CONFOCAL MICROSCOPY - CFM
fundamentals
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Over the years, confocal microscopy has become the method of choice for obtaining clear, three-dimensional optical images of sample structures. It has been used for imaging almost everything - from studying biological samples such as cells, to fluorescence measurements, to examining physical structures like semiconductor quantum dots, NEMS/MEMS devices and for the emerging area of nano-optics. The confocal imaging system achieves out-of-focus rejection by two strategies (schematically illustrated in the figure below):

1. By illuminating a single point of the specimen with a focussed beam. Thus, the illumination intensity drops rapidly above and below the plane of focus.

2. By using of a blocking pinhole in the conjugate plane to the specimen that eliminates the degrading out-of-focus information.

By scanning many thin sections through the sample, a very clean three-dimensional image can be obtained. Confocal imaging can offer another advantage in favourable situations (small pinhole size, bright specimen): the obtained resolution can be better than with any microscope operated conventionally. In practice, the best horizontal resolution of a confocal microscope is about 0.4 µm, and the best vertical resolution is about 1.4 µm, assumed an excitation wavelength of 630 nm and a numerical aperture of 0.65 at 633 nm.

This resolution can be further improved by combining confocal microscopy with solid immersion lens microscopy, where light is focused inside a high refractive-index lens close to the sample. This offers a method for achieving resolution well below the diffraction limit in air. With an illumination wavelength of 633?nm, a lateral resolution of 160?nm could be achieved in the attocube systems application lab. Combining these techniques, major improvement of resolution and light throughput are achieved in addition to offering a very simple experimental setup compared to other high resolution optical techniques, e.g. Scanning Near-field Optical Microscopy (SNOM).

Cryogenic Confocal Microscopy

To improve the image quality in high resolution microscopy, confocal microscopy is often used at cryogenic temperatures. Thus, combination of high resolution power, clear optical spectrums and reduced thermal noise can be achieved. Spectral lines become sharper as thermal broadening is reduced due to lower thermal energy present in the system. Optical signals become stronger as quantum efficiency is improved due to less scattering and non-radiative recombinations. For many optical microscopy applications, cryogenic temperatures are therefore inevitably required.

These advantages are profitable particularly for high resolution optical spectroscopy of semiconductor structures or single molecule detection. Thus, investigation of the emitted optical energy of the sample due to changes in the surrounding material, applied voltages, or the deposited optical energy becomes feasible.

Additionally, high spatial resolution and sharp spectral lines are a prerequisite for investigating photon anti-bunching (single photon emitters).

attocube systems CFMs

A decade of experience in low temperature optics led to the development of three complementary confocal microscope systems optimized for different applications. All attocube microscope systems are compatible with vacuum environments as well as high magnetic fields.

attoCFMI:
This system based on free beam optics has been developed to offer highest flexibility to the customer. The modular beam splitter head is mounted outside of the cryostat and, thus, the experiment can be adjusted to the customer’s requirements.

attoCFMII:
The key-feature of this system is its unreached stability allowing ultra stable long-term investigations. This is also the most compact system which is available for 2 inch and 1 inch (attoCFM?IIxs) bore size cryostats.

attoCFMIII:
This is the system of choice for customers interested in reflection as well as transmission microscopy. Similar to the attoCFM?II, it is also a fiber-based system.