Sculpting Super-Fast Light Pulses

Schematic of the pulse shaping setup. An incoming pulse of light diffracts off a grating, which disperses the pulse into its various frequencies. A parabolic mirror then redirects the dispersed light into a silicon surface etched with millions of tiny pillars. The nanopillars are specifically designed to simultaneously and independently shape such properties of each frequency component as its amplitude, phase or polarization. A second parabolic mirror and diffraction grating then recombines the separated components into a newly formed pulse. (Source: T. Xu, Nanjing U.)

Controlling the properties of ultrafast light pulses is essential for sending infor­mation through high-speed optical circuits and in probing atoms and molecules that vibrate thousands of trillions of times a second. But the standard method of pulse shaping is costly, bulky and lacks the fine control scientists increa­singly need. In addition, these devices are typically based on liquid crystals that can be damaged by the very same pulses of high intensity laser light they were designed to shape. Now, researchers at the National Institute of Standards and Tech­nology (NIST) and the University of Maryland’s NanoCenter in College Park have developed a novel and compact method of sculpting light.

They first deposited a layer of ultra­thin silicon on glass, just a few hundred nano­meters thick, and then covered an array of millions of tiny squares of the silicon with a protective material. By etching away the silicon surrounding each square, the team created millions of tiny pillars, which played a key role in the light sculpting technique. The flat, ultrathin device is a meta­surface, which is used to change the properties of a light wave traveling through it. By carefully designing the shape, size, density and distri­bution of the nano­pillars, multiple properties of each light pulse can now be tailored simul­taneously and inde­pendently with nanoscale precision. These properties include the amplitude, phase and polari­zation of the wave.

“We figured out how to indepen­dently and simul­taneously manipulate the phase and amplitude of each frequency component of an ultrafast laser pulse,” said Amit Agrawal. “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.” When a light wave travels through a set of the silicon nano­pillars, the wave slows down compared with its speed in air and its phase is delayed – the moment when the wave reaches its next peak is slightly later than the time at which the wave would have reached its next peak in air. The size of the nano­pillars determines the amount by which the phase changes, whereas the orien­tation of the nano­pillars changes the light wave’s polari­zation. When a device known as a polarizer is attached to the back of the silicon, the change in polarization can be translated to a corres­ponding change in amplitude.

Altering the phase, amplitude or polarization of a light wave in a highly controlled manner can be used to encode information. The rapid, finely tuned changes can also be used to study and change the outcome of chemical or biological processes. For instance, altera­tions in an incoming light pulse could increase or decrease the product of a chemical reaction. In these ways, the nanopillar method promises to open new vistas in the study of ultrafast pheno­menon and high-speed communi­cation. “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,” said Lezec.

A typical ultrafast laser light pulse lasts for only a few femto­seconds, too short for any device to shape the light at one particular instant. Instead, Agrawal, Lezec and their colleagues devised a strategy to shape the indi­vidual frequency components or colors that make up the pulse by first separating the light into those components with an optical device called a diffrac­tion grating. When directed into the nanopillar-etched silicon surface, different frequency components struck different sets of nanopillars. Each set of nano­pillars was tailored to alter the phase, intensity or electric field orienta­tion of components in a particular way. A second diffraction grating then recombined all the components to create the newly shaped pulse.

The researchers designed their nanopillar system to work with ultrafast light pulses composed of a broad range of frequency components that span wave­lengths from 700 nanometers to 900 nanometers. By simul­taneously and inde­pendently altering the amplitude and phase of these frequency components, the scientists demon­strated that their method could compress, split and distort pulses in a controllable manner. Further refinements in the device will give scientists addi­tional control over the time evolution of light pulses and may enable researchers to shape in exquisite detail indi­vidual lines in a frequency comb, a precise tool for measuring the frequencies of light used in such devices as atomic clocks and for identi­fying planets around distant stars. (Source: NIST)

Reference: S. Divitt et al.: Ultrafast optical pulse shaping using dielectric metasurfaces, Science 364, eaav9632 (2019); DOI: 10.1126/science.aav9632

Link: Photonics and Plasmonics Group, National Institute of Standards and Technology NIST, Boulder, USA

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