structures. The optical BIC states in a wave system have been widely discussed and utilized. The quasi-BIC effect was first reported in passive systems in the form of a 1D line-and-space periodic structure. [6] Subsequently, some passive devices utilizing BIC modes supported in metallic metasurfaces, [7,8] dielectric metasurfaces, [9][10][11] and photonic elements [12,13] are demonstrated. In an active device, a BIC mode can support narrow-linewidth lasing with a smaller device size down to a few dozen periods of the BIC structure, which makes it possible for the laser device to be integrated at high density onto a chip. [14] Due to these advantages of BIC in light confinement, BIC-supported lasers in the NIR range were realized. [14][15][16][17] The lasing emission is observed from a nanoarray structure having feature sizes smaller than 10 µm. Later, BIC-supported lasers were realized in the visible range. [18][19][20][21][22][23][24] A highspeed optical switch with vortex lasing emission is also realized via BICs in the visible range. However, a BIC-supported nanolaser emitting in the ultraviolet (UV) has not yet been reported.With wavelengths ranging from 200 to 400 nm, UV nanolasers hold important applications in high-resolution bioimaging, laser therapy, spectroscopy, lithography, and optical storage. GaN with a wide direct bandgap of 3.41 eV at room temperature is the technological material employed for UV laser diodes. With the advanced of growth techniques such as metal-organic chemical vapor deposition (MOCVD) and vapor phase epitaxy growth method, [25,26] high-quality GaN can be readily deposited so that GaN-based UV lasers have been realized using singlecrystal GaN film, [27,28] Fabry-Perot nanowire cavities, [29][30][31][32][33] whispering-gallery-mode cavities, [34][35][36][37][38] and vertical cavity surface emitting lasers (VCSEL). [39,40] For random scattering lasers, Fabry-Perot lasers, and whispering-gallery-mode lasers, out-ofplane directional emission lasing remains difficult to achieve. In contrast, VCSELs are designed for out-of-plane emission but require a relatively large cavity to support lasing action. BICbased lasers have the potential to realize highly directional emissions with small device sizes.Here, we demonstrate a BIC-based UV laser with directional emission and tunable emission wavelength that is fabricated on a standard GaN thin film without any etching step. A 1D periodic resist structure supporting the BIC mode was fabricated directly on the GaN thin film by a single-step electron-beam (e-beam) Optical bound states in the continuum (BICs), realizing substantial suppression of out-of-plane radiative losses, have been utilized to realize strong light confinement and optical modes with high quality-factor (Q). Lasing actions with narrow linewidths based on optical BIC modes have been demonstrated in the near-infrared and the visible ranges, but BIC-based lasers in the ultraviolet (UV) region have not been reported. As light sources possessing wavelengths at the UV scale are esse...
