  Optically active spin defects in wide-bandgap materials have many potential
applications in quantum information and quantum sensing. Spin defects in
two-dimensional layered van der Waals materials are just emerging to be
investigated. Here we demonstrate that optically-addressable spin ensembles in
hexagonal boron nitride (hBN) can be generated by femtosecond laser
irradiation. We observe optically detected magnetic resonance (ODMR) of hBN
spin defects created by laser irradiation. We show that the creation of spin
defects in hBN is strongly affected by the pulse energy of the femtosecond
laser. When the laser pulse number is less than a few thousand, the pulse
number only affects the density of defects but not the type of defects. With
proper laser parameters, spin defects can be generated with a high probability
of success. Our work provides a convenient way to create spin defects in hBN by
femtosecond laser writing, which shows promising prospects for quantum
technologies.
