What Is Photonics?

Photonics is the science and technology of generating, detecting, guiding, and controlling photons to transmit information, perform sensing, or deliver energy. Where electronics uses electrons as its information carrier, photonics uses photons, particles of light that travel at the speed of light in vacuum and interact very weakly with matter compared to electrons. This makes photonic systems capable of transmitting data at extraordinary bandwidths and over long distances with low loss. The discipline encompasses lasers, optical fibers, waveguides, detectors, modulators, and the integrated circuits that combine these components on a chip.

The field took its modern form after the demonstration of the laser in 1960 and the development of low-loss silica optical fiber in the 1970s. These two inventions made long-haul fiber-optic communication possible and seeded a technology ecosystem that now underlies global internet infrastructure. Subsequent decades added semiconductor lasers, erbium-doped fiber amplifiers, dense wavelength-division multiplexing, and silicon photonics, each extending the capacity and reducing the cost of photonic systems.

Silicon Photonics and Photonic Integrated Circuits

Silicon photonics uses standard CMOS fabrication processes to build photonic components, including waveguides, modulators, and photodetectors, on silicon substrates. Because silicon has a high refractive index and low absorption in the near-infrared wavelengths used for telecommunications, it confines and guides light efficiently. Photonic integrated circuits (PICs) combine multiple optical functions on a single die, analogous to electronic integrated circuits but operating with light. High-volume foundry production of PICs is enabling transceiver modules for data center interconnects at costs that were previously unattainable. Research on silicon photonics scaling and co-integration with electronics is published in IEEE Journal of Lightwave Technology, the field's primary archival journal.

Nanophotonics and Photonic Crystals

Nanophotonics studies the interaction of light with structures whose dimensions are comparable to or smaller than the wavelength of light. Photonic crystals are periodic dielectric structures that create photonic bandgaps, ranges of frequency in which light cannot propagate, analogous to electronic bandgaps in semiconductors. They enable highly selective optical filters, low-threshold lasers, and slow-light waveguides. Plasmonics, a closely related subfield, uses metal nanostructures to confine electromagnetic fields to volumes far below the diffraction limit through coupling to collective electron oscillations called surface plasmons. Plasmonic sensors can detect single molecules by their effect on the resonance condition. A survey of nanophotonic and plasmonic systems appears in Nature Photonics reviews on nanoscale light-matter interaction.

Biophotonics

Biophotonics applies optical techniques to biological and medical problems. Optical coherence tomography (OCT) provides depth-resolved cross-sectional imaging of tissue at micrometer resolution, making it the standard of care for retinal imaging and coronary artery assessment. Two-photon microscopy uses pulsed near-infrared lasers to image deep into scattering tissue with subcellular resolution. Photodynamic therapy uses photosensitizer molecules activated by specific wavelengths to generate reactive oxygen species that destroy tumor cells. NCBI PubMed Central research on biophotonic techniques reviews clinical translation of optical imaging and therapy methods.

Microwave Photonics

Microwave photonics uses photonic components to generate, process, and distribute microwave and millimeter-wave signals. Optical fiber carries RF signals from a central hub to distributed antenna units with lower loss and greater immunity to electromagnetic interference than coaxial cable. Photonic analog-to-digital conversion, optical beamforming for phased-array antennas, and photonic frequency synthesis are active research directions motivated by 5G and radar system requirements.

Photochromism

Photochromism is the reversible change in a material's optical properties, typically color or absorption spectrum, upon exposure to light. Photochromic molecules transition between two forms with different electronic structures when illuminated. Applications include self-darkening eyeglass lenses, optical data storage, and molecular switches for nanoscale devices.

Applications

• Long-haul fiber-optic communication carrying internet traffic across continents and ocean floors
• Data center interconnects using silicon photonic transceivers for high-bandwidth rack-to-rack links
• Lidar systems for autonomous vehicle ranging and 3D environment mapping
• Optical coherence tomography for noninvasive retinal and cardiac imaging in clinical settings
• Quantum key distribution using single-photon sources and detectors for secure communication
• Optical sensors for strain, temperature, and chemical detection in structural health monitoring