Basic Interaction of Light Fields with Particles

The motion of a charged particle in an electro-magnetic field is determined by the Lorentz force:

dp

dt =q(E+ve×B) (III.1)

In the non-relativistic case v c this reduces to dp/dt = qE, as B = E/c E and thus the second term can be neglected. By integrating this equation one can obtain the maximum velocity an electron can achieve in an E-field as described in II.1: ve,quiv =

eEA/(ω0m). ve,quivis also called quiver velocity. If this velocity approachesc, neglecting

thev×B-term is not justified anymore.

In terms of the vector potential Athe equation of motion has the form:

∂p ∂t +(v· ∇)p=e ∂A ∂t −v× ∇ ×A ! (III.2)

Here, the Coulomb gauge∇A=0 was used and the identityd/dt = ∂/∂t+v× ∇.

The normalized laser vector potential a0 ≡ eA/mec < 1 is considered as a rough dis-

tinction between the classical a0 < 1 and the relativistic regime a0 > 1. With a0 = 1, a

classical calculation would lead tove,quiv= c. From (II.12) it also follows

a0 = eA mc = λ 2π eE mec2 (III.3) which illustrates that the energy an electron can gain from the electric field of the laser pulse equals the rest energy of an electron fora0 =1.

III.1.1.1 A Single Electron in a Plane Wave

Although this is the most basic light-particle interaction, all other effects such as the pon- deromotive force, self-focusing or driving a plasma wave are in the end based on the behavior of each single electron in a light wave with relativistic intensities. For a com- prehensive understanding of these collective processes, the basic characteristics of this simple single-electron motion will be discussed.

According to the Noether theorem1_{the two symmetries of a plane wave (two-dimensional}

structure and invariance under t → t− x/c) correspond to two conservation laws for the electron motion:

• The transverse momentum p⊥is always conserved.

p_⊥+qA⊥=const1 for an initialp0 =0→const1= 0 (III.4)

It follows p⊥/mc=aand thus2

γ⊥=

1+a21/2 (III.5)

• For the longitudinal momentum pkit holds

E−cpk= const2 for an initialp0 =0→ const2=mc2 (III.6)

The energy of a relativistic electron in the coordinate system of the laser pulse can be written as: E =γmc2= q (mc2₎2+_p2 kc2+ p 2 ⊥c2 (III.7)

1_{Noether’s theorem: Every differentiable symmetry of the action of a physical system has a corresponding} conservation law. The action of a physical system is the time-integral over the Lagrangian. Symmetry under a time shift gives conservation of energy, symmetry under translation in space gives conservation of momentum and rotation symmetry gives conservation of angular momentum.

2_{The relativisticγis defined as:}

γ= p 1 1−β2 = s 1+ p mec !2 and β=v c

and with equations (III.4) and (III.6) it follows Ekin =mc2(γ−1)= pkc= p2⊥ 2m = e2A2⊥ 2m (III.8)

With the definition of the normalized vector potential (III.3) and regarding the fact that a= a⊥= (ax,ay,0) it follows

γ =1+ a

2

2 (III.9)

With the aid of the derived constants of motion the integration of the equation of mo- tion (III.1) is easily done and the trajectory of a single electron in a light field (linear polarization inx-direction) is obtained as follows:

x(τ)= ca0 ω sin (ωτ) withτ= t− z(τ) c y(τ)= 0 z(τ)= ca 2 0 4 " τ+ 1 2ωsin (2ωτ) # (III.10)

This trajectory consists of a drift in the light-propagation directionzdri f t(t)= a₀2(a2₀+4)−1ct

and a figure-8 motion in this drift framex=a0cω−1sin (ωτ),z−zdri f t =a0c(8ω)−1sin (2ωτ).

The electron can only gain energy from the transverse electric field. Forv≈ cthev× B- force directs this motion in the propagation direction of the laser. Still, if one imposes a symmetric temporal envelope on the electric field, the electron is at rest again after the pulse has passed. No energy is transferred. However, if this assumption of spatially uni- form light fields with a slowly varying temporal envelope is violated, the electron can indeed gain energy. For example tightly focused laser beams with strong intensity gra- dients can ”repel” electrons from the high-intensity regions. The corresponding force is described in detail in the next section. It could be experimentally verified [46] that the angleθunder which electrons are scattered out of an intense laser focus is the same angle under which an electron moves in a plane wave (with (III.8)):

tanθ= p⊥ pk = s 2 γ−1 (III.11)

III.1.1.2 Ponderomotive Force

The interaction of a single electron with an electro-magnetic wave becomes more inter- esting, if, instead of a plane wave, one considers a spatially and temporally limited pulse e.g. with a Gaussian envelope as (II.4). As will be seen, although in principle following the electric field (quiver motion), electrons drift away from regions of higher intensity. In the limitv cthe equation of motion (III.1) for an electron in a light wave polarized along thex-direction and propagation alongzreduces to

∂vx

∂t =− e

The Taylor expansion of an electric field as in (II.1) gives

Ex(x,t)= Ex,A(x,t) cos (ϕ)+x

∂Ex,A(x,t)

∂x cos (ϕ)+. . . (III.13)

withϕ = ω0t−kzz. To the lowest order the electron directly follows the field and moves

with the quiver velocityve,quivas defined before. However, from the cycle-averaged equa-

tion of motion for the second order field *∂v(2) x ∂t + T = * e2 m2ω2_c2Ex,A ∂Ex,A(x,t) ∂x cos (ϕ)2 + T = e2 4m2ω2 ∂E2 x,A ∂x (III.14)

the non-relativistic ponderomotive forceFp =mh∂vx/∂tican be determined (here already

for the general 3D case):

Fp =−

e2

4mω2 0

∇E2_A (III.15)

It is obvious that the ponderomotive force is proportional to the gradient of the intensity

I ∝ E2_A. Furthermore the ponderomotive force is a conservative force that can be derived from a potentialUpviaFp =−∇Upwith

Up =

e2

4mω2 0

E2_A (III.16)

It should be noticed that the ponderomotive potential is not only proportional to the inten- sity but to Iλ2₀.

In the relativistic case v ≈ cthe equation of motion is best used in the form of (III.2). Assuming that again the motion can be separated into a fast oscillating part, that directly follows the vector potential p = eA (III.4) and a slow component, the relativistic pon- deromotive force can be determined (withγ =(1+(p/mc)2₎1/2_(III.5)):

Fp,rel =−

e2m

2γ ∇A

2 ₌₋

mc2∇γ (III.17)

For a detailed derivation of the relativistic ponderomotive force see e.g. [47].