Bulk Transparent Materials Research Articles

Fabrication of multifunctional components in a glass chip with femtosecond laser direct writing

Femtosecond lasers have opened up new avenues in materials processing due to their unique characteristics of ultrashort pulse widths and extremely high peak intensities. 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 multiphoton absorption to be induced in materials that are transparent to the laser wavelength. More interestingly, irradiation with tightly focused femtosecond laser pulses inside transparent materials provides a facile route to modify the interior of transparent materials in a spatially selective manner, enabling three-dimensional (3D) fabrication and integration of multifunctional micro-/nano-structures and components in a monolithic substrate. For instance, femtosecond laser pulses have been used to write optical waveguides in both passive and active materials by locally modifying their refractive indices. In combination with wet chemical etching, femtosecond laser direct writing has also been used to fabricate microfluidic structures, including microchannels and chambers, microvalves, and micropumps. The same technique has been extended to fabricate free-space optics such as micromirrors and micro-optical lenses in glass materials. By virtue of its unique ability to build different types of functional components into a monolithic substrate, femtosecond laser direct writing offers a flexible approach to fabricate a wide variety of integrated devices and microsystems. Although femtosecond laser micromachining have indeed shown extreme flexibility for fabrication and integration of 3D multifunctional micro- components in bulk transparent materials, several major issues still exist, such as the limited size of the microfluidic structures, the limited fabrication resolution, high surface roughness, and so on. This review focuses primarily on the recent efforts to tackle the two issues as mentioned above. By use of femtosecond laser direct writing in porous glass immersed in water followed by post-annealing, we demonstrated microfluidic channels with nearly unlimited lengths and arbitrary 3D geometries. By controlling the laser peak intensity and polarization, a single nanoplane with sub-50-nm feature size could be achieved inside porous glass based on these strategies, several functional devices and their applications have been demonstrated, including 3D passive microfluidic mixer and an integrated micro-nanofluidic system for single DNA analysis. Furthermore, we demonstrate fabrication of 3D whispering gallery microcavities with quality (Q)-factors on the level of ~1×106 in glass chips by femtosecond laser direct writing followed by CO2 laser reflow.

Femtosecond Laser Fabrication of Periodical Structures in Bulk of Transparent Dielectrics

We present results of experiments in development technology of precision micromachining of materials by focusing the femtosecond laser radiation into the bulk of transparent materials. Conditions of the realization and some key parameters of special regime of micromachining have been defined. It implements at certain ratio between scanning speed and repetition rate of femtosecond laser pulses. Under mentioned conditions the area destroyed by laser radiation moves along the optical axis near focus region in both directions, forming (during transverse scanning) in a bulk of the sample “periodical” structures. In results of our investigation we demonstrate precision cutting crystals, glasses, polymers and creating cylindrical cavities 1÷2 microns in diameter with aspect ratio more than 200 directed along the axis of laser beam.

Purification, organophilicity and transparent fluorescent bulk material fabrication derived from hydrophilic carbon dots

High-efficiency oil/water interfacial self-assembly strategy has been used for the first time to purify hydrophilic CDs prepared by hydrothermal method.

Interferometric characterization of pulse front tilt of spatiotemporally focused femtosecond laser pulses.

We report on an experimental measurement of the pulse front tilt (PFT) of spatiotemporally focused femtosecond laser pulses in the focal plane in both air and bulk transparent materials, which is achieved by examination of the interference pattern between the spatiotemporally focused pulse and a conventional focused reference pulse as a function of time delay between the two pulses. Our simulation results agree well with the experimental observations.

Tunable Multicolored Femtosecond Pulse Generation Using Cascaded Four-Wave Mixing in Bulk Materials

This paper introduces and discusses the main aspects of multicolored femtosecond pulse generation using cascaded four-wave mixing (CFWM) in transparent bulk materials. Theoretical analysis and semi-quantitative calculations, based on the phase-matching condition of the four-wave mixing process, explain the phenomena well. Experimental studies, based on our experiments, have shown the main characteristics of the multicolored pulses, namely, broadband spectra with wide tunability, high stability, short pulse duration and relatively high pulse energy. Two-dimensional multicolored array generation in various materials are also introduced and discussed.

Formation Dynamics of Ultra-Short Laser Induced Micro-Dots in the Bulk of Transparent Materials

In this paper, we study the formation dynamics of ultra-short laser-induced micro dots under the surface of transparent materials. Laser-induced micro dots find their application in direct part marking, to address full life cycle traceability. We first demonstrate the possibility of direct laser part marking into the cladding of an optical fiber. Then, we monitor the laser affected zone with the help of a time-resolved phase contrast microscopy setup in a fused silica substrate. We show that the transient energy relaxation processes affect the host material over a region that exceeds the micro dot size by several micrometers.

Corrigendum to “Formation Dynamics of Ultra-Short Laser Induced Micro-Dots in the Bulk of Transparent Materials” [PHPRO 41 (2013) 769–773

The authors regret that the Acknowledgements section was incomplete in this article. The correct Acknowledgement section is presented below. The authors would like to apologise for any inconvenience caused. Acknowledgements This study was funded through the project “16891 BG – microdots”, financed by the Forschungsvereinigung Feinmechanik Optik und Medizintechnik e.V. (FOM) through the Arbeitsgemeinschaft industrieller Forschung (AiF) in the frame of the program “Förderung der industriellen Gemeinschaftsforschung (IGF)” from the ministry for economy and technology (BMWi), in application of a decision from the German parliament.

Optically Generated Sub-100 nm Structures for Biomedical and Technical Applications

Abstract Near infrared femtosecond laser - material interactions such as multiphoton ionization and plasma formation provide the possibility to perform 3D nanoprocessing in a variety of organic and non-organic materials. Sub-100 nm structures that are more than one order smaller than the laser wavelength and therefore far below Abbes diffraction limit can be generated on the surface and inside the bulk. Compared to conventional extreme ultraviolet lithography, multiphoton technology provides the chance to produce 3D nanofeatures much simpler, less expensive, more flexible, and even inside transparent bulk material. Rapid prototyping and low power nanosurgery are two examples of applications of this novel nonlinear nanotechnology tool. This paper provides an overview of projects within Germany regarding to multiphoton generation of sub-100 nm structures.

Indentation device for in situ Raman spectroscopic and optical studies

Instrumented indentation is a widely used technique to study the mechanical behavior of materials at small length scales. Mechanical tests of bulk materials, microscopic, and spectroscopic studies may be conducted to complement indentation and enable the determination of the kinetics and physics involved in the mechanical deformation of materials at the crystallographic and molecular level, e.g., strain build-up in crystal lattices, phase transformations, and changes in crystallinity or orientation. However, many of these phenomena occurring during indentation can only be observed in their entirety and analyzed in depth under in situ conditions. This paper describes the design, calibration, and operation of an indentation device that is coupled with a Raman microscope to conduct in situ spectroscopic and optical analysis of mechanically deformed regions of Raman-active, transparent bulk material, thin films or fibers under contact loading. The capabilities of the presented device are demonstrated by in situ studies of the indentation-induced phase transformations of Si thin films and modifications of molecular conformations in high density polyethylene films.

Single-pulse ultrafast laser imprinting of axial dot arrays in bulk glasses

Ultrafast laser processing of bulk transparent materials can significantly gain flexibility when the number of machining spots is increased. We present a photoinscription regime in which an array of regular dots is generated before the region of main laser focus under single-pulse exposure in fused silica and borosilicate crown glass without any external spatial phase modulation. The specific position of the dots does not rely on nonlinear propagation effects but is mainly determined by beam truncation and is explained by a Fresnel propagation formalism taking into account beam apodization and linear wavefront distortions at the air/glass interface. The photoinscription regime is employed to generate a two-dimensional array of dots in fused silica. We show that an additional phase modulation renders flexible the pattern geometry.

Ultrafast Laser Induced Structural Modification of Fused Silica—Part I: Feature Formation Mechanisms

Nonlinear absorption of femtosecond-laser pulses enables the induction of structural changes in the interior of bulk transparent materials without affecting their surface. This property can be exploited for transmission welding of transparent dielectrics, three dimensional optical data storages, and waveguides. In the present study, femtosecond-laser pulses were tightly focused within the interior of bulk fused silica specimen. Localized plasma was formed, initiating rearrangement of the network structure. Features were generated through employment of single pulses as well as pulse trains using various processing conditions. The change in material properties were studied through employment of differential interference contrast optical microscopy and atomic force microscopy. The morphology of the altered material as well as the nature of the physical mechanisms (thermal, explosive plasma expansion, or in-between) responsible for the alteration of material properties as a function of process parameters is discussed.

Ultrafast Laser Induced Structural Modification of Fused Silica—Part II: Spatially Resolved and Decomposed Raman Spectral Analysis

Nonlinear absorption of femtosecond laser pulses enables the induction of structural changes in the interior of bulk transparent materials without affecting their surface. In the present study, femtosecond laser pulses were tightly focused within the interior of bulk fused silica specimen. Localized plasma was formed, initiating rearrangement of the random network structure. Cross sections of the induced features were examined via decomposition of spatially resolved Raman spectra and a new method for the quantitative characterization of the structure of amorphous fused silica was developed. The proposed method identifies the volume fraction distribution of ring structures within the continuous random network of the probed volume of the target material and changes of the distribution with laser process conditions. Effects of the different process conditions and the material response to different mechanisms of feature generation were discussed as well.

Spectroscopic studies on glassy Ni(II) and Co(II) polyphosphate coacervates

Transparent amorphous bulk materials have been prepared through the coacervation process of sodium polyphosphate and Ni 2+ and Co 2+ chloride solutions. Structural and spectroscopic properties were analyzed by X-ray diffraction, thermogravimetric analysis, UV–vis, infrared and Raman spectroscopic techniques. Different optical properties and water absorption tendencies were observed for the polyphosphate coacervates. The symmetric P–O b and P–O t stretching modes on the Raman spectra for the coacervates and the sodium polyphosphate revealed the coordination processes of the polyphosphate chains to the metal ions, including the effects of the water coordination outside the polyphosphate cages, connecting the adjacent chains. Based on data collected from the electronic spectra, these materials can present important technological applicability. Being transparent materials, these glasses can be used as absorption filters with pass-band between 600 and 500 nm for the Ni coacervate, and above 600 nm for the Co coacervate.

Luminescence and Scintillation Properties at the Nanoscale

This contribution is a review of the luminescence and scintillation properties of nanoparticles (NP), particularly doped insulators. Luminescence spectroscopy is an appropriate tool to probe matter at the nanoscale. Luminescence is also the last stage of the scintillation process. Specific surface and structural effects occurring in NP are reported. Their consequences on the NP luminescence properties are discussed. Parts of the effects are related to the preparation method. On the other hand some intrinsic properties of the nanostructures which can modify the optical properties are described: quantum confinement and dielectric confinement. The response under high-energy excitation is also discussed. It appears that their size can be used as a tool to describe the spatial distribution of electronic excitations induced by the relaxation process after the high-energy excitation. Finally, potentiality to grow transparent bulk materials based on small Nps agglomeration via soft chemistry route is presented. It is a promising approach toward the development of scintillating materials.

Femtosecond bulk transparent material processing and recovery

Femtosecond lasers have a unique ability of processing bulk transparent materials for various applications such as micromachining, waveguide manufacturing, and photonic bandgap structures, just to name a few. These applications depend on the formation of micron or submicron size features are known to be index modifications to the bulk substrate [2, 11], which were thought to persist indefinitely. However, it has been observed that some of these bulk transparent materials recover or "heal" with time. This "healing" process is studied and quantified using Nomarski Differential Interference Contrast optical microscopy and diffraction efficiency measurements of micro-machined gratings. We find healing to be accelerated in dye doped polymers.

Etching and forward transfer of fused silica in solid-phase by femtosecond laser-induced solid etching (LISE)

We present a femtosecond laser-based technique for etching and forward transfer of bulk transparent materials in solid-phase. Femtosecond laser pulses with λ = 800 nm were focused through a fused silica block onto an absorbing thin film of Cr. A constraining Si wafer was pressed into tight contact with the Cr film to prevent lift-off of the film. A combination of the high temperature and pressure of the Cr, and compressive stress from the Si, resulted in etching of smooth features from the fused silica by cracking. Unlike in conventional ablative or chemical etching, the silica was removed from the bulk as single solid-phase pieces which could be collected on the Si. Using this so-called laser-induced solid etching (LISE) technique, 1–2 μ m deep pits and channels have been produced in the silica surface, and corresponding dots and lines deposited on the Si. The threshold fluence for etching was found to be ≈ 0.4 J/cm 2 with ≈ 130 fs duration pulses. The morphology of the etched features are investigated as functions of fluence and exposure to multiple pulses.

Size Corrections during Ultrafast Laser Induced Refractive Index Changes in Bulk Transparent Materials

Three-dimensional structuring of bulk dielectric materials usually requires focusing through air- dielectric interfaces. Consequently, depth-dependent spherical aberration appears. This determines an elongation of the energy deposition area and restricts the structuring accuracy. We discuss here strategies for counteracting wavefront distortion effects which occur during ultrafast laser induced changes of refractive index. The proposed approaches are based on programmable spatio-temporal pulse shaping and have the objective of concentrating the laser energy on minimal spatial scales. Using adaptive spatial tailoring of ultrashort laser pulses, spherical aberrations can be dynamically corrected, in synchronization with the writing procedure. This facilitates optimal writing of homo- geneous longitudinal waveguides over significant lengths. We also show that temporal forming of ultrafast laser pulses restricts the energy spread, leading to a higher confinement. We indicate the role of reduced nonlinearity in plasma formation as a control factor coupling the spatial and tempo- ral response of the material. The size corrections enable higher processing accuracy.

Dynamic ultrafast laser spatial tailoring for parallel micromachining of photonic devices in transparent materials

Femtosecond laser processing of bulk transparent materials can generate localized positive changes of the refractive index. Thus, by translation of the laser spot, light-guiding structures are achievable in three dimensions. Increasing the number of laser processing spots can consequently reduce the machining effort. In this paper, we report on a procedure of dynamic ultrafast laser beam spatial tailoring for parallel photoinscription of photonic functions. Multispot operation is achieved by spatially modulating the wavefront of the beam with a time-evolutive periodical binary phase mask. The parallel longitudinal writing of multiple waveguides is demonstrated in fused silica. Using this technique, light dividers in three dimensions and wavelength-division demultiplexing (WDD) devices relying on evanescent wave coupling are demonstrated.

Femtosecond index change mechanisms and morphology of SiC crystalline materials

Femtosecond lasers have a unique ability of processing bulk transparent materials for various applications such as micromachining, waveguide manufacturing, and photonic bandgap structures just to name a few. These applications depend on the formation of micron or submicron size features that are known to be index modifications to the bulk substrate [H. Guo, H. Jiang, Y. Fang, C. Peng, H. Yang, Y. Li, Q. Gong, J. Opt. A: Pure Appl. Opt. 6 (2004) 787]. To the best of our knowledge the physical understanding of how these index-modified features are formed is still unknown, but many good theories exist such as Petite et al. [G. Petite, P. Daguzan, S. Guizard, P. Martin, in: IEEE Annual Report Conference on Electrical Insulation and Dielectric Phenomena, vol. 15, IEEE, 1995, pp. 40–44] or Tien et al. [A. Tien, S. Backus, H. Kapteyn, M. Murnane, G. Mourou, Phys. Rev. Lett. 82 (1999) 3883]. In this Letter the question on the physical cause for index changes is investigated by the combined efforts between Wright–Patterson AFB (WPAFB) and the University of Dayton (UD) using numerous imaging equipment such as TEM, AFM, NSOM, Nomarski microscopy, X-ray crystallography, Raman spectroscopy, and even diffraction efficiency experiments. With all the combined imaging equipment this research is able to present valuable data and deduce plausible theories of the physics of the index modification mechanism.

Ultrafast laser writing of homogeneous longitudinal waveguides in glasses using dynamic wavefront correction

Laser writing of longitudinal waveguides in bulk transparent materials degrades with the focusing depth due to wavefront distortions generated at the air-dielectric interface. Using adaptive spatial tailoring of ultrashort laser pulses, we show that spherical aberrations can be dynamically compensated in optical glasses, in synchronization with the writing procedure. Aberration-free structures can thus be induced at different depths, showing higher flexibility for 3D processing. This enables optimal writing of homogeneous longitudinal waveguides over more significant lengths. The corrective process becomes increasingly important when laser energy has to be transported without losses at arbitrary depths, with the purpose of triggering mechanisms of positive refractive index change.