This proposal aims at demonstrating experimentally the implementation of a super-continuum source based on enhanced nonlinearity in near-transition ferroelectric supercrystals for chromatic aberration-free pump-and-probe ultrafast spectroscopy.
Objectives of the project are:
O1. Design and demonstration of a pump-and-probe scheme based on a super-continuum source obtained by a nanodisordered perovskite.
O2. Demonstration of chromatic aberration-free spectroscopy based on giant broadband optical refraction.
The expected outcome is a method to overcome one of the principal challenges in state-of-the-art ultrafast spectroscopy: dispersion, chirp, and chromatic aberration in super-continuum pump-and-probe schemes. Such effects generate undesired nonlinear artifacts, hampering the detection and the interpretation of ultrafast time-resolved spectra, which require the use of pulses with an ultra-broad spectral bandwidth to ensure a femtosecond time resolution.
To achieve this goal, the proposal pools together the Ultrafast Spectroscopy Group (USG) and the Nonlinear Photonics Group (NPG) at the Physics Department, both active in the experimental study and development of innovative optics and materials.
The underlying idea is based on giant broadband refraction in the visible (n>25), a phenomenon observed when light propagates along the strings of 3D topological defects (spontaneous polarization vortices) that form as a nanodisordered ferroelectric KTN (potassium-tantalate-niobate, KTa(1-x)Nb(x)O3) is cooled through its room-temperature Curie point. Giant refraction causes an enhanced optical nonlinearity that allows constraint-free wavelength conversion and supercontinuum generation through nonlinear Cherenkov radiation, without chromatic dispersion and walk-off. The project will design and demonstrate ultrafast pulsed-laser experiments that use this enhanced optical nonlinearity as an aberration-free super-continuum source in cutting-edge spectroscopy.
Dispersion and chromatic aberration are ones of the most detrimental effect for ultrafast time-resolved inquiries. The propagation of a transform limited on dispersive material alters the temporal profile of the light and is responsible for pulse broadening. In addition, the synthesis of ultrashort pulses in nonlinear crystals can be strongly affected by chromatic aberration. For example, in supercontinuum generation self phase modulation causes the spectral broadening of a femtosecond pump along the nonlinear material and is commonly exploited for generating spectrally ultra-broadband (from 400 to 1200nm) pulses. Critically, the spectral components red and blue shifted with respect to the pump pulse central wavelength are generated at the end of the nonlinear crystal, when the pulse has experienced dispersion and self-focusing. This results in the generation of a supercontinuum characterized by spectral components that are inhomogeneously spatially distributed.
If accepted, this proposal will be the first experimental realization of a dispersion-free super-resolved pump-probe setup. This will be the key to :
1) Performing pump-probe experiments with unprecedented temporal resolution taking advantage of a dispersion-free super-continuum source.
2) Building on an aberration-free super-continuum source to develop a chirped-based impulsive Raman spectroscopy setup able to record in a single shot time-domain Raman spectra without the use of a delay line.
In a recent work, we introduced an experimental scheme for the realization of chirped-based Impulsive Stimulated Raman Scattering (ISRS) experiments, for recording the time-domain Raman information without scanning the pump and probe delay and hence, ensuring acquisition times two orders of magnitude faster. Briefly, ISRS is a powerful technique able to real time monitor vibrational oscillations in photo-excited systems: by combining a pump and a probe pulse for stimulating and then probing Raman coherences, ISRS can directly access atomic motions and molecular properties. At odds with frequency-domain Raman approaches, ISRS typically requires long acquisition times since the system response has to be measured by scanning a sequence of time delays between pump and probe pulses. In the chirped-based scheme that we have implemented, since different probe wavelengths interact with samples at different time delays, the evolution of the stimulated vibrational coherences is encoded in the probe spectrum. Due to the chromatic dispersion, different colors of the probe pulse can have a different spatial overlap with the pump pulse and hence this technique cannot be exploited over broad spectral range, limiting hence its application. If successful, this proposal will provide the chance to synthesize optimally shaped chromatic aberration-free probe pulse for the application of ISRS for probing irreversible processes, such as phase transition, non-reversible chemical reactions or phenomena accompanied by sample damaging.
The present proposal will join the expertise of two leading groups in the field of time-resolved spectroscopy and nonlinear optics to develop a beyond-state-of-the art chirp/aberration-free spectroscopic paradigm.
On a general perspective, in view of the wide use within the scientific community of ultrafast time-resolved spectroscopic approaches used for tackling highly debated topics [C. Ferrante et al. JACS, 142, 2285, (2020), R. Kusaka et al., Nat. Chem. volume 13, pages 306¿311 (2021)], we believe that the whole scientific community will benefit from the outcomes of this proposal.