Nanopillars shape light accurately for practical applications

Imagine being able to form a pulse of light in every imaginable way – by compressing it, stretching it, dividing it in two, changing its intensity, or changing the direction of its electric field.

Controlling the properties of high-speed light pulses is essential for sending information through high-speed optical circuits and for detecting atoms and molecules that vibrate thousands of billions of times per second. However, the standard method of pulse shaping, which uses devices called spatial light modulators, is expensive, bulky and lacks the precise control that scientists need more and more. In addition, these devices are generally based on liquid crystals that can be damaged by the same high intensity laser light pulses for which they were designed.

Researchers from the National Institute of Standards and Technology (NIST) and the NanoCenter at College Park of the University of Maryland have come up with a new, compact method of light sculpture. They first deposited a layer of ultra-thin silicon on glass, a thickness of a few hundred nanometers (one-billionth of a meter), and then covered with a network of millions of tiny squares silicon of a protective material. By eliminating the silicon that surrounds each square, the team created millions of tiny pillars, which played a key role in the light sculpture technique.

The flat and ultra-thin device is an example of a metasurface used to modify the properties of a light wave that passes through it. By carefully designing the shape, size, density and distribution of the nanopillars, it is now possible to customize several properties of each light pulse simultaneously and independently with a nanoscale accuracy. These properties include amplitude, phase, and polarization of the wave.

A light wave, a set of oscillating electric and magnetic fields oriented perpendicular to each other, has peaks and valleys similar to those of an oceanic wave. If you are in the ocean, the frequency of the wave is the frequency with which the peaks or troughs exceed you, the amplitude is the height of the waves (from the hollow to the top) and the phase corresponds to your position relative to the peaks. and hollows.

"We have discovered how to independently and simultaneously manipulate the phase and amplitude of each frequency component of a high-speed laser pulse," said Amit Agrawal, NIST and NanoCenter. "To achieve this, we used carefully designed sets of silicon nanopillars, one for each constituent color of the pulse, and an integrated polarizer made at the back of the device."

When a light wave passes through a series of silicon nanopillaries, it slows down relative to its speed in the air and its phase is delayed – the moment when the wave reaches its peak is slightly later than the moment it would have occurred. reached its next summit in the air. The size of the nanopillars determines the amount of phase change, while the orientation of the nanopillars changes the polarization of the light wave. When a device called polarizer is attached to the back of the silicon, the polarization change can be reflected by a corresponding change in amplitude.

Changing the phase, amplitude or polarization of a light wave in a very controlled manner can be used to encode information. Fast and accurate changes can also be used to study and modify the results of chemical or biological processes. For example, changes in an incoming light pulse could increase or decrease the product of a chemical reaction. In this way, the nanopillar method promises to open up new perspectives in the study of the ultra-fast phenomenon and high-speed communication.

Agrawal, with NIST's Henri Lezec and their collaborators, describe the results online today in the newspaper Science.

"We wanted to extend the impact of metasurfaces beyond their usual application – by altering the shape of an optical wavefront – and instead using them to alter the way the impulse bright varies over time, "said Lezec.

A typical ultrafast laser light pulse lasts only a few femtoseconds, one thousandth of a trillion seconds, too short to allow a device to shape the light at a given moment. Instead, Agrawal, Lezec, and their colleagues devised a strategy for shaping individual frequency components or the colors that make up the pulse by first separating light into these components with an optical device called a grating. of diffraction.

Each color has a different intensity or amplitude – similar to how a musical harmonic is composed of many individual notes with different volumes. When they are directed to the silicon surface etched by nanopillets, different frequency components strike different sets of nanopillars. Each set of nanopillars has been designed to change the phase, intensity or orientation of the electric field (polarization) of the components in a particular way. A second diffraction grating then recombined all the components to create the newly formed impulse.

The researchers designed their nanopillar system to work with ultra-fast pulses (10 femtoseconds or less, equivalent to one hundredth of a trillion seconds) composed of a wide range of frequency components spanning wavelengths. from 700 nanometers (visible red light) to 900 nanometers (near infrared). By simultaneously and independently modifying the amplitude and phase of these frequency components, scientists have demonstrated that their method can compress, divide, and distort pulses in a controllable manner.

The enhancements to the device will give scientists additional control over the evolution of light pulses over time and may enable researchers to trace individual lines in a frequency comb, a precise tool for measuring the frequencies of the light. light used in devices such as to identify planets around distant stars.