How Rabi frequency scales with laser intensity?

• Mark Mendl
In summary: The real intensity is given byIn summary, the Rabi frequency scale with the laser intensity if the atom is in a model where the electric field amplitude is proportional to the square modulus of the electric field.
Mark Mendl

Homework Statement

It's not a homework, I'm just studying for the exam, but one question that I'm not sure of the answer is:
"In the semiclassical model of a two-level atom interacting with a monochromatic field, how does the Rabi frequency scale with the laser intensity"

Homework Equations

Also, V=d*E0 (I'm not sure how to put formulas here, sorry)

The Attempt at a Solution

Do I assume no damping? Still, I think something is missing...

Mark Mendl said:
In the semiclassical model of a two-level atom interacting with a monochromatic field, how does the Rabi frequency scale with the laser intensity
Well you can say ##V^2 \propto I## since intensity is proportional to the square modulus of the electric field amplitude ##E_0##. Several approximations assumed in arriving in that expression of Rabi frequency are:
1) Two participating levels approximation: Generally in tackling problems of time-dependent Hamiltonian, one uses the eigenstates of the time-independent part of the Hamiltonian as the bases into which the true wavefunction of the system is expanded, clearly there will be an infinite number of terms. However it can be simplified in the case where the frequency of the interacting light is close to the frequency difference of certain pair of eigenstates, and we can approximate the true wavefunction as a linear combination of only those two coupled eigenstates of the time-independent part of the Hamiltonian.
2) Dipole approximation: the "size" of the participating wavefunctions from point 1) be much smaller than the wavelength of the light the atom in question is interacting with.

Last edited:
Thank you for the answer, but for exemple in a more specific question in this model :
"We place a two-level atom with transition energy ħ*ω0 in a monochromatic laser field with
frequency ω. The transition dipole moment is d = 3ea0. a0= 5.29 x1011 m.
(a) Calculate the laser intensity needed to achieve a Rabi frequency Ω1/2π = 1MHz.
So, can I just use the formula above for Rabi frequency (ignoring damping) and the relation between the field and intensity? This would give me something like I=0.896 Js-1m-2

As far as I know, the Rabi oscillation frequency was originally derived in the quantum picture of light-matter interaction where only the atom is treated quantum mechanically, I have never known people incorporated the damping term in the Hamiltonian. May be you confused it with the purely classical Lorentz model.
ALso, you have to calculate the 'real' intensity, not just the square of E field. For plane wave in a medium of refractive index ##n##, the time averaged intensity is ##\frac{1}{2} \epsilon_0 nc |E_0|^2##.

1. What is Rabi frequency?

Rabi frequency is a term used in quantum mechanics to describe the strength of the interaction between an atom or molecule and an electromagnetic field. It is named after physicist Isidor Rabi and is measured in units of frequency.

2. How does Rabi frequency relate to laser intensity?

Rabi frequency is directly proportional to the intensity of the laser. This means that as the intensity of the laser increases, the Rabi frequency also increases.

3. What is the mathematical relationship between Rabi frequency and laser intensity?

The mathematical relationship between Rabi frequency and laser intensity is given by the equation Ω = μE/h, where Ω is the Rabi frequency, μ is the transition dipole moment of the atom or molecule, E is the electric field strength of the laser, and h is Planck's constant.

4. How does Rabi frequency affect the behavior of atoms or molecules?

The Rabi frequency determines the rate at which an atom or molecule will transition between different energy levels when exposed to a laser. A higher Rabi frequency means a faster transition rate, and vice versa.

5. Is there a limit to how high the Rabi frequency can be?

There is no theoretical limit to how high the Rabi frequency can be. However, practical limitations such as laser power and the ability to accurately control the laser intensity can affect the maximum achievable Rabi frequency.

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