reactions—say x could get converted to y, or x could get converted to z, each at the same rate—then if you have a test tube–sized sample, you will end up with a something that is half y and half z. But if you only have four molecules in a droplet, then perhaps they will all convert to y, and that's that: there's no z to be found."

In their experiments on the biochemical oscillator, however, Winfree and his colleagues discovered that this source of noise—stochastic reaction dynamics—was relatively small compared to a source of noise that they did not anticipate: partitioning effects. In other words, the molecules that were captured in each droplet were not exactly the same. Some droplets initially had more molecules, while others had fewer; also, the ratio between the various elements was different in different droplets. So even before the differential timing of reactions could create stochastic dynamics, these tiny populations of molecules started out with dissimilar features. The differences between them were then further amplified as the biochemical reactions proceeded.

"To make an artificial cell work," says Winfree, "you need to know what your sources of noise are. The dominant thought was that the noise you're confronted with when you're engineering nanometer-scale components has to do with randomness of chemical reactions at that scale. But this experience has taught us that these stochastic reaction dynamics are really the next-level challenge. To get to that next level, first we have to learn how to deal with partitioning noise."

For Winfree, this is an exciting challenge: "When I program my computer, I can think entirely in terms of deterministic processes. But when I try to engineer what is essentially a program at the molecular scale, I have to think in terms of probabilities and stochastic (random) processes. This is inherently more difficult, but I like challenges. And if we are ever to succeed in creating artificial cells, these are the sorts of problems we need to address."

Coauthors of the paper, "Diversity in the dynamical behaviour of a compartmentalized programmable biochemical oscillator," include Maximilian Weitz, Korbinian Kapsner, and Friedrich C. Simmel of the Technical University of Munich; Jongmin Kim of Caltech; and Elisa Franco of UC Riverside. The project was funded by the National Science Foundation, UC Riverside, the European Commission, the German Research Foundation Cluster of Excellence Nanosystems Initiative Munich, and the Elite Network of Bavaria.

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Cynthia Eller
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A New Laser for a Faster Internet

A new laser developed by a research group at Caltech holds the potential to increase by orders of magnitude the rate of data transmission in the optical-fiber network—the backbone of the Internet.

The study was published the week of February 10–14 in the online edition of the Proceedings of the National Academy of Sciences. The work is the result of a five-year effort by researchers in the laboratory of Amnon Yariv, Martin and Eileen Summerfield Professor of Applied Physics and professor of electrical engineering; the project was led by postdoctoral scholar Christos Santis (PhD '13) and graduate student Scott Steger.

Light is capable of carrying vast amounts of information—approximately 10,000 times more bandwidth than microwaves, the earlier carrier of long-distance communications. But to utilize this potential, the laser light needs to be as spectrally pure—as close to a single frequency—as possible. The purer the tone, the more information it can carry, and for decades researchers have been trying to develop a laser that comes as close as possible to emitting just one frequency.

Today's worldwide optical-fiber network is still powered by a laser known as the distributed-feedback semiconductor (S-DFB) laser, developed in the mid 1970s in Yariv's research group. The S-DFB laser's unusual longevity in optical communications stemmed from its, at the time, unparalleled spectral purity—the degree to which the light emitted matched a single frequency. The laser's increased spectral purity directly translated into a larger information bandwidth of the laser beam and longer possible transmission distances in the optical fiber—with the result that more information could be carried farther and faster than ever before.

At the time, this unprecedented spectral purity was a direct consequence of the incorporation of a nanoscale corrugation within the multilayered structure of the laser. The washboard-like surface acted as a sort of internal filter, discriminating against spurious "noisy" waves contaminating the ideal wave frequency. Although the old S-DFB laser had a successful 40-year run in optical communications—and was cited as the main reason for Yariv receiving the 2010 National Medal of Science—the spectral purity, or coherence, of the laser no longer satisfies the ever-increasing demand for bandwidth.

"What became the prime motivator for our project was that the present-day laser designs—even our S-DFB laser—have an internal architecture which is unfavorable for high spectral-purity operation. This is because they allow a large and theoretically unavoidable optical noise to comingle with the coherent laser and thus degrade its spectral purity," he says.

The old S-DFB laser consists of continuous crystalline layers of materials called III-V semiconductors—typically gallium arsenide and indium phosphide—that convert into light the applied electrical current flowing through the structure. Once generated, the light is stored within the same material. Since III-V semiconductors are also strong light absorbers—and this absorption leads to a degradation of spectral purity—the researchers sought a different solution for the new laser.

The high-coherence new laser still converts current to light using the III-V material, but in a fundamental departure from the S-DFB laser, it stores the light in a layer of silicon, which does not absorb light. Spatial patterning of this silicon layer—a variant of the corrugated surface of the S-DFB laser—causes the silicon to act as a light concentrator, pulling the newly generated light away from the light-absorbing III-V material and into the near absorption-free silicon.

This newly achieved high spectral purity—a 20 times narrower range of frequencies than possible with the S-DFB laser—could be especially important for the future of fiber-optic communications. Originally, laser beams in optic fibers carried information in pulses of light; data signals were impressed on the beam by rapidly turning the laser on and off, and the resulting light pulses were carried through the optic fibers. However, to meet the increasing demand for bandwidth, communications system engineers are now adopting a new method of impressing the data on laser beams that no longer requires this "on-off" technique. This method is called coherent phase communication.

In coherent phase communications, the data resides in small delays in the arrival time of the waves; the delays—a tiny fraction (10-16) of a second in duration—can then accurately relay the information even over thousands of miles. The digital electronic bits carrying video, data, or other information are converted at the laser into these small delays in the otherwise rock-steady light wave. But the number of possible delays, and thus the data-carrying capacity of the channel, is fundamentally limited by the degree of spectral purity of the laser beam. This purity can never be absolute—a limitation of the laws of physics—but with the new laser, Yariv and his team have tried to come as close to absolute purity as is possible.

These findings were published in a paper titled, "High-coherence semiconductor lasers based on integral high-Q resonators in hybrid Si/III-V platforms." In addition to Yariv, Santis, and Steger, other Caltech coauthors include graduate student Yaakov Vilenchik, and former graduate student Arseny Vasilyev (PhD, '13). The work was funded by the Army Research Office, the National Science Foundation, and the Defense Advanced Research Projects Agency. The lasers were fabricated at the Kavli Nanoscience Institute at Caltech.

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Monday, April 7, 2014
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Everyone Starts Small: How Metals Learn to Behave

Watson Lecture Preview

On Wednesday, February 12, Assistant Professor of Aerospace Dennis Kochmann will explain how controlling a material's complex structural details from the atomic scale up can affect its behavior in everyday life. The talk begins at 8:00 p.m. in Caltech's Beckman Auditorium. Admission is free.

Q: What do you do?

A: We study the mechanics of materials. Specifically, we're making computer models that start with collections of individual atoms, and we're trying to extend those models all the way up to the scale of visible objects. We also have a lab where we make new materials and test them to see if they behave the way the models predicted. And eventually, once we have that understanding, we'll try to go back downward—if we want a material with certain properties, how do we make it?

Most innovation nowadays depends on new materials. In energy, in space travel, in biomedicine, many of the challenges involve finding a material that can meet certain conditions or that we can use in an extreme environment. It used to be that whenever you needed a specific material, you would tell your colleagues, "I want a material that has this and this and this property," and they'd look in various handbooks and try to find something that met your conditions. Nowadays we do it online, but it's basically the same process. In the future, we want to be able to tell our computer, "Okay, I want this and this and this," push a button, and an hour later the material is delivered to you. We're far away from this, of course.

It's much more complicated than just looking at the periodic table and throwing atoms together. Getting something on the atomic level is just one challenge. We also need to bridge the scales from atoms to the visible, macroscopic scale that we can see with the naked eye. There are many, many levels in between. Atoms form crystals. The crystals have defects. The defects arrange into networks. So if you put any material under a microscope, as you zoom in closer and closer you'll find that on each level there's a very specific pattern. These very specific structures and the things going on at each level are what give the material its unique properties.

 

Q: What gets you excited about this?

 

A: Well, as any researcher would say, it's doing something nobody has ever done before. In our case, making new, peculiar materials, or materials with extreme properties.

For example, we are designing materials that are pretty boring under ambient conditions, but if you tweak the temperature or the electric field the material suddenly gets 100 times stiffer and 100 times better at damping out vibrations. So you can control these properties with the push of a button. Usually, stiffness and damping are mutually exclusive. On the atomic scale, if you want something stiff and strong you need a perfect crystal. If you want high damping, there must be mechanisms at the microscale such as crystal defects that somehow absorb energy. Materials that do both are of great interest in applications such as aerospace—airplanes and spacecraft need materials that can withstand extreme conditions while absorbing vibrations, because there's a lot of vibration going on in them.

 

Q: How did you get into this line of work?

A: Before I went to college, this was absolutely not my goal. I started playing classical music on the organ when I was a teenager, and so I was trying to decide between music and chemistry. Then someone told me that engineering was a safer career, so I ended up in majoring in mechanical engineering. I wanted to be a design engineer, but by coincidence I was offered a job in solid mechanics, which is the mechanics of materials. By yet another coincidence I ended up at the University of Wisconsin–Madison, where I worked with Roderic Lakes and Walter Drugan—pioneers in the design of extreme materials. So as is often in life, it was many small events and little coincidences.

 

Q: Do you still play the organ?

A: I do. I'm very lucky. There's a church up in Altadena that lets me go up there every Friday morning for two hours and practice.

 

Q: Do you plan on giving a recital any time soon?

A: [laughs] No.

 

Named for the late Caltech professor Earnest C. Watson, who founded the series in 1922, the Watson Lectures present Caltech and JPL researchers describing their work to the public. Many past Watson Lectures are available online at Caltech's iTunes U site.

 

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Douglas Smith
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