What laser is used 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.
Helium Neon HeNe lasers (λ=632.8nm) were the first commercial lasers, and 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. Traditionally the first light scattering experiments were conducted using 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, meaning only 1 out of every 10-20 photons arriving at the light sensitive area would be recorded. PMTs come in different peak detection efficiencies, so some are better at green than at red wave lengths.
When the solid state avalanche photo diode detectors APDs were developed, this 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% – which meant 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. The Zetasizer Nano family was designed based on these principles, with the additional bonus of a significant sensitivity increase due to the optical non-invasive backscattering design (plus the expanded application range to more concentrated samples) .
At the time the first Zetasizer Nano was developed, 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 diode lasers are needed we do offer them with the green Zetasizer (λ=532nm) and in 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 favored choice.
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