Photonic Crystal Surface
A photonic crystal surface is a planar, periodically patterned dielectric (or semiconductor) layer whose lattice period is on the order of the optical wavelength. The 2D periodicity creates a photonic band structure in the plane of the wafer. At certain high-symmetry points (especially the Γ point), light forms a large-area standing wave that can also diffract out of the surface, normal to the chip. That combination—in-plane feedback plus vertical radiation—is the basis of the photonic crystal surface-emitting laser (PCSEL) and of resonant photonic-crystal surfaces used for sensing.
A photonic crystal itself is a periodic refractive-index lattice. Making it a surface (a thin slab or a patterned layer just under the wafer face) is what allows wafer-scale surface emission and easy access to the evanescent field above the chip.
How it works in lasers (PCSEL):
A typical PCSEL stack is:
Cladding and electrodes for current injection
A quantum-well (or similar) gain layer
A 2D photonic-crystal layer: a square or triangular lattice of air holes (or buried dielectric posts) with period a≈λ/na
Emission through the top surface
In-plane Bloch waves couple at 180° and 90° so the whole aperture oscillates as one coherent mode. First-order diffraction at Γ sends that mode vertically. Because the resonator is lateral and large (hundreds of micrometers to millimeters), single-mode power can be far higher than a VCSEL of similar beam quality, with divergence often well under 1° and no external collimator.
Modulated photonic-crystal surfaces (M-PCSELs) shift hole positions periodically so the same cavity also steers or shapes the far field (doughnut, multi-spot, scanned beam) without external optics.
Fabrication usually means epitaxial growth, etching the lattice, then semiconductor regrowth to bury the holes—or a buried-dielectric lattice to keep the holes from filling during growth.
Compared with other diodes:
Edge emitters: high power, poor raw beam, need FAC/SAC optics
VCSELs: surface emission, easy arrays, limited single-mode power
PCSELs: surface emission and large-area single mode / high brightness
CW single-mode powers of tens of watts and brightness around 1 GW cm−2 sr−1 have been reported on large-aperture devices.
Photonic-crystal surfaces beyond lasers:
The same class of surface—a 1D or 2D resonant grating / photonic-crystal slab—supports guided-mode resonances or Bloch surface waves. Binding on the top face shifts the resonance wavelength; that is used for label-free biosensing and photonic-crystal enhanced imaging.
Technical knobs:
Lattice type and hole shape set polarization, beam profile, and coupling strength.
Lattice constant sets wavelength (GaAs ~940 nm, InP 1.3–1.55 µm, GaN visible/UV).
Aperture size trades power against higher-order modes; graded lattices and engineered Hermitian / non-Hermitian coupling help keep single-mode operation as the chip grows.
Surface emission is wafer-testable, like a VCSEL.
Applications:
Lidar and 3D sensing: high brightness, narrow divergence, optional on-chip beam steering.
Material processing and illumination: compact high-brightness sources that rival bulky lasers.
Communications: high-power single-mode surface emitters at telecom wavelengths.
Beam-engineered sources: radial/azimuthal polarization, structured beams.
Biosensing and microscopy: resonant photonic-crystal surfaces as label-free transducers.
Arrays: many emitters on one wafer for power scaling or multi-beam systems.
A photonic crystal surface is a wavelength-scale 2D lattice on a chip face that both confines light in the plane and radiates it from the surface—used as a large coherent laser cavity or as a resonant sensing interface.