Optically induced metastability in untwinned single-domain YBa2Cu3O7.

Raman spectra of untwinned YBa{sub 2}Cu{sub 3}O{sub 7} with {ital T}{sub {ital c}}=91 K indicate a 2.2-eV resonance for incident light polarized in the CuO chain direction. The resonance is characterized by the appearance of nominally forbidden phonon modes at 232 and 597 cm{sup {minus}1}. Below 100 K the forbidden modes can be optically bleached if the photon energy exceeds 2.2 eV. The resulting optically induced low-temperature state is metastable; upon warming the crystal to {ital T}{gt}200 K the forbidden modes reappear as before.