A method for the production of a holographic sensor which comprises a support medium supporting a reflection hologram wherein the support medium interacts with its physical or chemical environment to create an optical response which is a change in one or more optical properties of the hologram, the method comprising the steps of: a) introducing a colloidal dispersion of a recording material into the support medium; and b) ablating the colloidal particles of the recording material using a pulsed laser to form the holographic element in the support medium. The method of production can be used to introduce a reflection holographic grating into a hydrophobic support medium, in particular, polydimethylsiloxane (PDMS), which possesses an extraordinary ability to swell in the presence of both liquid and/or gaseous low molecular weight hydrocarbons and organic solvents and thus has many applications as a holographic sensor.