On August 21, NTT announced that, for the first time in the world, it simultaneously controlled the polarization and wavefront shape through a process called "high-harmonic generation" that uses an intense laser light to convert the wavelength of light, which had previously been difficult to control. The control was facilitated by taking advantage of the regularity of the atomic arrangement called "symmetry" in solid crystals, where wavelength conversion occurs. Additionally, they discovered a law for light conversion that serves as the basis for controlling the polarization and wavefront shape of the generated light. NTT expects these results to lead to new applications in various fields of optical technology in the future, including spectroscopy, laser processing, optical tweezers, and optical communications. A part of this research was supported by a grant-in-aid for scientific research from the Japan Society for the Promotion of Science (JSPS).
The control of the key parameters of laser light, such as color (wavelength and frequency), intensity, phase, polarization, and wavefront shape, enables an extremely wide range of optical applications in areas such as optical communications, science, industry, and medicine. In recent years, research on the optical response of materials irradiated with intense laser light has been conducted, leading to the development of laser processing and wavelength conversion technologies. Wavelength conversion is an important technology for producing laser light with the desired wavelength.
At the forefront of this research is the wavelength conversion process known as "high-harmonic generation," which is also the principle behind the generation of attosecond pulses that won the Nobel Prize in 2023. NTT Basic Research Laboratories has been researching high-harmonic generation for many years and has been exploring new optical technologies that can be applied to future precision optical measurements and high-speed optical devices by controlling the generated harmonics.
As a result, they achieved control of optical parameters such as the frequency, intensity, and phase of light by shortening the pulse length and wavelength and increasing the output power of high harmonics. The company aims to control all parameters of high harmonics by controlling the important optical parameters such as the polarization and wavefront shape. For the generation of high harmonics, there was no unified understanding regarding the conversions of these parameters in laser light, and control remained a challenge.
In this research, NTT focused on light states known as circular polarization and optical vortex as means to characterize the polarization and wavefront shape of laser light, respectively. By taking advantage of the symmetry of solid crystals and creating optical vortices using circularly polarized light, they succeeded in simultaneously controlling the polarization and wavefront shape of laser light converted through high-harmonic generation. Furthermore, the conversion rules are based on a generic law that reflects and determines the symmetry of solids.
In the experiment, high harmonics were generated in a solid crystal, and observations demonstrated that it was possible to control the circular polarization of various converted wavelengths of light and state of the optical vortex. A circularly polarized Gaussian driving beam of intense infrared femtosecond laser light with a wavelength of 2,500 nm was generated and focused onto a 2-mm-thick uniaxial gallium selenide (GaSe) crystal using a lens with a focal length of 6 mm to generate high harmonics.
The spatial shape of harmonic beams was confirmed by decomposing the red, orange, and blue lights into their polarization components, which correspond to frequencies many times higher than the frequency of the focused laser light, and then photographing the generated light with a camera. As a result, the harmonics of light with various wavelengths, such as those of red, orange, and blue lights, were obtained, and the spatial shape of the beam was observed to depend on the wavelength and polarization components.
Comparing the cases with and without light focusing, the spatial shape of the observed harmonics was significantly different because of the difference in the spatial distribution of the polarization of the infrared light interacting with the solid crystal. When not focused, harmonics were generated according to well-known laws for order and polarization, where only the cross-sectional shape of a normal beam was observed. Meanwhile, on focusing the beam, cross-sectional beam shapes, such as doughnut-like and windmill-like beam shapes, were observed.
The doughnut-like shape (3rd order) indicates a single optical vortex state, while the windmill shape (4th order) indicates multiple different optical vortices occurring simultaneously. These observations reveal that the selective circular polarization and optical vortex state are simultaneously controlled according to a single conversion law derived in this study.
The proposed law is a general law that can be used to determine the types of the polarization and wavefront shape characteristics of light generated during the conversion of the wavelength of laser light using solid-state crystals. It is an important discovery for the development of fundamental optical technology.
Journal Information
Publication: Science Advances
Title: High-harmonic spin-orbit angular momentum generation in crystalline solids preserving multiscale dynamical symmetry
DOI: 10.1126/sciadv.ado7315
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.