Generation of high quality tunable Bessel beams using a liquid-immersion axicon

By immersing a conventional glass axicon in index-matching liquid, we generated high quality, tunable, quasi-non-diffracting Bessel beams. The aberrations resulting from the roundness of the axicon tip are minimized when a large base angle is used in liquid-immersion. This configuration also allows coarse and fine tunability through changing the liquid and adjusting the temperature, respectively. Our experimental results match very well with calculated intensity profiles. We succeeded to generate two-meter long plasma channels in air by focusing femtosecond laser pulses with the liquid-immersion axicon.