Eureka!
To study these particles, the idea is to set up a prototype of a microscope to investigate such a particle: the helical particle is attached to a substrate and the helical probe light is focused on the helix along its axis. Then, we collect the scattered light either in forward (transmission) and backward (reflection) direction, and we analyze this scattered light regarding its helical Figure 3: In the planned experiments we generate helical light with
structure. This will be done in an analogous way to how light is
different parameters, focus it on the helical model particle, and analyze
analyzed conventionally with respect to its spectral content (its
the scattered light.
color): there, one uses a diffraction grating in a spectrometer to split up the light, separating different wavelengths, which
to find a simple exact theory. On the other hand, it is exactly
in turn can be detected by a CCD-camera (color-blind). For
this complication that will allow us to obtain more information
analysis of the scattered helical light, we use a special diffrac-
about the scatterer, the helical particle.
tion grating that splits up light depending on its helicity, and then a CCD (also helical-light blind!) can record the individual
We have already pointed out that the interesting helical structu-
components. By repeating this experiment with different kinds
res are small, i.e., on the order of a wavelength. To investigate
of input helical light and recording the scattered helical-light
such a microstructures, we obviously need to strongly focus the
components, a scattering matrix can be set up which describes
helical light. The description of strongly focussed (helical) light
all aspects of the helical particles’ scattering properties.
itself already requires numerical calculations, because in the case of strong focusing, the polarization of the light interacts
The vision
with the helical phase structure and vice versa.
The ultimate goal is to demonstrate application of this method to biological microscopy: Can we learn more about biological
Experiments
structures using helical-light microscopy? The microscopic
These complications allow in my opinion no other conclusion
structure of tissue clearly is important since, for instance,
than that the best entry into this new field is to do experiments!
alterations can indicate pathological modifications. Since lots of
We want to do model experiments from which we want to
tissue shows a certain degree of helical symmetry, investigation
deduce simple rules, which should describe the observed results
using helical light could be very interesting. A major difference
well. The development of an exact theory (like Mie theory for
compared to conventional microscopy is that here, a certain
spherical droplets) is for the future. For our model experiments
volume will be probed at once, since helical-light scattering
we need (i) helical light and (ii) the helical model particle. He-
happens clearly in 3D. Even if we could only make a small step
lical light has been produced for the first time here in Leiden,
towards a microscopy application, we would be very satisfied.
and 18 years ago this was a challenging task itself; nowadays it can be produced quite easily using a reprogrammable optical device, a spatial light modulator. This lets us basically shape
Wolfgang Löffler
any kind of helical light using a conventional laser as the light source. We can modulate the light’s twist, left or right, how fast it twists and by changing the laser frequency we can change the pitch. The type of high-quality micrometer-scale helical model particle that we need does not yet exist, but we have a novel idea how to make them, indeed using helical light and photolithography. We will fabricate small dielectric helices with a pitch of the order of a micrometer, and the other dimensions will be of the order of micrometers, too. 8
Eureka! Universiteit Leiden
After his masters in physics, Wolfgang Löffler received his Ph.D. summa cum laude at the University of Karlsruhe in Germany. At the moment he is a Postdoc in the Quantum Optics and Quantum Information group in Leiden. His current research interest is Graded Refractive Index optics.