Ultrafast Long-Distance Electron-Hole Plasma Expansion in GaAs Mediated by Stimulated Emission and Reabsorption of Photons Tinkara Troha, 1 Filip Klimovič , 2 Tomáš Ostatnický , 2 Filip Kadlec , 1 Petr Kužel, 1 and Hynek Němec 1,* 1 Institute of Physics ASCR, Na Slovance 2, 182 00 Prague 8, Czech Republic 2 Charles University, Faculty of Mathematics and Physics, Ke Karlovu 3, 121 16 Prague 2, Czech Republic (Received 23 June 2022; revised 13 March 2023; accepted 3 May 2023; published 31 May 2023) Electron-hole plasma expansion with velocities exceeding c=50 and lasting over 10 ps at 300 K was evidenced by time-resolved terahertz spectroscopy. This regime, in which the carriers are driven over > 30 μm is governed by stimulated emission due to low-energy electron-hole pair recombination and reabsorption of the emitted photons outside the plasma volume. At low temperatures a speed of c=10 was observed in the regime where the excitation pulse spectrally overlaps with emitted photons, leading to strong coherent light-matter interaction and optical soliton propagation effects. DOI: 10.1103/PhysRevLett.130.226301 Propagation speed of electrons in a crystal is a crucial parameter that determines the bandwidth of many elec- tronic devices. Unlike in vacuum where the only limit is the speed of light c, electron velocity v g ¼ð1=ℏÞð∂E=∂kÞ in crystals is controlled by their electronic band structure. The slope of the dispersion EðkÞ is limited by the interaction of electron orbitals forming the conduction band and by its periodicity in the k space, implying that maximum electron transport velocity (for any kind of transport including the ballistic one) does not exceed a few times 10 6 m=s ≈ c=100 in known crystals. The spatiotemporal dynamics of photoexcited electron- hole plasma (EHP) in semiconductors have attracted attention for decades as they frequently reveal velocities exceeding the ambipolar diffusion [1]. Depending on the excitation conditions, several phenomena have been iden- tified, including Fermi pressure [2–4], thermodiffusive transport [5], screening of electron-phonon interaction [6], and stimulated emission recombination and reabsorption [7,8]. As anticipated, the observed expansion rates have not exceeded the above limit imposed by the band structure. Here, we demonstrate that stimulated emission and subsequent reabsorption of low-energy photons in degen- erate EHP can dramatically enhance plasma expansion rates, resulting in effective velocities exceeding c=50 at room temperature. Free electrons and holes thus reappear together in the originally unexcited parts deep in the GaAs wafer, which may be effectively viewed as an ambipolar charge transport. Similar interconnection between elec- tronic and photonic aspects proved recently useful in organic perovskite solar cells [9] but it may be also a factor for fast photoconductive semiconductor switches. Below 100 K, the EHP surface shifts at much higher velocity ∼c=10 , reflecting propagation of an optical soliton in the highly excited semiconductor. In our experiment, dense EHP was created by absorption of a short intense optical pulse with photon energy slightly above the band gap. The photon fluence was high enough to cause absorption saturation due to the filling of states (the absorption length l 0 is thus considerably longer than the linear absorption depth), and to activate also the two- photon absorption [10]. The photoexcited area (∅≥ 2 mm) was fundamentally larger than the photoexcited depth to reduce possible edge effects. The EHP thickness was then probed by a delayed THz pulse, taking advantage of the high reflectivity of metalliclike EHP surface for long-wavelength radiation. The employed “time-of-flight” method [10,11] uses counterpropagating optical pump and THz probe pulses separated by a controlled time delay t . The time required by the probe to pass from the back surface of a 0.7 mm thick (100)-GaAs, reflect on the EHP, and propagate to the back surface again is measured: the temporal shift ΔθðtÞ of the output terahertz pulse represents plasma expansion by a distance cΔθðtÞ=ð2η THz Þ (η THz ¼ 3.6 is the terahertz refractive index of GaAs [12]). Experiments were performed with two different femto- second systems: multipass amplifier Quantronix ODIN (800 nm, 1 kHz rep. rate, 55 fs pulse length) and regenerative amplifier Spitfire ACE (800 nm, 5 kHz, 40 fs). For more details on the methods, see Secs. S1 and S2 in [13]. The expansion dynamics of the EHP layer observed at room temperature [Fig. 1(a)] reveal an initial short delay (∼1 ps) independent of the excitation fluence, followed by a fast onset of EHP expansion, thus indicating that the EHP initial state is static, rather than having nonzero kinetic energy like in Refs. [29,30]. The expansion rate reaches its maximum shortly after the very beginning of the system evolution and then it monotonically slows down. The prominent effect is then the strong dependence of the initial velocity and the expansion distance on the excitation PHYSICAL REVIEW LETTERS 130, 226301 (2023) 0031-9007=23=130(22)=226301(6) 226301-1 © 2023 American Physical Society