Search for: All records

In classical macroscopic ultrafast optical four-wave mixing signals, phase matching selects three classes of light–matter interaction pathways: double quantum coherence; non-rephasing; and photon-echo. Multiple pathways contribute to each of these signals. We show that a coincidence-double-heterodyne detection scheme that employs two classical and two vacuum fields can isolate a single pathway contribution to each of these signals. We further demonstrate the advantage of the proposed technique by comparing it with the classical photon-echo signal for a model Frenkel-exciton dimer.

We propose a time–frequency resolved spectroscopic technique which employs nonlinear interferometers to study exciton–exciton scattering in molecular aggregates. A higher degree of control over the contributing Liouville pathways is obtained as compared to classical light. We show how the nonlinear response can be isolated from the orders-of-magnitude stronger linear background by either phase matching or polarization filtering. Both arise due to averaging the signal over a large number of noninteracting, randomly oriented molecules. We apply our technique to the Frenkel exciton model which excludes charge separation for the photosystem II reaction center. We show how the sum of the entangled photon frequencies can be used to select two-exciton resonances, while their delay times reveal the single-exciton levels involved in the optical process.

Electro‐optic sampling has emerged as a new quantum technique enabling measurements of electric field fluctuations on subcycle time scales. In a second‐order nonlinear material, the fluctuations of a terahertz field are imprinted onto the polarization properties of an ultrashort probe pulse in the near infrared. The statistics of this time‐domain signal are calculated, incorporating the quantum nature of the involved electric fields right from the beginning. A microscopic quantum theory of the electro‐optic process is developed adopting an ensemble of noninteracting three‐level systems as a model for the nonlinear material. It is found that the response of the nonlinear medium can be separated into a conventional part, which is exploited also in sampling of coherent amplitudes, and quantum contributions, which are independent of the state of the terahertz input. Interactions between the three‐level systems which are mediated by terahertz vacuum fluctuations are causing this quantum response. Conditions under which the classical response serves as a good approximation of the electro‐optic process are also determined and how the statistics of the sampled terahertz field can be reconstructed from the electro‐optic signal is demonstrated. In a complementary regime, electro‐optic sampling can serve as a spectroscopic tool to study the pure quantum susceptibilities of matter.