Polarizer – Advanced DLS – get extra insight with polarization and FL filter

by Ulf Nobbmann, Thursday, 25th January 2024

Thanks for joining the recent Advanced DLS webinar on the use of polarizer and fluorescence filter with the Zetasizer! We had a few questions that I thought to summarize in this post.

Fluorescence in DLS

In samples with fluorescent components, the material absorbs light or a photon of one wavelength and then emitts it with a different wavelength (at a somewhat random time, the fluorescence life time, later). This fluorescent light (FL) is not suited for dynamic light scattering (DLS) since it is not coherent and therefore contributes just noise to the signal. We can improve this with a narrow band filter to only select the scattered light (with the same wavelength as the input laser). This can dramatically improve the quality of the correlation function for quantum dots (see application note) .

a fluorescence filter can dramatically reduce the influence of

Polarization in DLS

In some samples, the polarization state of the incoming laser light may change during the scattering. This can have several reasons, for example

  • optically active materials rotate the polarization of transmitted light
  • internal stress in solid materials can depolarize light
  • some materials exhibit birefringence
  • Flare from surfaces can be depolarized
  • Multiple scattering can be depolarized

In a conventional “ideal” light scattering setup, the polarization of the scattered light remains the same (parallel) as the incoming laser polarization. For convenience, all polarizations are often detected. However in some cases, data may improve with a polarization filter. The vertical polarizer selects only those photos with the same polarization as the incoming laser photons.

Image showing photons with random polarization emanating from turbid, multiple scattering sample, and a filter selects onla the vertical parallel component. This can help improve data quality when there is some turbidity or flare from plastic cuvettes

VV vertical polarization : improve data!

When we select a vertical polarization filter, we limit the collected light to that part, which has the same polarization as the input. This can help eliminate any artifacts life for example flare or scattering from internal thickness variations in the wall of a plastic cuvette. In the example below, the variation of data is greatly improved by the selection of the vertical filter. This improves the data quality so much to be on-par with the quality of glass cuvettes.

vertical polarizer VV detection can improve data quality as in this series of plastic cuvette size measurements, where the variation in results is much lower, almost even better than glass cuvettes

But is there any downside to this? The overall detected scattering intensity is lower. This may lead to a potential decrease or slight reduction in the sensitivity of the system.

How can you tell artifacts?

This questions is tricky and refers to the example application note of colloidal gold: when we detect all polarizations we detect two size peaks, when we use vertical polarization (parallel) we detect two size peaks. And even when we use horizontal polarization (perpendicular) we see again two size peaks – the important observation: the area under the smaller size peak is very different. This indicates that there is some component (at smaller size) that causes depolarized light. In the case of colloidal gold this can be due to particle rotation, and we occasionally observe this in real colloidal gold samples. The fact that the ratio is different for the polarization points to rotation or shape effects.

Colliodal gold shows an artifact peak at small size. due to rotation, this is enhanced with horizontal polarization detection in red in this image.

Do I need my own polarizer?

No need to provide polarization filters for your system. The Zetasizer Advance series includes polarizers for backscattering detection. Three optical filters are part of the system: the FL filter, the vertical polarization filter and the horizontal polarization filter. These are integrated into the hardware.

Does multiple scattering always depolarize the light?

Yes, for true multiple scattering the scattered light will be depolarized. For single scattering (that is for dilute samples) the polarization typically stays the same. So a polarizer will only see light in the parallel polarization component. Yes, in principle, if all scattering events stayed in the same traditional optical plane then there could be light scattering paths maintaining the same polarization. However, for turbid samples the likelihood of the polarization plane changing increases dramatically. And in the “ideal” case (“ideal” meaning perfect multiple scattering), the polarization is randomized so much, that the intensity signal detected from the parallel polarizer and from the perpendicular polarizer component would be the same.

As an aside, that is why the intercept of a perfectly multiple scattering sample will have a reduced value: It is only half that of perfect single scattering. You could think of the depolarized signal to be composed of two indepdendent polarized components. But that is porbably more than you wanted to know about polarizers.