Broadband near UV to visible optical activity measurement using self-heterodyned method

Mapping the broadband circular dichroism of copolymer films with supramolecular chirality in time and space

Measurements of the electronic circular dichroism (CD) are highly sensitive to the absolute configuration and conformation of chiral molecules and supramolecular assemblies and have therefore found widespread application in the chemical and biological sciences. Here, we demonstrate an approach to simultaneously follow changes in the CD and absorption response of photoexcited systems over the ultraviolet-visible spectral range with 100 fs time resolution. We apply the concept to chiral polyfluorene copolymer thin films and track their electronic relaxation in detail. The transient CD signal stems from the supramolecular response of the system and provides information regarding the recovery of the electronic ground state. This allows for a quantification of singlet-singlet annihilation and charge-pair formation processes. Spatial mapping of chiral domains on femtosecond time scales with a resolution of 50 μm and diffraction-limited steady-state imaging of the circular dichroism and the circularly polarised luminescence (CPL) of the films is demonstrated.

Phase-stable optical activity measurement by common-path spectral interferometry

A robust optical activity (OA) spectrometer covering the visible and near-infrared regimes was designed and built via a combination of a linear polarizer and a birefringent plate. The OA spectrometer relies on common-path spectral interferometry, where the two interfering fields travel common optical paths, and ensures signal reproducibility over several hours. By detecting OA without polarization switching, the data acquisition time is shortened to 1 s, enabling real-time monitoring of the chiral complex formation. The present configuration also allows OA measurement with broadband pulses, which is promising for probing ultrafast circular dichroism and optical rotatory dispersion.

Femtosecond Vibrational Sum-Frequency Generation Spectroscopy of Chiral Molecules in Isotropic Liquid

Vibrationally resonant optically active (VOA) sum-frequency generation (SFG) is a second-order nonlinear process sensitive to the stereospecific vibrational structure of chiral molecules. We demonstrate that a femtosecond VOA SFG signal can be measured in the isotropic bulk of a chiral liquid. The chiral, achiral, and VOA SFG spectra of R- and S-limonene and their racemic mixture in the C-H stretching frequency region are characterized. In particular, it is shown that the observed circular intensity difference (CID) signal, which can provide distinguishable stereochemical vibrational information between enantiomers, arises from interference of the electric-dipole allowed antisymmetric Raman tensor-induced and Raman optical activity (ROA) tensor-induced SFG fields. Furthermore, we show that the CID and linear polarization intensity difference (LID) SFG spectra are connected to the real and imaginary parts of the effective chiral VOA SFG susceptibility, respectively. We anticipate that the present technique will be of use in transient chiroptical spectroscopy and stereochemical vibrational imaging studies.

Efficient polarization direction measurement by utilizing the polarization axis finder and digital image processing

We report a new quantitative measurement method of polarization direction based on the polarization axis finder (PAF) and digital image processing. The PAF acts as an azimuthal analyzer to determine the polarization direction of linearly polarized light and form an "hourglass" intensity pattern, and a digital camera is employed to record the pattern, and then the lightwave's polarization direction is obtained accurately by analyzing the pattern with a specially designed digital image processing algorithm. The quantitative measuring experiments have been carried out by validating the standard quartz plates with known rotation angles, and the measurement accuracy has reached 0.01°.

Time-domain measurement of optical activity by an ultrastable common-path interferometer

We introduce a novel configuration for the broadband measurement of the optical activity of molecules, combining time-domain detection with heterodyne amplification. A birefringent common-path polarization-division interferometer creates two phase-locked replicas of the input light with orthogonal polarization. The more intense replica interacts with the sample, producing a chiral free-induction decay field, which interferes with the other replica, acting as a time-delayed phase-coherent local oscillator. By recording the delay-dependent interferogram, we obtain by a Fourier transform both the circular dichroism and circular birefringence spectra. Our compact, low-cost setup accepts ultrashort light pulses, making it suitable for measurement of transient optical activity.

Near shot-noise limited time-resolved circular dichroism pump-probe spectrometer

We describe an optical near shot-noise limited time-resolved circular dichroism (TRCD) pump-probe spectrometer capable of reliably measuring circular dichroism signals in the order of μdeg with nanosecond time resolution. Such sensitivity is achieved through a modification of existing TRCD designs and introduction of a new data processing protocol that eliminates approximations that have caused substantial nonlinearities in past measurements and allows the measurement of absorption and circular dichroism transients simultaneously with a single pump pulse. The exceptional signal-to-noise ratio of the described setup makes the TRCD technique applicable to a large range of non-biological and biological systems. The spectrometer was used to record, for the first time, weak TRCD kinetics associated with the triplet state energy transfer in the photosynthetic Fenna-Matthews-Olson antenna pigment-protein complex.

Optical discrimination of racemic from achiral solutions

We demonstrate purely optical discrimination between achiral and racemic solutions by selectively triggering an asymmetric photoreaction with femtosecond laser pulses.

Precise and rapid detection of optical activity for accumulative femtosecond spectroscopy

We present polarimetry, i.e. the detection of optical rotation of light polarization, in a configuration suitable for femtosecond spectroscopy. The polarimeter is based on common-path optical heterodyne interferometry and provides fast and highly sensitive detection of rotatory power. Femtosecond pump and polarimeter probe beams are integrated into a recently developed accumulative technique that further enhances sensitivity with respect to single-pulse methods. The high speed of the polarimeter affords optical rotation detection during the pump-pulse illumination period of a few seconds. We illustrate the concept on the photodissociation of the enantiomers of methyl p-tolyl sulfoxide. The sensitivity of rotatory detection, i.e. the minimum rotation angle that can be measured, is determined experimentally including all noise sources to be 0.10 milli-degrees for a measurement time of only one second and an interaction length of 250 μm. The suitability of the presented setup for femtosecond studies is demonstrated in a non-resonant two-photon photodissociation experiment.

Single-shot electronic optical activity interferometry: power and phase fluctuation-free measurement

We demonstrate that a single-pulse characterization of electronic optical activity-free induction decay, which carries information on both circular dichroism and optical rotatory dispersion, is experimentally feasible. Employing a self-referencing scheme, we show that a highly reliable interferometric chiroptical measurement free from power and phase fluctuations is achievable on a shot-by-shot basis.