Light-bending silicon chip shows projector potential

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

Traditional projectors pass a beam of light through an image, using lenses to map each point of the picture to corresponding, expanded points on a screen. The Caltech chip is claimed to eliminate 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 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,’ Hajimiri said in a statement. 

Using phase shifters, the OPA chip slows down or speeds up the timing of the waves, thereby 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 and not mechanically it can create a 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, said Hajimiri.

‘The new thing about our work is really that we can do this on a tiny, one-millimetre-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,’ said 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 simple, namely a triangle and single letters. However, the researchers are currently experimenting with larger chips that include more light-delivering array elements that 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,’ said Firooz Aflatouni, another of the paper’s coauthors. ‘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 said. 

However, although the chip could easily be incorporated into a cell phone, he points out that a tiny projection device can have many application - including light-based radar systems (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.