Showing papers by "Min Gu published in 2005"
TL;DR: In this article, a photopolymerizing material suitable for stereolithography of complex submicrometer-sized three-dimensional structural elements is presented, which allows further research in the field of nanofabrication and -technology as it surpasses current material limitations.
Abstract: This paper provides a photopolymerizing material suitable for stereolithography of complex submicrometer-sized three-dimensional (3D) structural elements to a broad scientific public. Here, we present the formulation of a polymer (LN1 resin) that allows further research in the field of nanofabrication and -technology as it surpasses current material limitations. The polymer consists of multifunctional acrylate oligomers as binder, polyfunctional monomers, and a photoinitiator (PI). The chemistry to form 3D structures is based on photopolymerization of the acrylate system initiated by free-radical species that are triggered by two-photon absorption of a PI. Important parameters of photocuring, such as the effects of PI concentration, temperature, and light intensity, were studied using photocalorimetry. The thermal stability of the material was tested using thermal gravimetric analysis, providing key information for electronic and photonic applications. Photonic-crystal structures generated from this resin exhibiting photonic stop gaps in near-infrared- and telecommunication-wavelength regions are presented.
124 citations
TL;DR: A nonlinear optical microscope that adopts double-clad photonic crystal fibers for single-mode illumination delivery and multimode signal collection and high resolution three-dimensional second harmonic generation images from rat tail tendon demonstrate the effectiveness of the system.
Abstract: We report on a nonlinear optical microscope that adopts double-clad photonic crystal fibers for single-mode illumination delivery and multimode signal collection. It is demonstrated that two-photon fluorescence and second harmonic generation signals can be simultaneously collected in such a microscope with axial resolution of 2.8 µm and 2.5 µm, respectively. The delivery and detection efficiencies of the photonic-crystal-fiber-based microscope are significantly improved by approximately 3 times and 40 times compared with those in the single-mode fiber-optic microscope. The high resolution three-dimensional second harmonic generation images from rat tail tendon demonstrate the effectiveness of the system.
99 citations
TL;DR: Microchannels are fabricated in a poly(methyl methacrylate) substrate by high repetition rate, nanojoule femtosecond laser pulses, resulting in smooth walled cylindrical channels.
Abstract: Microchannels are fabricated in a poly(methyl methacrylate) substrate by high repetition rate, nanojoule femtosecond laser pulses. The mechanism for channel fabrication is based on the localized heating of the substrate due to the high repetition rate of the laser, resulting in smooth walled cylindrical channels. Microchannels with diameters of 8 - 20 microm can be fabricated at 800 microm/s using 80 fs pulses at a repetition rate of 80 MHz and energy of 0.9 nJ/pulse.
73 citations
TL;DR: It is demonstrated both theoretically and experimentally that the introduction of radially polarized beam illumination combining with an annular beam illumination exhibits advantages in two aspects: corrects the focus elongation and splitting in a focused evanescent field associated with a linearly polarized beam.
Abstract: In this paper, a tightly focused evanescent field produced by a total internal reflection objective lens under the illumination of a radially polarized beam generated using a single liquid crystal phase modulator is investigated. The field distributions have been directly mapped by a scanning near-field optical microscope. It is demonstrated both theoretically and experimentally that the introduction of radially polarized beam illumination combining with an annular beam illumination exhibits advantages in two aspects. On one hand, it corrects the focus elongation and splitting in a focused evanescent field associated with a linearly polarized beam. On the other hand, it significantly improves the lateral localization to approximately a quarter of the illumination wavelength, which is less than half of the size that is achievable under linearly polarized illumination.
66 citations
TL;DR: In this paper, the intensity distributions of a tightly focused evanescent field generated by a center blocked high numerical aperture (1.65) objective lens are investigated by a scanning near-field optical microscope.
Abstract: Intensity distributions of a tightly focused evanescent field generated by a center blocked high numerical aperture (1.65) objective lens are investigated by a scanning near-field optical microscope. The pure focused evanescent field is mapped and a splitting phenomenon of the focal spot along the direction of polarization, caused by depolarization, is observed not only on the interface, where the evanescent field is generated, but also in the parallel planes away from the interface. The decaying nature of the focused evanescent field shows good agreement with the theoretical predication, indicating that the field is purely evanescent and does not contain a significant contribution from the propagating component. It is found in our experiment that the light coupling efficiencies of the longitudinal polarization component Ez and the transverse polarization component Ex to the fiber probe differ by a factor of 3.
52 citations
TL;DR: In this paper, the authors compare decreasing fluorescence signals from ensembles with the blinking statistics of individual nanocrystals in various environments, and find that the ensemble decay follows a power-law of time, the exponent being the difference of the power law exponents of on-and off-time distributions.
Abstract: We compare decreasing fluorescence signals from ensembles with the blinking statistics of individual nanocrystals in various environments. For most substrates, the ensemble decay follows a power-law of time, the exponent being the difference of the power-law exponents of on- and off-time distributions. The decay exponent is also found to depend on substrate. We discuss possible mechanisms for this dependence, in conjunction with previously published models.
43 citations
TL;DR: In this article, a femtosecond laser-driven microexplosion method was used to generate quasispherical voids in an anisotropic Fe:LiNbO3 crystal with a high refractive index.
Abstract: Smooth voids are achieved in an anisotropic Fe:LiNbO3 crystal with a high refractive index by use of a femtosecond laser-driven microexplosion method. Due to the anisotropy of the crystal, the maximum fabrication depth and the fabrication power threshold are different in different crystal directions, indicating that the direction perpendicular to the crystal axis is more suitable for thick three-dimensional structure fabrication. The dependence of the threshold power on the illumination wavelength shows that the microexplosion mechanism is caused by a two-photon absorption process. As a result, a near threshold fabrication method can be used to generate quasispherical voids.
37 citations
TL;DR: A compact second-harmonic-generation (SHG) microscope based on a three-port single-mode fiber coupler is presented and it is demonstrated that SHG and two-photon fluorescence images under parallel and perpendicular laser excitation polarization can be simultaneously obtained.
Abstract: We present a compact second-harmonic-generation (SHG) microscope based on a three-port single-mode fiber coupler. The fiber coupler is used to deliver a near-infrared ultrashort-pulsed laser beam as well as to collect the SHG signal in the visible wavelength range. The SHG microscope exhibits an axial resolution of 1.8 µm, which shows a slight enhancement of the optical sectioning effect compared with that for two-photon excitation at the same excitation wavelength. It is also demonstrated that SHG and two-photon fluorescence images under parallel and perpendicular laser excitation polarization can be simultaneously obtained.
32 citations
TL;DR: In this paper, an optically tunable three-dimensional photonic crystal that exhibits main gaps in the 3-4-μm range was manufactured via a femtosecond direct writing technique.
Abstract: In this letter, we report on an optically tunable three-dimensional photonic crystal that exhibits main gaps in the 3–4μm range. The photonic crystal is manufactured via a femtosecond direct writing technique. Optical tuning is achieved by a luminary polling technique with a low-power polarized laser beam. The refractive index variation resulting from liquid-crystal rotation causes a shift in the photonic band gap of up to 65 nm with an extinction of transmission of up to 70% in the stacking direction. Unlike other liquid-crystal tuning techniques where a pregenerated structure is infiltrated, this optical tuning method is a one-step process that allows arbitrary structures to be written into a solid liquid-crystal-polymer composite and leads to a high dielectric contrast.
31 citations
TL;DR: Results demonstrate an intrinsic trade-off between signal collection and axial resolution in two-photon fluorescence microscopy based on the separation distance of a single-mode optical fiber coupler and a gradient-index rod lens.
Abstract: We report on the experimental investigation into the characterization of two-photon fluorescence microscopy based on the separation distance of a single-mode optical fiber coupler and a gradient-index (GRIN) rod lens. The collected two-photon fluorescence signal exhibits a maximum intensity at a defined separation distance (gap length) where the increasing effective excitation numerical aperture is balanced by the decreasing confocal emission collection. A maximum signal is found at gap lengths of approximately 2, 1.25, and 1.75 mm for GRIN lenses with pitches of 0.23, 0.25, and 0.29 wavelength at 830 nm. The maximum two-photon fluorescence signal collected corresponds to a threefold reduction of axial resolution (38.5 microm at 1.25 mm), compared with the maximum resolution (11.6 microm at 5.5 mm), as shown by the three-dimensional imaging of 10 microm beads. These results demonstrate an intrinsic trade-off between signal collection and axial resolution.
30 citations
TL;DR: The inadequacy of the optical trapping model based on ray optics in the case of describing the Optical trapping performance of annular and doughnut laser beams is discussed and the vectorial diffraction theory needs to be employed for the determination of the trapping force exerted on small particles.
Abstract: The inadequacy of the optical trapping model based on ray optics in the case of describing the optical trapping performance of annular and doughnut laser beams is discussed. The inadequacy originates from neglecting the complex focused field distributions of such beams, such as polarization and phase, and thus leads to erroneous predictions of trapping force. Instead, the optical trapping model based on the vectorial diffraction theory, which considers the exact field distributions of a beam in the focal region, needs to be employed for the determination of the trapping force exerted on small particles. The theoretical predictions of such a trapping model agree with the experimentally measured results.
TL;DR: In this paper, three-dimensional photonic crystals with a tunable wavelength range of over 70nm of stop gaps between 3 and 4μm have been generated in a liquid crystal-polymer composite.
Abstract: Electrically tunable three-dimensional photonic crystals with a tunable wavelength range of over 70nm of stop gaps between 3 and 4μm have been generated in a liquid crystal-polymer composite. The photonic crystals were fabricated by femtosecond-laser direct writing of void channels in an inverse woodpile configuration with 20 layers providing an extinction of infrared light transmission of 70% in the stacking direction. Stable structures could be manufactured up to a liquid crystal concentration of 24%. Applying a direct voltage of several hundred volts in the stacking direction of the photonic crystal changes the alignment of the liquid crystal directors and hence the average refractive index of the structure. This mechanism permits the direct tuning of the photonic stop gap.
TL;DR: In this article, spherical void dots with a diameter of 1.2-1.8 μm have been generated in a solid polymer material by use of the ultrafast laser driven micro-explosion method.
Abstract: Spherical void dots with a diameter of 1.2–1.8 μm have been generated in a solid polymer material by use of the ultrafast laser driven micro-explosion method. Micron-sized structures with a face-centered cubic lattice stacked in the [100] and [111] directions have been fabricated. Confocal microscopic images show the high uniformity of the fabricated structures. Photonic stopgaps with a suppression rate of approximately 70% as well as the second-order stopgaps have been observed in both directions. It is shown that the dependence of the stopgaps on the illumination angle in the [100] direction is significantly different from that in the [111] direction.
TL;DR: In this article, a minimum value of the refractive-index contrast for opening both partial and full two-dimensional spectral gaps for both TM-and TE-polarized waves was calculated.
Abstract: We study the band-gap properties of two-dimensional photonic crystals created by a lattice of rods or holes conformed in a symmetric or asymmetric triangular structure. Using numerical plane-wave method, we calculate a minimum value of the refractive-index contrast for opening both partial and full two-dimensional spectral gaps for both TM- and TE-polarized waves. We also analyze the effect of ellipticity of rods and holes and their orientation on the threshold value and the relative size of the band gaps.
TL;DR: In this paper, a single-monomer acrylate resin (SR348, Sartomer) with an aromatic ketone photoinitiator (Irgacure 369, Ciba) has been developed to produce photonic crystal structures by two-photon photopolymerisation.
Patent•
15 Apr 2005
TL;DR: In this paper, an optically active element is formed by creating a matrix (1) in which a polymer dispersed liquid crystal is dispersed, and generating one or more void structures (2, 3) in the matrix.
Abstract: An optically active element, such as a photonic crystal, is formed by creating a matrix (1) in which an optically active material is dispersed, and generating one or more void structures (2, 3) in the matrix. The matrix (1) may comprise polymer dispersed liquid crystal. The void structures (2, 3) may be generated by laser ablation. Properties of the optically active element may be tuned by thermal effects, or via the application of electric, magnetic, or polarised electromagnetic fields. The element may be adapted for use in beam steering, fluid detection, tunable lasers, polarisation multiplexing, and optical switching.
TL;DR: In this article, the authors studied the thermal decomposition of the acryl-ate-based photopolymer LN1 for microfabrication with dynamic thermogravimetric analysis experiments.
Abstract: Summary: Kinetics of thermal decomposition of the acryl-ate-based photopolymer LN1 for microfabrication was stud-ied with dynamic thermogravimetric analysis experiments.The kinetic parameters including the activation energy, pre-exponential factor, and rate constant, which were calculatedby the Flynn and Wall method, are presented. The resultsindicate that thermal degradation of the material consistsof three distinct reaction stages. A mechanism of thermaldecomposition was proposed, which is in good agreementwith the experimental results. The lifetime of the materialwas estimated by the method of Toop, giving the key in-formationformicroelectronicsandphotonicapplications. Inaddition, the curing behavior of the system was studied bydielectric analysis providing valuable information for pro-cessing and fabrication control.Conversion time versus temperature (a), percentage con-versionversus time (b). Macromol. Chem. Phys. 2005, 206, 1659–1664 DOI:10.1002/macp.200500136 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
TL;DR: In this article, the authors studied the bandgap properties of two-dimensional photonic crystals created by a lattice of rods or holes conformed in a symmetric or asymmetric triangular structure.
Abstract: We study the bandgap properties of two-dimensional photonic crystals created by a lattice of rods or holes conformed in a symmetric or asymmetric triangular structure. Using the plane-wave analysis, we calculate a minimum value of the refractive index contrast for opening both partial and full two-dimensional spectral gaps for both TM and TE polarized waves. We also analyze the effect of ellipticity of rods and holes and their orientation on the threshold value and the relative size of the bandgap.
TL;DR: The FDTD method is more suitable than the RO method for the stress study of erythrocytes because it automatically takes into account the diffraction and interference effects of the light wave.
Abstract: The surface stress on the real shape (biconcave disklike) of an erythrocyte under laser irradiation is theoretically studied according to the finite-difference time-domain (FDTD) method. The distribution of the surface stresses depends on the orientation of erythrocytes in the laser beam. Typically when the erythrocyte was irradiated from the side direction (the laser beam was perpendicular to the normal of the erythrocyte plane), the surface stresses were so asymmetrical and nonuniform that the magnitude of the surface stress on the back surface was three times higher than that on the front surface, and the highest-to-lowest ratio of the stress reached 16 times. For comparison, the surface stress was also calculated according to the ray optics (RO) method. The tendency of the stress distribution from the RO calculation was roughly similar to that of the FDTD method. However the RO calculation produced some unphysical results, such as the infinite stress on some surface region and the zero stress on the most parts of the erythrocyte surface, which is due to the neglecting of light diffraction. The results obtained from the FDTD calculation are believed quantitatively reliable, because the FDTD method automatically takes into account of the diffraction and interference effects of the light wave. Thus, the FDTD method is more suitable than the RO method for the stress study of erythrocytes.
TL;DR: Planar dielectric microcavities embedded in woodpile void channel photonic crystals with stop bands in the stacking direction ranging from 4.3 to 4.8 microm in wavelength were generated by femtosecond-laser direct writing in a solid polymer.
Abstract: Planar dielectric microcavities embedded in woodpile void channel photonic crystals with stop bands in the stacking direction ranging from 4.3 to 4.8 µm in wavelength were generated by femtosecond-laser direct writing in a solid polymer. Infrared transmission spectra revealed fundamental and second-order modes crossing the stop gap region with a free spectral range of 430 nm on varying the microcavity size from 0.3 to 2.25 µm. Supercell calculations confirmed the cavity size dependence of highly localized cavity modes, whereas the angle of incidence was accounted for using a simple Fabry-Perot model.
TL;DR: An anomaly in the intensity distribution at the focal region of a Laguerre-Gaussian beam, when such a beam is focused by a high numerical aperture objective lens through an index-mismatched interface satisfying the total internal reflection condition, is demonstrated.
Abstract: We demonstrate theoretically and experimentally an anomaly in the intensity distribution at the focal region of a Laguerre-Gaussian beam, when such a beam is focused by a high numerical aperture objective lens through an index-mismatched interface satisfying the total internal reflection condition An asymmetric rotation of the focal field arising from the interplay of the phase shift induced by the total internal reflection and the helical phase of the Laguerre-Gaussian beam has been experimentally observed by a scanning near-field optical microscope A cross-section analysis shows that the experimental results match well with the theoretical predictions
TL;DR: A theoretical approach is presented to characterize the refractive index of the material for calculating the bandgap spectra, and it is confirmed that the wavelengths of the observed bandgaps are in good correlation with the analytical predictions.
Abstract: We report on the fabrication and characterization of void-based body-centered-cubic (bcc) photonic crystals in a solidified transparent polymer by the use of a femtosecond laser-driven microexplosion method. The change in the refractive index in the region surrounding the void dots that form the bcc structures is verified by presenting confocal microscope images, and the bandgap properties are characterized by using a Fourier transform infrared spectrometer. The effect of the angle of incidence on the photonic bandgaps is also studied. We observe multiple stop gaps with a suppression rate of the main gap of 47% for a bcc structure with a lattice constant of 2.77 µm, where the first and second stop gaps are located at 3.7 µm and 2.2 µm, respectively. We also present a theoretical approach to characterize the refractive index of the material for calculating the bandgap spectra, and confirm that the wavelengths of the observed bandgaps are in good correlation with the analytical predictions.
TL;DR: In this article, an analysis of the optical properties of photonic woodpile structures is presented, showing that the structure exhibits strong anisotropic properties at frequencies slightly above the band gap in the (Γ-X) direction, leading to an extreme sensitivity to the angle and the frequency of the incident light.
Abstract: An analysis of the optical properties of photonic woodpile structures is presented. We demonstrate large superprism phenomena inside polymeric woodpile structures having a refractive index of less than n=1.6. Due to the low contrast in refractive indices the structures investigated do not possess a complete photonic band gap. Nevertheless, their photonic band structures show strong anisotropy at frequencies slightly above the band gap in the (Γ-X) direction, leading to an extreme sensitivity to the angle and the frequency of the incident light in the propagation direction inside the crystal. Furthermore, if the woodpile structure is arranged in a prism-like shape, the transmitted beam outside the crystal shows a strong sensitivity to the frequency and angle of the incident light.
TL;DR: In this paper, a ray optics model was proposed to calculate the trapping force on a dielectric particle located on the interface between two media and illuminated by a focused evanescent field beam.
Abstract: We propose a ray optics model to calculate the trapping force on a dielectric particle located on the interface between two media and illuminated by a focused evanescent field beam. Such a focused evanescent beam is produced by a high numerical aperture objective with a central obstruction whose size satisfies the total internal reflection condition on the interface. The dependence of the lifting force on the obstruction size, the particle size, and the distance of the particle from the interface is revealed.
TL;DR: In this article, it was demonstrated that the polarisation states of pulsed and continuous wave illumination over the broad wavelength range can be preserved in a conventional fused fiber coupler. And the result is of importance for fiber-optic second harmonic generation microscopy which requires the polarization preservation to extract molecular organization of a sample.
Abstract: We report on the experimental investigation carried out in the polarisation characteristics of femtosecond pulsed and continuous wave beams propagating through a 1×2 fibre coupler. It is demonstrated that the polarisation states of pulsed and continuous wave illumination over the broad wavelength range can be preserved in a conventional fused fibre coupler. Furthermore, the fibre coupler acts as a low-pass spectral filter in the visible wavelength range with a splitting ratio between 99.6/0.4 and 99.7/0.3. The result is of importance for fibre-optic second harmonic generation microscopy which requires the polarisation preservation to extract molecular organization of a sample.
28 Dec 2005
TL;DR: In this paper, the authors used femtosecond laser pulses with energy of 0.9 nJ per pulse and a 80 MHz repetition rate at a wavelength of 750 nm to fabricate straight microchannels in a PMMA substrate.
Abstract: Femtosecond laser pulses with energy of 0.9 nJ per pulse and a 80 MHz repetition rate at a wavelength of 750 nm were used to fabricate straight microchannels in a PMMA substrate. The size and shape of the microchannels can be controlled by changing the fabrication parameters of speed, the number of fabrication repeats and delay in-between fabrication repeats. It has been proposed that the absorption of energy in the focal region modifies the density of the polymer matrix, which after annealing the sample above the glass transition temperature results in the formation of the microchannels. Diffusion of heat through the substrate is a uniform process which has the effect of creating symmetrically shaped channels. This fabrication method is expected to have applications in the fabrication of
microstructures or microfluidic devices in polymer substrates.
TL;DR: In this paper, a nonlinear optical microscope that adopts double-clad photonic crystal fibers was presented, and the detection efficiency was significantly improved by approximately 40 times compared with that in the single-mode fiber-optic microscope.
Abstract: We report on a nonlinear optical microscope that adopts double-clad photonic crystal fibers. The detection efficiency is significantly improved by approximately 40 times compared with that in the single-mode fiber-optic microscope.
10 Jul 2005
TL;DR: In this article, the spectral encoding capability of semiconductor nanocrystals and its application to high-density optical data storage was discussed and the effect of polymer matrix and energy transfer between the NCs were also studied.
Abstract: In this paper, we discuss the spectral encoding capability of semiconductor nanocrystals (NCs) and its application to high-density optical data storage. The effect of polymer matrix and energy transfer between the NCs are also studied.
TL;DR: In this paper, the near field optical tweezers using focused evanescent illumination have been used to construct an optical micromanipulation setup with great flexibility and accuracy, which would find applications in optical nanometry.
Abstract: In this paper, we report on near field optical tweezers using focused evanescent illumination. The intensity pattern of the focused evanescent field has been characterized using a SNOM. The near field trapping efficiency has been experimentally and theoretically investigated using both TEM 00 and TEM 01 beams. The trapping efficiency was found to be polarization dependent and also changed for the two modes of illumination used. By incorporating the near field and far field optical tweezers one could construct an optical micromanipulation setup with great flexibility and accuracy, which would find applications in optical nanometry. By coupling the techniques like morphology dependent resonance (MDR), one could employ near field tweezers for near field sensing and for characterization of microfluidic channels.
05 Dec 2005
TL;DR: In this paper, femtosecond-laser direct writing in a solid polymer was used to generate tuneable microcavities embedded in woodpile photonic crystals.
Abstract: Tuneable microcavities embedded in woodpile photonic crystals were generated by femtosecond-laser direct writing in a solid polymer. Analogous to a simple Fabry-Perot etalon cavity size and angular dependence were observed experimentally and confirmed using supercell calculations. The addition of one and two-dimensional lattices to the cavity allows for additional fine-tuning