Formation of a surface having a periodic subwavelength structure (SWS) is a well-known technique for reducing the
Fresnel reflection of transmissive optical elements. We fabricated an anti-reflective structured (ARS) surface with a twodimensional
periodic structure with the period of 300 nm on an optical glass by a precision molding process using a
silica glass mold coated with a thin carbon film. The surface structure was formed on the mold using reactive ion etching
(RIE) with fluorocarbon plasma. A thin chromium film was patterned using electron beam lithography and a wet etching
process. The anti-reflective surface was formed on a phosphate glass with a deformation point of 412°C and a refractive
index of 1.60 at a wavelength of 462 nm. The phosphate glass was molded at 420°C for 510 s under a pressure of 5 MPa.
The height of the periodic structure on the mold was 550 nm and the height of that on the formed glass was 480 nm.
Therefore, the filling rate of the phosphate glass to the mold was 87%. The surface reflectance of the glass was estimated
as 0.56% at a wavelength of 462 nm, which was approximately 1/10 that of the optically polished surface.
A self-trapped filament of ultrashort laser pulses can induce a several-hundred-micron-long region of refractive-index change in silica glass. The maximum refractive-index change and the diameter of refractive-index change are approximately 0.01 and 2 μm, respectively. The filament is 10 - 500 μm long along the pulse propagation axis and its length depends mainly on the numerical aperture of the focusing lens. In this paper, we present the fabrication experiment of volume gratings induced in silica glass by a self-trapped filament of ultrashort pulses. When the 150-μm-long filament was translated perpendicular to the optical axis by 300 μm, a layer of refractive-index change with the thickness of 2 μm was induced. We stacked the layers with a period of several microns and fabricated volume gratings. We entered a He-Ne laser beam at the wavelength of 632.8 nm to the grating with the Bragg angle to measure the diffraction efficiency. The maximum diffraction efficiency was 74.8% with the grating that had the period of 3 μm, and the thickness of 150 μm.
Permanent refractive-index change can be induced by a self-trapped filament of infrared femtosecond laser pulses in silica glass. We present the fabrication of waveguides, couplers, and gratings by use of the self-trapped filament. The self-trapped filament with 30 μm long was found to bend from the direction of the incident laser pulses by two-dimensional translation. This technique leads to a formation of a permanent curved waveguide. We fabricate a 2-mm long directional coupler with the core diameter of 2 μm to split the coupled beam into 1:1. We also show the fabrication of gratings. Finally, we investigate the dependence of refractive-index change on polarization of incident laser pulses.
We have already shown that the refractive-index change is induced by a self-trapped filament of ultrashort laser pulses in silica glass. In this paper, we investigate the dependence of refractive-index change on polarization of incident laser pulses in silica glass. In the experiment, we focused linearly polarized pulses inside a sample of silica glass. We polished the sample and etched by 5% HF solution to observe the cross-sectional view of the regions of refractive-index change that are perpendicular to the filament. The observation with a scanning-electron microscope shows that the cross-section is elliptical and the long axis was parallel to polarization direction of incident laser pulses. The ellipticity was 0.85. We fabricated gratings to estimate the index ellipsoid of the region of refractive-index change. We confirmed that the index ellipsoid was uniaxial and negative. The optic axis was parallel to the axis of the filament and the birefringence was 1×10-3.
When femtosecond laser pulses are tightly focused inside the bulk of transparent materials, the intensity in a focal volume becomes high enough to produce submicrometer-scale structural modifications. This damage was shown to be a cavity or a void surrounded by densified material. An array of voids can be used as optical data storages or gratings. We showed the control experiment of the positions and shapes of voids insider transparent materials with femtosecond laser pulses. We have demonstrated the experiments involving optical movement of a void along the optical axis by translation of the focal spot with femtosecond laser pulses. Irradiation of femtosecond laser pulses moves a void inside calcium fluoride and silica glass without any mechanical translations of the optical system up to 2 micron. In this paper, we show that the shapes of voids can be controlled by the spatial profile of incident laser pulses. Finally we show that the fabrication of a Fresnel lens inside silica glass.
We have been studying the refractive index changes and vacancies that are induced in silica glass by the irradiation of ultrashort laser pulses. By scanning the laser beam in the glass we can form 3-D shape of waveguides, arrays of tiny vacancies, called voids, and long holes with microscopic diameters. In this paper, we report on the asymmetry of the waveguide formed by linearly polarized ultrashort pulses. The formation of the photo-induced waveguide is normally accompanied by the filamentation, the self-trapping of laser beam due to nonlinear optical effects. The asymmetric cross-section of the waveguide structures explains properly the illusory birefringence of photo-induced waveguides observed earlier. The cross-sectional forms of the waveguides were observed by polishing and etching the cross-psections. We also report the possibility of forming asymmetric shapes of voids. The asymmetry of voids results from the beam profile. We controlled the profile by inserting apertures before the focusing lens. The asymmetry leads to the polarization dependence of diffraction from the array of voids. We also report on the formation of Bragg grating in glass. The Bragg gratings were formed in soda-lime glass. We succeeded in forming a series of three Bragg gratings. The formation of grating inside glass was confirmed by diffraction experiments and chemical etching of polished cross-sections.
When femtosecond laser pulses are tightly focused inside the bulk of transparent materials, the intensity in a focal volume become high enough to produce submicrometer-scale structural modifications. The modifications has been applied to fabricate 3D photonic structures. Tightly-focused femtosecond laser pulses create voids, which are surrounded by densified material. In this paper we show that the shapes of voids can be controlled by the spatial profile of incident laser pulse. We also show that the diffraction intensities due to the fabricated arrays of voids depend on the polarization-states of the readout beam. Finally, we demonstrate that irradiation of femtosecond laser pulses moves a void inside calcium fluoride and silica glass without any mechanical translations of the optical system. In situ observation revealed that a void moves towards incident direction of laser pulses as long as 2 micron.
Large-scale integration of compact photonic devices may be the key technology for the future information age. If optical-waveguide structures with small radius of curvatures are integrated inside a small glass tip along with active devices, a paradigm of microphotonics inside glass may become possible. We will review the fabrication techniques for the waveguides and photonic-band structures inside glass with the ultrashort laser pulses and present our recent results of fabrication experiments. The experimental studies include fabrication of birefringent waveguides, formation of small vacancies called voids, and drilling of a narrow but long hole from the rear side of a glass. We will show the birefringence properties of waveguides that are induced by the lienarly-polarized ultrashort laser pulses of approximately 100 fs duration. Birefringence of the waveguides depends on the polarization states of the fabrication beam. The experimental studies on the stabilities of a void during the fabrication process are also presented. We will show that the voids move during the fabrication process although the beam and sample are fixed. It will be shown that the void moves from pulse to pulse toward the upstream direction along the optical axis. We can currently drill a long hole of more than 200 microns with a diameter of several microns. We also present the results of numerical analysis of optical propagation through photonic structures that are based on the array of voids.
Many researchers have investigated the interaction of femtosecond laser pulses with a wide variety of materials. The structural modifications both on the surface and inside the bulk of transparent materials have been demonstrated. When femtosecond laser pulses are focused into glasses with a high numerical aperture objective, voids are formed. We report first observation that a bubble, which is called void moves under irradiation of femtosecond laser pulses inside silica glass and calcium fluoride. In situ observation reveals that the void moves towards incident direction of laser pulses as long as 5 micron by successive laser pulses without any mechanical translations.
Refractive-index change is induced by a self-trapped filament of ultrashort laser pulses in a wide variety of glasses. We investigated the dependence of refractive-index change on polarization state of incident ultrashort laser pulses in silica glass. We fabricated birefringent structures and estimated the refractive-index change.
Many researchers have investigated the interaction of femtosecond laser pulses with a wide variety of materials. The structural modifications both on the surface and inside the bulk of transparent materials have been demonstrated. When femtosecond laser pulses are focused into glasses with a high numerical-aperture objective, voids are formed. We demonstrate that one can seize and move voids formed by femtosecond laser pulses inside silica glass and also merge two voids into one. We also present clear evidence that a void is a cavity by showing a scanning-electron-microscope image of cleft voids: we clove through the glass along a plane that includes the laser-ablated thin line on the surface and the voids formed inside. The optical seizing and merging of voids are important basic techniques for fabricate micro-optical dynamic devices, such as the rewritable 3-D optical storage.
Recently, the structural modifications of glass by focusing femtosecond laser pulses have been demonstrated. We present photo-induced structural changes in silica glass with femtosecond laser pulses. We investigated the relationship between the formation of filaments and local refractive index changes in silica glass. In situ observation revealed the coincidence between the location of filament and that of refractive index change. The observation also showed that the region of refractive index change elongates toward the upstream direction of laser pulses with the exposure time. The region of refractive index change was several hundred- micron long and the diameter was smaller than two microns. The length of the region was dependent on the numerical aperture of focusing lenses. The refractive index change was confirmed to be as large as 0.01 by three different methods. We fabricated a 2-mm waveguide by translating the sample along the optical axis.
We studied the behavior of filaments that are formed in the beam of ultrashort laser pulses via self-focusing effect. A refractive index change of 0.01 was confirmed by three independent image-analysis techniques. The waveguide structures were a few to 500 micrometers long depending on the numerical aperture (NA) of the ultrashort pulses. All the diameters were approximately two micrometers. We identified the conditions to produce one filament in one beam and produced waveguides using this single filament. By moving the focus along the optical axis, a longer straight waveguide can be produced. We also demonstrate that one can seize and translate voids formed by ultrashort laser pulses inside silica glass and can also merge two voids into one. We further present a clear evidence of a void and its surrounded region by showing scanning-electron-microscopic image of cleaved voids: we cleaved through the glass along a plane that includes the laser-ablated thin line on the surface and the voids formed inside.
We succeeded in fabricating small Bragg reflectors in glass and will present this experiment. We found that the photo- induced refractive index change caused by 1-mJ 130-fs laser pulses is elongated along the optical axis compared with the original intensity distribution near the focus of the fabrication beam. The length and radius of the index change were approximately 30 and 2 micrometers, respectively. We consider that the self-focusing due to the self-phase modulation resulted in this form. This structure is suited for the fabrication of small Bragg reflectors in glass. We fabricated 1 mm X 1 mm X 30 mm Bragg reflectors. By analyzing the diffraction efficiencies of higher order diffraction beams we estimated the refractive index distribution. The thickness and spacing of the periodic laminar structure were 1.5 and was 3.8 micrometers, respectively. The maximum index change was 0.0015. We note that we observed a quite large amount of scattering loss (approximately 15%) in the grating. We fabricated also a 1 mm X 1 mm X 60 mm grating by writing two layers of gratings and obtained a diffraction efficiency of 13%. Recent experimental results of photorefractive waveguides will also be reviewed.
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