Ph.D Thesis

Ph.D StudentChuntonov Lev
SubjectRational Femtosecond Coherent Control of Multiphoton
Excitations: From Weak-Field to Intermediate-Field
DepartmentDepartment of Chemistry
Supervisor ASSOCIATE PROF. Zohar Amitay
Full Thesis textFull thesis text - English Version


Femtosecond coherent control manipulates photo-induced quantum interferences and state-to-state transition probabilities by optically shaping the femtosecond pulse. Rational control is based on identifying first the interfering pathways and their interference mechanisms. This PhD research studies theoretically and experimentally rational femtosecond phase control of multiphoton excitation processes in the weak and intermediate field regimes, and its application to photo-induced analog coherent information processing.

For achieving high degree of selectivity in complicated excitation scenarios, a new scheme for extended multichannel selective control based on symmetry properties is presented and implemented. Here, it is applied to atomic non-resonant two-photon absorption channel which is coherently incorporated in the resonance-mediated (2+1) three-photon absorption channel. By proper pulse shaping, utilizing the invariance of the two-photon absorption to specific phase transformations of the pulse, the three-photon absorption is tuned independently over an order-of-magnitude yield range for any possible two-photon absorption yield. The model system is atomic sodium (Na).

In order to open the door for rational control in the intermediate-field regime

of considerable excitation yields, femtosecond control of two-photon absorption is studied in detail in the powerful frequency domain using an extended fourth-order perturbative description that is introduced and developed here. Based on this new intermediate-field analysis and its experimental verification, it is identified that the two-photon absorption is enhanced by any shaped pulse having a spectral phase that is anti-symmetric around one-half of the two-photon transition frequency and a spectrum that is asymmetric around it (red or blue-detuned according to the system). Theoretical and experimental results are obtained for Na.

An additional case involving two types of multiphoton transitions is femtosecond absorption in the single-cycle regime. The corresponding ultra-broad pulse spectrum generally induces transitions involving only absorbed photons or both absorbed and emitted photons. It is theoretically shown how in this regime the carrier-envelope phase can serves as a new additional control parameter.

The multiphoton control capabilities are also used for implementing innovative photo-induced analog coherent information processing, which generally might be a basis for future "smart hardware". Here, two computational tasks are demonstrated experimentally, with the input information being encoded into the pulse's spectral phases and the computational task being carried out by the multiphoton non-linear response of the irradiated physical system. One computational task is the global classification of an unknown pattern according to its anti-symmetric nature. The other task is a single-shot calculation of the cross-correlation function between two encoded numerical functions.