Attosecond electron localization in metallic titanium

In the 1980s, rapid progress in picosecond and femtosecond ultrafast lasers has enabled to start to bridge the gap between electronics and optics with the optical generation of terahertz frequencies for the investigation of ever faster physical processes and device performance. More recently with full electric field control within few-cycle pulses [1] we have continued to fully bridge this gap approaching the petahertz regime. A number of pioneering publications demonstrated that attosecond carrier transport can be resolved with attosecond transient absorption spectroscopy (ATAS) [2-7] and an attosecond interferometry technique at solid surfaces [8,9]. 

The process of photo-induced electronic screening modification is a universal phenomenon in solids, the degree of which depends on the strength of electron interaction and the excitation strength. It is for example the first stage of an ultrafast electronic phase transition in VO₂ [10] and allows to manipulate the electronic properties of solids potentially on a sub-cycle timescale. Transition metals are the key elements in such electronic phase transitions due to their localized d- and f- valence electrons, thus a notable screening modification may happen upon their excitation. However, attosecond time resolution is required to investigate the highly non-equilibrium electronic state during the laser-matter interaction prior to electron thermalization.

Here, we apply attosecond transient absorption spectroscopy to Ti. First, we verify the observation of well-understood phenomena related to the static absorption spectra, acoustic wave excitation and electron thermalization dynamics in metal foils. Then, in order to probe the screening effects, we use a single attosecond pulse (SAP) tuned close to the collective absorption resonance of titanium around its inner-shell transition corresponding to the M_2,3 edge. In contrast to previous studies of sub-cycle oscillatory dynamics in semiconductors and dielectrics [5-7] we observe a quasi-instantaneous response to the laser fluence. Ab-initiocalculations support our observations and allow us to pin-point the origin of our observations, which is the screening modification due to light-induced ultrafast electron localization on the d-orbitals of Ti. 

Reference
[458] M. Volkov, S. A. Sato, F. Schlaepfer, L. Kasmi, N. Hartmann, M. Lucchini,  L. Gallmann, A. Rubio, U. Keller 
Download"Attosecond screening dynamics mediated by electron-localization" (PDF, 2.2 MB)vertical_align_bottom 
Nature Physics, vol. 15, pp. 1145-1149, 2019 
Supplementary information: Downloaddownload (PDF, 1.6 MB)vertical_align_bottom 
doi: external page10.1038/s41567-019-0602-9call_made

ETH News 
English https://www.phys.ethz.ch/news-and-events/d-phys-news/2019/08/how-light-steers-electrons-in-metals.html 
German https://www.phys.ethz.ch/de/news-und-veranstaltungen/d-phys-news/2019/08/wie-licht-elektronen-in-metallen-dirigiert.html

NCCR MUST Highlight on August 5, 2019
external pagehttp://www.nccr-must.ch/highlights.htmlcall_made
external pagehttp://www.nccr-must.ch/highlights/willenberg-keller_6122019.htmlcall_made

^{[1] H. R. Telle, G. Steinmeyer, A. E. Dunlop, J. Stenger, D. H. Sutter, U. Keller, "Carrier-envelope offset phase control: A novel concept for absolute optical frequency measurement and ultrashort pulse generation", Appl. Phys. B 69, 327 (1999)
[2] M. Schultze, K. Ramasesha, C. D. Pemmaraju, S. A. Sato, D. Whitmore, A. Gandman, et al., "Attosecond band-gap dynamics in silicon", Science 346, 1348 (2014)
[3] H. Mashiko, K. Oguri, T. Yamaguchi, A. Suda, H. Gotoh, "Petahertz optical drive with wide-bandgap semiconductor", Nature Physics 12, 741 (2016)
[4] M. Schultze, E. M. Bothschafter, A. Sommer, S. Holzner, W. Schweinberger, M. Fiess, et al., "Controlling dielectrics with the electric field of light", Nature 493, 75 (2013)
[5] M. Lucchini, S. A. Sato, A. Ludwig, J. Herrmann, M. Volkov, L. Kasmi, et al., "Attosecond dynamical Franz-Keldysh effect in polycrystalline diamond", Science 353, 916 (2016)
[6] F. Schlaepfer, M. Lucchini, S. A. Sato, M. Volkov,L. Kasmi, N. Hartmann, A. Rubio, L. Gallmann, U. Keller, "Attosecond optical-field-enhanced carrier injection into the GaAs conduction band", Nature Physics 14, 560 (2018)
[7] S. A. Sato, M. Lucchini, M. Volkov, F. Schlaepfer, L. Gallmann, U. Keller, A. Rubio, "Role of intraband transitions in photocarrier generation", Phys. Rev. B 98, 035202 (2018)
[8] R. Locher, L. Castiglioni, M. Lucchini, M. Greif, L. Gallmann, J. Osterwalder, M. Hengsberger, U. Keller, "Energy-dependent photoemission delays from noble metal surfaces by attosecond interferometry", Optica 2, 405 (2015)
[9] M. Lucchini, L. Castiglioni, L. Kasmi, P. Kliuiev, A. Ludwig, M. Greif, J. Osterwalder, M. Hengsberger, L. Gallmann, U. Keller, "Light-matter interaction at surfaces in the spatiotemporal limit of macroscopic models", Phys. Rev. Lett. 115, 137401 (2015)
[10] D. Wegkamp, M. Herzog, L. Xian, M. Gatti, P. Cudazzo, C. L. McGahan, R. E. Marvel, R. F. Haglund, A. Rubio, M. Wolf, and J. Stähler, "Instantaneous band gap collapse in photoexcited monoclinic VO2 due to photocarrier doping", Phys. Rev. Lett. 113(21), 216401 (2014).}