Pump–probe spectroscopy

Also called ultrafast spectroscopy, time-resolved spectroscopy

In plain words

A way to film events that last a millionth of a millionth of a second. A first, strong flash of laser light – the pump – disturbs the sample; a second, weaker flash – the probe – arrives a precisely set moment later and measures how the sample looks then. Repeating this with the delay changed step by step builds up a slow-motion film of how the electrons and atoms settle back. The delay is set by sending one flash along a longer path: a tenth of a millimetre more makes it a third of a picosecond later.

Going deeper

A strong pump pulse disturbs the sample and a weak probe pulse, sent along a longer path, looks at it a set time later. Stepping the delay and repeating builds a film of the response: the electrons settle within femtoseconds, the atoms vibrate and relax over picoseconds.

Light as a stopwatch

No electronic detector is fast enough to follow electrons that relax in less than a picosecond, so pump–probe experiments use light itself as the clock. One laser pulse is split in two. The strong part excites the sample; the weak part is sent along a path whose length a motorised mirror changes, and arrives later by the extra distance divided by the speed of light – 0.3 micrometres per femtosecond. Each measurement records the change in the probe at one delay, and many repetitions at many delays add up to a time trace, much as a strobe light turns a fast motion into a series of frozen frames.

The pulses set the resolution: a few tens of femtoseconds is routine, a few femtoseconds possible. The method assumes the sample returns to the same state between one pulse and the next; when the pump leaves something lasting behind, the experiment has to be adapted.

What it has shown in 2D materials

In of MoS₂ and WS₂ a hole crosses from one layer to the other within about 50 femtoseconds, faster than many processes inside either layer – the first step towards an . In crystals such as TbTe₃ and -TaS₂, time-resolved watches the gap of the ordered state collapse within a fraction of a picosecond after the pump, faster than the lattice can follow, which separates the electronic part of the order from the part carried by the atoms.

In Ta₂NiSe₅, a candidate , the gap narrows or widens depending on how hard the crystal is pumped, which is used to argue about what holds the gap open. Terahertz probes add the conductivity of the excited carriers, and diffraction probes add the motion of the atoms themselves.

Switching, not just watching

A strong enough pump can push a material into a state it would not reach in equilibrium. In 1T-TaS₂ a single 35-femtosecond pulse switches the crystal into a metallic ‘hidden’ state that persists at low temperature until it is erased, for example by warming – the basis of the ultrafast memory devices proposed for it.

Such results need care. The pump also heats the sample, and the average heating from thousands to millions of pulses a second can mimic or hide an effect, so careful experiments vary the repetition rate and the pulse energy, and check that the sample has fully recovered before the next pulse arrives.

For specialists

A stroboscopic measurement in which an ultrashort pump pulse excites the sample and a delayed probe pulse records the change, as a function of a delay set by an optical delay line, with a time resolution limited by the pulse durations – tens of femtoseconds routinely, a few with dedicated sources. The probe can be optical reflectivity or transmission, terahertz conductivity, photoemission (time-resolved ARPES), or X-ray or electron diffraction, giving access to carrier thermalisation and cooling, formation, , coherent , and the melting, recovery or switching of ordered phases such as charge density waves and excitonic insulators. Its power lies in separating processes by timescale: electrons respond within femtoseconds, the lattice within picoseconds.

Where this comes from

Transient electronic structure and melting of a charge density wave in TbTe₃ Schmitt et al. · Science 321, 1649 (2008) cited by 505
Ultrafast charge transfer in atomically thin MoS₂/WS₂ heterostructures Hong et al. · Nature Nanotechnology 9, 682 (2014) cited by 2,318
Ultrafast switching to a stable hidden quantum state in an electronic crystal Stojchevska et al. · Science 344, 177 (2014) cited by 725
Ultrafast electronic band gap control in an excitonic insulator Mor et al. · Physical Review Letters 119, 086401 (2017) cited by 187

Excitonic insulator

A state in which electrons and the holes they leave behind bind into pairs so strongly that the material opens a gap and stops conducting – not because of its chemistry, but because the pairs have all settled together into one shared, collective state. Telling this apart from an ordinary distortion of the crystal is notoriously difficult.

A ground state predicted for a semimetal with small band overlap, or a semiconductor whose gap is smaller than the exciton binding energy, in which spontaneous exciton condensation reconstructs the bands and opens a gap. The experimental difficulty is that the transition is usually accompanied by a lattice distortion of the same symmetry, so photoemission, ultrafast and pressure experiments have to separate an electronic condensate from a conventional Peierls-like instability – the argument that surrounds Ta₂NiSe₅ and 1T-TiSe₂.

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