Multiphoton absorption with Bessel Gaussian beams

This study investigates multiphoton absorption in thin, homogeneous nonlinear materials using Gauss–Bessel beams as the incident laser source. Multiphoton absorption processes are crucial for advanced photonic applications including nonlinear microscopy, optical limiting and photodynamic therapy, yet theoretical frameworks for structured beams remain limited. We derive an analytical expression for the normalized optical transmittance for arbitrary order n of nonlinearity under the weak nonlinearity approximation. Numerical simulations focusing on two-photon absorption at 800[Formula: see text]nm wavelength reveal the unique characteristics of Gauss–Bessel beams in nonlinear interactions. Our simulations investigate Rayleigh lengths ranging from 0.407 to 3.979[Formula: see text]mm, demonstrating that the valley width increases by approximately a factor of 10 across this range. Our results demonstrate that these beams maintain consistent maximum absorption of approximately 0.28 across various Rayleigh parameter values, indicating sustained high nonlinear interaction efficiency over extended distances. This constant absorption efficiency contrasts markedly with conventional Gaussian beams, which exhibit decreasing valley depth for larger Rayleigh lengths. This property, combined with a controllable interaction region width, offers significant advantages over conventional Gaussian beams in nonlinear optics applications. The extended depth of focus and uniform nonlinear response of Gauss–Bessel beams suggest improved robustness and flexibility in optical devices, reducing alignment sensitivity and increasing working distances in nonlinear setups.