An optical switch comprising: a plate (20, 120); a first slit (23, 123) formed in the plate (20, 120); and a second slit (25, 125) formed in the plate (20, 120) separately from the first slit (23, 123); wherein: the optical switch comprises a phase difference generator (35A, 35B, 35C) configured to receive input light, to output to the first slit the input light as first light (Ls), and to respond to a control signal to output selectively to the second slit the input light as second light (Lc) with a phase difference relative to the first light; the plate comprises a conductive metal or a semiconductor and has a nanometre sized thickness, for example up to 250 nm or 300 nm; a plurality of first grooves (30, 130) are formed on the plate (20, 120), on a first side of the first slit (23, 123), between the first slit (23, 123) and the second slit (25, 125); and a plurality of second grooves (32, 132) are formed on the plate (20, 120) on a second side of the first slit (23, 123), opposite the first side; the width of each of the first and second slits, and the width and the depth of each of the first and second grooves, is in the range of about 1/2 λ to about 1/20 λ, wherein λ is a wavelength of the first light; wherein the optical switch is configured such that the first light incident on the first slit is transmitted by dipoles generated at the first slit and is focussed by dipoles generated at the first and second grooves (30, 130); and whereby the focusing of the transmitted first light is selectively turned on and off by the control signal causing the second light (Lc) selectively to be output to the second slit and thereby to weaken the dipoles generated at the first and second grooves and thus to diffuse the transmitted first light. a plate (20, 120); a first slit (23, 123) formed in the plate (20, 120); and a second slit (25, 125) formed in the plate (20, 120) separately from the first slit (23, 123); the optical switch comprises a phase difference generator (35A, 35B, 35C) configured to receive input light, to output to the first slit the input light as first light (Ls), and to respond to a control signal to output selectively to the second slit the input light as second light (Lc) with a phase difference relative to the first light; the plate comprises a conductive metal or a semiconductor and has a nanometre sized thickness, for example up to 250 nm or 300 nm; a plurality of first grooves (30, 130) are formed on the plate (20, 120), on a first side of the first slit (23, 123), between the first slit (23, 123) and the second slit (25, 125); and a plurality of second grooves (32, 132) are formed on the plate (20, 120) on a second side of the first slit (23, 123), opposite the first side; the width of each of the first and second slits, and the width and the depth of each of the first and second grooves, is in the range of about 1/2 λ to about 1/20 λ, wherein λ is a wavelength of the first light; wherein the optical switch is configured such that the first light incident on the first slit is transmitted by dipoles generated at the first slit and is focussed by dipoles generated at the first and second grooves (30, 130); and whereby the focusing of the transmitted first light is selectively turned on and off by the control signal causing the second light (Lc) selectively to be output to the second slit and thereby to weaken the dipoles generated at the first and second grooves and thus to diffuse the transmitted first light.