Report Outline

This broadband polarizer has to be build, as the intensity of the pulses is very high and their spectrum too broad for a regular polarizer. The intensity of the lightpulses coming into the COLTRIMS setup is controlled by an iris in front of the COLTRIMS chamber. However, the intensity going into the chamber cannot be controlled completely by this iris, as the beam has an intensity prole that is dependent on the distance to the center of the beam.

Mostly, this intensity dependence is assumed to be Gaussian. Another dis-advantage of the iris controlling the intensity of the beam is the fact that the iris changes the size of the beamspot going into the chamber, complicating the interaction with the molecular beam in the COLTRIMS apparatus. By blocking part of the beam, the iris will also introduce a small phase shift, changing the polarization of the beam, which is another unwanted eect as the beam going into the COLTRIMS chamber should be perfectly linearly po-larized. Therefore, the aim is to build an optical setup before the COLTRIMS chamber which can control the intensity of the beam, without changing the size of the beamspot, and the polarization of the laserlight. This control of the polarization is necessary because the interesting inelastic scattering processes only can occur in the case the laser light is perfectly linearly polar-ized. The broadband polarizer should also have a minimal pulse broadening, as the pulse duration in the COLTRIMS setup should be as short as possible.

The idea is thus to build a broadband polarizer in front of the COLTRIMS chamber, as the intensity of the laser pulses will be very high and their fre-quency spectrum very broad. The preliminary design of this broadband polarizer consists of an halfwave plate followed by two germanium plates on which the incoming laserbeam will bounce o at Brewster's angle. The polarization of the laserbeam is thus controlled by the reection of the two germanium plates at Brewster's angle, as this only leaves the s-polarized part of the reection. The choice for germanium is mainly due to its high reec-tivity and its characteristics in the used frequency spectrum. The intensity of the light can be controlled by the halfwave plate by changing the angle between the axis of the halfwave plate and the polarization of the incoming light.

1.3 Report Outline

As the aim of this report is stated, there rst is some need for knowlegde on the lasersystem. The theory and experimental setup of the femtosecond laser system are discussed in Chapter 2. Then Chapter 3 will discuss the theory used for the calculations of characterizing the broadband polarizer.

Chap-ter 4 will elaborate on the experimental setup and design of this broadband polarizer and discuss which parameters can be experimentally veried and how this can be done. Chapter 5 will present the calculations performed to theoretically describe the broadband polarizer. The results of the measure-ments will be presented in Chapter 6, where they will also be compared to the theoretical calculations. Chapter 7 will then show a summary of activities, overall conclusions and recommendations for future work.

Chapter 2

A few-cycle pulsed laser system

In the last decades pulsed laser systems have evolved and been greatly im-proved. Pulse lengths have become much shorter and the intensities of the system much higher, both by orders of magnitudes. All these developments started with the technique of laser mode-locking. [3, 4] In this chapter the developments that led to the current state-of-the-art systems are discussed, followed by a description of the system used in this project.

2.1 Mode-locking

The phenomenon of laser mode-locking is based on the principle of super-position of waves. Inside the laser cavity a large amount of longitudinal phase-locked laser modes exist alongside and most of the time all those dif-ferent modes cancel each other by superposition. This means there is a net laser eld of zero. On some brief moments of time however, all those dierent modes do not cancel, but add up coherently. For these moments very short light pulses will leave the laser cavity along the longitudinal axis. Because these brief moments occur regularly, the laser produces a regular train of light pulses. The phenomenon of laser mode-locking is visualized in Figure 2.1. It can be seen that an increase in the number of locked modes leads to shorter pulses.

Mode-locking of a laser can be achieved in two distinct ways: active and passive mode-locking. Active methods use an external signal to induce mod-ulation of the light in the laser cavity, while passive methods make use of some kind of material in the laser cavity which causes self-modulation of the light. In the 1980's laser pulses shorter than 100 fs were reached with

pas-7

Figure 2.1: Visualization of the pulse length of a mode-locked laser, which is dependent on the roundtrip time in the cavity and the number of modes. [3]

sive mode-locking, but were limited by the response time of the absorption process. Another step towards shorter pulses was made with the discovery of the optical Kerr-eect. [4]

2.1.1 Kerr-lens mode-locking

The optical Kerr eect describes a change in refractive index of materials dependent on the intensity of the incoming light. This non-linear eect is only visible for high intensity electromagnetic elds, such as laser beams. [5]

The refractive index of a material then becomes:

n = n₁+ n₂(I) (2.1)

where n1 is the linear refractive index of the material and n2(I)the intensity-dependent non-linear refractive index due to the optical Kerr eect. The consequence of the optical Kerr eect is that parts of the laser beam with high intensities suer less loss than those with lower intensities. Applying this eect to a Gaussian laser beam with high enough intensity, the middle part with highest intensity will experience a higher refractive index than the outer part of the beam with lower intensity, those thus experiencing a rela-tively lower refractive index. So, the middle part of the beam will be more focussed than the outer part of the beam. This eect is called self-focussing.

[6] [7]

By introducing an aperture in the laser cavity, the cavity modes without self-focussing will be (partially) blocked. This will thus create extra energy loss for the cavity modes without self-focussing. In this way the introduced aperture will work like a high-intensity pass lter. This use of the self-focussing eect is called Kerr-lens mode-locking and is visualized in Figure