Fe b 20 00 Ultrafast optical nonlinearity in quasi-one-dimensional Mott-insulator Sr 2 CuO

We report strong instantaneous photoinduced absorption (PA) in the quasione-dimensional Mott insulator Sr2CuO3 in the IR spectral region. The observed PA is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculations based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole-coupling between nearly degenerate oneand two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of Sr2CuO3 for all-optical switching. 78.47. +p,42.65. Re Typeset using REVTEX 1 Nonlinear optical materials with large nonlinear coefficient, fast response-time, low loss and operability at room-temperature will be indispensable for next generation high speed network systems in which clock recovery and buffering at terabit/second rate will be performed by all-optical switches [2]. These devices should operate in the transparent region below the fundamental band edge, where optical nonlinearity is mainly associated with twophoton absorption (TPA). Real carrier excitation, inevitable due to TPA, is considered as the major limiting factor here. Although terabit/second all-optical switches have been already demonstrated with conventional inorganic band semiconductors [3], considerable effort is necessary to overcome the difficulty associated with long carrier lifetime. Large optical nonlinearity and sub-picosecond response times are observed in organic π-conjugated polymers [4,5], but further improvements in sample quality and morphology would be required for actual applications. Parallel development of novel nonlinear optical inorganic materials that on the one hand possess the large nonlinearity and ultrafast recovery times of the organics, and the intrinsic robustness and superior thermal conductivity of inorganics on the other, is highly desirable. This is especially so, in view of the fact that interconnecting inorganic optoelectronic components and the existing inorganics-based electronics would be simpler. To date, no such inorganic semiconductor is known. In the present Letter we report the observation of large ultrafast optical nonlinearity in a novel strongly correlated inorganic semiconductor that is intrinsically different from conventional inorganic band semiconductors. The specific material we have studied is Sr2CuO3, the mechanism of optical nonlinearity in which is shown to be substantively different from that in conventional semiconductors. We show that the optical nonlinearity in Sr2CuO3 is a consequence of very large transition dipole coupling between the optical state and a nearly degenerate two-photon state, while the unusually fast ground state recovery ( ∼ 1 ps) is perhaps due to nonradiative relaxation through very wide (∼ 1 eV) and gap-less spin excitations band [6–8]. We demonstrate room-temperature photon-gate operation using double pulses at the optical fiber communication window. Taken together, these results introduce a novel opportunity for terabit/sec optoelectronics using quasi-1D cuprates. 2 Sr2CuO3 is a quasi-1D Cu-O linear chain compound, whose structure is shown in Fig. 1(a). The material is a prototypical strongly correlated charge-transfer insulator within the Zaanen-Sawatzky-Allen scheme [9]. Optical absorption in Sr2CuO3 involves charge-transfer excitation of a Cu-hole to an O-site, with an absorption band-edge at ∼ 1.6 eV and the band maximum at 2 eV (Fig. 1(b)). In addition to the high energy charge excitations, there exist low energy spin excitations in Sr2CuO3, which correspond to those of a nearly ideal 1D Heisenberg chain with intrachain exchange integral J ∼ 2000 − 3000 K [8,10]. The interchain coupling in this system is rather weak, as is evidenced by the occurrence of 3D long-range antiferromagnetic coupling only below TN = 5 K [11]. The large intrachain exchange coupling introduces the possibility of ultrafast nonradiative relaxation from the optical state involving the emission of multiple spinons, in analogy with the ultrafast relaxations via nonradiative channels involving multiple magnons in YBa2Cu3O6.2 and Nd2CuO4 [6,7]. The occurrence of the spin excitations within the optical gap is a direct consequence of the strongly correlated nature of copper oxides. Such intrinsic intra-gap states are absent in the conventional semiconductors, in which photogenerated carriers relax mainly by slow radiative channels. We present here the results of time-resolved femtosecond pump-probe measurements on a single crystal of Sr2CuO3, grown by the traveling-solvent floating-zone method [10]. A thin flake with a thickness of ∼ 50 μm was cleaved out with the bc-plane for transmission measurement. Optical pulses are provided by a system based on amplified mode-locked Ti:sapphire laser and optical parametric generator supplemented with sum and difference frequency generation. This system generates pulses with the center photon energy of 0.2-2.0 eV (λ = 0.6 − 6 μm) and temporal width ∼ 200 fs. In our experiment, we measure the differential transmission ∆T/T (where T is the transmission in the absence of the pump beam), as a function of the delay time of the probe with respect to the pump. In Fig. 1(c) we show the temporal evolutions of ∆T/T measured for different pump and probe photon energies measured at low-temperature (∼ 10 K). The intensity of the pump pulse is ∼ 2 GW/cm. The observed PA consists of two components. A sharp peak-like 3 structure, corresponding to the coherent TPA at the sum-frequency of the pump and the probe (see below), appears immediately after the excitation. The temporal width of this peak corresponds to the width of pump and probe pulses, ∼ 200 fs. The second component of the absorption change consists of an exponentially decaying tail. The decay time of this second component, 1− 3 ps, is attributed to the life-time of the charge-transfer exciton. We found that the PA magnitude is proportional to the pump pulse energy up to ∼ 5 GW/cm, indicating that the PA is associated with the third-order optical nonlinearity of the material. The peak structure in the differential transmission is strongly suppressed when the pump photon energy is 3.1 eV, which is far above the absorption edge of ∼ 1.6 eV. This is because the linear absorption of the pump pulse prevents the coherent interaction between pump and probe pulses. In contrast, the coherent signal is very much pronounced when the pump frequency is tuned below the absorption edge. Furthermore, the width of the peak structure, ∼ 1 ps, is obviously broader than that of the pump and the probe pulse when the pump photon energy was at 1.55 eV. This broadening is due to the strong group velocity dispersion near the absorption edge. We note that these features hardly change at room temperature. We define ∆α(ω) as the frequency–dependent pump–induced change in the absorption coefficient of the peak structure. In Fig. 2(a) we show the spectrum of the TPA coefficient β(ω) = ∆α(ω)/I for pump photon energies of 0.7 eV, 1.1 eV, and 1.55 eV (∼ 10 K). The sum of the pump energy and the maximum of the probe energy is nearly the same in all cases, confirming that the peak structure in Fig. 1(c) arises from the direct TPA to an excited state at ∼ 2.1 eV. In Fig. 2(b), we plot β(ω) against the sum of the pump and probe photon energies. The dashed line shows the linear absorption spectrum of Sr2CuO3 . The TPA band nearly overlaps the linear absorption spectrum. In order to clarify the origin of the observed TPA band in Fig. 2(b) we consider the Cu-O chain within the two-band extended Hubbard model [12],