Effect of decreasing pressure on soliton self-compression in higher-order modes of a gas-filled capillary

We numerically investigate soliton self-compression in the higher-order modes of a gas-filled capillary with decreasing pressure. demonstrate four times enhancement compression pressure compared to equivalent constant case HE 12 mode, reaching sub-cycle duration 1.85 fs at its output. Moreover, negative gradient effectively suppresses intermodal coupling later stage compressor, which helps maintain high output mode purity. These findings are direct benefit for applications that require ultrashort light pulses unconventional spatial beam profiles, including nonlinear frequency conversion, microscopy, micromachining, and particle manipulation.