Optical Vortices Revolutionize Optics, Extending Understanding of Light-Matter Interaction across Scales

Light, the cornerstone of our perception, now serves as a remarkably versatile tool for scientific advancement, and researchers are continually exploring its fundamental properties. Bikash K. Das and Marcelo F. Ciappina, from the Guangdong Technion-Israel Institute of Technology and Technion-Israel Institute of Technology, investigate optical vortices, beams of light characterised by twisted phase fronts and a central intensity null, which represent a new dimension in our understanding of light’s behaviour. These unique light fields carry angular momentum, fundamentally altering how light interacts with matter and opening up possibilities across diverse fields. This work revisits the core concepts of optical vortices, detailing methods for their generation and detection, and importantly, demonstrates how exploiting these beams in nonlinear optical regimes enhances phenomena such as harmonic generation, potentially leading to breakthroughs in data transmission, microscopy, and particle manipulation.

Controlling High-Harmonic Generation with Laser Pulses

Scientists are actively investigating high-harmonic generation (HHG), a process where intense laser pulses create extreme ultraviolet and soft X-ray radiation. Current work explores manipulating HHG by precisely controlling the characteristics of the driving laser, such as its polarization and waveform, and by carefully selecting the gas used in the process. A major trend involves shifting from gas-phase HHG to solid-state HHG, as solids offer the potential for more efficient and compact sources of extreme ultraviolet light. Studies demonstrate that HHG in solids occurs through different mechanisms than in gases, including transitions between electronic bands and the acceleration of electrons within those bands.

Investigations into various solid-state materials, including crystals, semiconductors, and two-dimensional materials like graphene, reveal how material properties influence HHG efficiency. Scientists are developing techniques to control HHG in solids by manipulating laser polarization, crystal orientation, and laser pulse shape. This control is crucial for maximizing the efficiency of HHG and tailoring the properties of the generated radiation. HHG is a key enabler of attosecond science, which studies the dynamics of electrons on incredibly short timescales. Ongoing research explores how many-body effects, or interactions between electrons, influence HHG in solids, and how HHG can be used to modify the properties of materials.

Generating and Manipulating Optical Vortex Beams

Scientists are extensively studying optical vortices, beams of light characterized by a twisted phase and a central intensity null, and their potential applications across diverse fields. Researchers have developed several techniques to generate these beams, employing both conventional optical components and modern digital devices. Diffraction holography, where computer-generated holograms create the desired beam profile, is a prominent method. Beyond holography, researchers explore mode conversion techniques, demonstrating that a pair of cylindrical lenses can transform higher-order Gaussian modes into Laguerre-Gaussian modes carrying orbital angular momentum.

This conversion relies on the Gouy phase shift and astigmatism, leveraging the relationship between different Gaussian modes. Furthermore, scientists utilize spatial light modulators and metasurfaces to dynamically control the phase and amplitude of the incident beam, creating twisted beams with tailored properties. To verify successful generation, scientists meticulously analyze the beam’s spatial profile using imaging techniques, observing the characteristic ring-shaped intensity distribution and confirming the presence of a phase singularity at the beam’s center. By measuring the topological charge, researchers validate the orbital angular momentum carried by the generated vortex beams.

Optical Vortices Sorted by Angular Momentum

Scientists have achieved precise control and measurement of light’s orbital angular momentum (OAM), a property linked to the twisted phase of light beams known as optical vortices. Researchers developed methods to not only generate these vortex beams but also to accurately determine their topological charge, a key parameter defining the beam’s twist. One technique involves transforming the spatial coordinates of light using optical elements, effectively sorting different OAM modes by focusing them at distinct positions on a detector. Further refinement involves utilizing cylindrical and convex lenses to detect the topological charge of vortex beams. By reversing the process used to generate these beams, scientists convert an input vortex beam into a Hermite-Gaussian mode, allowing the topological charge to be determined by counting the intensity minima within the resulting pattern. Measurements also reveal a rotational Doppler effect, where the frequency of light shifts due to the interaction with rotating objects.

Vortex Beams and Enhanced Light-Matter Control

This review comprehensively examines optical vortices, demonstrating how their orbital angular momentum (OAM) provides an additional means of controlling light-matter interactions in both linear and nonlinear optical regimes. The work revisits established classes of vortex beams, including Laguerre-Gaussian, Bessel-Gauss, Perfect Optical Vortex, and Lorentz-Gauss beams, and details their generation, detection, and propagation characteristics. Particular attention is given to nonlinear optical processes, where vortex beams have yielded new understanding of light-matter coupling at high intensities. In perturbative regimes, these beams have been used to investigate harmonic generation, four-wave mixing, and self-focusing, with the conservation and transfer of OAM playing a crucial role in determining the characteristics of generated fields. Recent studies have also explored high-order harmonic generation (HHG) driven by vortex beams in both gaseous and solid-state media, revealing coherent transfer of OAM to emitted harmonics. The authors acknowledge that further research is needed to fully explore the interplay between beam topology and crystal symmetry in condensed-matter systems, potentially leading to new approaches for controlling angular-momentum transport and topological states.