BEGIN:VCALENDAR
VERSION:2.0
PRODID:icalendar-ruby
CALSCALE:GREGORIAN
METHOD:PUBLISH
BEGIN:VTIMEZONE
TZID:Europe/Vienna
BEGIN:DAYLIGHT
DTSTART:20270328T030000
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=3
TZNAME:CEST
END:DAYLIGHT
BEGIN:STANDARD
DTSTART:20261025T020000
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
TZNAME:CET
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
DTSTAMP:20261007T163316Z
UID:1794216600@ist.ac.at
DTSTART:20261109T103000
DTEND:20261109T113000
DESCRIPTION:Speaker: Chao Shen\nhosted by Maximilian Jösch\nAbstract: It i
 s a basic property of electromagnetic waves that they propagate. Yet the i
 mplications of this fact are often glossed over when it comes to nonlinear
  terahertz (THz) spectroscopy of bulk dielectrics\, with propagation effec
 ts being given marginal attention. In thick samples\, the THz fields\, opt
 ical probe\, and nonlinear polarization all propagate with different veloc
 ities\, experiencing different dispersion\, absorption and birefringence. 
 This gives rise to complicated nonlinear optical interactions prone to mis
 interpretation in the absence of a comprehensive theoretical framework. Th
 is thesis combines two-dimensional THz Kerr-effect spectroscopy (2D-TKE)\,
  two-dimensional THz-field-induced second-harmonic generation (2D-TFISH)\,
  and propagation-resolved numerical modeling to interpret time-resolved TH
 z-pump/optical-probe spectroscopies and track nonlinear THz polariton tran
 sport in bulk dielectrics.In birefringent LaAlO$_3$\, we use 2D-TKE data a
 nd four-wave-mixing simulations to separate propagation-induced electronic
  features from the intrinsic Raman-mediated Kerr response. We unambiguousl
 y identify the $0.86~\\mathrm{THz}$ and $0.36~\\mathrm{THz}$ oscillations 
 in TKE as arising from phase-matched co- and counter-propagating THz-pump/
 optical-probe coupling\, whereas the $1.1~\\mathrm{THz}$ oscillation arise
 s from the $E_g$ Raman phonon in LaAlO$_3$ excited through THz two-photon 
 absorption.In KTaO$_3$\, we identify the origin of the non-oscillatory par
 t of the TFISH signal by combining single-pulse and 2D-TFISH measurements 
 with simulations based on a Maxwell--Duffing model. We show that propagati
 ng soft-mode polaritons produce non-local transient polarization inside th
 e sample. Spatial integration of the TFISH response at different locations
  acts to single out a polariton mode with a wavevector corresponding to th
 e phase mismatch between the probe and its second harmonic. A Maxwell--Duf
 fing model reproduces the temperature and field-strength dependence of bot
 h the single-pulse and 2D-TFISH signals\, showing that the non-oscillatory
  component arises purely from nonlinear polariton propagation effects with
 out invoking transient ferroelectricity. The 2D data also rule out the pro
 posed mechanisms based on relaxational polar nanoregions or hot phonons.Fi
 nally\, we develop a spatiotemporal method for tracking soft-mode phonon-p
 olariton transport in bulk SrTiO$_3$ using 2D-TKE. The nonlinear Kerr sign
 al is shown to probe the spatial overlap between two independent polariton
 ic wave packets. Then\, using the less anharmonic $A_{2u}$ polariton as a 
 moving reference\, we track the field-dependent evolution of $E_u$ polarit
 ons. We observe that anharmonic effects lead to the formation of additiona
 l polarization accumulation near the sample entrance and an approximately 
 30\\% increase in the effective velocity of the leading peak of the $E_u$ 
 polariton. A minimal Maxwell--Duffing model captures many qualitative feat
 ures of the 2D-TKE signal\, including the entrance-layer response\, but pr
 edicts a substantially weaker velocity renormalization than observed exper
 imentally. These results establish 2D-TKE as a protocol to trace field-tun
 able polariton transport in an anharmonic medium\, while revealing behavio
 r beyond the Maxwell--Duffing description.Together\, these results establi
 sh propagation as a crucial part of understanding nonlinear THz spectrosco
 py of bulk dielectrics. Propagation effects can mimic intrinsic material r
 esponse but\, when identified\, become a sensitive probe of nonlinear latt
 ice dynamics and transport in quantum materials.
LOCATION:Sunstone Bldg / Ground floor / Big Seminar Room A (I23.EG.102) \, 
 ISTA
ORGANIZER:
SUMMARY:Chao Shen: Propagation-Resolved Nonlinear Terahertz Spectroscopy of
  Bulk Dielectrics
URL:https://talks-calendar.ista.ac.at/events/6683
END:VEVENT
END:VCALENDAR