We explore a new class of distributed feedback (DFB) structures that employ the
recently-developed concept of parity-time (PT) symmetry in optics. We show that, based on
PT-symmetric pure reflective volume gratings, a vertical surface-emitting cavity can be
constructed. We provide a detailed analysis of the threshold conditions as well as the
wavelength and angular spectral characteristics using the Kogelnik coupled-wave
approximation, backed up by an exact solution of the Helmholtz equation. We show that such
a PT-symmetric cavity can be configured to support one and only one longitudinal mode,
leading to inherently single-mode lasing.