A shielding apparatus (14, 44) for passively attenuating electromagnetic radiation comprising: a plurality of cells (16, 82) with each cell comprising a plurality of resonators which are spaced from one another; wherein the plurality of cells (16, 82) are arranged in a plurality of unit cells (30, 80) with each unit cell comprising a common loop (32, 84) which surrounds at least two adjacent cells (16, 82) of the plurality of cells (16, 82); the plurality of unit cells (30, 80) each having an asymmetric structure so that the shielding apparatus has a negative refractive index for at least one selected frequency whereby electromagnetic radiation at the at least one selected frequency is attenuated; characterized by: the plurality of unit cells (30, 80) each comprise a first cell having a first pair of adjacent resonators (20, 22, 24, 26, 28, 32, 84) and a second cell having a second pair of adjacent resonators (20, 22, 24, 26, 28, 32, 84) wherein a spacing between the first pair of adjacent resonators is different from a spacing between the second pair of adjacent resonators whereby each of the plurality of unit cells has the asymmetric structure; and wherein each unit cell (30, 80) comprises at least two additional resonators surrounding the common loop (32, 84) and the additional resonators are split ring resonators comprising a loop with two open ends forming a gap, wherein the gap in a first additional resonator is positioned at an opposite end of the unit cell (30, 80) to the gap in a second additional resonator. a plurality of cells (16, 82) with each cell comprising a plurality of resonators which are spaced from one another; wherein the plurality of cells (16, 82) are arranged in a plurality of unit cells (30, 80) with each unit cell comprising a common loop (32, 84) which surrounds at least two adjacent cells (16, 82) of the plurality of cells (16, 82); the plurality of unit cells (30, 80) each having an asymmetric structure so that the shielding apparatus has a negative refractive index for at least one selected frequency whereby electromagnetic radiation at the at least one selected frequency is attenuated; characterized by: the plurality of unit cells (30, 80) each comprise a first cell having a first pair of adjacent resonators (20, 22, 24, 26, 28, 32, 84) and a second cell having a second pair of adjacent resonators (20, 22, 24, 26, 28, 32, 84) wherein a spacing between the first pair of adjacent resonators is different from a spacing between the second pair of adjacent resonators whereby each of the plurality of unit cells has the asymmetric structure; and wherein each unit cell (30, 80) comprises at least two additional resonators surrounding the common loop (32, 84) and the additional resonators are split ring resonators comprising a loop with two open ends forming a gap, wherein the gap in a first additional resonator is positioned at an opposite end of the unit cell (30, 80) to the gap in a second additional resonator. the plurality of unit cells (30, 80) each comprise a first cell having a first pair of adjacent resonators (20, 22, 24, 26, 28, 32, 84) and a second cell having a second pair of adjacent resonators (20, 22, 24, 26, 28, 32, 84) wherein a spacing between the first pair of adjacent resonators is different from a spacing between the second pair of adjacent resonators whereby each of the plurality of unit cells has the asymmetric structure; and wherein each unit cell (30, 80) comprises at least two additional resonators surrounding the common loop (32, 84) and the additional resonators are split ring resonators comprising a loop with two open ends forming a gap, wherein the gap in a first additional resonator is positioned at an opposite end of the unit cell (30, 80) to the gap in a second additional resonator.