Perovskite materials prepared in the form of solution-processed nanocrystals and used in top-down fabrication techniques are very attractive to develop low-cost and high-quality integrated optoelectronic circuits. Particularly, integrated miniaturized coherent...

Solution‐process perovskite quantum dots (QDs) are promising materials to be utilized in photovoltaics and photonics with their superior optical properties. Advancements in top‐down nanofabrication for perovskite are thus important for practical photonic and plasmonic devices. However, different from the chemically synthesized nano/micro‐structures that show high quality and low surface roughness, the perovskite QD thin film prepared by spin‐coating or the drop‐casting process shows a large roughness and inhomogeneity. Low‐roughness and low‐optical loss perovskite QD thin film is highly desired for photonic and optoelectronic devices. Here, this work presents a pressure‐assisted ligand engineering/recrystallization process for high‐quality and well‐thickness controlled CsPbBr3 QD film and demonstrates a low‐threshold and single‐mode plasmonic lattice laser. A recrystallization process is proposed to prepare the QD film with a low roughness (RMS = 1.3 nm) and small thickness (100 nm). Due to the low scattering loss and strong interaction between gain media and plasmonic nanoparticles, a low lasing threshold of 16.9 µJ cm−2 is achieved. It is believed that this work is not only important to the plasmonic laser field but also provides a promising and general nanofabrication method of solution‐processed QDs for various photonic and plasmonic devices.

Extensive studies on lead halide perovskites have shown that these materials are excellent candidates as gain mediums. Recently, many efforts have been made to incorporate perovskite lasers in integrated optical...

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...

The recent development of perovskite-based lasers showcased the outstanding optical properties of the material such as high absorption coefficient and high quantum yield. The lasers were demonstrated in the form of nanowires and nanoplates, which are difficult to be integrated on a chip in the form of high-density arrays due to the difficulties in positioning them on the chip. The solution to this problem should be to use the well-known lithography process in the fabrication process of the lasers. In this work, we demonstrate several perovskite-based plasmonic lasers that were fabricated by using the lithographic in-mold patterning method that relies on the electron beam lithography process. The lasers utilized CsPbBr3 perovskite nanocrystals as the gain material and plasmonic distributed Bragg reflector grating structure as the optical feedback provider to achieve a low lasing threshold of 42.5 μJ/cm2 with a linewidth of 0.6 nm (FWHM) at room temperature. The use of the lithographic process in the fabrication of the lasers makes it possible to fabricate and integrate them on a chip in a relatively high-density manner, so that they can be used extensively in quantum optics and on-chip integrated photonics applications.