Ultrafast dynamics in solids probed by femtosecond time-resolved broadband electronic sum frequency generation

Abstract

Time-resolved sum-frequency generation is an established tool to investigate ultrafast vibrational dynamics with surface and interface specificity. We present an extension of the technique to the regime of electronic transitions based on a compressed white light continuum. It is used to probe the non-equilibrium dynamics of excitonic resonances in the non-centrosymmetric oxide ZnO, where we demonstrate that we can probe transient changes of the electronic sum frequency generation signal as small as 0.6%.Second-order non-linear optical spectroscopies base on the wave-mixing phenomena that can occur in a material due to the second-order non-linear polarizationacting as the source term for a third electric field E(ω s ) at the sum-or difference-frequency ω s = ±ω 1 ± ω 2 as given by energy and momentum conservation 1-3 . The amplitude of this field depends on the third-rank tensor χ (2) , which reflects the symmetry of the material and vanishes for systems with inversion symmetry. Additionally, P (2) is enhanced whenever at least one of the incoming or generated electric fields is resonant with an optical transition between electronic or vibrational energy levels in a material 3 . Thus, second order non-linear optical effects constitute a unique tool for the spectroscopy of electronic and vibrational states with symmetry specificity. In particular, for centrosymmetric materials and in the dipole approximation, second-order non-linear spectroscopies are intrinsically surface and interface specific, since these break the inversion symmetry 2 . Also, they are sensitive to phase transitions between centrosymmetric and non-centrosymmetric phases as well as between different non-centrosymmetric crystal structures 4 . The combination of non-linear optical spectroscopies with a third laser beam in a "pump-probe" scheme additionally allows to achieve time resolution in the femtosecond range and thereby access the ultrafast timescales of electronic and vibrational processes following photoexcitation in most materials. The sample is photoexcited by a first laser pulse, the "pump", which alters the optical and electronic properties. These pump-induced changes affect the polarization and can be monitored by the nonlinear probe as a function of the time delay after photoexcitation. In this way, second-order non-linear optical effects can be used to probe the dynamics of processes such as solvation 5 , molecular orientation and motion 6-9 , The challenge of non-linear optical spectroscopy is that second-order susceptibilities are 9 to 11 orders of magnitude smaller than linear susceptibilities and, when aiming at time-resolved measurements, the expected pumpinduced signal variation is usually 10% of this already small signal. In other words, for typical pulse energies of 1-5 µJ and at 40 kHz repetition rate, the change of the number of photons impinging on the detector is on the order of few tens of photons per second of exposure. In order to minimize the acquisition time and to optimize the signal to noise ratio and energy resol...

Revealing the competing contributions of charge carriers, excitons, and defects to the non-equilibrium optical properties of ZnO

“…1(d), where we compare a luminescence spectrum (light blue) of the single crystal sample with eSFG spectra measured for two polarization configurations as illustrated by the inset (red spectrum: S , orange spectrum: P ). In eSFG, 43 a white light continuum in the visible range [1.8–2.3 eV, cf. Fig.…”

“…The generated eSFG intensity was dispersed in a spectrometer (Andor Shamrock 303i) and detected in an electron multiplied charge coupled device (EMCCD). [43] As illustrated by the inset in Figure 1b, in both experiments, tr-EST and tr-eSFG, the white light was incident at an angle of 45° (purple). In tr-EST, the pump beam was almost collinear with the white light (40°), as was the upconverting beam (43-44°) of the eSFG.…”

“…1(b)]. The generated eSFG intensity was dispersed in a spectrometer (Andor Shamrock 303i) and detected in an electron multiplied charge coupled device (EMCCD) 43 Fig.…”

“…This band width further broadens our spectra, as shown in Figure 1d, where we compare a luminescence spectrum (light blue) of the single crystal sample to eSFG spectra measured for two polarization configurations as illustrated by the inset (red spectrum: s, orange spectrum: p). In eSFG, [43] a white light continuum in the visible range (1.8 -2.3 eV, cf. Figure 1c) is up-converted with 1.55 eV (800 nm) photons, covering an eSFG photon energy range from 3.35 to 3.85 eV.…”

Ultrafast dynamics of hot charge carriers in an oxide semiconductor probed by femtosecond spectroscopic ellipsometry

“…Beyond this, optical spectroscopy accesses a convolution of joint density of states, electron and hole populations, and transition matrix elements via the complex, frequency-dependent dielectric function (DF). For excited systems, conventional transient spectroscopy has been performed at different spectral ranges [19,20,21,22]; time-resolved sum-frequency generation can be used to probe the dynamics of electronic transitions after excitation [23]. In general, a challenge is not only to achieve high time-resolution but to discriminate different processes triggered by the excitation [15,24].…”