HeNe Laser and APD in the Zetasizer
What laser is in the Zetasizer Nano series? This question came to me via one of our Malvern account managers, so I thought a brief explanation could help clarify which laser and which detector is used in the systems.
The Laser
Helium Neon HeNe lasers (λ=632.8nm) were the first commercial lasers. They have superior beam qualities (for example in stability, mode, and coherence length). Since they have been around for decades, they are also reliably durable. A typical HeNe laser lifetime reaches up to ~20,000 hours. Traditionally, the first light scattering experiments utilized various gas lasers and photo multiplier tubes PMTs as detectors. The typical quantum efficiency of these PMT detectors was somewhere in the 5-10% range. This means we record only 1 out of every 10-20 photons arriving at the light sensitive area. PMTs come in different peak detection efficiencies, so some are better at green than at red wave lengths.
The Detector
The development of solid state avalanche photo diode detectors APDs had a dramatic influence on the design of many light scattering systems. The unique performance specification of APDs is their remarkable quantum efficiency of up to ~65% . This yields an order of magnitude higher sensitivity, albeit limited to in the red wave length range. Thus many systems took advantage of the reliability of the red HeNe laser combined with the high quantum efficiency of the APD.
Laser and Dectetor combined
The Zetasizer Nano family uses the above design principles and an additional bonus: There is a significant sensitivity increase due to the optical non-invasive back scattering design. (This further expanded the application range to more concentrated samples) .
At that time, the combination of HeNe laser and APD detectors was simply the best possible combination. The design choice has proven
itself in the number of systems and publications produced citing them since.
There are also good diode lasers available, with the advantage of more compact size and good energy efficiency. Early on, the first higher power models suffered from limited life time and fluctuating intensity output, coupled with inferior beam quality parameters, and relatively high cost compared to gas lasers. Many of these issues have now been overcome. If your experiment requires diode lasers we do offer the green Zetasizer (λ=532nm) and the microV (λ=830nm) as well as the APS (λ=830nm) systems. Historically though, the HeNe laser (λ=632.8nm) of the Zetasizer Nano series has been the most favored choice.
Further Resources
- Specification and description of the Zetasizer range
- Technical Note: Achieving high sensitivity at different scattering angles with different optical configurations
- Comparison of different systems : Sample measurements performed to demonstrate Zetasizer performance
Previously
- On the origin of persistent nanoparticles in sugar solutions
- Tips and Tricks for Nanoparticle Characterization
- Which size is right: intensity volume number distributions