Showing papers by "Min Gu published in 2003"

Focusing of doughnut laser beams by a high numerical-aperture objective in free space.

Abstract: We report on, in this letter, a phenomenon that the central zero-intensity point of a doughnut beam, caused by phase singularity, disappears in the focus, when such a beam is focused by a high numerical-aperture objective in free space. In addition, the focal shape of the doughnut beam of a given topological charge exhibits the increased ring intensity in the direction orthogonal to the incident polarization state and an elongation in the polarization direction. These phenomena are caused by the effect of depolarization, associated with a high numerical-aperture objective, and become pronounced by the use of a central obstruction in the objective aperture.

Two-photon fluorescence endoscopy with a micro-optic scanning head

Abstract: A major obstacle in the race to develop two-photon fluorescence endoscopy is the use of complicated bulk optics to transmit an ultrashort-pulsed laser beam and return the emitted fluorescence signal. We describe an all-fiber two-photon fluorescence microendoscope based on a single-mode optical fiber coupler, a microprism, and a gradient-index rod lens. It is found that the new endoscope exhibits an axial resolution of 3.2 μm and is capable of imaging transverse cross sections of internal cylindrical structures as small as approximately 3.0 mm in diameter. This device demonstrates the potential for developing a real-time diagnostic tool for biomedical research without the need for surgical biopsy and may find applications in photodynamic therapy, microsurgery, and early cancer detection.

Comparison of three-dimensional imaging properties between two-photon and single-photon fluorescence microscopy

Abstract: Summary The imaging performance in single-photon (l-p) and two­ photon (2-p) fluorescence microscopy is described. Both confocal and conventional systems are compared in terms of.the three-dimensional (3-D) point spread function and the 3-D optical transfer function. Images of fluorescent sharp edges and layers are modelled •. giving resolution in transverse and axial directions. A comparison of the imaging properties is also given for a 4Pi confocal system. Confocal 2-p 4Pi fluorescence microscopy gives the best axial resolution in the sense that its 3-D optical transfer function has the strongest response along the axial direction.

Experimental investigation of single voxels for laser nanofabrication via two-photon photopolymerization

Abstract: We report critical roles that are played by laser system parameters in two-photon laser nanofabrication, which are not significant in rapid prototyping at larger scale, including: (i) polarization-induced lateral deshaping of volume elements, voxels, the primitive building block of micronanostructures, and (ii) lateral size reduction of voxels at low numerical aperture lens focusing due to thresholding effect. Also interesting is (iii) simultaneous recording of zeroth- and higher-order diffraction patterns, which was not hindered by the two-order light intensity difference by taking the advantage of the large dynamic exposure time range of general resins, a concept that is proposed in contrast to dynamic power range.

Multiple higher-order stop gaps in infrared polymer photonic crystals.

Abstract: Engineering of stop gaps between higher photonic bands provides an alternative to miniaturization of photonic crystals. Femtosecond laser microfabrication of highly correlated void channel polymer microstructures results in photonic crystals with large stop gaps and a multitude of higher-order gaps in the mid- and near-infrared spectral regions. The gap wavelengths obey Bragg's law. Consistent with theory, varying the woodpile structure unit cell allows for tuning the number of higher-order gaps, and transitions from mere resonant Bragg scattering to stop band total reflection are observed.

Void channel microstructures in resin solids as an efficient way to infrared photonic crystals

Abstract: Microvoid channels were generated by local melting in a solidified polymer resin sample moving perpendicular to the focus of a high numerical-aperture objective under visible femtosecond-pulsed illumination. Channel size, surface quality, and high density channel vicinity depended on laser intensity and scanning speed. Electron microscope images revealed elliptical channel cross sections of 0.7–1.3 μm in lateral diameter and an elongation in the focusing direction of approximately 50%. A 20 layer woodpile-type photonic crystal structure with a 1.7 μm layer spacing and a 1.8 μm in-plane channel spacing provided a sharp peak in reflection and a suppression of infrared transmission in the stacking direction by 85% at wavelength 4.8 μm with a gap/midgap ratio of 0.11.

Penetration depth of single-, two-, and three-photon fluorescence microscopic imaging through human cortex structures: Monte Carlo simulation.

Abstract: Penetration depth is investigated in terms of the performance of transverse image resolution and signal level in human cortex under single-, two-, and three-photon fluorescence microscopy. Simulation results show that, in a double-layer human cortex structure consisting of gray and white matter media, the signal level is strongly affected by the existence of the white matter medium under three-photon excitation. Compared with three-photon excitation, two-photon excitation keeps a better signal level and sacrifices a slight degradation in image resolution. In a thick gray matter medium, a penetration depth of 1500 μm with a near-diffraction-limited resolution is obtainable under three-photon excitation. It is also demonstrated that the numerical aperture has a slight influence on image resolution and signal level under two- and three-photon excitation because of the nonlinear nature in the excitation process.

Two-photon fluorescence scanning near-field microscopy based on a focused evanescent field under total internal reflection.

Abstract: We present two-photon fluorescence near-field microscopy based on an evanescent field focus produced by a ring beam under total internal reflection. The evanescent field produced by this method is focused by a high-numerical-aperture objective, producing a tightly confined volume that can effectively induce two-photon excitation. The imaging system is characterized by the two-photon-excited images of the nanocrystals, which show that the focused evanescent field is split into two lobes because of the enhancement of the longitudinal polarization component at the focus. This feature is confirmed by the theoretical prediction. Unlike other two-photon near-field probes, this method does not have the heating effect and requires no control mechanism of the distance between a sample and the probe.

Effect of ultrashort pulsed illumination on foci caused by a Fresnel zone plate

Abstract: The focal distribution produced by a zone plate under ultrashort pulsed laser illumination is investigated under the Fresnel approximation. A comparison of the diffraction patterns in the focal region between pulsed and continuous-wave illumination shows that the focal shape produced by a zone plate can be significantly altered when an ultrashort pulse is shorter than 100 fs. In particular, the focal width in the axial and the transverse directions is increased by approximately 5% and 85%, respectively, from continuous-wave illumination to 10-fs pulsed illumination.

Effect of numerical aperture on the spectral splitting feature near phase singularities of focused waves

Abstract: We demonstrate that because of the depolarization effect associated with a high-numerical-aperture lens, the recently predicted spectral splitting phenomenon near phase singularities of focused waves [G. Gbur, T. D. Visser, and E. Wolf, Phys. Rev. Lett. 88, 013901 (2002)] disappears when the numerical aperture is higher than critical values that are different between the incident polarization direction and the axial direction.

Three-dimensional localisation of fluorescence resonance energy transfer in living cells under two-photon excitation

Abstract: Three-dimensional (3-D) imaging of fluorescence resonance energy transfer (FRET) in human cells under two-photon excitation was demonstrated in this study. A sample was prepared by expressing a donor and an acceptor in living cells and using an antibody to secure the proximity of contact between the donor and the acceptor. The quenching of fluorescence emission of a donor in the double-labelled cells indicates the presence of FRET that occurred in these living cells. Because of the quadratic relation of the excitation power, 3-D localisation of FRET becomes possible.

Monte Carlo simulation of multiphoton fluorescence microscopic imaging through inhomogeneous tissuelike turbid media.

Abstract: Image resolution and signal level in fluorescence microscopy through inhomogeneous turbid media consisting of scatterers of multiple sizes under single- (1p), two- (2p), and three-photon (3p) excitation have been investigated based on a modified Monte Carlo model. The effects of the size distribution and the concentration distribution of scattering particles are explored. Simulation results reveal that the size and the concentration distribution both have an impact on image formation in media consisting of small particles and that 3p excitation has the most significant impact. In media with scatterers of a large size, both size and concentration distributions lead to a slight effect. Image formation in a mixed medium containing small and large scattering particles is more affected by the large particles.

Three-dimensional optical-transfer-function analysis of fiber-optical two-photon fluorescence microscopy.

Abstract: The three-dimensional optical transfer function is derived for analyzing the imaging performance in fiber-optical two-photon fluorescence microscopy. Two types of fiber-optical geometry are considered: The first involves a single-mode fiber for delivering a laser beam for illumination, and the second is based on the use of a single-mode fiber coupler for both illumination delivery and signal collection. It is found that in the former case the transverse and axial cutoff spatial frequencies of the three-dimensional optical transfer function are the same as those in conventional two-photon fluorescence microscopy without the use of a pinhole.However, the transverse and axial cutoff spatial frequencies in the latter case are 1.7 times as large as those in the former case. Accordingly, this feature leads to an enhanced optical sectioning effect when a fiber coupler is used, which is consistent with our recent experimental observation.

Tripartite fiber-coupled fluorescence instrument

Abstract: A non-linear fluorescence microscope or endoscope includes: (a) a source of coherent light suitable to initiate non-linear fluorescence in a sample of interest located at a sample region; (b) a first light conductor positioned to receive at its terminal light from the light source and direct the light to a coupler; (c) a second light conductor located to receive light introduced to the coupler from the first conductor and direct the light to the sample via a sample end of the second conductor, the sample end also located to receive fluoresced light from the sample such that the second conductor may direct the fluoresced light to the coupler; and (d) a third light conductor located to direct fluoresced light from the coupler to a light detection device via a terminal of the third light conductor.

Review of Optical Data Storage

Abstract: As the computer industry grows, so will the requirements for data storage. Magnetic memory has been the most stable method in terms of capacity and recording/reading speed. However, we have reached the point where a substantial increase in the capacity cannot be produced without increasing the size of the system. When compact discs (CDs) were introduced in the 1980s they revolutionized the concept of data storage. Since their inception, the capacity requirements have far exceeded what is available on a compact disc, and they are now following the same path as magnetic memories. Following this trend, it could be assumed that digital versatile discs or digital video discs (DVDs) have a limited lifetime as a storage medium. In fact it has been noted that the maximum capacity of DVDs will be reached in 3-5 years. The question then is what comes next. This chapter aims to illustrate the technology involved in current optical storage methods as well as to introduce several new concepts of optical storage. It is envisaged that a storage system based on either solid immersion lens, holography or three-dimensional bit recording could be the way of the future.

Scanning total internal reflection fluorescence microscopy

Abstract: We report on a new form of total internal reflection fluorescence microscopy. Instead of using a prism, an objective of numerical aperture 1.65 is used under ring beam illumination. As a result, the propagating component of the illumination wave is suppressed and a focused evanescent spot is produced with strength of 20 times stronger than that in the prism. A near-field image is obtained by the scanning of a sample illuminated by the evanescent focal spot. The new imaging system has been successfully used for characterizing CdSe quantum dot nanoparticles and will be useful in nano-fabrication and single-molecular detection.

Two-Photon Optical Storage in Photorefractive Polymers in the Near-Infrared Spectral Range

Abstract: We report the use of a polymer-based photorefractive material for three-dimensional bit optical data storage using near-infrared illumination. The research was conducted using photorefractive materials that were fabricated in two polymer matrices: poly(N-vinylcarbazole) (PVK) and poly(Methyl Methacrylate) (PMMA). The recording samples also consisted of the following compounds in various proportions: 2,5-dimethyl-4-(p-nitrophenylazo)anisole (DMNPAA), 2,4,7-trinitro-9-fluorenone (TNF) and N-ethylcarbazole (ECZ). Two-photon excitation was used as the recording mechanism to achieve rewritable bit data storage in a photorefractive polymer. As a result of two-photon excitation, the quadratic dependence of the excitation on the incident intensity produces an excitation volume that is confined to the focal region in both the transverse and axial directions. The use of ultrashort pulsed lasers, while effective, is not a practical solution for an optical data storage system. This research demonstrates the ability to produce three-dimensional rewritable bit data storage using continuous-wave illumination. Using this technology it has been possible to achieve a density of 88 Gbits/cm3, which in the future could be increased to 3.5 Tbits/cm3.

Two-dimensional near-infrared photonic crystal fabrication by generation of void channels in solid resin

Abstract: Two-dimensional (2D) triangular void channel photonic crystals with different lattice constants stacked in two different directions were fabricated by using femtosecond laser micro-explosion in solid polymer material Fundamental and higher-order stop gaps were observed both in the infrared transmission and reflection spectra There is an approximately linear relationship between the gap position and the lattice constant The suppression of the fundamental gap is as high as 70% for 24-layer structures stacked in the ?-M direction

Compensation for depolarisation by a fibre coil in the presence of self-phase modulation

Abstract: We report on the dependence of the degree of polarisation of an 80 fs pulsed laser beam propagating through a length of a single-mode birefringent fibre on the illumination power. Due to the birefringence and the nonlinear effect of self-phase modulation, the measured depolarisation dependence is oscillatory. It is, however, demonstrated that the oscillatory depolarisation can be compensated for by introducing a loop into the fibre geometry, which maintains the degree of polarisation of the pulsed beam as high as 90%.

Height controllable two-dimensional photonic crystal structures fabricated with two-photon photopolymerisation

Abstract: In this paper photonic crystal structures were fabricated with a shot by shot method. By utilizing two-photon photopolymerisation feature size can be decreased to less than 300 nm. Feature height can be accurately controlled by using different sized obstructions.

New features of light focusing by high numerical-aperture objective

Abstract: In modern optical microscopy, a high numerical-aperture objective plays an important rule for high-resolution imaging and laser trapping. However, a high numerical-aperture objective results in a depolarisation process [1]. As a result, a linear polarised beam is depolarised into three components of the electromagnetic wave in the focal region. One of the components is along the direction of the beam propagation, i.e. along the axial direction. The contribution of the longitudinal component can be enhanced if the incident illumination over the lens aperture is centrally obstructed. As a result, the focal spot in free space is split into two peaks with a separation determined by the size of the obstruction [2] (see Fig. 1). (b) (a) Fig. 1 The focal shapes of a high numerical-aperture objective obstructed by an opaque disk. The normalized size of the obstruction is ε: (a) ε = 0; (b) ε = 0.5. Another feature caused by the longitudinal component is that the recently predicted spectral splitting phenomenon near phase singularities of focused waves [G. Gbur, T. D. Visser and E. Wolf, Phys. Rev. Lett. 88, 013901 (2002)] disappears when the numerical aperture is higher than critical values that are different between the incident polarization direction and the axial direction [3]. If the phase of the field over the lens aperture is also apodised, the focal shape of a high numerical-aperture objective becomes more complicated. For example, when the phase over the lens aperture possesses a helical structure, i.e. when the illumination on the lens has a screw optical singularity, the doughnut focal shape, which occurs for a low numericalaperture objective, does not necessarily exist when the numerical aperture is large [4]. In this presentation, the new features mentioned above will be discussed. [1] Min Gu, Advanced Optical Imaging Theory, Springer Verlag, 2000. [2] J. W. M. Chon, X. Gan, Min Gu , Splitting of the focal spot of a high numerical-aperture objective in free space , Appl. Phys. Lett. 81 (2002), 1576-1578. [3] D. Ganic, James Chon, Min Gu, Effect of numerical aperture on anomalous behaviour of spectra near phase singularities of focused waves, Appl. Phys. Lett., (2003), in press. [4] D. Ganic, X. Gan, Min Gu, Focusing of doughnut laser beams by a high numericalaperture objective in free space, submitted to Appl. Phys. Lett., (2003).

Use of quantum dot nanocrystals for spectrally encoded optical data storage

Abstract: In this paper, we demonstrate the spectral encoding capability of quantum dot nanocrystals (QDs). We mixed two different sizes of QDs in a resin, and two-photon excitation at various wavelengths showed that the relative intensities of the fluorescence from two QDs varied significantly. Such a difference in level of absorption at different wavelengths allows one to record information only on one size of QDs, without affecting the other sized QDs. This property can be applied to optical data storage, so that storage density can be greatly improved.

Multiple higher-order bandgaps in infrared polymer photonic crystals

Abstract: We present a new type of polymer photonic crystals which feature a multitude of higher-order bandgaps in the mid and near infrared spectral region. The crystals are void channel microstructures. Photonic band structure calculations are carried out using an iterative eigensolver program. Their results are consistent with the Fourier transform infrared spectra measured in transmission and reflection and allow for the determination of structural parameters such as the channel cross section and the effective refractive index of the polymer material after fabrication. With increasing ratio of the layer spacing to the in-plane channel spacing the number of higher-order bandgaps rises, and transitions from mere resonant angle Bragg reflections to bandgap total reflection are observed. We emphasize the enormous potential for applications of these highly correlated infrared polymer photonic crystals.

Femtosecond fabrication of three-dimensional photonic crystals in polymers

Abstract: We report on the successful femtosecond fabrication of polymer-based micro-void channel photonic crystals with a high degree of perfection. In woodpile-type structures large main bandgaps in the mid infrared wavelength region and numerous higher-order gaps are observed.

Two-dimensional near-infrared void channel photonic crystal fabricated in solid resin

Abstract: Photonic stop gaps of two-dimensional triangular photonic crystals fabricated by laser scanning of void channels in solid resin are examined in two different directions. Fundamental and higher-order gaps are observed in infrared transmission and infrared reflection.

Recent development of fibre-optic two-photon fluorescence endoscopy devices

Abstract: We will give an overview of our recent on fibre-optic two-photon fluorescence endoscopy devices. The main component of these devices is a single fibre coupler which acts as a low pass filter. The first device involves a lens-based probe head and the second includes a prism-based probe head.

Femtosecond fabrication of waveguide-like micro-structures in a photorefractive polymer

Abstract: In this letter, we report on, for the first time, the successful femtosecond micro-fabrication of continuous waveguide-like channels in the photorefractive polymer consisting of the nonlinear chromophore 2,5-dimethyl-4-(p-nitrophenylazo)anisole (DMNPAA), the photosensitive compound 2,4,7-trinitro-9-flourenone (TNF), and the plasticiser N-ethylcarbazole (ECZ) all doped in the polymer matrix poly(methyl methacrylate) (PMMA). These channels are caused by the change in refractive index as a result of the localised heating of the polymer and therefore have an important potential for micro-photonic devices in future.