Setup

The laser¹ beam was directed, without any further parallelization (divergence

<0.5mrad), over two mirrors to the prism – sample cell combination as presented schematically in Fig. 6.3 (a). On its way the laser beam is passing two stops which are not limiting the beam but just removing unwanted laser reflections. The prism – sample cell combination (see Fig. 6.3 (c)) is placed on top of a goniometer² which can be rotated with a minute accuracy. By rotating the goniometer we control the angle on which the parallel laser beam is hitting the hypotenuse of the prism. After being totally reflected at the hypotenuse the laser beam leaves the

1Coherent Verdi, working at 532nm.

2Newport, Cat. No. M-UTR80.

prism via a dove prism, attached with highly viscous immersion oil³ to the prism, in order to avoid unwanted reflections from the prism surfaces. After leaving the dove prism the laser beam is directed into a beam dump.

On top of the goniometer, where the prism – sample cell combination is sitting, another goniometer is placed on which the receiver optics (seen on left in Fig. 6.3 (d)) are attached with an arm so that it can be rotated around the sample cell, its rotation axis being in the middle of sample cell. The receiver optics⁴ consists of a polarizer, a lens and a single-mode fiber and are tilted slightly away from the plane of incoming laser beam in order to minimize the possible collection of reflected unscattered light.

The receiver optics are also used to align the whole setup. This is done so that prior to a measurement a probe laser beam⁵ is coupled into the detection fiber from the PMT-end (see Fig. 6.3 (b)). The probe laser beam, emerging out of the receiver optics, shows us the reversed path of the scattered light and can therefore be used in the setup alignment. This is done so that the probe beam is matched with the evanescent wave at the surface of the sample cell. Furthermore the reflections of the probe beam from the sample cell are used to determine that the upper goniometer is at the symmetry axis of the glass semicylinder. Typically the alignment of the setup was done so that first the prism was adjusted into the desired penetration depth (laser beam slightly over the critical angle) and the receiver optics were turned perpendicular to the prism hypotenuse (seen by the reflection from the hypotenuse). After that the goniometer, where the receiver optics were attached, was turned into the desired measurement angle. In the final step the receiver optics were adjusted with X-Y stages to be perpendicular to the semicylinder so that the beam is going through the glass without refraction.

After that the overlapping of probe beam and total reflected incident laser spot at the sample surface was once again checked. Once the setup was adjusted the probe laser was turned off, the single-mode fiber was connected to the PMT⁶ and the scattered light was analyzed with a correlator⁷.

The sample cell (see Fig. 6.3 c)) was constructed from a prism⁸ on which the coverslip was glued with optical adhesive⁹ and cured with UV light for roughly two hours. The rest of the sample cell was constructed from two glass pieces: a glass semicylinder¹⁰, through which the scattering signal was measured and from a small piece of glass which was glued under the semicylinder in order to make

3Cargille, type NVH.

4Microbench components, Linos.

5Helium Neon laser 05 LHP 151, Melles Griot working at wavelength 632.8 nm.

6ALV / SO-SPID Single Photon Detector.

7ALV-5000/E Correlator.

8Edmund optics, Cat. No. L32−3364, height 25 mm, width 25 mm and hypotenuse 35.4 mm.

9Norland optical adhesive No 72.

10Prepared at the glass workshop at the university of Konstanz.

PMT CPU

Figure 6.3: a) The EWDLS setup: The laser is directed over two mirrors to the prism – sample cell combination. The prism – sample cell combination is sitting on top of a goniometer. By varying the angle of the incoming laser light the penetration depth can be varied. In order to avoid back reflections within the prism a dove prism is attached to the prism guiding away the laser beam.

Scattered light is collected with a single-mode fiber, detected with PMT and analyzed with a correlator. b) The principle of alignment: laser is coupled into the fiber and the beam emerging out of the receiver optics is used to align the setup. c) Prism – sample cell combination in close-up: a microscope coverslip is glued on the hypotenuse of the prism with optical glue and the rest of the sample cell is then glued with two component silicone rubber on top of this coverslip. d) Setup in use: On the left side is the receiver optics, in the middle are the two goniometers on top each other and between the goniometers is the prism - sample cell combination glued together with the dove prism. The upper goniometer can be moved with two translation stages in X and Y direction in order to fine tune the alignment and the lower goniometer is movable in Z direction in order to change the measurement spot at the sample surface.

the whole construction water proof. The semicylinder and the small bottom piece were glued to rest of the prism with two component silicone rubber and the glue was always left to dry overnight.

We were using the coverslips as an active surface, instead of prism hypotenuse surface directly which is more common in EWDLS experiments (see for example [78], [79] or [80]). This is done because of three reasons: (i) the functionalization of the coverslips is much easier and faster than the functionalization of the whole prism, (ii) it is also much cheaper to use the same prism and just vary the surface and finally (iii) the quality of the coverslip surface in terms of roughness is very good (see chapter 6.4.4). The coverslip can be removed from the prism after the UV curing by incubating the prism overnight in methylene chloride.

After thorough washing with ethanol, cleaning with isopropanol and lens cleaning tissues, the prism can be reused.