Metasurface
A patterned layer on a thin transparent carrier, working in transmission by default. Overlays the same lens-array, grating, steering, and speckle functions as the SLM, with an optional undiffracted zeroth order — but the phase profile is fixed at fabrication rather than programmable.
Open in the canvas →In the real world
A metasurface is a flat optic that works by pattern rather than by shape. Instead of bending light with a curved glass surface, it carries an array of meta-atoms — pillars, fins, or slots smaller than the wavelength — each imposing its own local phase delay on the light passing through. Choose the phase at every point across the aperture and you choose what the surface does.
Because the structures are subwavelength, they do not diffract individually; the surface behaves as a continuous phase profile Φ(x). What steers the light is the gradient of that profile, which generalises Snell's law: a phase that varies along the surface adds momentum to the transmitted beam.
What makes the idea powerful is that the profile is arbitrary. The same fabrication process yields a lens, a blazed grating, a beam splitter, a vortex plate carrying orbital angular momentum, a polarisation-selective element that does different things to each state, or a hologram — decided entirely by the pattern. And because the whole optic is a film a fraction of a micrometre thick on a carrier, it replaces components that would otherwise be centimetres of glass, which is why metasurfaces are pursued for phone cameras, endoscopes, AR displays, and satellite instruments.
The costs are real. Efficiency is finite, so some light leaves undiffracted in the zeroth order along with scatter and reflection. Most designs are strongly chromatic, since the phase is set for one wavelength and every other colour sees the wrong profile. Many are polarisation-sensitive by construction. And the pattern is fixed at fabrication — which is the sharp distinction from an SLM, whose liquid crystal lets the same aperture display a new profile thousands of times a second. A metasurface trades that programmability for being thin, passive, fast at the speed of light, and needing no drive electronics.
In OpticalSetup
The metasurface is drawn as what it is: a patterned layer on a thin transparent carrier. It is transmissive by default, since that is how these optics are almost always used, with a toggle for the reflective case.
It shares its phase engine with the SLM — deliberately, because a phase profile does the same thing to a ray whether liquid crystal or etched silicon put it there. The difference between the two elements is what they represent, not what the light does. So the same stack of optical functions applies, up to four, in order:
- Lens array — divides the aperture into lenslets, the multi-focus profile a metasurface array carries.
- Grating — a fixed blazed deflector with chosen line density and orders. Verified against the grating equation: 600 lines/mm at 532 nm sends the first order to 18.61°, exactly arcsin(λ/d).
- Beam steer — the constant phase gradient of the generalised Snell's law above, the simplest metasurface there is.
- Speckle / diffuser — a randomised profile.
Layers compose in sequence, so a steer of 5° followed by a 600 lines/mm grating puts the output at 23.98° — the two sines adding, as they should.
The undiffracted 0th order toggle models finite efficiency: turn it on and the chosen fraction leaves along the original path while the patterned light carries the rest, so a design can be drawn with its leakage and a beam dump put where the waste goes. It is off by default so a teaching figure stays clean.
For the specific case of a focusing metasurface with a wavelength-dependent focal length, use the metalens instead — it models the diffractive f(λ) = f₀λ₀/λ scaling that this element's geometric layers do not.
No phase map is computed and no field is propagated. The layers are geometric ray operations standing in for what a profile does, so there are no meta-atoms, no subwavelength geometry, no fill factor, and no diffraction calculation — an aperture that would be far too small to work in reality traces exactly like a large one. The steering here is also achromatic where a real metasurface is strongly chromatic: only the grating layer disperses, through the grating equation, while a steer or lens-array layer treats every wavelength alike. Polarisation is untouched, though polarisation sensitivity is a defining property of many real designs, and the zeroth-order fraction is a number you set rather than one derived from the structure. Efficiency, scatter, substrate reflections, and fabrication tolerance are all absent.