Quantum weak value amplified terahertz chiroptical measurement

Abstract

A precise method for phase and amplitude detection in both the time and frequency domains of terahertz spectroscopy based on the weak-value amplification technique is proposed and demonstrated. Within the weak-value amplification scheme, the imaginary weak value enhances variations in the terahertz phase signals, whereas the real weak value amplifies changes in the terahertz amplitude signals. By employing various postselections in the terahertz weak measurement procedure in detecting minute changes of the phase and amplitude of the terahertz wave, we achieved a phase change range from −0.0187 rad to 0.0183 rad with an interval of 0.004 rad and an amplitude change range from −0.0238 rad to 0.0228 rad with an interval of 0.0056 rad. This results in a phase and amplitude measurement resolution of 10−4 rad in the time domain. In the frequency domain, E $\left\vert E\right\vert $ spectra are calculated to assess phase and amplitude variations with respect to frequency or wavelength. We apply these methods to chiral detection, particularly in measuring optical activity such as circular dichroism (CD) and optical rotatory dispersion (ORD). Despite challenges such as strong terahertz wave absorption in aqueous solutions and weak optical responses from natural chiral materials in the terahertz band, we successfully conducted chiroptical spectroscopy on a relatively large volume (2.3 mL) of liquid (R)- and (S)-limonene, as well as lactose tablets with varying mass fractions. Furthermore, the carrier-envelope phase (CEP) shift, defined for one- or few-cycle time-domain terahertz pulses, was effectively achieved through the manipulation of a pair of terahertz polarizers in the terahertz beam path. Notably, when ϕ CEP = 0, a 150 % increase in the absorption coefficient of lactose was observed when weak measurement techniques were employed, compared to conditions without such measurements. This effort yielded THz-ORD and THz-CD spectra, demonstrating the potential of our methods to overcome traditional limitations and provide new insights into the optical response, dynamic properties, and low-frequency vibrational modes of biomolecules and materials in low-energy states, ultimately facilitating the identification of chiral stereoisomers.