Cryogenic freezing - use liquid nitrogen - natural leaves
Cryogenics is the science that addresses the production and effects of very low temperatures. The word originates from the Greek words 'kryos' meaning "frost" and 'genic' meaning "to produce." Under such a definition it could be used to include all temperatures below the freezing point of water (0 C).
Nitrogen boiling point is -196 C. So liquid nitrogen can use as a cryogenic liquid.