May 3 2019
Envisage shaping a light pulse in any plausible way—stretching it, compressing it, dividing it in two, changing the direction of its electric field, or altering its intensity.
(Image credit: S. Kelley/NIST)
The characteristics of ultrafast light pulses must be controlled for transferring data via high-speed optical circuits and in exploring molecules and atoms that are capable of vibrating thousands of trillions of times per second.
However, the traditional technique of pulse shaping—that is, utilizing devices called spatial light modulators—is bulky, expensive, and does not have the fine control which is increasingly required by researchers. Added to this, such devices are usually predicated on liquid crystals that can be impaired by the very same high-intensity laser light pulses they were designed to sculpt.
Now, at the University of Maryland’s NanoCenter in College Park and the National Institute of Standards and Technology (NIST), scientists have come up with a new, compact technique of shaping light. To achieve this, the researchers initially deposited an ultrathin silicon layer on glass, measuring only billionths of a meter or a few hundred nanometers thick, and subsequently used a protective material to cover an array of millions of minute squares of the silicon. Next, the researchers etched away the silicon enclosing each square and produced millions of microscopic pillars, which considerably contributed to the light sculpting method.
An example of a metasurface is the flat, ultrathin device, which is utilized to alter the characteristics of a light wave passing through it. If the density size, shape, and distribution of the nanopillars are meticulously designed, various properties of every light pulse can now be customized at the same time and separately with nanoscale accuracy. Phase, amplitude, and polarization of the wave constitute these properties.
A light wave is a sequence of oscillating magnetic and electric fields that are oriented at right angles to one another. It contains troughs and peaks, analogous to an ocean wave. If individuals are standing in the ocean, the phase is where they are relative to the troughs and peaks; the frequency of the wave is how frequently the troughs or peaks move past them, and the amplitude refers to the waves’ height (trough to peak).
We figured out how to independently and simultaneously manipulate the phase and amplitude of each frequency component of an ultrafast laser pulse. To achieve this, we used carefully designed sets of silicon nanopillars, one for each constituent color in the pulse, and an integrated polarizer fabricated on the back of the device.
Amit Agrawal, Visiting Fellow, Photonics and Plasmonics Group, NIST and NanoCenter.
Whenever a light wave passes via a set of the silicon nanopillars, the wave has a tendency to slow down in comparison to its speed in air and its phase is also delayed—the time when the wave arrives its next peak is somewhat later than the moment at which the wave would have arrived its subsequent peak in air. The nanopillars’ size establishes the amount through which the phase alters, while the nanopillars’ orientation alters the polarization of the light wave.
When a device, called a polarizer, is fixed to the rear of the silicon, the variation in polarization can be translated to an equivalent variation in amplitude.
Information can be encoded by changing the polarization, amplitude, or phase of a light wave in a highly regulated way. In addition, the quick and finely adjusted changes can be used for examining and altering the outcome of biological or chemical processes. For example, changes in an incoming light pulse might either decrease or increase the product of a chemical reaction. In this fashion, the nanopillar technique could present new opportunities in the analysis of high-speed communication and ultrafast phenomenon.
Together with Henri Lezec of NIST and their collaborators, Agrawal has recently described the study findings in the journal, Science.
We wanted to extend the impact of metasurfaces beyond their typical application—changing the shape of an optical wavefront spatially—and use them instead to change how the light pulse varies in time.
Henri Lezec, Project Leader, Photonics and Plasmonics Group, Physical Measurement Laboratory, NIST.
A standard ultrafast laser light pulse will last for just one-thousandth of a trillionth of a second, or a few femtoseconds. This time is very short for any device to sculpt the light at one specific instant. Therefore, Lezec, Agrawal¸ and their colleagues developed a method to mold the separate colors or frequency components that constitute the pulse by initially isolating the light into those components using a diffraction grating, a kind of optical device.
Every color possesses a different amplitude or intensity—akin to the way a musical overtone is composed of several single notes having varying volumes.
When different frequency components are directed inside the nanopillar-etched silicon surface, they hit different sets of nanopillars. Every set of nanopillars was customized to change the intensity, phase, or the components’ electric field orientation (polarization) in a specific manner. Then, all the components are recombined by a second diffraction grating to produce the newly sculpted pulse.
The unique nanopillar system designed by the scientists operates with ultrafast light pulses (10 femtoseconds or less, corresponding to one-hundredth of a trillionth of a second) made up of a wide range of frequency components spanning wavelengths from 700 nm (visible red light) to 900 nm (near-infrared). By independently and concurrently changing the phase and amplitude of these frequency components, the researchers showed that their technique can distort, split, and compress pulses in a controllable way.
More improvements in the device will not only give researchers more control over the time evolution of light pulses but may also allow them to sculpt separate lines in a frequency comb in remarkable detail—an accurate tool for detecting planets around remote stars and for determining the frequencies of light employed in these devices like atomic clocks.