Subfield: Modern Physics

THEORETICAL PROBLEM 2 

DOPPLER LASER COOLING AND OPTICAL MOLASSES

The purpose of this problem is to develop a simple theory to understand the so-called "laser cooling" and "optical molasses" phenomena. This refers to the cooling of a beam of neutral atoms, typically alkaline, by counterpropagating laser beams with the same frequency. This is part of the Physics Nobel Prize awarded to S. Chu, P. Phillips and C. Cohen-Tannoudji in 1997.

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The image above shows sodium atoms (the bright spot in the center) trapped at the intersection of three orthogonal pairs of opposing laser beams. The trapping region is called "optical molasses" because the dissipative optical force resembles the viscous drag on a body moving through molasses.

In this problem you will analyze the basic phenomenon of the interaction between a photon incident on an atom and the basis of the dissipative mechanism in one dimension.

PART I: BASICS OF LASER COOLING

Consider an atom of mass $m$ moving in the $+x$ direction with velocity $v$. For simplicity, we shall consider the problem to be one-dimensional, namely, we shall ignore the $y$ and $z$ directions (see figure 1). The atom has two internal energy levels. The energy of the lowest state is considered to be zero and the energy of the excited state to be $\hbar a_{0}$, where $\hbar=h / 2 \pi$. The atom is initially in the lowest state. A laser beam with frequency $a_{L}$ in the laboratory is directed in the $-x$ direction and it is incident on the atom. Quantum mechanically the laser is composed of a large number of photons, each with energy $\hbar \boldsymbol{c}_{L}$ and momentum $-\hbar q$. A photon can be absorbed by the atom and later spontaneously emitted; this emission can occur with equal probabilities along the $+x$ and $-x$ directions. Since the atom moves at non-relativistic speeds, $v / c \ll 1$ (with $c$ the speed of light) keep terms up to first order in this quantity only. Consider also $\hbar q / m v \ll 1$, namely, that the momentum of the atom is much larger than the



momentum of a single photon. In writing your answers, keep only corrections linear in either of the above quantities.

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internal energy levels of the atom

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Fig. 1 Sketch of an atom of mass $m$ with velocity $v$ in the $+x$ direction, colliding with a photon with energy $\hbar c_{L}$ and momentum $-\hbar q$. The atom has two internal states with energy difference $\hbar \boldsymbol{c}_{0}$.

Assume that the laser frequency $a_{L}$ is tuned such that, as seen by the moving atom, it is in resonance with the internal transition of the atom. Answer the following questions:

1. Absorption.
Context question:
1a Write down the resonance condition for the absorption of the photon.
Context answer:
\boxed{$\omega_{L}\left(1+\frac{v}{C}\right)$}


Context question:
1b Write down the momentum $p_{a t}$ of the atom after absorption, as seen in the laboratory.
Context answer:
\boxed{$m v-\frac{\hbar \omega_{L}}{c}$}


Context question:
1c Write down the total energy $\varepsilon_{a t}$ of the atom after absorption, as seen in the 0.2 laboratory.
Context answer:
\boxed{$\frac{m v^{2}}{2}+\hbar \omega_{L}$}


Extra Supplementary Reading Materials:

2. Spontaneous emission of a photon in the $-x$ direction.

At some time after the absorption of the incident photon, the atom may emit a photon in the $-x$ direction.

Question: 2a Write down the energy of the emitted photon, $\varepsilon_{p h}$, after the emission process in the $-x$ direction, as seen in the laboratory.

Solution:
First, one calculates the energy of the emitted photon, as seen in the lab reference frame. One must be careful to keep the correct order; this is because the velocity of the atom changes after the absorption, however, this is second order correction for the emitted frequency:

$\omega_{p h} \approx \omega_{0}\left(1-\frac{v^{\prime}}{c}\right) \quad$ with $\quad v^{\prime} \approx v-\frac{\hbar q}{m}$

thus,

$$
\begin{aligned}
\omega_{p h} & \approx \omega_{0}\left(1-\frac{v}{c}+\frac{\hbar q}{m c}\right) \\
& \approx \omega_{L}\left(1+\frac{v}{c}\right)\left(1-\frac{v}{c}+\frac{\hbar q}{m c}\right) \\
& \approx \omega_{L}\left(1+\frac{\hbar q}{m c}\right) \\
& \approx \omega_{L}\left(1+\left(\frac{\hbar q}{m v}\right)\left(\frac{v}{c}\right)\right) \\
& \approx \omega_{L}
\end{aligned}
$$

Final answer: $\hbar \omega_{L}$