Magnetic-free chiral eigenmode spectroscopy for simultaneous sensitive measurement of optical rotary dispersion and circular dichroism

Chirality, defined by Lord Kelvin, refers to the geometric symmetry property of an object that cannot be superposed onto its mirror image using rotations and translations. The material’s chirality can be probed with light as the optical activity: optical rotary dispersion (ORD) and circular dichroism (CD). It is still challenging to yield extremely sensitive ORD and CD for very weak chirality and measure both simultaneously. Cavity ringdown polarimetry has been reported to improve ORD detection sensitivity with the absence of equally important CD signature, at the price of high cavity finesse near 400, frequency-locking sophistication, and large magnetic field. Here, we report a unique recipe to demonstrate the simultaneous measurement of ORD and the CD by separately observing the chiral eigenmode spectra from a bowtie optical cavity with a finesse about 30, without resorting to frequency locking or magnetic field. We obtain a sensitivity of \(\sim 2.7\times 10^{-3} \text {deg}/\sqrt{\text {Hz}}\) for ORD, \(\sim 8.1 \times 10^{-6} /\sqrt{\text {Hz}}\) for CD, and a spectral resolution of \(0.04~\text {pm}\) within a millisecond-scale measurement. We present a cost-effective yet ultrasensitive account for chiral chromatography, the conformational dynamics and chiroptical analysis of biological samples which particularly exhibit weak and narrow spectral signals.