We review recent progress in application of femtosecond laser nanostructuring of fused silica. The tight control of nanostructures’ properties through writing parameters is demonstrated implementing elements with unique optical properties, which can be widely used in material processing, microscopy, optical trapping and manipulation.

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Polarization sensitive elements fabricated by femtosecond laser nanostructuring of glass [Invited]

We demonstrate maskless, single-step fabrication of strongly birefringent Fresnel zone plates by focusing of femtosecond laser pulses deep within silica substrates. The process allows us to produce alternate zone rings directly by inducing a local refractive-index modification of the order of n~10-2 . The embedded zone plates shown in this Letter exhibit efficiencies that vary by as much as a factor of ~6 for orthogonal polarizations. Focal lengths of primary and secondary foci are shown to compare well with theory.

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Birefringent Fresnel zone plates in silica fabricated by femtosecond laser machining

In the past centuries, the scale of engineering optics has evolved toward two opposite directions: one is represented by giant telescopes with apertures larger than tens of meters and the other is the rapidly developing micro/nano-optics and nanophotonics. At the nanoscale, subwavelength light-matter interaction is blended with classic and quantum effects in various functional materials such as noble metals, semiconductors, phase-change materials, and 2D materials, which provides unprecedented opportunities to upgrade the performance of classic optical devices and overcome the fundamental and engineering difficulties faced by traditional optical engineers. Here, the research motivations and recent advances in subwavelength artificial structures are summarized, with a particular emphasis on their practical applications in super-resolution and large-aperture imaging systems, as well as highly efficient and spectrally selective absorbers and emitters. The role of dispersion engineering and near-field coupling in the form of catenary optical fields is highlighted, which reveals a methodology to engineer the electromagnetic response of complex subwavelength structures. Challenges and tentative solutions are presented regarding multiscale design, optimization, fabrication, and system integration, with the hope of providing recipes to transform the theoretical and technological breakthroughs on subwavelength hierarchical structures to the next generation of engineering optics, namely Engineering Optics 2.0.

Subwavelength Artificial Structures: Opening a New Era for Engineering Optics.

A switchable Fresnel zone plate lens is demonstrated using a polymer-stabilized liquid crystal. The fabrication process is relatively simple and the device can be operated below 10 volts with fast response time. Such a device works well for a linearly polarized light.

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Switchable Fresnel lens using polymer-stabilized liquid crystals

We demonstrated a highly efficient, polarization-independent and electrically tunable Fresnel lens based on dye-doped liquid crystal using double-side photoalignment technique. The maximum diffraction efficiency reaches 37%, which approaches the theoretical limit ~41%. Such a lens functions as a half-wave plate, and this feature could be well preserved under the applied voltage. In addition, the device is simple to fabricate, and has fast switching responses between focusing and defocusing state.

Highly efficient and polarization-independent Fresnel lens based on dye-doped liquid crystal

The dispersion characteristics of a diffractive micro-Fresnel lens (MFL) which is mass-produced for a laser diode collimator are examined for novel applications. The MFL's resolving power (Rayleigh criterion for resolution) at the design wavelength is calculated to be comparable to that of conventional dispersing prisms of the same size. A simple spectrometer with an MFL (4.5 mm diameter, 14.83 mm focal length at 0.78 μm design wavelength) and a movable pinhole (PH), is fabricated. The optimum PH diameter for both resolving power and light efficiency is 5 m, and the resolution and efficiency at the design wavelength are estimated to be 3.7 nm and 85%, respectively. The measurable spectral range determined from both the diffraction efficiency and the free spectral range of the MFL almost covers the entire visible region. The absorption spectrum of KMnO4 solution is roughly obtained using the spectrometer. The space between the MFL and the PH, and the PH's movable length, which are required for spectroscopy, are as short as 20 and 7 mm, respectively. The simple and straight-line-structured spectroscopic system utilizing a more cost-efficient MFL may lead to the realization of less expensive and compact spectrometers.

Spectrometer employing a micro-Fresnel lens

The secondary emission RSE emitted after resonant excitation of F center in KCl has been measured over the whole Stokes range at 80 K. The degree of polarization P defined for the RSE intensities parallel and perpendicular to resonant light polarization is measured over the same range. The spectrum of P is divided into three successive regions; the depolarization range at the one-phonon Raman scattering, the near plateau range, and the depolarization range down to vanishing. The former two are found to depend on the resonant photon energy. The latter may be caused by the level crossing of 2 s - and 2 p -like electronic states during non-radiative process.

The Resonant Secondary Emission of the F Center in KCl at 80 K

The Stark effects in F center emission have been investigated for KBr, KI, RbBr, and RbI crystals as a function of emission wave-length and polarization, electric field amplitude, and sample temperature, with using a PbS IR detector. Observed results are qualitatively similar to those measured by Bogan and Fitchen for several other alkali halides. The Stark polarization and the first moment change (a red shift) of the emission band are quadratic in electric field strength; their coefficients are tabulated. Two different theoretical forms of the temperature dependence of the Stark polarization P ( T ), which have been derived by Bogan and Fitchen, and Ham and Grevsmuhl, respectively, are found to be invalid to explain the experimental data consistently. A phenomenological form to describe P ( T ) is proposed by introducing a new parameter.

Stark Effects in F Center Emission in KBr, KI, RbBr, and RbI Crystals

Relaxation of optically excited F centers

Solids of seven all-trans isomeric carotenoids, β-carotene, β-apo-8′-carotenal, astaxanthin, canthaxanthin, spheroidene, lycopene and zeaxanthin, were studied in the forms of KBr disks, aggregates, thin films and single crystals in an effort to understand and control the molecular arrangement of the solids. Optical absorption and resonance Raman scattering were adopted to characterize the arrangements. Card-packed arrangements and head-to-tail arrangements were realized by changing either the end groups or the growth process. These studies, which reveal the elementary engineering of solid growth and its characterization, may open up carotenoid solid engineering to develop useful applications of the materials in the fields of electronics, photonics and mechanical engineering. Copyright © 2001 John Wiley & Sons, Ltd.

Yuzo Mori

Papers

Spectrometer employing a micro-Fresnel lens

The Resonant Secondary Emission of the F Center in KCl at 80 K

Stark Effects in F Center Emission in KBr, KI, RbBr, and RbI Crystals

Relaxation of optically excited F centers

Introductory studies on the growth and characterization of carotenoid solids: an approach to carotenoid solid engineering