Researchers at Columbia Engineering have developed a new class of integrated optical devices — “leaky wave surfaces” — that can convert light initially confined in an optical waveguide into a random free-space optical pattern. These devices are the first to demonstrate simultaneous control of all four optical degrees of freedom, namely amplitude, phase, elliptical polarization, and polarization orientation. – world record. Because the devices are so thin, transparent, and compatible with photonic integrated circuits (PICs), they can be used to improve optical displays, LIDAR (light detection and ranging), optical communications, and quantum optics.
“We are excited to find an elegant solution for the interaction between free-space optics and integrated photonics—these two platforms have been traditionally studied by investigators from various subfields of optics and have resulted in commercial products that meet very different needs,” said Nanfang Yu, an associate professor of applied physics and applied mathematics who is a lead In research on nanodevices. “Our work points to new avenues for creating hybrid systems that use the best of both worlds — free-space optics for wavefront shaping of light and compact photonics for photonic data processing — to address many emerging applications such as quantum optics, optogenetics, sensor networks, inter-chip communications, and holographic displays.”
Interconnection of free space optics and integrated photonics
The main challenge of interconnecting PICs with free-space optics is to convert a simple waveguide mode confined within a waveguide—the athenian ridge defined on a chip—into a wide free-space waveguide with a complex wavefront, and vice versa. Yu’s team tackled this challenge by building on their invention last fall of “non-local metasurfaces” and extending the functionality of the devices from controlling light waves in free space to controlling directed waves.
Specifically, they expanded the mode of the input waveguide by using the tapering of the waveguide in the slab waveguide mode—a sheet of light propagating along the length of the slab. “We realized that the slab waveguide mode can be decomposed into two perpendicular standing waves—waves reminiscent of those generated by plucking a thread,” said Heqing Huang, a doctoral student in Yu’s lab and co-first author of the study published today. in Nature’s Nanotechnology. Therefore, we designed a ‘leaky wavefront’ consisting of two sets of rectangular apertures that have a sub-wavelength off each other to independently control these two standing waves. The result is that each standing wave is converted into a surface emission with amplitude and polarization; the emission component merges. surfaces together into a single free-space wave with fully controllable amplitude, phase, and polarization at every point on the wavefront.”
From quantum optics to optical communications to 3D holographic screens
Yu’s team has experimentally demonstrated several leaky wave metasurfaces that can convert a waveguide mode propagating along a waveguide with a cross-section of the order of one wavelength into free-space emission with a wavefront modeled over an area of about 300 wavelengths in the telecom. The wavelength is 1.55 microns. These include:
Leaky wave metals produce a focal spot in free space. Such a device would be ideal for forming a low-loss, high-capacity free-space optical link between PIC slices; It would also be useful for an integrated optogenetic probe that produces focused beams to optically stimulate neurons located far from the probe.
An Aleaky-wave optical grid generator that can produce hundreds of focal points that form a Kagome grid pattern in free space. In general, the subtle surface of the leaky wave can produce complex, aperiodic, three-dimensional optical lattices to trap cold atoms and molecules. This ability will enable researchers to study exotic quantum optical phenomena or perform quantum simulations that have not yet been easily achieved with other platforms, and enable them to significantly reduce the complexity, size, and cost of atomic array-based quantum devices. For example, the leakage wave metasurface can be directly integrated into the vacuum chamber to simplify the optical system, making portable quantum optics applications, such as atomic clocks, possible.
A leaky wave vortex beam generator produces a beam with a key-shaped wavefront. This could lead to a blank optical link between buildings that relies on PICs to process information carried by light, with light waves with modulated wavefronts used for high-capacity communication.
A leaky-wave hologram can simultaneously displace four distinct images: two at the device plane (in two orthogonally polarized states) and two at a distance in free space (also in two orthogonally polarized states). This function can be used to create lighter, more comfortable augmented reality glasses and more realistic 3D 3D screens.
Device fabrication was performed in the Columbia Nano Initiative Cleanroom, and in the NanoFabrication Advanced Science Research Center at the City University of New York Graduate Center.
Yu’s current demonstration is based on a simple platform of polymer and silicon nitride materials at near-infrared wavelengths. His team next plans to demonstrate devices based on the more robust silicon nitride platform, which are compatible with foundry fabrication protocols and tolerate high optical power operation. They also plan to demonstrate designs for highly efficient production and operation at visible wavelengths, which are better suited for applications such as quantum optics and holographic rendering.
The study was supported by the National Science Foundation (Grant No. QII-TAQS-1936359 (HH, YX, NY) and ECCS-2004685 (SCM, C.-CT, NY)), and the Air Force Office of Scientific Research (No. FA9550-16). -1-0322 (New York)) and the Simons Corporation (ACO and AA). SCM acknowledges support from the NSF Graduate Research Fellowship Program (grant number DGE-1644869).