Photoinduced Electronic & Structural Dynamics

• Photoinduced electronic and structural dynamics are the ultrafast processes where photon absorption reshapes both electronic distributions and atomic positions on femtosecond to picosecond timescales.
• Advanced spectroscopies and diffraction techniques, coupled with ab initio simulations, are employed to resolve transient modifications in potential energy surfaces and nucleation kinetics.
• These dynamics underpin practical applications such as nonthermal phase transitions, coherent phonon generation, and the optical control of electronic, structural, and topological states.

Photoinduced electronic and structural dynamics refer to the coupled, nonequilibrium evolution of electronic populations and atomic lattice coordinates in molecules and solids following the absorption of photons. When a material absorbs ultrafast optical pulses, energy is first injected into the electronic degrees of freedom, often redistributing carriers nonthermally, which subsequently alters the interatomic potential energy surface (PES). This electronic reconfiguration can displace, modulate, or even remove energy barriers to structural change, driving atomic motion on femtosecond to picosecond timescales. These processes underpin photoinduced phase transitions, ultrafast symmetry breaking or restoration, nonthermal metallization, coherent phonon generation, and the emergence of metastable, non-equilibrium phases.

1. Fundamental Principles of Photoinduced Dynamics

In both condensed matter and molecular systems, photon absorption excites electrons from bonding to antibonding orbitals or between crystal-field levels, promoting a nonthermal carrier distribution with a characteristic lifetime $\tau_\mathrm{exc}$. This drives a modification of the electronic free energy:

$$F_\mathrm{el}(T_\mathrm{el}, \{u_i\}) = E_\mathrm{coh}(T_\mathrm{el}, \{u_i\}) - T_\mathrm{el} S_\mathrm{el}(T_\mathrm{el})$$

where $\{u_i\}$ are nuclear displacements (lattice or molecular coordinates), and $T_\mathrm{el}$ is the effective electronic temperature. Filling of antibonding states raises the cohesive energy, weakening the lattice and softening or erasing minima or barriers between competing crystal structures. For sufficiently strong excitation, the PES is reshaped such that the system can cross over to a new phase on ultrafast timescales—the hallmark of photoinduced structural transitions (Bennemann, 2010).

Photoinduced changes in the chemical potential, $\Delta\mu$, and cohesive energy, $\Delta E_\mathrm{coh}$, control nucleation barriers for phase transitions, as expressed in the rate formula:

$$\Gamma \propto \exp \left[-\frac{\Delta G}{kT}\right], \qquad \Delta G_{\rm photo} = \Delta G_0 + \alpha F$$

where $F$ is the absorbed fluence and $\alpha$ quantifies the influence of excited-state energetics on the nucleation landscape.

2. Spectroscopic and Energy-Resolved Probes

Ultrafast pump–probe spectroscopies resolve the time evolution of electronic and structural order parameters. All-optical measurements of transient reflectivity or transmission track sub-picosecond closure or filling of electronic gaps, carrier dynamics, and coherent oscillations of zone-center phonons. Femtosecond (fs) electron diffraction and time-resolved X-ray or XUV absorption (XAS, XANES) offer atomic-scale quantification of nuclear rearrangement and element/site-selective changes in valence configurations (Lemke et al., 2015, Liu et al., 2021). In MeV ultrafast electron diffraction, the separation of elastic (structural) and inelastic (electronic-state) scattering allows direct mapping of excited electronic populations and their coupling to specific nuclear trajectories (Wang et al., 22 Feb 2025). Advanced protocols combine these tools with ab initio calculations (TDDFT, Ehrenfest or Born–Oppenheimer MD) to reconstruct the time-dependent PES topology and reaction coordinates (Nicholson et al., 2018, Qi et al., 2021).

Element specificity is critical for deconvolution of concurrent processes. For example, transient XUV absorption at the M-edges of transition metals tracks both electron and polaron formation, while K-edge hard X-rays probe shifts in local coordination or oxidation state, with sub-100 fs temporal resolution (Liu et al., 2021).

3. Archetypes of Photoinduced Structural Response

3.1. Nonthermal Melting and Barrier Suppression

In covalent solids and molecular clusters, promoting a critical fraction $\xi$ of electrons from valence to conduction bands can obliterate the barrier separating ordered and disordered phases in the PES, enabling nonthermal melting or graphite/graphene formation from diamond on timescales $\lesssim 100$ fs (Bennemann, 2010).

3.2. Insulator–Metal and Symmetry-Breaking Transitions

Photoexcitation of correlated insulators containing Peierls or Mott order can drive insulator–metal transitions via distinct pathways. For example, in the spinel CuIr$_2$S$_4$, electronic gap collapse (photoinduced Mott transition) occurs at a threshold fluence $F_\mathrm{elec}\sim 0.6$ mJ/cm$^2$ within $\sim$500 fs, prior to and largely decoupled from the slower ($\gtrsim$0.6 ps) suppression of Peierls-type structural order, which is governed by first-order nucleation kinetics and a separate critical fluence $F_\mathrm{struct}\sim 3$ mJ/cm$^2$ (Naseska et al., 2021). This dynamical decoupling produces intermediate states where the electronic system is transiently metallic but the lattice retains broken symmetry.

In contrast, Ta$_2$NiSe$_5$—an excitonic insulator—displays a strong blocking mechanism: because absorption saturates at the $\Gamma$ point before the threshold for structural rearrangement is reached, the system cannot be optically driven into the high-T orthorhombic phase, highlighting self-protection of the ground state against nonthermal switching (Mor et al., 2018).

3.3. Coherent Phonons and Symmetry Engineering

Short pulses often impulsively excite coherent optical phonons that modulate the lattice along specific symmetry coordinates, e.g., octahedral rotation in Sr$_2$RhO$_4$ (Lee et al., 2019) and Peierls–shear double-well distortions in Td-MoTe$_2$/WTe$_2$ (Qi et al., 2021). The generation mechanism (displacive or impulsive) and phase response encode details of the underlying symmetry, electron–phonon coupling, and the type of structural transition (order–disorder vs. displacive).

Concurrent manipulation of coupled order parameters—exemplified by the simultaneous suppression of intralayer Peierls distortion and interlayer shear in Td-1T$'$ MoTe$_2$/WTe$_2$—enables ultrafast traversal of multidimensional PES landscapes and tuning of topological features (e.g., Weyl nodes, higher-order boundary states), supporting a universal picture of multidimensional symmetry engineering (Qi et al., 2021).

3.4. Strongly Correlated and Multifunctional Materials

Time-resolved diffraction and transport in quantum materials such as chalcogenide phase-change compounds (e.g., Ge$_2$Sb$_2$Te$_5$) and multiferroics (e.g., TbMnO$_3$) capture ultrafast suppression of local Peierls distortions and the key role of electronic–structural bottlenecks in the evolution of metastable states (Qi et al., 2021, Abreu et al., 2022). In chalcogenides, incoherent suppression of randomly oriented local distortions leads to homogeneous cubic geometries within 0.3 ps, while in multiferroics, the lattice strain induced by electronic excitation evolves on a tens-of-ps timescale—slower than pure electronic processes—establishing a synchronization of demagnetization and structural expansion via exchange modulation.

4. Energy Transfer, Kinetic Bottlenecks, and Phase Nucleation

Structural transitions are often limited not by thermodynamic energy absorption but by the kinetics of nucleation and growth of the new phase. In CuIr$_2$S$_4$, even when the absorbed energy exceeds the latent enthalpy difference between the low-T insulating and high-T metallic phase, the transformation is limited by the stochastic nucleation rate $I(T)$ and the reversion rate $\Gamma$:

$\frac{dn}{dt} = I(T) (1 - n) - \Gamma n$

where $n(t)$ is the metallic volume fraction. Delays of 50–100 ps result, and the full structural conversion is kinetically impeded on ultrafast timescales despite high local excitation (Naseska et al., 2021). Similarly, in Ta$_2$NiSe$_5$, optical absorption saturation precludes the accumulation of critical excitation required for the phase transition, demonstrating that carrier density and not fluence alone determines structural reconfigurability (Mor et al., 2018).

5. Case Studies Across Material Classes

Material System	Dominant Effect / Order Parameter	Key Timescale	Coupling Regime
CuIr$_2$S$_4$ (Peierls, 3D)	Electronic gap vs. structural Peierls	0.5 ps (elec.),	Electronic–lattice decoupling; nucleation bottleneck
		0.6 ps+ (struct.)
Ge$_2$Sb$_2$Te$_5$ (phase-change)	Local Peierls distortion	0.3 ps	Bond-length incoherence
Td-MoTe$_2$/WTe$_2$ (topol.)	Peierls + shear double-well traversal	0.3–0.7 ps	Multimode symmetry engineering
Sr$_2$IrO$_4$ (Mott)	Coherent acoustic phonon propagation	27–100 ps	Neutral exciton–phonon
ZrTe$_5$ (top. ins./DSM)	Te-Te bond elongation & SOC	3–6 ps (lattice),	SOC-controlled, low DOS
		160 ps (spin)
TbMnO$_3$ (multiferroic)	Tensile strain, demagnetization	20–30 ps	Spin–lattice exchange

These, among many other documented examples, illustrate the diversity of mechanisms, observables, and kinetics in photoinduced electronic and structural dynamics. Timescales range from sub-100 fs for pure electronic redistributions and PES barrier collapse to tens or hundreds of picoseconds for bottlenecked energy transfer or relaxation pathways involving collective spin or orbital degrees of freedom.

6. Toward Optically Controllable Functionality

The integration of photoinduced dynamics with macroscopic control strategies leverages strong coupling between carriers, lattice, spin, and orbital degrees of freedom to engineer ultrafast switching of metastable states, topological properties, or functional polarization. In rhombohedral bilayer MoS$_2$, photoinduced carrier injection triggers giant ultrafast photostrictive lattice expansion and polarization enhancement on the timescale of interlayer phonons, with bandgap renormalization, opening avenues for memory and optoelectronic devices controlled by light-induced strain (Yang et al., 30 May 2025). In MAPI perovskites, sub-ps internal vibrational coherence transfer stabilizes large polarons, vastly improving carrier lifetimes and device photostability (Duan et al., 2020).

Developments in ultrafast spectroscopy, time-resolved diffraction, and first-principles simulation continue to reveal how electronic excitation reshapes free-energy landscapes and enables unprecedented control of structural, electronic, and topological order on ultrashort timescales. These advances redefine the landscape of non-equilibrium phase transitions, symmetry switching, and energy conversion in complex quantum materials.