An array of more than 4,000 optical antennas working in unison has been demonstrated on a millimetre-scale silicon chip. The result highlights the remarkable capabilities of optical integration in silicon. See Letter p.195
In his Nobel lecture in 1909, physicist Ferdinand Braun remarked1 that: “It had always seemed most desirable to me to transmit the waves, in the main, in one direction only”. He was referring to radio waves, which had hitherto been known to radiate only in a doughnut shape away from the emitter, as described by Heinrich Hertz two decades earlier. The invention for which Braun was recognized was the phased array — the idea of connecting up multiple antennas fed from a common source and using the relationship between the antennas' phase (where a wave's peaks and troughs lie) to enhance the emission of radio waves in a given direction. His work was instrumental in the development of radio transmitters, and led to other inventions such as the television antenna and radar. The phased-array concept is now also extensively used in astronomy, where the information from multiple telescopes is collected in phase to extend the resolution of the system. On page 195 of this issue, Sun et al.2 introduce an integrated optical phased array that combines the equivalent of more than 4,000 telescopes onto a single chip smaller than a fingernail.
The creation of phased arrays in the optical domain has been an active area of research for some time, as it is motivated by the need to improve applications such as optical beam steering and imaging. To achieve the highest performance, the challenges are to integrate a large number of antennas onto a single chip, to place them close together and to fully control the phase on each element. The short optical wavelength makes it easy, in principle, to address the first challenge and to build large arrays on a small footprint in an integrated format. The other two challenges are much more difficult to meet, however, because the optical couplers that feed a controlled amount of power into each antenna and the phase shifters needed to control the phase on each element tend to be tens or hundreds of wavelengths long, which requires a large distance between each antenna. Therefore, the latest realizations of integrated optical phased arrays have been relatively large and have featured a maximum of only 4 × 4 antennas3,4.
The architecture now introduced by Sun et al.2 addresses all of these requirements and is scalable. By carefully designing the optical coupler, phase shifter and gratings that act as radiating antennas, the team has managed to produce a phased array of 64 × 64 antenna units, or 'pixels', on a footprint of a little more than 0.576 × 0.576 millimetres, with each pixel covering 9 × 9 micrometres — equivalent to a pixel side length of six optical wavelengths. Most impressive are the accuracy and control that the researchers were able to exercise over the array's optical functions, especially those of the optical couplers.
The device's accuracy and performance are highlighted by the well-balanced power output achieved across the array. Equally, the fact that a phase shift of π radians can be attained on a pixel size a few wavelengths across (in contrast to the tens or hundreds of wavelengths previously required), and with relatively low power and crosstalk, points to the remarkable capabilities of nanophotonic concepts that are now available. Fabrication of the array, as a photonic circuit, using only silicon microelectronics processing techniques — a concept pioneered5 by the IMEC microelectronics centre in Leuven, Belgium — is also a testimony to the high performance that can be achieved by using silicon as a photonic material. The successful demonstration of the authors' array shows that the large-scale integration of photonic circuitry in silicon has well and truly arrived.
So where do we go from here? One interesting idea is that of imaging through light-scattering media6, such as roughened glass or biological tissue. Such imaging uses the adaptive-optics principle of adjusting the phase of an optical wave locally so as to compensate for the distortion caused by the medium. The authors' optical phased array now makes available the phase control and large pixel number that this application requires. To achieve imaging through time-varying light scatterers, such as turbulent liquids, a fast time-response of the system is also essential; the small and efficient tuning of each antenna demonstrated by Sun et al. can also meet this requirement. Another application is in sensor networks, in which communication between randomly distributed sensors is based on optical beams. Such networks require accurate control over the direction of each individual beam, which can now be achieved with the authors' array.
Other applications in imaging, such as projection television, involve visible optical wavelengths that are shorter than the near-infrared wavelengths used by the authors. Operation at these wavelengths would require further miniaturization of the array's pixel size and waveguides that are not made of silicon, because silicon absorbs light in the visible range. Also, the multiple images seen in the device's radiation pattern (see Fig. 2 of the paper2) are due to the fact that the pixels are multiple wavelengths apart. To reduce the number of images, ideally to a single one, it will be necessary to make the pixels even smaller and to move them closer together. This might be achievable by stacking the optical coupler, phase shifter and grating antenna on top of one another, thereby integrating the array in three dimensions. Clearly, further miniaturization and three-dimensional integration are exciting challenges that can now be considered seriously.
Most remarkably, even a century ago Braun had already thought ahead, and considered whether the antenna effects he described might also be observable in the optical domain. He wondered1 whether the concepts he had studied with radio waves could “also be linked up with optical phenomena, though this can hardly be experimentally verified in this field”. Now, Sun et al. have provided us with just such an experimental demonstration. Braun would surely have been pleased indeed.
Braun, K. F. http://nobelprize.org/nobel_prizes/physics/laureates/1909/braun-lecture.pdf 239–241 (1909).
Sun, J. et al. Nature 493, 195–199 (2013).
Van Acoleyen, K., Rogier, H. & Baets, R. Opt. Express 18, 13655–13660 (2010).
Doylend, J. K. et al. Opt. Express 19, 21595–21604 (2011).
Čižmár, T., Mazilu, M. & Dholakia, K. Nature Photon. 4, 388–394 (2010).
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IEEE Journal of Selected Topics in Quantum Electronics (2014)