Lez #37+38 Double sided Feynman diagrams: linear absorption and pump-probe (3rd order)

  We introduced the double sided Feynman diagrams to describe the perturbative expansion of the polarization, end explicitly evaluated the linear absorption of a two level system.

We started to extend the same approach to third order processes. Among them, pump-probe experiments with the first 2 simultaneous interactions with the pump, and the third interaction with a probe. This originates stimulated emission (SE), bleaching and excited state absorption (ESA). We commented on the differences between bleaching and SE. 

Lez #35+36 Radiation - matter, rabi oscillations, perturbative expansion

   Today we recalled the basic concepts of quantum mechanics, introducing the density matrix. We stressed the importance  of density matrix to describe statistical mixtures, and how this is not possible through an eigenstates superposition (for instance a two level system).

We discussed how to solve Bloch equation and Rabi oscillation, introducing the rotating wave approximation to neglect some terms in the integration.

We then introduced Shroedinger, Heisenberg and Interaction schemes (in this latter both eigenfunctions and operators evolve, and the temporal evolution related to the unperturbed Hamiltonian is removed from the wavefunction). Then it makes sense a perturbative expansion of the eigenfunctions and of the density matrix in the Interactions scheme. Finally, we went back to the Shroedinger scheme.

Please refer to the first part of the notes added to classroom.

Lez #33+34 Introduction to spontaneous vs stimulated Raman processes, classical approach

  We introduced the field-induced polarization, in both the spontaneous and stimulated cases. We discussed the origin of spontaneous Raman, and why it is incoherent, linear and homodine. As for the selection rules we recalled some basic notions from struttura della materia class. Then we moved to the stimulated Raman, a third order process for which we evaluated different contributions to the polarization. See the material on classroom.

Lez #31+32 Laser Principles II

 We first finalized the laser lecture introducing the solid state laser principle. Then mode locking for pulsed laser generation, active and passive methods, gain and losses compensation.  

Lez #29+30 Laser Principles I

Recap on Absorption, Spontaneous and stimulated emission. Laser principles, treshold condition. Passive resonator. Is a laser superior to a conventional sources? Trading spatial and temporal coherence with flux. Statistical properties of light. How to make a laser: strategies for 2/3/4 level systems, complete equation with lasing dependent inversion.

Lez #27+28 ESONERO 1

Lez #25+26 NOPA II

 We started again from the Phase Matching (PM) condition and the necessity of using birefringent nonlinear crystals to satisfy it. Specifically, we saw in detail the use of BBO crystals (negative uniaxial) to achieve Type I PM (ooe), in collinear and non-collinear configurations. We saw how the “magic angle” can be determined by means of a numerical script, by drawing the phase-matching curves as a function of the signal wavelength and crystal angle Ɵ for different values of the noncollinearity angle α. We saw that different pumping wavelengths (515 and 400 nm) feature different magic angles, corresponding to distinct values of Ɵ. We also evaluated the wavevector mismatch as a function of ʎ and Ɵ, at the magic angle, and quantified the deviation from perfect phase-matching caused by fixing a specific value of Ɵ.

Finally, we explored the NOPA architecture's technical layout, detailing the various optical components and the pivotal role of the delay line in tuning the pump-signal delay inside the nonlinear crystal. We noted that with a chirped seed, amplification is restricted entirely to the spectral components that temporally overlap with the pump. As a result, simply moving the delay line enables a tunable spectral shift of the amplified pulse across the phase-matching region.