Optical Microscopes5. March 2021
There exists a broad variety of optical microscopes and we own several types. Here the microscopes we require for the fabrication and characterization of the electro-optical nano-antennas are presented. We start with with the simplest (and cheapest) one and moving up to more sophisticated ones.
Stereo microcopes are good if you want to have a quick look with an overview magnification on your sample. They work in reflection mode, i.e., the sample is placed on the dish and you observe it from above and usually they have a built-in illumination either from below and/or the side.
We use a “BRESSER Researcher ICD LED 20x-80x Stereo Microscope” with a magnification of 4×10* for processing our samples. After FIB milling for example the samples have to be cleaned from remaining “gold dirt” and we do that by putting a droplet of gold etching solution onto the flakes for a couple of seconds. This microscope helps a lot for placing the droplet properly.
* 4×10 means a magnification of 4 times from the objective and 10 times from the ocular (eyepiece), i.e., all together 40x. More advanced microscope have a tubus lens in between which can give you an additional boost of, e.g., 1.5x or 2.0x times. When utilizing a camera the definition of the magnification gets more tricky as you have to factor in the pixel size on the monitor vs the sensor, digital zoom and so on.
Next up are upright microscopes. They also work in reflection but with much higher magnifications (e.g. 50x2x22) and the sample can be illuminated directly through the objective to provide enough light. Furthermore, in many designs an exposure from the bottom using a condensor is possible. This is handy for acquiring dark-field images where only edges of objects light up.
We own a Olympus BX51 upright microscope for quickly assessing our samples, transfering flakes and acquiring shiny reflection images. To achieve that we have a non-standard stage, ultra-long-distance objectives, specific filters and DSLR on the top.
Inverted optical microscopes
Inverted optical microscopes are very common in biology and other fields. The are designed upside down, i.e., the condensor is on the top, the objectives are on the bottom and the samples are placed on top of them. Therefore, transparent samples are required but the benefit is that they allow manipulating the sample from the top without disturbing the view from the bottom. Furthermore, immersion oil objectives can easily be used to boost the magnification and resolution to its limits (e.g. 100×1.5×22 @ NA 1.45).
We own an Olympus IX70, which is compatible to the BW51, and is also used for the transfer but most importantly for determining the thickness of the flakes.
Confocal optical microscopes are usually based on inverted optical microscopes. As the name suggests they utilize two focii — the excitation and the detection focus — which overlap and help to reduce noise levels and slightly increase the resolution.
We use with the Nikon TE2000 another inverted microscope often in confocal mode. It is part of a whole setup and upgraded with a scanning stage, different (laser) light sources, single-photon detectors and a state-of-the-art spectrometer.