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)
Link: Photonics and Plasmonics Group, National Institute of Standards and Technology NIST, Boulder, USA
Speak Your Mind