Low temperature optical microscope for simultaneous electrical transport measurements

We discuss the design and performance of a liquid nitrogen-cooled optical microscopy system that also allows for si-multaneous transport measurements. With a cold-finger based design, we have been able sample temperature between100 K to 400 K with controlled ramp rate (0.1 K/min to 42 K/min) and thermal stability (±5 mK) at any given tempera-ture. The cryostat forms a part of a modular and versatile experimental set up, particularly suitable for low temperaturebright-field reflection microscopy under Köhler illumination. With careful selection of optical components, an activefocus correction system to account for the thermal expansion, along with effective vibration isolation, a sub-micronspatial resolution has been achieved.