Circular Dichroism Spectroscopy Using Coherent Laser-Induced Thermal Gratings

Abstract

A new pulsed four-wave mixing technique for the detection and real-time measurement of circular dichroism (CD) in liquid samples is demonstrated. The technique is based on the formation and detection of transient thermal gratings formed by the interference of two laser beams whose polarizations are controlled and modulated using a photoelastic modulator. Through an internal heterodyne process, coherent thermal gratings interfere to greatly enhance a weak circular dichroism signal. By measuring the ratio of the difference of scattered laser light from two different polarizations, ΔS, to the average amount of scattered light, S ave, one can determine the value of Δε/ε for the compound under investigation. Samples of chiral camphorquinone are used to demonstrate the technique in the ultraviolet region of the spectrum. These studies of camphorquinone, which has a value of Δε/ε ≈ 10-3 at 266 nm, produce values of ΔS/S ave which are orders of magnitude larger than Δε/ε and which approach 2 (the mathematical limit for ΔS/S ave). Possible extensions of the technique for measuring CD in very small sample volumes and monitoring time-dependence are discussed.

Precise alignment of a longitudinal Pockels cell for time-resolved circular dichroism experiments

“…From Eqs. (8) and (11) we can estimate that this limit corresponds to a relative artifact of a few 10 Ϫ5 when the pump-induced change of the probe transmission is equal to 10%, which is lower than our noise level. This procedure therefore allows us to dispense with the artifact signals that are due to misalignment of the PC.…”

“…Various techniques have been proposed: an ellipsometric method with a strained plate that introduces right and left ellipticity, 5,6 a splitbeam method, 7 or laser-induced thermal gratings. 8 A review of these techniques as well as other related ones can be found in Ref. 9.…”

Recent advances in ultrafast time-resolved chirality measurements: perspective and outlook

“…For the camphorquinone 5‐8 ns excitation pulses and for the Λ‐(+) 589 ‐[Co(en) 3 ]Br 3 20 ns excitation pulses were used to write the grating and the scattered light of the continuous wave probe was detected with a photomultiplier tube and monitored with a digital oscilloscope. The continuous probe laser was chosen to avoid the problems associated with shot‐to‐shot power fluctuations of pulsed lasers which could overshadow the weak signal .…”

“…To increase the signal to noise, a heterodyne technique can be employed, in which the polarization grating is superimposed with an intensity grating . This can be accomplished by using elliptically polarized light for one of the grating polarizations, fast switching between opposite ellipticity, and calculating the difference (ΔS) of the scattered signal and dividing it by its average (S avg ) for the two elliptical polarizations.…”

Laser wave-mixing optical method for sensitive detection of analytes in microarrays and microchips

“…[8][9][10][11][12][13][14][15] This wave-mixing method is inherently suitable for interfacing to microchannels and microarrays since the probe volume, i.e., overlap of the two input laser beams, is very small. Figure 1 shows a schematic diagram of a forwardscattering wave-mixing optical setup.…”