Abstract
Spectrally resolved measurements of optical activity, such as circular dichroism (CD) and optical rotatory dispersion (ORD), are powerful tools to study chiroptical properties of (bio)molecular and nanoplasmonic systems. The wider utilization of these techniques, however, has been impeded by the bulky and slow design of conventional spectropolarimeters, which have been limited to a narrowband scanning approach for more than 50 years. In this work, we demonstrate broadband measurements of optical activity by combining a balanced detection scheme with interferometric Fourier-transform spectroscopy. The setup utilizes a linearly polarized light field that creates an orthogonally polarized weak chiral free-induction-decay field, along with a phase-locked achiral transmitted signal, which serves as the local oscillator for heterodyne amplification. By scanning the delay between the two fields with a birefringent common-path interferometer and recording their interferogram with a balanced detector that measures polarization rotation, broadband CD and ORD spectra are retrieved simultaneously with a Fourier transform. Using an incoherent thermal light source, we achieve state-of-the-art sensitivity for CD and ORD across a broad wavelength range in a remarkably simple setup. We further demonstrate the potential of our technique for highly sensitive measurements of glucose concentration and the real-time monitoring of ground-state chemical reactions. The setup also accepts broadband pulses and will be suitable for broadband transient optical activity spectroscopy and broadband optical activity imaging.
Highlights
Resolved measurements of optical activity, such as circular dichroism (CD) and optical rotatory dispersion (ORD), are powerful tools to study chiroptical properties ofmolecular and nanoplasmonic systems
In this work we introduce an approach to the sensitive measurement of broadband optical activity, which combines a noise-canceling balanced detection scheme with interferometric FT spectroscopy
The essence of our approach is a highly sensitive measurement of polarization rotation thanks to the combination of (i) balanced detection; (ii) broad spectral coverage afforded by the FT time-domain interferometry; and (iii) intrinsic interferometric stability of the common-path heterodyne detection scheme
Summary
Resolved measurements of optical activity, such as circular dichroism (CD) and optical rotatory dispersion (ORD), are powerful tools to study chiroptical properties of (bio)molecular and nanoplasmonic systems. CD spectroscopy is performed in resonance with electronic/vibrational transitions and is routinely employed to determine the handedness and structural organization of chemical, biological, and material systems.[7,8] ORD, on the other hand, has the advantage that it is nonzero outside the absorption band and can distinguish chiral molecules even in nonresonant conditions Both CD and ORD spectra are enantio-differentiating in their sign and can be correlated to the absolute molecular configurations in condensed phases through ab initio quantum-chemical calculations.[9,10]. CD and ORD represent the imaginary and real parts of a complex chiroptical susceptibility, just as ordinary absorption and dispersion correspond to the imaginary and real parts of the complex refractive index As such, they are related by a Kramers−Kronig transform; yet their measurement typically requires two separate optical setups. Standard spectropolarimeters offer high sensitivity, their scan rate is quite low due to the narrowband serial approach, which requires long acquisition times
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