Embedded anisotropic microreflectors by femtosecond-laser nanomachining

Abstract

Directly written embedded structures created within fused silica by a femtosecond Ti:sapphire laser are observed to strongly reflect blue light. Reflection emerges only in a direction parallel to the polarization axis of the writing laser. This anisotropic-effect is caused by a periodic modulation of refractive index of amplitude ⌬nϳ10 Ϫ2 with a characteristic period ⌳ϳ150 nm over a spot size ϳ1.5 m. We show that the origin of the anisotropic reflection is the primary cause of other anisotropic phenomena reported in recent experiments. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1492004͔The use of a femtosecond laser source to directly write structures deep within transparent media has recently attracted much attention due to its simplicity compared to lithographic methods, and its capability for writing in three dimensions. [1][2][3] Tight focusing of the laser into the bulk of material causes nonlinear absorption only within the focal volume, depositing energy that induces a permanent material modification. 4,5 A variety of photonic devices have already been created by translating a sample through the focus of a femtosecond laser. 6,7 Although molecular defects caused by such intense irradiation have been identified in fluorescence, electron-spin resonance and other studies, 8 the mechanism of induced modifications in glass is still not fully understood. Moreover, such structures in Ge-doped silica 9 and other glass materials 10 show an unexpected anisotropic light scattering which is dependent on the plane of light polarization. This has been interpreted in terms of photoelectrons moving along the direction of light polarization inducing index inhomogenities. More recently, uniaxial birefringence imprinted in structures written within fused silica plates has been observed but the origin of this anisotropic phenomenon remains a mystery. 11 In this letter, we will describe a further anisotropic property observed in silica after being irradiated by a femtosecond laser-strong reflection from the modified region occurring only along the direction of polarization of the writing laser. Our analysis suggests that this effect is also the primary cause of all previously reported anisotropic phenomena.A regeneratively amplified mode-locked Ti:sapphire laser ͑150 fs pulse duration, 250 kHz repetition rate͒ operating at ϭ850 nm was used to directly write microstructures into a fused silica plate (40ϫ40ϫ1 mm). In the experimental arrangement ͑Fig. 1͒, the collimated laser passes through an electronic shutter, variable neutral density filter and halfwave plate before a dichroic mirror reflecting only in the 400-700 nm region. The infrared laser light travels through the mirror and is focused through a 50 ϫ(numerical apertureϭ0.55) objective into the bulk of the sample, down to a beam waist diameter estimated to be ϳ1.5m. The silica sample is mounted on a computer-controlled linear-motor three-dimensional translation stage ͑of 20 nm resolution͒. To simultaneously observe the writing process, a charge coupled device...

Extraordinary stability of anisotropic femtosecond direct-written structures embedded in silica glass

“…6 It has been shown that femtosecond laser writing in wide band gap materials can induce an optically isotropic positive index change 1 as high as 3 ϫ 10 −2 ͑type I_fs͒. However, when the material is irradiated above a certain intensity threshold ͑which depends on the material, laser wavelength and pulse duration͒ we observe that the femtosecond written structures are characterized by anisotropic scattering, 7 anisotropic reflection, 8 strong birefringence and average negative index change, 9 due to the formation a self-assembled periodic nanostructures. 10 During the energy absorption process, tunneling, multiphoton and avalanche ionization produce free electrons within the focus of the ultrashort pulsed laser.…”

Self-assembled sub-wavelength structures and form birefringence created by femtosecond laser writing in glass: properties and applications

“…Later uniaxial birefringence imprinted in structures written within fused-silica plates was observed. 7), 8) Anisotropic reflection was then observed and explained in terms of the formation of a self-organized nanograting within the focal volume, 9) and finally the existence of the self-induced nanograting was proved by direct imaging. 10), 11) During the energy absorption process, tunneling, multiphoton and avalanche ionization produce free electrons within the focus of the ultrashort pulsed laser.…”

“…14) The extraordinary stability of these nanogratings has also been reported. 15) Embedded micro-reflectors, 9) retardation plates, 13) and micro-fluidic channels 16) based on these nanostructures have been reported; however their optimization requires a systematic characterization of the properties of the periodic structures ver- Fig. 1.…”

Ultrafast light-matter interaction in transparent medium

“…Interestingly, at intermediate energies, birefringent index changes are induced. It has been shown that the anisotropy is caused by self-organized structures with subwavelength periodicity, the so-called nanogratings [9] that are oriented perpendicular to the polarization of the writing beam [10]. Since their discovery, much attention has been given to use nanogratings for the fabrication of microfluidic channels [11].…”