Polarization-dependent ablation of silicon using tightly focused femtosecond laser vortex pulses Cyril Hnatovsky 1, *, Vladlen G. Shvedov 1 , Natalia Shostka 1,2 , Andrei V. Rode 1 , and Wieslaw Krolikowski 1 1 Laser Physics Centre, The Australian National University, Canberra ACT 0200, Australia 2 Department of Physics, Taurida National V. Vernadsky University, Simferopol 95007 Crimea, Ukraine *Corresponding author: chn111@physics.anu.edu.au Received October 27, 2011; revised November 29, 2011; accepted November 29, 2011; posted November 30, 2011 (Doc. ID 157048); published January 13, 2012 We demonstrate experimentally that, in a tight focusing geometry, circularly polarized femtosecond laser vortex pulses ablate material differently depending on the handedness of light. This effect offers an additional degree of freedom to control the shape and size of laser-machined structures on a subwavelength scale. © 2012 Optical Society of America OCIS codes: 320.2250, 140.3390, 320.7160, 140.3440, 140.3300. Focused femtosecond laser pulses are now routinely used to micromachine or change properties of different materials. By focusing the pulses very tightly, and simul- taneously keeping the light intensity in the focal region near the optical breakdown threshold, the structural changes can be spatially localized to tens of nanometers [ 1]. So far, this combined approach has been applied for high-density optical recording [ 2], fabrication of three- dimensional nanofluidic channels [ 3], single-cell surgery [ 4], and synthesis of novel material phases [ 5] using lin- early polarized Gaussian beams. Focusing an electromagnetic wave inevitably changes its initial state of polarization [ 6]. A wave polarized along the x direction after focusing produces a focal electric field distribution with nonzero y and z components. This “depolarization effect” scales with the NA of the focusing optics and becomes pronounced, for instance, in the case of radially polarized light when the z component in the tight focus (i.e., high NA) can dominate [ 7]. Tightly fo- cused radially polarized beams and beams carrying the optical angular momentum have already found several applications, including orientational imaging of single mo- lecules [ 8], Raman spectroscopy [ 9], second-harmonic generation [ 10], and particle trapping and manipulation [ 11]. Studies on the use of such beams in the femto- second laser micromachining domain have started only recently [ 12– 14]. In this Letter we investigate how the vectorial nature of tightly focused femtosecond laser vortex pulses affects their interaction with crystalline silicon (Si), an impor- tant material for the semiconductor industry. Specifi- cally, we demonstrate that circularly polarized vortex pulses of the opposite handedness but with identical in- tensity distribution at the entrance pupil of the focusing optics can produce ablation features that differ dramati- cally in shape and size. The experimental setup is schematically shown in Fig. 1. The linearly polarized output beam of a femtosecond Ti: sapphire amplifier with a central wavelength of λ  775 nm is first spatially filtered and attenuated. An optical vortex with the topological charge m  1 is generated with the fused silica segmented phase mask (M). The vor- tex beam is expanded with the Galilean telescope (T) and then spatially filtered using the pinhole (P) to suppress the parasite modes caused by the diffraction on the bound- aries of the segments of M. After recollimation the beam is transformed into an either left- or right-handed circu- larly polarized beam with the quarter-wave plate λ∕4. The negative corrector lens (C) of focal distance -200 mmis placed in front of the microscope objective (O) to com- pensate for spherical aberration, which is introduced when finite conjugate distance optics (Nikon M Plan 100×, 210∕0, NA  0.9) is used for focusing collimated beams. The pulse duration after the λ∕4 is ∼200 fs (FWHM) based on collinear autocorrelation measurements. In our experiments we ablated Si in air using either fixed spot irradiation or by moving the sample perpendi- cular to the laser beam propagation direction z. In both cases we performed ablation when the spin and orbital angular momentum of the pulses, which are defined by the handedness of the circular polarization σ and the sign of m, respectively, were either parallel or anti- parallel. According to the theory [ 15], these two situa- tions correspond to quite different focal intensity distributions in the regime of tight focusing. The simulated intensity distributions in vacuum for the longitudinal (E z ) and transverse (E ⊥ ) electric field com- ponents in the focal plane of a NA  0.9 aplanatic objec- tive for m  1, σ  -1, and m  1, σ  1 are presented in Figs. 2(a) and (b), respectively. The simulations are based on the vectorial Debye integral with a Laguerre– Gaussian amplitude function [ 6, 15]. The complex ampli- tudes of the electric field components at any point in the image space, which is not too close to the exit pupil [ 6], can then be written as Fig. 1. (Color online) Setup for femtosecond laser micro- machining. M, segmented phase mask; T, 2.5× Galilean tele- scope; P, pinhole; L, collimating lens; λ∕4, quarter-wave plate; C, corrector lens; O, microscope objective; Si, silicon wafer. 226 OPTICS LETTERS / Vol. 37, No. 2 / January 15, 2012 0146-9592/12/020226-03$15.00/0 © 2012 Optical Society of America