New Method Maps Optical Absorption of Molecules

A new method has been developed to map the optical absorption of molecules in the electric field of an ultrashort terahertz pulse, which helps determine the strength and dynamics of electric interactions. Molecules in water and other polar media are subject to strong electric forces that originate from their liquid environment. At ambient temperature, this environment undergoes ultrafast structural fluctuations.

The Stark effect, or the spectral shift of optical transitions in an external electric field, is a fundamental quantum effect in light-matter interaction that provides information on atomic and molecular properties. However, the Stark effect has primarily been studied under stationary conditions to elucidate the time-averaged behavior of a single quantum system and/or an ensemble. Time-resolved measurements, on the other hand, allow for observing transient properties and give insight into processes on an atomic scale.

Methodology

Scientists from Max Born Institute in Berlin and Ludwig-Maximilians-Universität in Munich have used strong electric fields in the terahertz frequency range to modify the optical absorption of dye molecules in liquid solution and follow the ultrafast absorption changes in time. They have reported in The Journal of Physical Chemistry Letters that interaction with a THz pulse of a 1-ps duration broadens the electronic absorption spectrum of the molecules substantially. This transient effect provides quantitative insight into the coupling of the molecules to the external electric field and, at the same time, allows for calibrating the electric field from the solvent.

An ultrashort THz pulse interacts with a solution of the dye betaine-30. The THz electric field acting on the molecules is enhanced with the help of a metallic antenna structure and reaches a maximum value of 3.6 megavolts/cm, corresponding to approximately one third of the fluctuating field from the solvent. The momentary change of molecular absorption is monitored by probe pulses of a 100-fs duration. The time evolution is recorded by changing the delay between the two pulses.

Results

The time evolution of the THz electric field shows that the absorption change of the dye solution is plotted as a function of frequency and wavelength. The solid blue line represents the stationary absorption spectrum in the absence of a THz field. The transient absorption decrease in the center of the stationary spectrum and the absorption increase on the low- and high-frequency wings correspond to a transient spectral broadening, induced by the THz electric field. In time, this broadening follows the THz intensity, while a contribution of the solvent to the absorption changes is absent. On the ultrashort time scale of the measurement, the solvent is structurally “frozen.”

In the liquid solution, there exists a disordered ensemble of dye molecules, each molecule possessing a permanent electric dipole moment. The interaction of such dipoles with the THz electric field shifts the electronic transition between the ground state S0 and the first excited state S1 in frequency. The interaction strength and, thus, the sign and amount of spectral shift are determined by the projection of the THz field onto the direction of molecular dipole moment. As a result, the transient spectra reflect the orientally averaged behavior of the dye molecules.

A quantitative analysis of the spectral broadening gives the electric coupling strengths and allows for an experimental calibration of the electric fields in the solution. Beyond this basic insight, the ultrafast and fully reversible character of the field-induced absorption changes may lead to applications in optical switches and modulators.

A new method has been developed to map the optical absorption of molecules in the electric field of an ultrashort terahertz pulse, which helps determine the strength and dynamics of electric interactions. The method has been used to modify the optical absorption of dye molecules in liquid solution and follow the ultrafast absorption changes in time. The results of the study provide quantitative insight into the coupling of the molecules to the external electric field and allow for calibrating the electric field from the solvent. The ultrafast and fully reversible character of the field-induced absorption changes may lead to applications in optical switches and modulators.