Electromagnetic wave oscillating devices and a method of producing the same

A device comprising a non-linear optical crystal, said device oscillating an electromagnetic wave having a target frequency of 0.1 THz to 30 THz from said crystal to outside of said crystal:

• said device comprising a main body comprising said non-linear optical crystal and a sub-wavelength grating structure formed on said main body;
• wherein said sub-wavelength grating structure comprises protrusions periodically arranged in first and second directions on said main body, respectively, first grooves each provided between said adjacent protrusions and extending in said first direction, and second grooves each provided between said adjacent protrusions and extending in said second direction,
• wherein each of said protrusions comprises an upper end, a pair of first faces opposing in said first direction with each other, and a pair of second faces opposing in said second direction with each other,
• wherein a width of said first face is made smaller from said main body toward said upper end of said protrusion, a width of said second face is made smaller from said main body toward said upper end of said protrusion, said first groove forms a curved profile in a cross section cut along a direction of width of said first groove, and said second groove forms a curved profile in a cross section cut along a direction of width of said second groove.