Wanji Yu

Polarization-multiplexed diffractive optical elements fabricated by subwavelength structures.

Polarization-multiplexed phase-only diffractive optical elements with subwavelength structures are proposed and fabricated. The differences among the phase modulations result from the differences among the effective indices exhibited in the subwavelength structures with various filling factors and surface profiles, and the phase retardations are obtained by the relief depth of the structures. The polarization-selective property is achieved by the polarization dependence of the effective indices exhibited in the one-dimensional subwavelength structures and the polarization independence exhibited in the two-dimensional structures. Additionally, the polarization contrast of our polarization-multiplexed elements, defined as the cross talk between the two polarization incidences, is independent of the relief depth. The principle of the polarization multiplexing by use of the subwavelength structures is described, and the fabrication results for the polarization-multiplexed computer-generated holograms are demonstrated.

Fabrication of multilevel phase computer-generated hologram elements based on effective medium theory

A conventional method to synthesize diffractive optical elements and computer-generated holograms (CGHs) with high diffraction efficiency relies on an increase of phase levels. To fabricate such a device, one should perform electron-beam (e-beam) lithography with multiple-dose exposures or multiple-step photolithography. Here we describe a one-step method, which is based on the effective medium theory, for the fabrication of a multilevel phase CGH. The phase modulations required in cells of a CGH are constructed by means of dividing these cells into fine (subwavelength) structures. The surface features of these fine structures control their corresponding indices, and their values can be calculated according to the effective medium theory. By proper selection of the fine structures, based on the requirements of the phase modulation of the cells, a CGH with multilevel phases is synthesized when a binary structure is relieved on the dielectric material. Then the CGH can be fabricated by direct e-beam lithography or one-step photolithography through an amplitude mask followed by an ion-etching treatment. The experimental results showed that the reconstructed wave field is in good agreement with that simulated by a computer, indicating the effectiveness of the proposed method.

Reduced wavelength-dependent quarter-wave plate fabricated by a multilayered subwavelength structure

We developed a novel fabrication method of a reduced wavelength-dependent quarter-wave plate (QWP) based on form birefringence of a multilayered subwavelength structure. The multilayered structure was constructed by depositing a high-refractive-index thin film on a subwavelength-structured substrate with a low refractive index. The surface structure of the substrate was shallow enough to be formed by a mass replication technology. A high-refractive-index subwavelength grating was formed on ridges of the substrate by sputtering Zn2SnO4 (refractive index of 2.03 at a wavelength of 633 nm). Moreover, since the grooves of the high-refractive-index grating were very deep and narrow, the dispersion of form birefringence suppressed the dependency of phase retardance on the wavelength of light in a limited spectral region. The phase retardance of the fabricated QWP was 89 degrees at a 633 nm wavelength and 79 degrees at a 785 nm wavelength.

Synthesis of Wave Plates Using Multilayered Subwavelength Structure

The form-birefringence effect exhibited in the multilayered subwavelength structure (SWS) has been applied for the realization of wave plates. The SWS was obtained by coating a thin film of a material with a high refractive index onto a base SWS fabricated on a substrate with low refractive index. The multilayer SWS fabricated by this coating process exhibits strong form birefringence compared with that of the base SWS. This fact results in less depth required to realize wave plates than that with only the base SWS. In this study, a quarter-wave plate for the operating wavelength of a He-Ne laser (λ= 632.8 nm) was designed and fabricated using the above described multilayered SWS. The experimental results demonstrated that the fabricated device is effective as a quarter-wave plate and gives transmission efficiencies near 90% for both transverse electric (TE) and transverse magnetic (TM) waves.

Application of ultrafast time-to-two-dimensional-space-to-time conversion (I): Time-varying spectral modulation for arbitrary ultrafast signal generation

Time-varying spectral modulation for arbitrary ultrafast signal generation is demonstrated as one of applications of ultrafast time-to-two-dimensional-space-to-time conversion. The continuous sequence of a spectral-decomposed pulse is generated from a seed ultrashort pulse and its spectrum on each timing is independently modulated in the space domain. As a result of preliminary experiments, the ultrashort pulse sequence composed of four pulses of a 1-ps interval with three wavelength channels can be generated after time-varying spectral modulation.

Optical Elements with Subwavelength Structured Surfaces

Several optical elements with subwavelength structured (SWS) surfaces have been developed. The SWS has optical features of artificial refractive index, form birefringence, resonance and band-gap effects. This paper describes some applications of form birefringent optical elements and a resonant reflection element. A form-birefringent quarter-wave plate was realized by sputtering the high refractive-index thin film on a SWS substrate. The wavelength dispersion of form birefringence restrains the phase retardance from depending on the wavelength of light. An array of form-birefringent wave-plates is useful for the real-time imaging polarimetry. We developed a real-time polarization imaging system for the visible light. A guided-mode resonant grating with a PLZT wave-guide was designed for optical switches. The PLZT is ferroelectric material with an electro-optic (EO) effect. We made a feasibility study on the optical switching by numerical simulations.

Synthesis of polarization-selective optical components with multilayer subwavelength structures

In this presentation we propose the multi-layer subwavelength structures to realize the polarization-sensitive optical elements. The multi-layer structures are fabricated on such consideration that a periodic relief subwavelength structure (we call the base structure later) with depth D1 is first prepared in a substrate with refractive index n1 , then a dielectric material with a refractive index n2 is directionally coated on it with thickness D2 . With the above fabrication process the coated dielectric material alternatively covers on the ridges or fills in the grooves of base structure to form the multi-layer structures. The characteristics of the effective indices and the form-birefringence behaved in such structures were investigated. The form-birefringent wave plates and the blazed-binary polarization-selective diffractive optical elements for normal incidence were synthesized with the proposed structures. The optical behaviors included the phase retardation, diffraction efficiencies, and the polarization contrasts were numerically analyzed with rigorous electromagnetic algorithm. A quarter-wave plate for the operating wavelength of He-Ne laser (λ=632.8 nm) was fabricated and the significant optical performances was obtained.

Development of Deep Gratings for Wavelength Dispersion Applications

The deep gratings with ultrahigh spatial frequencies have been developed for wavelength dispersion applications. The diffraction performances, including diffraction efficiencies, wavelength dispersive abilities and wavelength band, have been theoretically investigated. The high diffraction efficiencies, wide wavelength dispersive abilities and broad wavelength range proved the significant advantages of the developed gratings for our applications. For the fabrication process, a method for synthesizing the grating profiles to produce optimal optical performances for practical applications and for easily achieving high fidelity by microfabrication technologies has been proposed. The experimental results show that the fabricated gratings using the synthesis method have diffraction efficiencies higher than 95%, while the wide wavelength dispersions are reserved, and low diffraction fluctuations are also reduced.

Sensitive Detection of Transitional Spectral Pattern Using Optical Spectrogram Scope

We propose a new method using analysis before acquisition (here, we call it the preprocessing method) to inhibit the influence of the properties of the detector in pulse measurement. We introduce concepts of optical signal processing into the pulse measurement system to realize the preprocessing method. Optical spectrogram scope (OSS) proposed by us is a pulse measurement system based on an optical correlation technique using an optical filtering technique, which enables the introduction of various processing methods into the OSS system by designing the optical filter. In this paper, we propose the preprocessing method using OSS, and demonstrate the sensitive detection of a transitional spectral pattern as an example of the pulse measurement using the preprocessing method.

Suppression of Crosstalk in Header Recognition Based on Optical Correlation using Multiple-Object Discriminant Filter

We demonstrate that crosstalk caused by matched filtering can be suppressed by designing a header recognition filter in the header recognition unit based on optical correlation processing. The header recognition filter is designed using a technique of a multiple-object discriminant filter (MODF). Results of preliminary experiments show that the designed header recognition filter can suppress crosstalk. The peak intensity ratio of correlation signals of a target header signal to those of nontarget ones is higher than 1.48:1 in the case of 4-bit header signals. The number of discriminable header signals could markedly increase using the designed header recognition filter.