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• Abstract

SECTION I

## INTRODUCTION

Ultrashort-pulse micro- and nanomachining has been an active research topic for almost 20 years now. In particular, the possibility to realize structures with minimized thermal and mechanical damage has attracted much attention. These features enable the generation of structures with a resolution on the order of the laser wavelength. By exploiting nonlinear effects even structures with dimensions well below the diffraction limit can be obtained (for an overview on recent research activities see, e.g., [1], a special issue covering results of a Priority Programme funded by the Deutsche Forschungsgemeinschaft). Moreover, the nonlinear absorption process enables a true 3-D energy confinement at the position of the focus inside transparent media, since this is the region with the highest intensity.

Covering all excellent work within this fascinating topic area would however be impossible due to the page limits of this paper. Thus, we have decided to focus on specific areas and to explicitly exclude others. As an example, we will not cover laser-induced periodic surface structures, although there has been significant progress throughout the past years. In addition, we will not cover special beam shaping approaches like the generation of Bessel beams or alike. For your reference, please refer to [2] for further details on this topic.

Within this paper, we will concentrate on transparent samples and consequently highly nonlinear interaction processes. The topics covered include recent advances in two-photon polymerization (2PP) as a means to generate nanoscale structures by additive manufacturing, the realization of self-organized periodic nanostructures, so-called nanogratings, slightly larger deterministic periodic refractive index changes as used for Bragg gratings, and, finally, the application of ultrashort pulses in biophotonics.

SECTION II

## 2PP

For 3-D nanomachining, 2PP has proven to be a very powerful technique. Ultrashort laser pulses are tightly focused using immersion microscope objectives into a photosensitive resin that is transparent for the applied laser wavelength. Curing of the resin only takes place in the focal region of the objective, where the intensities are sufficiently high to initialize a radical polymerization reaction via two-photon absorption (TPA). By 3-D scanning of the focus through the resin, complex 3-D microstructures can be fabricated with feature sizes smaller than 100 nm. However, it is currently under investigation if TPA is really the main mechanism for the generation of free radicals. Malinauskas et al. [3] showed that radical generation is primarily caused by avalanche ionization and not TPA for a low (1-kHz) repetition rate illumination with 800-nm pulses of 150-fs duration.

Nevertheless, the outstanding capability of 2PP in contrast to conventional lithographic techniques has led to various applications in photonics and biomedicine. For instance, Ergin et al. [4] realized an invisibility-cloaking structure by writing a woodpile photonic crystal that covered a bump in a gold reflector at optical wavelengths for viewing angles up to 60°. By writing scaffolds with selectively biofunctionalized parts, the shape and adhesion of cells cultured in that scaffolds could be controlled in three dimensions [5].

A challenging task is further improvement of the resolution of the structuring process. In particular, the axial resolution is an issue. Due to the shape of the focal intensity distribution of the microscope objectives, the ratio of axial to lateral resolution is limited to about 3 : 1. One promising technique to improve that ratio and the overall resolution of 2PP is inspired by nonlinear microscopy, the so-called stimulated-emission-depletion (STED) microscopy; a second laser beam is applied to deactivate the excited photoinitiator through stimulated emission, therefore reducing the polymerized volume. With the use of a depletion laser, the axial to lateral aspect ratio could be reduced to 1.6 [6]. Another possibility is the employment of amplitude filters in the aperture of the focusing optics, which also leads to a decrease in the axial dimension of the fabricated structures [7].

A further challenge is the correction of aberrations induced by the refractive index mismatch between photoresist and immersion oil of the objective. These aberrations, which are writing-depth dependent, are causing a significant decrease of resolution and intensity with increasing structure heights, complicating the homogeneous fabrication of structures with heights above approximately 10 $\mu\hbox{m}$ depending on the actual refractive index mismatch. Besides the well-established method of aberration correction with spatial light modulators, deformable mirrors, or a combination thereof [8], further possibilities are direct immersion into the polymer without the use of immersion oil [9] or to apply specially corrected focusing optics [10].

SECTION III

## NANOGRATINGS

One very active research topic is the formation of birefringent domains inside fused silica through the interaction of ultrashort laser pulses. The birefringence originates from self-organized periodic structures, so-called nanogratings, which depict a period of a few hundreds of nanometer, depending on the inscription wavelength. For the complete formation of these gratings, numerous pulses are required [11], and their orientation is perpendicular to the orientation of the laser polarization. This fact can be used to imprint the local polarization in the focal region into the processed material. Hnatovsky et al. demonstrated that this technique depicts subwavelength resolution, which enables even the imaging of complex focal polarization states [12]. In addition, Dai et al. reported an additional tilt of the orientation of the nanogratings. This shift is induced by the pulse front tilt of the processing pulses and allows the inscription of rotated nanogratings [13]. A setup to control the formation of nanogratings during the inscription was shown by Mauclair et al. [14]. Here, the diffraction pattern of the inscribed nanograting is analyzed during the inscription process. By changing the temporal pulse envelope, the grating period can be controlled. Lancry et al. have shown that the birefringence of the modified area can be enhanced by slightly doping of the glass [15].

Furthermore, the application of nanogratings for birefringent optical devices was the scope of multiple publications. The possibility to inscribe nanogratings even with repetition rates up to 10 MHz enables the time efficient realization of birefringent devices [11]. As the birefringence of nanogratings can be controlled precisely, several new birefringent devices [as an example see Fig. 1(a)] were realized, e.g., a polarization converter, which enables the generation of optical vortices [16] or integrated polarization beam splitters within fibers [17]. Another application was shown by Li et al., who inscribed multiple Bragg stop bands based on nanogratings in optical waveguides [18]. As the period of the nanogratings can be controlled by the processing parameters, the Bragg peak can be shifted and controlled, too.

Fig. 1. (a) Nanogratings with locally varying orientation inscribed into the bulk of fused silica by ultrashort laser pulses as an example for a complex polarization converter (courtesy of C. Vetter, Institute of Applied Physics, Friedrich-Schiller-Universität Jena, Germany). (b) Schematic illustration of nanomanipulations using gold nanoparticles and resonant illumination with a laser beam (courtesy of D. Yelin, Technion—Israel Institute of Technology, Haifa, Israel).

Despite the fascinating possibilities the artificial birefringence offers, the formation process of nanogratings is still not completely understood. Beresna et al. [19] proposed that excition–polaritons mediate the imprinting of nanogratings in fused silica. Their simulations and experimental data yield evidence that an initial polariton grating is frozen into the glass and remains as the well-known nanograting. In addition, Richter et al. [20] conducted a double-pulse experiment, from which they followed the importance of self-trapped excitions (STEs) for the cumulative action of isolated laser pulses. In addition, they could show that nanogratings in fused silica exhibit a large amount of dangling bond type defects, which also lead to a modification of the material. The actual constituents of the nanogratings have been revealed by small angle X-ray scattering (SAXS) and focused ion beam (FIB) milling. It has been proven that nanogratings consist of hollow cavities with a size of several tens of nanometer [21]. The sheet-like arrangement of these cavities forms the periodicity of the nanogratings.

SECTION IV

## BRAGG STRUCTURES

The interest in femtosecond written Bragg gratings has been steadily growing within the last years. Bragg gratings are periodic modulations of the refractive index of a transparent material like optical fibers and bulk glasses. For the inscription of fiber Bragg gratings (FBGs), there are mainly two different approaches used. The direct inscription with the Point-by-Point (PbP) technique and the phase mask technique. An overview on the fundamentals of fs-written gratings can be found in [22].

The use of the PbP technique allows the inscription of highly localized FBGs. This can be exploited for tailoring the coupling to specific cladding modes [23], a promising approach for sensing applications. The direct inscription method is very versatile concerning grating period and chirp. However, the high refractive index contrast is due to the formation of voids. Thus, these modifications have to deal with relatively strong scattering losses. Williams et al. [24] optimized the reflectivity of PbP gratings by reducing the losses, which is interesting for fiber laser applications. Furthermore, the direct inscription technique was used to build up a dual-wavelength waveguide laser in an Yb-doped phosphate glass [25]. Phase-shifted Bragg grating waveguides could be realized by Grenier et al. [26] enabling a better control in 3-D integrated optical circuits.

The drawback of the phase mask method of a fixed grating period provided by the phase mask could be overcome by using a deformed wave front of the inscription laser [27]. Thus, the period can be shifted and chirped. Furthermore, the temperature stability of fs-generated FBGs in an active Yb-doped fiber has been investigated [28]. Losses in the visible spectral range after grating inscription into the active fiber with UV fs radiation could be attributed to defects and color center formation. These defects and, consequently, the absorption could be reduced by an annealing process. Troy et al. [29] investigated the role of hydrogen loading of FBGs inscribed into fibers with different glass compositions. The combination of the phase mask inscription and selective chemical etching was used to fabricate a microhole array in a fiber, which can be used for sensing applications [30]. A monolithic narrow linewidth laser could be realized by two matched FBGs in a phosphate fiber [31]. These fibers can be highly doped with erbium. Thus, with very short cavity length, single-frequency fiber lasers can be built.

However, the gratings do not necessarily have to be inscribed into fibers. If a bulk substrate is used a volume Bragg grating (VBG) can be realized, offering applications e.g., for laser diode stabilization, integrated optics or high power beam combining. The fabrication of these gratings can also be achieved using the PbP inscription, the phase mask method, or an interferometric technique [32]. Recently, a 2-D VBG inscribed with a phase mask in fused silica could be realized [33].

SECTION V

## BIOPHOTONICS

In both cases, if the particles are positioned at the cell membrane, a transient opening can be achieved, and uptake of foreign molecules like DNA and RNA is enabled. Conjugation of the particle allows a cell-type specific targeting. The efficiencies of this so-called transfection strongly depend on laser and particle parameters, cell type, and the molecules to be delivered [45]. Several groups applied this technology for gene expression [35], [37], [41] and gene knockdown [36] in living cells with efficiencies and cell viabilities similar or higher than standard physical or chemical transfection methods. Certainly, this technique is as well suitable in cancer therapy to achieve selective cell killing by permanent perforation of the cells and, thus, induction of necrosis or apoptosis, as shown in carcinoma cells in vitro with in-resonance illumination [38].

As a possible advantage of the nanomanipulation of cells, this highly selective procedure might one day be applicable in vivo, using the spatial confinement of the laser irradiation within a tissue to very selectively influence gene expression within a living organism. Nevertheless, the challenges of intratissue labeling of cells with nanoparticles, as well as controlled energy deposition of laser radiation at different tissue depths, have to be tackled.

## Footnotes

Corresponding author: S. Nolte (e-mail: stefan.nolte@uni-jena.de)

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