There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

Highly sensitive, label-free biodetection methods have applications in both the fundamental research and healthcare diagnostics arenas. Therefore, the development of new transduction methods and the improvement of the existing methods will significantly impact these areas. A brief overview of the different types of biosensors and the critical parameters governing their performance will be given. Additionally, a more in-depth discussion of optical devices, surface functionalization methods to increase device specificity, and fluidic techniques to improve sample delivery will be reviewed.

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Label-free biological and chemical sensors

The rapid development of the femtosecond laser has revolutionized materials processing due to its unique characteristics of ultrashort pulse width and extremely high peak intensity. The short pulse width suppresses the formation of a heat-affected zone, which is vital for ultrahigh precision fabrication, whereas the high peak intensity allows nonlinear interactions such as multiphoton absorption and tunneling ionization to be induced in transparent materials, which provides versatility in terms of the materials that can be processed. More interestingly, irradiation with tightly focused femtosecond laser pulses inside transparent materials makes three-dimensional (3D) micro- and nanofabrication available due to efficient confinement of the nonlinear interactions within the focal volume. Additive manufacturing (stereolithography) based on multiphoton absorption (two-photon polymerization) enables the fabrication of 3D polymer micro- and nanostructures for photonic devices, micro- and nanomachines, and microfluidic devices, and has applications for biomedical and tissue engineering. Subtractive manufacturing based on internal modification and fabrication can realize the direct fabrication of 3D microfluidics, micromechanics, microelectronics, and photonic microcomponents in glass. These microcomponents can be easily integrated in a single glass microchip by a simple procedure using a femtosecond laser to realize more functional microdevices, such as optofluidics and integrated photonic microdevices. The highly localized multiphoton absorption of a tightly focused femtosecond laser in glass can also induce strong absorption only at the interface of two closely stacked glass substrates. Consequently, glass bonding can be performed based on fusion welding with femtosecond laser irradiation, which provides the potential for applications in electronics, optics, microelectromechanical systems, medical devices, microfluidic devices, and small satellites. This review paper describes the concepts and principles of femtosecond laser 3D micro- and nanofabrication and presents a comprehensive review on the state-of-the-art, applications, and the future prospects of this technology.

Femtosecond laser three-dimensional micro- and nanofabrication

Photonic lanterns are made by adiabatically merging several single-mode cores into one multimode core. They provide low-loss interfaces between single-mode and multimode systems, where the precise optical mapping between cores and individual modes is unimportant.

The photonic lantern

Femtosecond laser exposure produces form and stress birefringence in glasses, mainly controlled by laser polarization and pulse energy, which leads to challenges in certain applications where polarization mode dispersion or birefringence splitting is critical for the desired responses from optical devices. In this paper, parallel laser modification tracks with different geometries were applied to preferentially stress the laser-written waveguides and explore the possibility of tuning the waveguide birefringence in devices fabricated in bulk fused silica glass. Polarization splitting in Bragg grating waveguides showed the laser modification tracks to controllably add or subtract stress to the pre-existing waveguide birefringence, demonstrating independence from the nanograting induced form birefringence and the contributions from material stress. Stressing bars are shown that offer tunable birefringence in the range from ∼0 up to 4.35 × 10−4, possibly enabling great flexibility in designing polarization dependent devices, as well as making polarization independent devices.

Stress induced birefringence tuning in femtosecond laser fabricated waveguides in fused silica

Microfluidic channels and Bragg Grating Waveguides (BGWs) were simultaneously fabricated inside fused silica glass by means of femtosecond laser exposure followed by chemical etching. Evanescent field penetration of the waveguide mode into the parallel microfluidic channel induced Bragg resonant wavelength shifts to enable refractive index characterization of the fluidic medium in the 1 to 1.452 range. Laser exposure was optimized to fabricate devices with optically smooth channel walls and narrow Bragg resonances for high sensing response at 1560 nm wavelength. Reference gratings were also employed in the optical circuit for temperature and strain compensation. These devices open new directions for optical sensing in three-dimensional optofluidic and reactor microsystems.

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Femtosecond laser written optofluidic sensor: Bragg grating waveguide evanescent probing of microfluidic channel

Integrated polarization beam splitters based on birefringent directional couplers are demonstrated. The devices are fabricated in bulk fused silica glass by femtosecond laser writing (300 fs, 150 nJ at 500 kHz, 522 nm). The birefringence was measured from the spectral splitting of the Bragg grating resonances associated with the vertically and horizontally polarized modes. Polarization splitting directional couplers were designed and demonstrated with 0.5 dB/cm propagation losses and −19 dB and −24 dB extinction ratios for the polarization splitting.

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Femtosecond laser fabrication of birefringent directional couplers as polarization beam splitters in fused silica.

Femtosecond laser (300 fs, 500 kHz, 522 nm) fabrication of optical waveguides in bulk silica glass is extended to waveguide retarders. We study the merits of nanograting orientation (perpendicular or parallel to the waveguide) for generating high and low birefringence waveguides. This is used together with other exposure condition to control the waveguide birefringence between 10−5 and 10−4 permitting for the simultaneous fabrication of the waveguides and the tuning of the retardance demonstrating quarter and half-wave retarders in the 1200 nm to 1700 nm spectrum. The wavelength dependence of the birefringence is also characterized over a range of exposure conditions.

Femtosecond laser writing of waveguide retarders in fused silica for polarization control in optical circuits

Femtosecond laser-fabricated waveguides have been formed into helical paths throughout the cladding of single-mode optical fibers to demonstrate a strain-independent fiber torsion sensor A comparison between a Bragg grating sensor and a Mach–Zehnder based on helical waveguides (HWs) showed a much weaker twist sensitivity of 15 pm/(rad/m) for the grating in contrast with a value of 261 pm/(rad/m) for the interferometer The HW geometry provided an unambiguous determination of the rotational direction of the twist while facilitating a convenient and efficient means for optical coupling into the single-mode core of the fiber The flexible three-dimensional writing by the femtosecond laser fabrication method enabled the direct inscription of compact and robust optical cladding devices without the need for combining or splicing multiple-fiber segments

Jason R. Grenier

Papers

Stress induced birefringence tuning in femtosecond laser fabricated waveguides in fused silica

Femtosecond laser written optofluidic sensor: Bragg grating waveguide evanescent probing of microfluidic channel

Femtosecond laser fabrication of birefringent directional couplers as polarization beam splitters in fused silica.

Femtosecond laser writing of waveguide retarders in fused silica for polarization control in optical circuits

Fiber optic stress-independent helical torsion sensor.