Laser Rapid Prototyping of Photonic Band-Gap Microstructures

Recent progress in field-assisted additive manufacturing: materials, methodologies, and applications

Owing to its advantages of freedom to design, improved material utilization, and shortened lead time, additive manufacturing (AM) has the potential to redefine manufacturing after years of evolvement and opens new avenues to produce customized and complex-shaped products. Despite these benefits, AM still suffers problems stemmed from limited material selection, anisotropic material property, low production speed, coarse resolution, etc. In response to these problems, extensive attention has been drawn on integrating AM with fields, which mainly include magnetic field (MF), electric field (EF), and acoustic field (AF). These fields have been proved to be effective in tailoring microstructures, enhancing mechanical properties, focusing and sorting cells, serving as stimuli, etc., thus providing new opportunities to address existing problems and enable new functionalities of AM technologies. This paper presents a review on recent developments and major advances in MF-, EF-, and AF-assisted AM technologies and 4D printing method from aspects of materials, methodologies, and applications. In addition, current challenges and future trends of field-assisted AM technologies and 4D printing method are also outlined and discussed.

Controlled diffusion of nanoparticles by viscosity gradient for photonic crystal with dual photonic band gaps

Coalescence of droplets containing nanoparticles has been paid much attention regarding fabrication of functional photonic crystal (PC) patterns. However, most studies focus on the coalescence of droplets containing the same nanoparticles. Currently, an active challenge comes from the coalescence of droplets containing different nanoparticles due to the spontaneous mutual diffusion of different nanoparticles between coalescing miscible droplets driven by the released Gibbs free energy. Such diffusion breaks the self-assembly of nanoparticles into promising PCs with dual photonic band gaps (PBGs). In this work, a viscosity gradient was induced in coalescing droplets containing different nanoparticles to control the diffusion of nanoparticles and impede the diffusion across the coalescing interface. Nanoparticles diffused along the viscosity gradient to droplet surfaces and self-assembled into a period structure which enhanced the interaction of nanoparticles and contributed to impeding the random diffusion between droplets. At the same time, the high viscosity at the coalescing interface slowed down the horizontal movement of nanoparticles further and consequently the diffusion of nanoparticles across the interface was impeded. By use of such controlled diffusion of nanoparticles in the viscosity gradient, PCs with PBGs were achieved. These results demonstrate the controlled diffusion of nanoparticles during the coalescence of miscible droplets to facilely fabricate PCs with PBGs in the absence of an existing external field.

The Fabrication of Micro/Nano Structures by Laser Machining

Micro/nano structures have unique optical, electrical, magnetic, and thermal properties. Studies on the preparation of micro/nano structures are of considerable research value and broad development prospects. Several micro/nano structure preparation techniques have already been developed, such as photolithography, electron beam lithography, focused ion beam techniques, nanoimprint techniques. However, the available geometries directly implemented by those means are limited to the 2D mode. Laser machining, a new technology for micro/nano structural preparation, has received great attention in recent years for its wide application to almost all types of materials through a scalable, one-step method, and its unique 3D processing capabilities, high manufacturing resolution and high designability. In addition, micro/nano structures prepared by laser machining have a wide range of applications in photonics, Surface plasma resonance, optoelectronics, biochemical sensing, micro/nanofluidics, photofluidics, biomedical, and associated fields. In this paper, updated achievements of laser-assisted fabrication of micro/nano structures are reviewed and summarized. It focuses on the researchers' findings, and analyzes materials, morphology, possible applications and laser machining of micro/nano structures in detail. Seven kinds of materials are generalized, including metal, organics or polymers, semiconductors, glass, oxides, carbon materials, and piezoelectric materials. In the end, further prospects to the future of laser machining are proposed.

Effect of phenol red dye on monocrystal growth, crystalline perfection, and optical and dielectric properties of zinc (tris) thiourea sulfate

In this work, the growth of large size (∼25 × 29 × 5 mm and ∼25 × 24 × 6 mm) colorful single crystals of zinc (tris) thiourea sulfate (ZTS) in the presence of 0.05–2 wt% phenol red (PR) dye was achieved using a simple and low-cost technique. Powder X-ray diffraction patterns confirm the presence of PR dye, which is indicated by an enhancement of the Raman peak intensities, a shift in their position and the appearance of a few extra peaks. The quality of the grown crystals was assessed by high-resolution X-ray diffraction, which shows that the crystalline perfection of 1 wt% PR-dyed ZTS crystals is better than that of 2 wt% PR-dyed crystals. The measured UV–vis absorbance spectra show two additional, strong absorption bands at ∼430 and 558 nm in the dyed crystals, due to the presence of PR dye, along with a band at ∼276 nm which is present for all crystals but is slightly shifted for the dyed crystals. Photoluminescence spectra were recorded at two excitation wavelengths (λexc= 310 and 385 nm). The luminescence intensity is found to be enriched in dyed crystals, with some extra emission bands. An enhancement in the value of the dielectric constant and a.c. electrical conductivity was also observed in the dyed ZTS crystals.

Additive Manufacturing of Metal Structures at the Micrometer Scale

Currently, the focus of additive manufacturing (AM) is shifting from simple prototyping to actual production. One driving factor of this process is the ability of AM to build geometries that are not accessible by subtractive fabrication techniques. While these techniques often call for a geometry that is easiest to manufacture, AM enables the geometry required for best performance to be built by freeing the design process from restrictions imposed by traditional machining. At the micrometer scale, the design limitations of standard fabrication techniques are even more severe. Microscale AM thus holds great potential, as confirmed by the rapid success of commercial micro-stereolithography tools as an enabling technology for a broad range of scientific applications. For metals, however, there is still no established AM solution at small scales. To tackle the limited resolution of standard metal AM methods (a few tens of micrometers at best), various new techniques aimed at the micrometer scale and below are presently under development. Here, we review these recent efforts. Specifically, we feature the techniques of direct ink writing, electrohydrodynamic printing, laser-assisted electrophoretic deposition, laser-induced forward transfer, local electroplating methods, laser-induced photoreduction and focused electron or ion beam induced deposition. Although these methods have proven to facilitate the AM of metals with feature sizes in the range of 0.1-10 µm, they are still in a prototype stage and their potential is not fully explored yet. For instance, comprehensive studies of material availability and material properties are often lacking, yet compulsory for actual applications. We address these items while critically discussing and comparing the potential of current microscale metal AM techniques.

Numerical investigation of band gaps in 3D printed cantilever-in-mass metamaterials

In this research, the negative effective mass behavior of elastic/mechanical metamaterials is exhibited by a cantilever-in-mass structure as a proposed design for creating frequency stopping band gaps, based on local resonance of the internal structure. The mass-in-mass unit cell model is transformed into a cantilever-in-mass model using the Bernoulli-Euler beam theory. An analytical model of the cantilever-in-mass structure is derived and the effects of geometrical dimensions and material parameters to create frequency band gaps are examined. A two-dimensional finite element model is created to validate the analytical results, and excellent agreement is achieved. The analytical model establishes an easily tunable metamaterial design to realize wave attenuation based on locally resonant frequency. To demonstrate feasibility for 3D printing, the analytical model is employed to design and fabricate 3D printable mechanical metamaterial. A three-dimensional numerical experiment is performed using COMSOL Multiphysics to validate the wave attenuation performance. Results show that the cantilever-in-mass metamaterial is capable of mitigating stress waves at the desired resonance frequency. Our study successfully presents the use of one constituent material to create a 3D printed cantilever-in-mass metamaterial with negative effective mass density for stress wave mitigation purposes.

Simultaneous Dual Encoding of Three-Dimensional Structures by Light-Induced Modular Ligation

A highly efficient strategy for the simultaneous dual surface encoding of 2D and 3D microscaffolds is reported. The combination of an oligo(ethylene glycol)-based network with two novel and readily synthesized monomers with photoreactive side chains yields two new photoresists, which can be used for the fabrication of microstructures (by two-photon polymerization) that exhibit a dual-photoreactive surface. By combining both functional photoresists into one scaffold, a dual functionalization pattern can be obtained by a single irradiation step in the presence of adequate reaction partners based on a self-sorting mechanism. The versatility of the approach is shown by the dual patterning of halogenated and fluorescent markers as well as proteins. Furthermore, we introduce a new ToF-SIMS mode ("delayed extraction") for the characterization of the obtained microstructures that combines high mass resolution with improved lateral resolution.

3D printing of Al2O3 photonic crystals for terahertz frequencies

A simple 3D printing technique is used to fabricate three-dimensional photonic crystals made of Al₂O₃ and intended to be functional for terahertz frequencies. The process is completed by a thermal sintering to obtain compact structures.

Controlling Nanowire Growth by Light

Individual Au catalyst nanoparticles are used for selective laser-induced chemical vapor deposition of single germanium nanowires. Dark-field scattering reveals in real time the optical signatures of all key constituent growth processes. Growth is initially triggered by plasmonic absorption in the Au catalyst, while once nucleated the growing Ge nanowire supports magnetic and electric resonances that then dominate the laser interactions. This spectroscopic understanding allows real-time laser feedback that is crucial toward realizing the full potential of controlling nanomaterial growth by light.

A Review of Product Design for Additive Manufacturing

Additive Manufacturing (AM) is the digital manufacturing technology by which products are fabricated directly from computer models by selectively curing, depositing or consolidating materials in successive layers. The technology has provided an opportunity to rethink the methods of product design to maximize the product performance through the synthesis of material compositions, structure, and sizes. This overview is created to relate the unique capabilities of AM technologies and discuss the methods of product design. Finally, the current problems and difficulties in this field are discussed in this paper, and this paper proposes the development direction of the product design for additive manufacturing in the future.

Facile size-tunable fabrication of functional tin dioxide nanostructures by multiple size reduction lithography

A novel ultraviolet (UV)-assisted imprinting procedure that employs photosensitive tin(II) 2-ethylhexanoate is presented for the facile size-tunable fabrication of functional tin dioxide (SnO(2)) nanostructures by varying annealing temperatures. These imprinted SnO(2) nanostructures were also used as new masters for size reduction lithography. SnO(2) lines down to 40 nm wide were obtained from a silicon master with 200 nm wide lines by simply performing size reduction lithography twice. This leads to 80 and 87.5% reduction in the width and height of imprinted lines, respectively. An imprinted pattern annealed at 400 °C demonstrated transmittance greater than 90% over the range of 350-700 nm, which is high enough to make the pattern useful as a transparent SnO(2) mold. This demonstrated approach allows the accessibility to size-tunable molds, eliminating the need for conventional expensive imprinting masters with very fine structures, as well as functional SnO(2) nanostructures, potentially useful in applications where ordered surface nanostructures are required, such as photonic crystals, biological sensors, and model catalysts.

Fabrication of Nickel Nanostructure Arrays Via a Modified Nanosphere Lithography

In this paper, we present a modified nanosphere lithographic scheme that is based on the self-assembly and electroforming techniques. The scheme was demonstrated to fabricate a nickel template of ordered nanobowl arrays together with a nickel nanostructure array-patterned glass substrate. The hemispherical nanobowls exhibit uniform sizes and smooth interior surfaces, and the shallow nanobowls with a flat bottom on the glass substrate are interconnected as a net structure with uniform thickness. A multiphysics model based on the level set method (LSM) was built up to understand this fabricating process by tracking the interface between the growing nickel and the electrolyte. The fabricated nickel nanobowl template can be used as a mold of long lifetime in soft lithography due to the high strength of nickel. The nanostructure-patterned glass substrate can be used in optical and magnetic devices due to their shape effects. This fabrication scheme can also be extended to a wide range of metals and alloys.

Magnetic assembly of nonmagnetic particles into photonic crystal structures

We report the rapid formation of photonic crystal structures by assembly of uniform nonmagnetic colloidal particles in ferrofluids using external magnetic fields. Magnetic manipulation of nonmagnetic particles with size down to a few hundred nanometers, suitable building blocks for producing photonic crystals with band gaps located in the visible regime, has been difficult due to their weak magnetic dipole moment. Increasing the dipole moment of magnetic holes has been limited by the instability of ferrofluids toward aggregation at high concentration or under strong magnetic field. By taking advantage of the superior stability of highly surface-charged magnetite nanocrystal-based ferrofluids, in this paper we have been able to successfully assemble 185 nm nonmagnetic polymer beads into photonic crystal structures, from 1D chains to 3D assemblies as determined by the interplay of magnetic dipole force and packing force. In a strong magnetic field with large field gradient, 3D photonic crystals with high reflectance (83%) in the visible range can be rapidly produced within several minutes, making this general strategy promising for fast creation of large-area photonic crystals using nonmagnetic particles as building blocks.

Fabrication of functional silver nanobowl arrays via sphere lithography

We report a low-cost and high-throughput method to fabricate large-area silver nanobowl arrays via thermal evaporation of silver on a self-assembled monolayer of nanospheres. The nanobowl array is a hierarchical structure, composed of silver nanoparticles with average diameter size of ca.10 nm, which can serve as a reaction container and catalyst. The optical absorption spectra indicates that surface plasmon resonance of silver nanoparticles exists on the nanobowl array, and it can serve as an excellent surface enhanced Raman scattering (SERS)-active substrate.

Optical induction of three-dimensional photonic lattices and enhancement of discrete diffraction

We demonstrate experimentally the formation of three-dimensional (3D) reconfigurable photonic lattices in a bulk nonlinear crystal by employing the optical induction technique. Such 3D lattices are established by inducing 2D square lattices in two orthogonal directions. The induced 3D periodic index structures are monitored by plane-wave guidance and Brillouin zone spectroscopy. Enhanced discrete diffraction due to the waveguide modulation and coupling in 3D lattices is also observed.

Design and electrochemical fabrication of gold binary ordered micro/nanostructured porous arrays via step-by-step colloidal lithography

Colloidal lithography is a low-cost, high-throughput, facile nanofabrication technique capable of producing a large variety of nanostructured arrays. In this letter, we report a methodology, named step-by-step colloidal lithography, using electrochemical deposition as a fabrication technique to sculpture various hexagonally packed 2D-ordered gold binary micro/nanostructured porous arrays. By the designed fabrication routes, the structures of arrays and the morphology of the building blocks in the arrays can be easily controlled. Because of the feature of step-by-step fabrication, such a strategy will provide a versatile methodology not only for unitary components but also for binary and even multiplex materials, leading to heterostructured arrays with controlled compositions and block sizes. Such morphology and structure-controlled 2D binary porous arrays will exhibit the importance in building micro/nanostructured devices.

Large-scale fabrication of ordered metallic hybrid nanostructures

A low-cost and high-throughput method for the fabrication of large-area ordered hybrid metallic nanostructure arrays is presented. Each structure unit is a nanobowl with a hexagonal distributed pillar array upon it. A self-assembled monolayer of polystyrene (PS) nanospheres is used as a template. After thermal evaporation, electroforming and removal of the nanospheres and the conductive layer, ordered arrays of hybrid nickel nanostructures have been fabricated. Both nanobowl arrays and pillar arrays exhibit uniform sizes. Smooth interior surfaces were observed in the nanobowl arrays. The geometry of the structure can be tuned by controlling the thickness of the conductive layer. The approach presented in this paper can be extended to fabricate ordered hybrid nanostructures of a wide range of metals and alloys with controlled size.

Design of hybrid sol-gel films for direct x-ray and electron beam nanopatterning

New epoxy based sol-gel organic inorganic materials, showing lithographic resist-like properties without the addition of any photocatalysts, are presented. To obtain a material sensitive to radiation, specific sol-gel syntheses based on an organically modified alkoxide containing an epoxy ring, 3-glycidoxypropyltrimethoxysilane (GPTMS), have been developed. The synthesis and the patternability of hybrid materials have been obtained controlling the inorganic crosslinking degree and with an almost total absence of organic polymerization. Two examples of directly patternable hybrid films, called GB and GGe, have been synthesized using acidic (GGe) and basic (GB) conditions and obtaining different compositions. After electron beam lithography (EBL) or x-ray synchrotron radiation lithography (XRL) the polymerization of the organic component of the sol-gel film occurs, generating a hardening of the structure after post-exposure baking. The exposed polymerized material becomes insoluble, determining a negative resist-like behaviour of the film: the lithographic process of nanopatterning results from the dissolution of the unexposed areas in proper solvents (developers). Spatial resolution of the order of 200 nm is reported and a contrast of 2.2 is achieved. The novelty of this work is that epoxy based materials, which have enhanced thermomechanical stability with respect to the more usual acrylic based resins, are directly nanopatterned for the first time by electron beam (EB) and/or x-ray beam radiation exposure without the aid of catalysts for polymerization. In contrast to common resists that are sacrificial layers of the fabrication process, direct patternable sol-gel hybrids constitute the final material of the devices. In fact, an example of doping with a light emitting dye is reported together with the achievement of directly patterned structures by EBL and XRL.

Plasmon-assisted chemical vapor deposition

We introduce a new chemical vapor deposition (CVD) process that can be used to selectively deposit materials of many different types. The technique makes use of the plasmon resonance in nanoscale metal structures to produce the local heating necessary to initiate deposition when illuminated by a focused low-power laser. We demonstrate the technique, which we refer to as plasmon-assisted CVD (PACVD), by patterning the spatial deposition of PbO and TiO(2) on glass substrates coated with a dispersion of 23 nm gold particles. The morphology of both oxide deposits is consistent with local laser-induced heating of the gold particles by more than 150 degrees C. We show that temperature changes of this magnitude are consistent with our analysis of the heat-loss mechanisms. The technique is general and can be used to spatially control the deposition of virtually any material for which a CVD process exists.

Fabrication of metallic nanodots in large-area arrays by mold-to-mold cross imprinting (MTMCI)

We have developed a mold-to-mold cross imprint (MTMCI) process, which redefines an imprint mold with another imprint mold. By performing MTMCI on two identical imprint molds with silicon spacer nanowires in a perpendicular arrangement, we fabricated a large array of sub-30-nm silicon nanopillars. Large-area arrays of Pt dots are then produced using nanoimprint lithography with the silicon nanopillar mold.

Photonic band gap templating using optical interference lithography

We describe the properties of three families of inversion-symmetric, large photonic band-gap (PBG) template architectures defined by iso-intensity surfaces in four beam laser interference patterns. These templates can be fabricated by optical interference (holographic) lithography in a suitable polymer photo-resist. PBG materials can be synthesized from these templates using two stages of infiltration and inversion, first with silica and second with silicon. By considering point and space group symmetries to produce laser interference patterns with the smallest possible irreducible Brillouin zones, we obtain laser beam intensities, directions, and polarizations which generate a diamond-like (fcc) crystal, a novel body-centered cubic (bcc) architecture, and a simple-cubic (sc) structure. We obtain laser beam parameters that maximize the intensity contrasts of the interference patterns. This optimizes the robustness of the holographic lithography to inhomogeneity in the polymer photo-resist. When the optimized iso-intensity surface defines a silicon to air boundary (dielectric contrast of 11.9 to 1), the fcc, bcc, and sc crystals have PBG to center frequency ratios of 25%, 21%, and 11%, respectively. A full PBG forms for the diamond-like crystal when the refractive index contrast exceeds 1.97 to 1. We illustrate a non-inversion symmetric PBG architecture that interpolates between a simple fcc structure and a diamond network structure. This crystal exhibits two distinct and complete photonic band gaps. We also describe a generalized class of tetragonal photonic crystals that interpolate between and extrapolate beyond the diamond-like crystal and the optimized bcc crystal. We demonstrate the extent to which the resulting PBG materials are robust against perturbations to the laser beam amplitudes and polarizations, and template inhomogeneity. The body centered cubic structure exhibits the maximum robustness overall.

Defect modes in photonic crystal slabs studied using terahertz time-domain spectroscopy

We describe broadband coherent transmission studies of two-dimensional photonic crystals consisting of a hexagonal array of air holes in a dielectric slab in a planar waveguide. By filling several of the air holes in the photonic crystal slab, we observe the signature of a defect mode within the stop band, in both the amplitude and phase spectra. The experimental results are in reasonable agreement with theoretical calculations using the transfer matrix method.

Holographic creation of photonic crystals

We describe the creation of general photonic crystals by means of holography with an experimental demonstration. The recordings of periodic variations of amplitude and phase by the interference of coherent laser beams offer a natural means for the creation of one- two- or three-dimensional photonic crystals. Based on the principle of the interference of four noncoplanar beams, we present a comparative analysis of two different approaches for creating photonic crystals and use numerical simulated lattice structures to illustrate the differences between these two approaches. We then use a specific symmetrical optical architecture and select the proper approach to create holographic photonic crystals. The advantages and constraints of this holographic method are discussed.

Effects of polarization on laser holography for microstructure fabrication

We perform a kind of computer stimulation on the multi-laser-beam interference. Using this method, we picture the interference patterns and describe the influence of the polarization of lights upon the clarity of the pattern. We find out the relations between the polarization states of the lights for the case of the best pattern and provide an optimal solution of the polarization on holographic lithography technology, and experiential formulas. This kind of analysis will improve the fabrication of submicrometer periodic structure efficiently.

Interference of four umbrellalike beams by a diffractive beam splitter for fabrication of two-dimensional square and trigonal lattices

A simple optical interference method for fabricating two-dimensional square and trigonal lattices is demonstrated. A general formula for the interference contrast formed by two arbitrary polarized elliptical waves is deduced, the relation between wave vectors of incident light and the resultant pattern is analyzed, and polarization optimization of all beams to ensure uniform contrast is given.

Polarization optimization in the interference of four umbrellalike symmetric beams for making three-dimensional periodic microstructures

A systematic and comprehensive analysis of the interference of four umbrellalike beams (lFUB) is provided based on the reciprocal space theory. The concept of pattern contrast is extended to the case of the IFUB, and it is indicated that a uniform contrast for all the interference terms can be obtained by properly choosing the beam ratio and the polarization of each beam. Different polarization combinations, including linear light and linear light, circular light and circular light, and linear light and circular light, have been discussed for the purpose of maximum uniform contrast. It is shown that the use of circular light may generally improve the uniform contrast. This study may lay a theoretical foundation for holographic fabrication of three-dimensional (3D) periodic microstructures, such as simple cubic, body-centered cubic, face-centered cubic, or trigonal lattice.

Formation of three-dimensional periodic microstructures by interference of four noncoplanar beams

A newly reported method of making three-dimensional microstructures or photonic crystals by holographic lithography has some obvious advantages over other techniques with the same purpose. A systematic and comprehensive analysis of interference of four noncoplanar beams (IFNB) is provided. It shows that all 14 Bravais lattices can be formed by means of IFNB and gives explicit relationships between each lattice and the corresponding recording geometry. The concept of pattern contrast is extended to the case of IFNB, and it is indicated that a uniform contrast for each interference term can be obtained by properly choosing the beam ratio and polarization. A calculation algorithm is then developed to optimize the direction of polarization of each beam to ensure maximum uniform contrast. These results, verified by computer simulations, may lay a theoretical foundation for fabrication of photonic crystals with the approach of IFNB.

All fourteen Bravais lattices can be formed by interference of four noncoplanar beams

The interference of four noncoplanar beams (IFNB) is analyzed. It is shown that all 14 Bravais lattices can be formed by a holographic method of IFNB. The relationship among the three basis vectors of the lattice that are to be produced, the required wavelength, and the geometric arrangement of the four beams is derived. This analysis may lay the foundation for fabrication of three-dimensional photonic crystals by holographic lithography.

Formation of a microfiber bundle by interference of three noncoplanar beams

A systematic analysis of interference of three noncoplanar plane waves with identical frequency is provided. This analysis shows that a fiber bundle with spacing of the order of the wavelength, which one may conveniently control by changing the recording geometry, can be formed by this means. The relation between the incident light-wave vectors and the resultant pattern is analyzed. The concept of uniform contrast for an interference pattern is introduced, and the polarization optimization approach for each beam that ensures maximum uniform contrast for each beam is also given.

Fabrication of a submicrometer crystalline structure by thermoplastic holography

We report what we believe to be a novel method for fabrication of permanent submicrometer periodic structures by interference laser fields. The new method is holographic lithography combined with laser-induced thermoplastification. The crystalline structures that result from this new method not only can be maintained permanently after the optical field is evacuated but also can be rewritten by exposure of an inteference laser field for the second time. The process of fabrication is rapid, convenient, and effective.

Tight-binding description of the coupled defect modes in three-dimensional photonic crystals

We have experimentally observed the eigenmode splitting due to coupling of the evanescent defect modes in three-dimensional photonic crystals. The splitting was well explained with a theory based on the classical wave analog of the tight-binding (TB) formalism in solid state physics. The experimental results were used to extract the TB parameters. A new type of waveguiding in a photonic crystal was demonstrated experimentally. A complete transmission was achieved throughout the entire waveguiding band. We have also obtained the dispersion relation for the waveguiding band of the coupled periodic defects from the transmission-phase measurements and from the TB calculations.