The perfect interface between the free space and the integrated optical system
(Nanowerk News) Researchers at Columbia Engineering have developed a new class of integrated photonic devices–“leaky-wave metasurfaces”–that can transform light initially confined in an optical waveguide into arbitrary optical patterns in free space (Natural Nanotechnology, “A leaky wave metasurface for integrated photonics”).
This device was the first to demonstrate simultaneous control of all four optical degrees of freedom, namely, amplitude, phase, polarization ellipticity, and polarization orientation—a world record. Because these devices are ultra-thin, transparent, and compatible with photonic integrated circuits (PIC), they can be used to enhance optical displays, LIDAR (Light Detection and Range), optical communications, and quantum optics.
“We are very pleased to find an elegant solution to link free-space optics and integrated photonics – these two platforms have traditionally been studied by investigators from different optical subfields and have resulted in commercial products that meet completely different needs,” said Nanfang Yu, professor of physics. and applied mathematics which is a leader in nanophotonic device research. “Our work demonstrates new ways to create hybrid systems that make use of the best of both worlds—free-space optics for shaping light wavefronts and integrated photonics for optical data processing—to address many emerging applications such as quantum optics, optogenetics, sensor networks, communications. chip-to-chip, and holographic displays.”
Bridging free-space optics and integrated photonics
The main challenge in connecting PIC and free-space optics is to transform a simple waveguide confined within the waveguide – a thin defined ridge on the chip – into a broad free-space wave with a complex wavefront, and vice versa. Yu’s team tackled this challenge by expanding on their discovery last fall of “non-local metasurfaces” and extending the functionality of the device from controlling free-space light waves to controlling guided waves.
Specifically, they extended the input waveguide mode by using the waveguide taper to a slab waveguide mode – a sheet of light propagating along the chip. “We realized that the plate waveguide mode can be decomposed into two orthogonal standing waves—waves reminiscent of the waves generated by plucking a string,” said Heqing Huang, a PhD student in Yu’s lab and first co-author of the study, published today in Natural Nanotechnology. “Therefore, we designed a ‘leaky wave metasurface’ consisting of two sets of rectangular openings whose wavelengths offset each other to independently control these two standing waves. The result is that each standing wave is converted into a surface emission with independent amplitude and polarization; together, the two surface emission components combine to form one free space wave with fully controllable amplitude, phase, and polarization at any point above its wavefront.”
From quantum optics to optical communications to holographic 3D displays
Yu’s team experimentally demonstrated several leaky wave metasurfaces that can change the mode of a waveguide propagating along a waveguide with a cross section on the order of one wavelength to free space emission with a designer wavefront in an area of about 300 times the wavelength in the 1.55-wavelength telecommunication. micron. This includes:
Leaking wave metal that produces a focal point in empty space. Such a device would be ideal for forming low-capacity and high-capacity free-space optical links between PIC chips; it would also be useful for integrated optogenetic probes that generate a focused beam to optically stimulate neurons located far from the probe.
Leak wave optical lattice generators that can generate hundreds of focal points form a Kagome lattice pattern in free space. In general, a leaky wave metasurface can generate complex aperiodic and three-dimensional optical lattices to trap cold atoms and molecules. This capability will allow researchers to study exotic quantum optical phenomena or perform quantum simulations that were hitherto not easily achieved with other platforms, and enable them to substantially reduce the complexity, volume, and cost of quantum devices based on atomic arrays. For example, a leaky wave metasurface can be directly integrated into a vacuum to simplify optical systems, making portable quantum optical applications, such as atomic clocks, possible.
A leaky vortex beam generator that produces a beam with a corkscrew-shaped wavefront. This could lead to free-space optical links between buildings that rely on PICs to process light-carried information, while also using light waves with shaped wavefronts for high-capacity intercommunication.
A leak-wave hologram that can replace four distinct images simultaneously: two in the plane of the device (at two orthogonal polarization states) and another two at a distance in free space (also at two orthogonal polarization states). This function can be used to create lighter, more comfortable augmented reality glasses, and more realistic holographic 3D displays.
Fabrication of the device was performed in the cleanrooms of the Columbia Nano Initiative, and in the Advanced Science Research Center’s NanoFabrication Facility at the City University of New York Graduate Center.
The next step
Yu’s current demonstration is based on a simple polymer-silicon nitride material platform at near-infrared wavelengths. His team next plans to demonstrate a device based on a more robust silicon nitride platform, which is compatible with foundry fabrication protocols and tolerant of high optical power operation. They also plan to demonstrate designs for high throughput efficiency and operation at visible wavelengths, which are more suitable for applications such as quantum optics and holographic displays.