Abstract
Femtosecond-pulse-induced (Epump = 2.5 eV) picosecond infrared absorption is studied in the spectral region between 0.30 eV and 1.05 eV in LiNbO3:Mg. We find a non-instantaneous mid-infrared absorption peak in the time domain up to 1 ps and a broad-band, long-lived absorption (maximum at 0.85 eV, width ≈ 0.5 eV), for t > 1 ps. The modelling succeeds by considering small NbNb4+ electron polaron formation along the sequence: (i) two-photon injection of hot electron-hole pairs at Nb-O-octahedra, (ii) dissociation and electron cooling by electron-phonon-scattering, and (iii) electron self-localization by strong electron-phonon-coupling.
Highlights
Pulse-induced transient absorption (TA) is a widely accepted nonlinear optical phenomenon in lithium niobate, LiNbO3 (LN), prominently observed as green- or blue-induced infrared absorption (GRIIRA and BLIIRA) with a relaxation time of several seconds [1, 2]
The modelling succeeds by considering small Nb4N+b electron polaron formation along the sequence: (i) twophoton injection of hot electron-hole pairs at Nb-O-octahedra, (ii) dissociation and electron cooling by electron-phonon-scattering, and (iii) electron self-localization by strong electronphonon-coupling
Our analysis clarifies the picosecond transient absorption of femtosecond-pulse injected carriers during their relaxation to small free Nb4N+b electron polarons in the spectral range of 0.30 eV up to 1.05 eV. It extends the knowledge about the optical properties of small electron polarons to the ultrafast time regime that comprises small polaron formation
Summary
Pulse-induced transient absorption (TA) is a widely accepted nonlinear optical phenomenon in lithium niobate, LiNbO3 (LN), prominently observed as green- or blue-induced infrared absorption (GRIIRA and BLIIRA) with a relaxation time of several seconds [1, 2]. Due to a long-lived transient absorption, propagating laser pulses in lithium niobate foster to a great extent the appearance of laser-induced bulk- and surface damages, and limits the conversion efficiency mainly by damping [3, 4] It may cause phase-detuning via localized crystal heating and the thermo-optical effect [5]. Nearly nothing is known about the TA in the mid-IR (MIR) on the ultrafast time scale which is mandatory for the further improvement of high-power mid-infrared optical parametric oscillators (OPOs) This contribution focuses on the spectral detection of picosecond MIR-TA in LN:Mg related to small Nb4N+b free polaron (FP) formation in the range between 0.30 eV and 1.05 eV that is difficult to access from the experimental point of view. The obtained data set can be quantitatively modelled on the basis of theoretical expectations for the formation and absorption of small, free Nb4N+b polarons tested earlier using steady-state spectrophotometric techniques in thermally pre-treated Mg-doped LN and TA-measurements limited to near-IR frequencies [9, 15, 16]
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