Tunable OpticalBottle Beam through a Laser-WrittenDual-Layer Nonlinear Grating

Abstract We demonstrate a tunable optical bottle beam produced via second-harmonic (SH) generation in a laser-written dual-layer lithium niobate (LN) nonlinear grating structure. By properly designing the χ(2) structures in different layers, a Gaussian mode and a higher-order radial mode are generated at the SH wave. Owing to the Gouy phase, their superposition naturally evolves from constructive to destructive interference along the propagation direction, forming a well-defined bottle region. By tuning the pump wavelength, one can easily modulate the relative phase between the SH modes, enabling continuous postfabrication control of the bottle’s characteristics without altering the device geometry. Experimentally, as the pump wavelength increases from 825 to 845 nm, the bottle center shifts by 0.25 mm toward the sample and the bottle length decreases by 0.2 mm. This unique operation, i.e., pulling inward and axially compressing the bottle beam by simply increasing the pump wavelength, exhibits the tunable characteristics of the optical bottle beam. Our scheme offers a compact and dynamically tunable platform to generate three-dimensional structured fields for applications in optical trapping and manipulations.