A sample holder (10) comprises a sample chamber (33), a gas reservoir (32) and an upper layer (20) covering over the sample chamber (33) and gas reservoir (32), wherein a bottom surface of the upper layer (20) comprises a microstructure array (23) which overlies at least a portion of a top periphery of the sample chamber (33), and wherein the microstructure array (23) is in communication with a gas path which extends to the gas reservoir (32), to allow gas exchange between the sample chamber (33) and the gas reservoir (32).