Methods and devices for using photorefractive materials at infrared wavelengths

The method of writing a reflective hologram in a photorefractive crystal, wherein the hologram is reflective to an incident beam at a reading side at a wavelength longer than a photorefractive sensitivity range of the crystal, comprising the steps of:

• writing a plane holographic grating in the crystal, by illumination of the crystal with interfering beams at wavelengths lying in the photorefractive sensitivity range of the crystal, through at least one face substantially perpendicular to the holographic grating planes;
• and
• rendering the holographic grating substantially permanent.

An electromagnetic wave reflector device comprising a photorefractive crystal having a photorefractive sensitivity range of about 350 nm to 700 nm and a c-axis, the crystal having a holographic grating therein with planes substantially normal to the c-axis of the crystal, the grating being reflective to an incident beam through a face substantially normal to the c-axis of the crystal at a wavelength greater than 770 nm.

An electromagnetic wave reflector device comprising a photorefractive crystal having a photorefractive sensitivity in a first wavelength range, the crystal having a holographic grating therein which is reflective in a selected direction to incident beams in a wavelength range longer than the first range.

A method of writing reflection mode holographic gratings in a photorefractive material, comprising the steps of:

• writing plane holographic gratings in the material Bragg-matched for reflection at a desired infrared wavelength at a specific incidence angle, using shorter wavelength writing beams through a face of the material perpendicular to the grating planes, wherein the writing beams' wavelength is selected to be within the photorefractive sensitivity range of the material and to obtain a Bragg condition solution for the desired infrared wavelength;
• and
• controlling the angle and wavelength of the writing beams to obtain a desired grating spacing inside the material for substantially normal incidence reflection at the desired infrared wavelength.

A method of writing reflection mode holographic gratings in a photorefractive material, comprising the steps of:

• writing plane holographic gratings Bragg-matched for retro-reflection at a desired infrared wavelength in the material using shorter wavelength writing beams in reflection mode, wherein the writing beams' wavelength is selected to be within the photorefractive sensitivity range of the material and to obtain a Bragg condition solution for the desired infrared wavelength;
• and
• controlling the angle and wavelength of the writing beams to obtain a desired grating spacing inside the material for normal incidence reflection at the desired infrared wavelength.

A method for writing volume holographic gratings Bragg-matched for reflection at infrared wavelengths outside the photorefractive sensitivity range of a host material, comprising the steps of:

• illuminating the material using one or more visible wavelength plane writing beams incident upon side faces of the material parallel to the material's c-axis, so that a resulting grating period inside the material is matched for reflection at a desired infrared wavelength and incidence angle;
• and
• coating the material with an anti-reflection layer for the infrared wavelength on a c-axis face of the material and for the visible writing beam wavelength on the side faces of the material.