An elegant solution for bridging free-space optics and integrated photonics

Columbia Engineering researchers have developed a new class of integrated photonic devices–„leaky-wave metasurfaces”–that can convert light initially confined in an optical waveguide into an arbitrary optical shape in free space. These devices are the first to demonstrate simultaneous control of four optical degrees of freedom, namely amplitude, phase, polarization ellipticity and polarization orientation – a world record. Because the devices are extremely 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 to interface free-space optics and integrated photonics – two platforms that have traditionally been studied by investigators from different subfields of optics and have led to commercial products that address completely different needs.„said Nanfang Yu, associate professor of applied physics and applied mathematics, who is leading research on nanophotonic devices.”Our work points to new ways to develop hybrid systems that use free-space optics for shaping the wavefront of light and integrated photonics for optical data processing to address many emerging applications such as quantum optics, optogenetics, etc. Sensor Networks, Inter-Chip Communications and Holographic Displays.”

Bridging Free-Space Optics and Integrated Photonics

A key challenge in interfacing PICs and pre-space optics is to convert a simple waveguide mode confined within a waveguide—a thin ridge defined on a chip—into a broad free-space waveguide with a complex wavefront, and vice versa. Yu’s team overcame this challenge by developing their invention of „nonlocal metasurfaces,” extending the functionality of the devices from controlling free-space light waves to controlling guided waves.

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Specifically, they expanded the input waveguide mode into a slab waveguide mode using a waveguide taper — a sheet of light propagating on the chip. „We realized that the slab waveguide mode can be decomposed into two orthogonal standing waves—reminiscent of waves created by plucking a string.” Heqing Huang, a PhD student in Yu’s lab and co-first author of the study, was published today Nature Nanotechnology. „Therefore, to control these two standing waves independently, we designed a 'leakage-wave metasurface’ composed of two rectangular holes with a sub-wavelength offset from each other. As a result each standing wave is converted into a surface emission with independent amplitude and polarization; together, the two surface emission components are At every point on the wavefront they converge into a free-space wave with completely controllable amplitude, phase, and polarization.”

From quantum optics to optical communications to holographic 3D displays

Yu’s group experimentally demonstrated several leaky-wave metasurfaces, which can convert a waveguide mode propagating in a waveguide into free-space emission with a cross section on the order of a wavelength, a designer wavefront with an area of ​​about 300 times the telecommunication wavelength. 1.55 micron wavelength. These include:

A leakage-wave metals that creates a focal spot in free space. Such a device would be ideal for creating a low-loss, high-capacity free-space optical link between PIC chips; It is also useful for an integrative optogenetic probe, which generates focused beams to optically stimulate neurons located far from the probe.

A leaky-wave optical-lattice generator, capable of generating hundreds of focal points, creates a Kagome lattice pattern in free space. In general, leaky-wave metasurfaces can form complex aperiodic and three-dimensional optical lattices to trap cold atoms and molecules. This capability will enable researchers to study exotic quantum optical phenomena or perform quantum simulations not yet easily achievable with other platforms, and will significantly reduce the complexity, size and cost of atomic-array-based quantum devices. For example, a leaky-wave metasurface can be integrated directly into a vacuum chamber to simplify the optical system, making possible small quantum optical applications such as atomic clocks.

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A leaky-wave spiral-beam generator produces a beam with a corkscrew-shaped wavefront. This leads to a free-space optical link between buildings that relies on PICs to process information carried by light, while using light waves with shaped waveguides for high-efficiency interconnection.

A leaky-wave hologram capable of displacing four different images simultaneously: two in the device plane (in two orthogonal polarization states) and another spaced apart (in two orthogonal polarization states). This functionality can be used to create lighter, more comfortable augmented reality glasses and more realistic holographic 3D displays.

Creating the device

Device fabrication was carried out in the Columbia Nano Initiative Cleanroom and in the Nanofabrication Facility at the Advanced Science Research Center at the Graduate Center of the City University of New York.

Next steps

Current demonstration of UV is based on a simple polymer-silicon nitride materials platform at near-infrared wavelengths. His team plans to demonstrate devices based on a highly robust silicon nitride platform that is compatible with foundry fabrication protocols and tolerant of high optical power operation. They also plan to demonstrate designs for high output efficiency and functionality at visible wavelengths, well suited for applications such as quantum optics and holographic displays.

Source: https://www.engineering.columbia.edu/

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