Broadband ultrafast self-heterodyned chiro-optical spectroscopy

TL;DR

This work tackles the challenge of weak transient chiro-optical signals in ultrafast spectroscopy by introducing a broadband, self-heterodyned detection scheme that combines a common-path birefringent interferometer with a polarization bridge to measure time-resolved CD and ORD with ultrafast resolution. The method leverages time-domain Fourier-transform spectroscopy and balanced detection to reach near-shot-noise-limited sensitivity, enabling millidegree-level ΔCD/ΔORD signals across a broad spectral range (550–950 nm) in tens of seconds and with potential extension to wider bands and 2D spectroscopy. Demonstrations on a chiral plasmonic Au nano-helicoid array reveal ultrafast, pump-induced modulations linked to hot-carrier dynamics and near-field dipole interactions, while spin-selective excitation in a lead halide perovskite exposes broadband time-resolved Faraday-rotation-like signals tied to spin polarization and relaxation. The approach provides a simple, sensitive platform for broadband ultrafast chiro-optical spectroscopy with wide applicability in biochemistry, nanophotonics, and condensed-matter physics, and it can be extended to study enantioselective interactions and valley-spin phenomena with advanced pump schemes.

Abstract

Ultrafast chiro-optical spectroscopy provides unique access to the structural dynamics of molecules, spin-valley relaxation in semiconductors, and the non-equilibrium optical response of chiral nanophotonic systems. Yet, because chiral signals are intrinsically weak and time-resolved spectroscopy probes small photoinduced changes, transient chiro-optical responses are often difficult to isolate from parasitic achiral contributions. Here, we introduce a broadband ultrafast chiro-optical spectroscopy technique that integrates a birefringent common-path interferometer with an optical polarization bridge to sensitively detect photoinduced changes in the polarization state of light. Phase-sensitive self-heterodyned detection enables simultaneous measurement of transient circular dichroism and optical rotatory dispersion across a broad spectral range with ultrafast temporal resolution. Balanced detection suppresses excess laser noise, enabling exceptional sensitivity (<50 $μ$deg) close to shot-noise limit. We demonstrate this approach on an array of gold nano-helicoids, supported by a full-wave time-resolved model of the spatiotemporal dynamics of plasmonic non-equilibrium carriers and their associated optical nonlinearities. The model traces the system's transient chiro-optical response back to photoinduced modulations of the electric-magnetic dipole interaction in the nano-helicoid, elucidating the connection of near- and far-field dynamics in the non-equilibrium regime. We further investigate spin excitation, thermalization, and relaxation in a lead halide perovskite, establishing a novel approach to broadband time-resolved Faraday rotation. The simplicity, sensitivity, and wide applicability of this detection scheme provide a powerful platform for broadband ultrafast chiro-optical spectroscopy, opening new opportunities in biochemistry, solid-state physics, and nanophotonics.

Figures (5)

• Figure 1: Ultrafast chiro-optical spectroscopy setup. a. Working principle of the static chiro-optical measurements. After transmission through a chiral sample, the electric field of a linearly polarized probe pulse can be described as the sum of two components: achiral (AFID) and orthogonally polarized chiral (CFID) free induction decay, where the latter is much lower in amplitude (exaggerated in the figure for visualization). A common-path interferometer (CPI), constituted by a birefringent plate and a pair of birefringent wedges, scans the delay ($t$) between the two components. The combination of a Wollaston prism (WP) and a balanced photodetector (BPD) enables the detection of polarization rotation, measuring chiral interferograms upon insertion of the wedges. b. Ultrafast chiro-optical setup. The pump pulse repetition rate is halved by a Pockels cell (PC) and a polarizing beam splitter (PBS). A quarter wave plate can be inserted in the pump pulse path for circular polarization. A broadband probe pulse is linearly polarized by a Glan Taylor (GT) polarizer and delayed with respect to the pump by $\tau$. The detection scheme follows the procedure described in a, except that here we use a lock-in amplifier (LIA) to demodulate the signal. This allows us to extract the pump-induced change in the chiral interferogram, which we refer to as the differential chiral interferogram, $\Delta$I$_{chiral}$($t$,$\tau$). c. Example of a differential chiral interferogram and the associated $\Delta$CD and $\Delta$ORD spectra.
• Figure 2: Static chiro-optical response of plasmonic helicoids. a. Schematic of the system under study, consisting of an ordered square lattice of chiral Au nano-helicoids lying on a PDMS substrate. b. Spatial distribution of the (normalised) optical helicity density $h_d$ computed across one helicoid, evaluated at a wavelength of 655 nm in unperturbed conditions, for either LCP (left panel, black) or RCP (right panel, yellow) incoming light. Arrows indicate the electric field vectors, evaluated on the surface of the helicoid. c. Same as (b), shown as a top view (in a $xy$ plane at mid-height of the helicoids) across a few unit cells of the nanoparticle lattice. d. Experimental steady-state chiral interferogram. e. Measured (solid lines) and simulated (dashed lines) CD (blue) and ORD (orange) spectra of the plasmonic helicoid array in unperturbed conditions.
• Figure 3: Ultrafast chiro-optical response of plasmonic helicoids. a, b. Measured (a) and simulated (b) maps of the ultrafast $\Delta$CD, as a function of the probe wavelength and pump-probe delay. c, d. Measured (c) and simulated (d) maps of the ultrafast $\Delta$ORD, as a function of the probe wavelength and pump-probe delay.
• Figure 4: Spectro-temporal dynamics of chiral plasmonic helicoids. a, b. Spectral cuts of the measured $\Delta$CD (a) and $\Delta$ORD (b) spectra at selected pump-probe delays $\tau$ = $-1ps$ (blue), 0.5ps (purple), 2ps (red), 10ps (orange). c, d. Time traces of the measured $\Delta$CD (c) and $\Delta$ORD (d) at selected probe wavelengths $\lambda = 641nm$ (blue), 681nm (purple), 741nm (orange) for $\Delta$CD (c), $\lambda = 609nm$ (blue), 682nm (purple), 827nm (orange) for $\Delta$ORD (d). e, f. Same as a, b for the simulated $\Delta$CD (e) and $\Delta$ORD spectra (f). g, h. Same as (c, d) for the simulated $\Delta$CD (g, where a minor spectral shift is considered for the wavelength corresponding to the purple trace) and $\Delta$ORD (h) time traces.
• Figure 5: Ultrafast chiro-optical spectroscopy on lead halide perovskites . a. Scheme of the electronic bands close to the band edge, and the spin-polarized populations of electrons and holes upon excitation with circularly polarized light. b. TR-CD (solid lines) and TR-ORD (dashed lines) spectra recorded at 1 ps pump-probe delay for different pump light helicities (solid and dashed red lines for $\sigma^+$, solid and dashed blue lines for $\sigma^-$). c. TR-CD map upon $\sigma^+$ photoexcitation. d. Same as (c) for opposite pump pulse helicity. e. TR-ORD map upon $\sigma^+$ photoexcitation. f. Same as (e) for opposite pump pulse helicity. The employed pump fluence was 15 $\mu$J/$\mathrm{cm}^2$