Threshold characteristics in the emission from charged sources in periodic structures

It is shown that the amplitudes and phases of electromagnetic wave emitted by charged particles moving above periodic structures exhibit a characteristic behavior at the excitation threshold of waves of new orders. These characteristics are analogous to Wood's anomalies appearing in the diffraction of plane electromagnetic waves by periodic structures. Attention is given to the relation between amplitudes of waves of a repeating order with threshold S-matrix values and anomalies in waves of existing orders.