When you poke with three beams (wavevectors ( k_1, k_2, k_3 )), the polarization emits light in specific directions. The most famous is the :
[ k_signal = -k_1 + k_2 + k_3 ]
In nonlinear spectroscopy, you poke with (or more). The polarization wiggles in a complicated way, but the magic is: The signal is proportional to the third power of the electric field. (Hence, “nonlinear.”) Practical takeaway: You are not doing magic. You are hitting a molecule with three light pokes and listening to the echo of the polarization. Principle 2: The One Equation You Must Memorize (Fixed Version) Mukamel writes: ( S(t) = \int_0^\infty dt_3 \int_0^\infty dt_2 \int_0^\infty dt_1 R^(3)(t_1,t_2,t_3) E(t-t_3-t_2-t_1) E(t-t_3-t_2) E(t-t_3) )
In linear spectroscopy (absorption), you poke once, the polarization wiggles, and you measure the wiggle decay. Boring.
Confusing ( T_1 ) (population lifetime) and ( T_2 ) (dephasing time). Fix: ( T_2 ) = ( 1/( \textlinewidth ) ). ( T_1 ) = how long excited state lives. Always ( T_2 \le 2T_1 ). If your ( T_2 ) is shorter than ( 2T_1 ), you have pure dephasing.
When you poke with three beams (wavevectors ( k_1, k_2, k_3 )), the polarization emits light in specific directions. The most famous is the :
[ k_signal = -k_1 + k_2 + k_3 ]
In nonlinear spectroscopy, you poke with (or more). The polarization wiggles in a complicated way, but the magic is: The signal is proportional to the third power of the electric field. (Hence, “nonlinear.”) Practical takeaway: You are not doing magic. You are hitting a molecule with three light pokes and listening to the echo of the polarization. Principle 2: The One Equation You Must Memorize (Fixed Version) Mukamel writes: ( S(t) = \int_0^\infty dt_3 \int_0^\infty dt_2 \int_0^\infty dt_1 R^(3)(t_1,t_2,t_3) E(t-t_3-t_2-t_1) E(t-t_3-t_2) E(t-t_3) )
In linear spectroscopy (absorption), you poke once, the polarization wiggles, and you measure the wiggle decay. Boring.
Confusing ( T_1 ) (population lifetime) and ( T_2 ) (dephasing time). Fix: ( T_2 ) = ( 1/( \textlinewidth ) ). ( T_1 ) = how long excited state lives. Always ( T_2 \le 2T_1 ). If your ( T_2 ) is shorter than ( 2T_1 ), you have pure dephasing.
