Analysis of all-optically tunable functionalities in sub-wavelength periodic structures by the Fourier modal method

We propose the nonlinear Fourier Modal Method (FMM) [J. Opt. Soc. Am. B 31, 2371 (2014)] as a convenient and versatile numerical tool for the design and analysis of grating based next generation all-optical devices. Here, we include several numerical examples where the FMM is used to simulate all-optically tunable functionalities in sub-wavelength periodic structures. At first, we numerically investigate a 1-D periodic nonlinear binary grating with amorphous TiO2. We plot the diffraction efficiency in the transmitted orders against the structure depth for normally incident plane wave. Change in diffraction efficiencies for different incident field amplitudes are evident from the plots. We verify the accuracy of our implementation by comparing our results with the results obtained with the nonlinear Split Field-Finite Difference Time Domain (SF-FDTD) method. Next we repeat the same experiment with vertically standing amorphous Titanium dioxide (TiO2) nanowire arrays grown on top of quartz which are periodic in two mutually perpendicular directions and examine the efficiencies in the direct transmitted light for different incident field amplitudes. Our third example includes analysis of a form birefringent linear grating with Kerr medium. With FMM we demonstrate that the birefringence of such a structure can be tuned by all-optical means. As a final example, we design a narrow band Guided Mode Resonance Filter (GMRF). Numerical experiments based on the nonlinear FMM reveal that the spectral tunability of such a filter can be obtained by all-optical means.This work is partially supported by the Academy of Finland (contract 285880)

Avant-garde femtosecond laser writing

Recently discovered phenomena of quill and non-reciprocal femtosecond laser writing in glasses and crystals are reviewed. Common beliefs that laser writing does not change when reversing beam scan or propagation direction are challenged

Femtosecond laser printed microoptics in hydrogenated amorphous silicon

Conventional optics (e.g. lenses or mirrors) manipulates the phase via optical path difference by controlling thickness or refractive index of material. Recently, a promising type of optics emerged which exploits geometric phase shift, when a lightwave is transformed by parameter other than optical path difference, e.g. polarization. Here, wavefront is modified by introducing spatially varying anisotropy and is a result of Panchatraman-Berry phase [1]. Theoretically any phase pattern can be achieved solely by means of geometric phase with efficiencies reaching 100% [2]. This allows continuous optical phase shifts and without phase resets, in stark contrast to conventional elements, wherein phase profiles are encoded as discrete optical path variations in refractive index or thickness, limiting performance. The geometric phase optics is a promising alternative for controlling and manipulating light, but it stumbles on the lack of adequate fabrication technology

Ultrafast laser calligraphy

Control of structural modifications inside transparent materials by varying the direction of pulse front tilt is demonstrated, achieving a calligraphic style of writing. Anisotropic ultrafast laser cavitation in the irradiated region is observed

Recent advances in ultrafast laser micromachining: from optical materials to living cells

Modification of transparent materials with ultrafast lasers has attracted considerable interest due to a wide range of applications including laser surgery, integrated optics, optical data storage, 3D micro- and nanostructuring [1,2]. Three different types of material modifications can be induced with ultrafast laser irradiation in the bulk of a transparent material, silica glass in particular: an isotropic refractive index change (type 1); a form birefringence associated with self-assembled nanogratings and negative refractive index change (type 2) [3,4]; and a void (type 3). In fused silica the transition from type 1 to type 2 and finally to type 3 modification is observed with an increase of fluence. Recently, a remarkable phenomenon in ultrafast laser processing of transparent materials has been reported manifesting itself as a change in material modification by reversing the writing direction [5]. The phenomenon has been interpreted in terms of anisotropic plasma heating by a tilted front of the ultrashort laser pulse. Moreover a change in structural modification has been demonstrated in glass by controlling the direction of pulse front tilt, achieving a calligraphic style of laser writing which is similar in appearance to that inked with the bygone quill pen [6]. It has also been a common belief that in a homogeneous medium, the photosensitivity and corresponding light-induced material modifications do not change on the reversal of light propagation direction. More recently it has observed that in a non-centrosymmetric medium, modification of the material can be different when light propagates in opposite directions (KaYaSo effect) [7]. Non-reciprocity is produced by magnetic field (Faraday effect) and movement of the medium with respect to the direction of light propagation: parallel (Sagnac effect) or perpendicular (KaYaSo effect). Moreover a new phenomenon of ultrafast light blade, representing itself the first evidence of anisotropic sensitivity of isotropic medium to femtosecond laser radiation has been recently discovered [8]. We anticipate that the synergy of advances in ultrafast laser micromachining with state of the art imaging techniques such as digital holographic microscopy will open new opportunities in laser material processing, laser surgery and optical manipulatio

New phenomena in ultrafast laser interaction with matter

Modification of transparent materials with ultrafast lasers has attracted considerable interest due to a wide range of applications including laser surgery, integrated optics, optical data storage, 3D micro- and nano-structuring [1]. Three different types of material modifications can be induced with ultrafast laser irradiation in the bulk of a transparent material, silica glass in particular: an isotropic refractive index change (type 1); a form birefringence associated with self-assembled nanogratings and negative refractive index change (type 2) [2, 3]; and a void (type 3). In fused silica the transition from type 1 to type 2 and finally to type 3 modification is observed with an increase of fluence. Recently, a remarkable phenomenon in ultrafast laser processing of transparent materials has been reported manifesting itself as a change in material modification by reversing the writing direction [4]. The phenomenon has been interpreted in terms of anisotropic plasma trapping and heating by a tilted front of the ultrashort laser pulse. Moreover a change in structural modification has been demonstrated in glass by controlling the direction of pulse front tilt, achieving a calligraphic style of laser writing which is similar in appearance to that inked with the bygone quill pen [5]. It has also been a common belief that in a homogeneous medium, the photosensitivity and corresponding light-induced material modifications do not change on the reversal of light propagation direction. More recently we have observed that in a non-centrosymmetric medium, modification of the material can be different when light propagates in opposite directions (KaYaSo effect) [6]. Non-reciprocity is produced by magnetic field (Faraday effect) and movement of the medium with respect to the direction of light propagation: parallel (Sagnac effect) or perpendicular (KaYaSo effect). We anticipate that the observed phenomena will open new opportunities in laser material processing, laser surgery, optical manipulation and data storage

Ultrafast light blade: anisotropic sensitivity of isotropic medium to femtosecond laser radiation

Material processing with ultrafast lasers has recently attracted considerable interest mainly due to a wide range of applications including laser surgery and 3D micro- and nano-structuring. However, we have recently demonstrated that in-depth investigation of the processes occurring in condensed medium in presence of intense ultrashort laser pulses opens new insight on the light-matter interaction at high intensities. In particular, we discovered that photosensitivity of non-centrosymmetric crystal is not necessary reciprocal [1], i.e. the light-induced modification depends on light propagation direction. We observed also strong dependence of the isotropic glass modification on the orientation of writing direction relative to the direction of pulse front tilt (quill writing effect) [2]. These effects indicate that at high intensities, a homogeneous illumination can produce essentially inhomogeneous modification in the medium, i.e. the presence of the hidden anisotropy in the light-matter interaction at high intensity. However until now this anisotropy manifested itself only when laser beam moves with respect to the sample. The question of fundamental importance is can this anisotropy manifest itself in the conventional material processing, i.e. when femtosecond light beam interacts with non-moving sample. One may recall that until now only two types of optical anisotropy have been identified. The first one is attributed to anisotropy of material structure being inherent, e. g. in crystals or externally produced e. g. by stress. Another type assigned to anisotropy of geometric structure, e. g. anisotropy of Fresnel reflections or form birefringence, is produced by macro or sub-wavelength scale interfaces and gradients of material. Here we demonstrate experimentally that uniform illumination an isotropic homogeneous medium can give rise to its inhomogeneous modification, i.e. the new mechanism of the optical anisotropy that manifests itself at ultrahigh intensities. Specifically, we observe dependence of the refractive index and absorption coefficient induced by intense ultrashort light pulse in an isotropic glass on the orientation of the polarization plane azimuth. This new phenomenon originates from the absorption anisotropy of electron plasma produced by the femtosecond light pulse with tilted intensity front [3]. Our results present the evidence of new type of collisionless heating mechanism [4] arising at the oblique interface produced by the pulse front tilt. We would like to point out that the observed anisotropic photosensitivity is intrinsic property of the light-matter interaction at high intensities. We anticipate that the observed phenomenon, which offers the orientation of a light polarization plane relative to the direction of pulse front tilt as a new tool to control interaction of matter with ultrashort light pulses, will open new opportunities in laser material processing, optical manipulation and data storage. We refer the observed phenomenon as ultrafast light blade, drawing an analogy between material modification produced by ulrashort light pulse with tilted front and cutting with a blade