we report the lifetime enhancement of light-induced refractive index grating by applying a bias field during writing. in comparison with the lifetime of about 10 hours of the photorefractive grating prepared without a bias field, the lifetime of the grating with a 4 kv/cm bias field can be prolonged to 7.5 years, which is obtained from the dynamic behavior of grating visualized and monitored with digital holographic microscopy. the higher the bias field is applied, the longer the dark decay time of grating can be achieved. the enhanced lifetime of phase grating is attributed to polar nanoregions oriented by external field. this effect is of great significance for electro-holographic device applications. (c) 2017 optical society of america