Are the Attocube LT-APO cryogenic objectives with a numerical aperture of 0.82 actually suitable for work with single quantum emitters at liquid helium temperatures? If not, are there any other commercial alternatives?
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2 Answers
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Surprisingly, the Attocube LT-APO objectives seem have very low light collection efficiencies that belie their supposedly high NA. We compared two Attocube objectives with 0.82 NA with an Olympus objective with a 0.75 NA at room temperature, and found the Olympus objective had more than an order of magnitude greater collection efficiency. Confocal raster scans of single colloidal quantum dots on a glass substrate revealed a disparity between the peak intensity of the brightest dots of more than 50. Operation of the Attocube objective at 2 K indicates a comparable performance to the room temperature operation, giving poor S/N. Overall, I would strongly avoid these objectives if you are doing cryogenic confocal microscopy requiring high collection efficiencies.
Unfortunately, to the best of my knowledge, there are no commercial objectives rated to be suitable for cryogenic operation. However, there is a commercial objective with a 0.95 NA that is known to operate at this temperature, even though it is not rated for it. Furthermore, it maintains its high NA at 2 K. Although, the objective is no longer being manufactured and so may not be a viable option.
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$\begingroup$ I should add that there does seem to be a commercial objective rated for both vacuum and cryogenic conditions. Montana Instruments uses a Zeiss objective in their cryostat under vacuum. I have been informed that it is rated to 40 K. $\endgroup$ Commented Jan 31, 2023 at 11:39
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Pike Technologies sells ultra-high vacuum compatible high NA reflective objectives. I am using their 100x 0.80 NA reflective objective for cryogenic fluorescence microscopy though the objective itself remains at room temperature (no immersion). The best part of these is the bandwidth goes from around 0.2-20μm. Because it has monolithic design, I bet it would work just fine submerged in cryogen.
https://www.piketech.com/product/ir-microscope-objectives-high-na-and-broadband-ir/
