Polariton

In plain words

A wave that is part light and part motion in a material – electrons or atoms swinging back and forth – travelling as one. In thin crystals such waves can be squeezed to wavelengths many times shorter than the light that made them, so they can guide and focus infrared light into spaces far smaller than light normally allows.

Going deeper

Left: where light meets a vibration of the same frequency, the two mix into polaritons, and the lower branch carries the energy with a much shorter wavelength than light. Right: near-field microscopy launches polaritons from a sharp tip; they reflect from the flake’s edge and interfere, and the fringes, half a wavelength apart, give their wavelength and how far they travel.

Light that borrows a partner

Light passing through a material makes its charges move: electrons slosh in a metal, ions rock against each other in a polar crystal, are created and destroyed in a . Near the frequency of such a motion, light and the motion mix so strongly that neither exists on its own, and the wave that travels is part and part matter – a polariton. It moves much more slowly than light, so at the same frequency its wavelength is much shorter. That is how polaritons carry light below the diffraction limit.

Three kinds matter for . Plasmon polaritons ride on free carriers, in graphene and in metals. Phonon polaritons ride on optical lattice vibrations in polar crystals such as hBN and MoO₃, in the mid-infrared. Exciton polaritons form when a , with its strongly bound excitons, sits in an optical cavity.

Why van der Waals crystals make good hosts

Layered crystals respond very differently along and across their layers. Between its optical- frequencies, hBN responds like a metal in one direction and like an in the other, and light inside it can then travel only along cones – hyperbolic phonon polaritons, tightly confined, strongly directional and long-lived for a polariton. α-MoO₃ is within the plane as well, so its polaritons travel along some in-plane directions and not others, and two twisted sheets can steer them. In graphene the sets the plasma frequency, so a tunes the plasmons.

Thickness is a knob of its own. Imaging hBN of different thickness, Dai and colleagues showed that the polariton wavelength shrinks as the crystal is thinned down to a few layers – a direct handle on confinement that bulk materials do not offer.

Seeing them, and what they are for

Wavelengths this short cannot be reached with ordinary optics, so polaritons are launched and imaged with a sharp metal-coated tip in a scattering-type near-field microscope (s-SNOM). Infrared light focused on the tip launches a polariton; it travels to the flake’s edge, reflects and comes back, and the interference leaves fringes half a polariton wavelength apart. The fringe spacing gives the wavelength and the rate at which the fringes fade gives the loss – the two figures of merit.

Uses are at the research stage: mid-infrared of molecules through their vibrational fingerprints, nanoscale and lenses, and polaritons as probes – plasmon fringes in graphene map carrier density and stacking domains in twisted layers. Loss is the main limit; even the best phonon polaritons live for only picoseconds, which is why isotopically pure hBN, with fewer scattering centres, matters.

For specialists

A hybrid mode of photons with a polarisation excitation: free-carrier plasma oscillations (plasmon polaritons), optical phonons (phonon polaritons) or excitons (exciton polaritons). In crystals they confine light to wavelengths tens to hundreds of times below that in free space. hBN supports hyperbolic phonon polaritons in its two Reststrahlen bands, α-MoO₃ in-plane anisotropic ones and graphene gate-tunable plasmons; TMDC monolayers in optical cavities form exciton polaritons. They are imaged in real space by scattering-type near-field microscopy (s-SNOM), which launches them from a sharp tip and maps their interference fringes.

Where this comes from

Polaritons in van der Waals materials Basov, Fogler and García de Abajo · Science 354, aag1992 (2016) cited by 1,215
Tunable phonon polaritons in atomically thin van der Waals crystals of boron nitride Dai et al. · Science 343, 1125 (2014) cited by 1,354

Photon

The smallest possible amount of light – one indivisible packet of it. The packets of a given colour all carry the same energy, and blue ones carry more than red: that is why ultraviolet light gives you sunburn and red light, however bright, does not. A semiconductor absorbs only photons with enough energy to cross its band gap and gives light back at about the gap’s energy – which is why LEDs of different colours are made from different semiconductors.

The quantum of the electromagnetic field, with energy E = hν and momentum h/λ but no mass or charge. Visible photons carry about 1.7–3.1 eV, comparable to the band gaps of semiconducting TMDCs, but a momentum far smaller than the crystal momenta across the Brillouin zone, so optical transitions are vertical and an indirect gap needs a phonon to make up the difference. Single-photon emission is established by antibunching in photon-correlation measurements.

Full explanation

Metal, semimetal and insulator

The three basic ways a solid treats electricity, set by how its electrons fill the energies open to them. In a metal they fill a band of energies only partly, so the smallest push sets them moving, and it conducts at any temperature. In an insulator they fill a band completely and a wide gap separates it from the next, so nothing can move. A semimetal sits in between: two bands just touch or overlap a little, leaving a small number of mobile electrons and holes. NbSe₂ is a metal, graphene a semimetal, hBN an insulator.

A classification by where the Fermi level falls in the band structure. In a metal it crosses a partly filled band, leaving a Fermi surface, a carrier density of order one per atom (about 10²² cm⁻³) and a resistivity that falls on cooling. In a band insulator it lies in a gap of several electronvolts; a semiconductor is an insulator whose gap is small enough to be doped or gated. A semimetal has a slight overlap between conduction and valence bands (graphite, bismuth, bulk WTe₂) or bands that touch at isolated points (graphene), giving low and nearly equal densities of electrons and holes. Interactions can open gaps that band theory does not predict: Mott insulators from half-filled bands, excitonic insulators from semimetals.

Full explanation

Gating

Changing a material by putting a voltage on a nearby electrode – a gate – separated from it by an insulator. The voltage pulls electrons into the material or pushes them out, as in every transistor. Because a 2D material is so thin, the gate reaches all of it, so the number of electrons can be dialled up and down continuously: a single device can be turned from insulator to metal, its magnetism strengthened, or superconductivity switched on. Two gates, above and below, can also apply an electric field across the layer.

Electrostatic control of carrier density and electric field through a gate coupled capacitively across a dielectric: the induced density equals the gate capacitance times the voltage beyond threshold, divided by the electron charge, reaching a few 10¹³ cm⁻² with oxide or hBN gates before breakdown. Dual gating sets density and perpendicular displacement field independently, opening the gap of bilayer graphene and tuning moiré flat bands. Ionic-liquid and solid-electrolyte gating form an electric double layer about a nanometre thick and reach 10¹⁴–10¹⁵ cm⁻², enough to induce superconductivity in MoS₂ or raise the Curie temperature of Fe₃GeTe₂, but can intercalate or react electrochemically and work only while the ions are mobile. Hysteresis from traps, contact effects and quantum capacitance are the usual caveats.

Full explanation

Sensor

A device that turns something in its surroundings – a gas, a magnetic field, a pressure, a molecule in blood – into an electrical signal. 2D materials make unusually sensitive sensors because every atom is on the surface: a few molecules landing on a single layer change how well it conducts. Graphene also makes very good magnetic-field sensors. The hard part is not sensitivity but selectivity – responding to one thing and ignoring everything else – and staying stable for years.

Transducers in which a stimulus changes an electrical property of a 2D layer: chemiresistive and field-effect gas and humidity sensors, in which adsorbates transfer charge and shift the conductance (graphene registered single NO₂ molecules in 2007); Hall sensors, in which graphene’s low carrier density and high mobility give a large, temperature-stable Hall voltage; biosensors with receptors on a graphene or MoS₂ channel that bind target molecules; and strain, pressure and light sensors. Sensitivity comes from the all-surface geometry and the low density of states; the limits are selectivity, drift and hysteresis from humidity and contamination, recovery time, device-to-device variation and calibration, addressed by functionalisation, encapsulation and arrays read by pattern recognition.

Full explanation

Silicon photonics

Guiding light instead of electricity through tiny channels on a silicon chip, made with the same factories and tools as ordinary computer chips. Light carries data faster and with less loss over distance, so such chips already connect the servers in data centres. Silicon itself is poor at a few jobs – it hardly emits light, cannot detect the infrared light used in fibre networks, and switches light only weakly – and 2D materials laid on top of the light channels are one way to add the missing functions.

Photonic integrated circuits in silicon-on-insulator, in which submicron silicon waveguides, with their high index contrast to the buried oxide, route light at telecom wavelengths through splitters, rings and gratings made with CMOS tools; germanium detectors and silicon carrier-depletion modulators are standard, and lasers are bonded or grown from III–V semiconductors. 2D materials couple to the evanescent field of the waveguide: graphene gives broadband, fast modulators and photodetectors that can be integrated at wafer scale, thin MoTe₂ and black phosphorus emit and detect in the near infrared, and low-loss phase-change Sb₂S₃ and Sb₂Se₃ give non-volatile switches and tuning. The obstacles are wafer-scale transfer or growth, contacts, optical loss and reproducibility to foundry standards.

Full explanation