A third-harmonic generation method includes frequency-quadrupling a fundamental optical beam via cascaded second-harmonic generation to yield a fourth-harmonic optical beam. The method also includes down-converting the fourth-harmonic optical beam to yield a third-harmonic optical beam. A third harmonic generator includes a monolithic optical element having nonzero quadratic electric susceptibility to (i) frequency-quadruple a fundamental optical beam via cascaded second-harmonic generation, and (ii) down-convert a fourth-harmonic optical beam to yield a third-harmonic optical beam.