A structure comprising a substrate comprising a region of monocrystalline silicon, the region comprising: a first surface which is a crystal plane of the monocrystalline silicon; and a second surface (5210.4) which is a crystal plane of the monocrystalline silicon; wherein the first and second surfaces (5210.4) meet each other at an angle greater of 90°, the angle being measured through a space occupied by the monocrystalline silicon, and wherein: the second surface (5210.4) is a first reflector in a path of electromagnetic waves, the path having a first segment parallel to the first surface and a second segment passing through the first surface, the first and second segments meeting at the first reflector; and the structure further comprises a first transducer (120.2) meeting the second segment of the path, for receiving the electromagnetic waves from the path; the structure characterized in that: the region further comprises a third surface (5210.2) opposite to the second surface (5210.4), wherein the first and third surfaces meet each other at an angle greater of 90°, the angle being measured through a space occupied by the monocrystalline silicon; the third surface is a second reflector, and the path has a third segment passing through the first surface, the first and third segments meeting at the second reflector; and the structure further comprises a second transducer (120.1) meeting the third segment of the path, for transmitting the electromagnetic waves into the path. a first surface which is a crystal plane of the monocrystalline silicon; and a second surface (5210.4) which is a crystal plane of the monocrystalline silicon; wherein the first and second surfaces (5210.4) meet each other at an angle greater of 90°, the angle being measured through a space occupied by the monocrystalline silicon, and wherein: the second surface (5210.4) is a first reflector in a path of electromagnetic waves, the path having a first segment parallel to the first surface and a second segment passing through the first surface, the first and second segments meeting at the first reflector; and the structure further comprises a first transducer (120.2) meeting the second segment of the path, for receiving the electromagnetic waves from the path; the structure characterized in that: the region further comprises a third surface (5210.2) opposite to the second surface (5210.4), wherein the first and third surfaces meet each other at an angle greater of 90°, the angle being measured through a space occupied by the monocrystalline silicon; the third surface is a second reflector, and the path has a third segment passing through the first surface, the first and third segments meeting at the second reflector; and the structure further comprises a second transducer (120.1) meeting the third segment of the path, for transmitting the electromagnetic waves into the path. the second surface (5210.4) is a first reflector in a path of electromagnetic waves, the path having a first segment parallel to the first surface and a second segment passing through the first surface, the first and second segments meeting at the first reflector; and the structure further comprises a first transducer (120.2) meeting the second segment of the path, for receiving the electromagnetic waves from the path; the region further comprises a third surface (5210.2) opposite to the second surface (5210.4), wherein the first and third surfaces meet each other at an angle greater of 90°, the angle being measured through a space occupied by the monocrystalline silicon; the third surface is a second reflector, and the path has a third segment passing through the first surface, the first and third segments meeting at the second reflector; and the structure further comprises a second transducer (120.1) meeting the third segment of the path, for transmitting the electromagnetic waves into the path.