Rong-Chung Tyan

Design, Fabrication, and Characterization of Form-Birefringent Multilayer Polarizing Beam Splitter

Polarizing beam splitters that use the anisotropic spectral reflectivity (ASR) characteristic of high-spatial-frequency multilayer binary gratings have been designed, fabricated, and characterized. Using the ASR effect with rigorous coupled-wave analysis, we design an optical element that is transparent for TM polarization and reflective for TE polarization at an arbitrary incidence angle and operational wavelength. The experiments with the fabricated element demonstrate a high efficiency (97), with polarization extinction ratios higher than 220:1 at a wavelength of 1.523 m over a 20 angular bandwidth by means of the ASR characteristics of the device. These ASR devices combine many useful characteristics, such as compactness, low insertion loss, high efficiency, and broad angular and spectral bandwidth operations.

Polarizing Beam Splitter Based on the Anisotropic Spectral Reflectivity Characteristic of Form-Birefringent Multilayer Gratings

We introduce a novel polarizing beam splitter that uses the anisotropic spectral reflectivity (ASR) characteristic of a high-spatial-frequency multilayer binary grating. Such ASR effects allow us to design an optical element that is transparent for TM polarization and reflective for TE polarization. For normally incident light our element acts as a polarization-selective mirror. The properties of this polarizing beam splitter are investigated with rigorous coupled-wave analysis. The design results show that an ASR polarizing beam splitter can provide a high polarization extinction ratio for optical waves from a wide range of incident angles and a broad optical spectral bandwidth.

Fabrication, Modeling, and Characterization of Form-Birefringent Nanostructures

A 490-nm-deep nanostructure with a period of 200 nm was fabricated in a GaAs substrate by use of electron-beam lithography and dry-etching techniques. The form birefringence of this microstructure was studied numerically with rigorous coupled-wave analysis and compared with experimental measurements at a wavelength of 920 nm. The numerically predicted phase retardation of 163.3° was found to be in close agreement with the experimentally measured result of 162.5°, thereby verifying the validity of our numerical modeling. The fabricated microstructures show extremely large artificial anisotropy compared with that available in naturally birefringent materials and are useful for numerous polarization optics applications.

New fabrication techniques for high quality photonic crystals

We have developed new methods for the fabrication of high quality two-dimensional (2D) and three-dimensional (3D) photonic crystals. These techniques involve anisotropic etching and steam oxidation of AlAs mask layers. We have made manufacturable 2D photonic crystals with high aspect ratios for use as micropolarizers and have measured extinction ratios larger than 800 to 1 between TE and TM modes transmitted through these structures. The new Al2O3 mask fabrication technique also allows us to fabricate 3D structures with up to six repeating layers in depth and over 90% attenuation in the band gap region. Here, we show the fabrication details and performance of 2D and 3D photonic crystals.

Form-birefringent computer-generated holograms

Polarization-selective computer-generated holograms made with form-birefringent nanostructures were designed, fabricated, and evaluated experimentally at 1.5 microm. The fabricated element showed a large polarization contrast ratio (>250:1) and a high diffraction efficiency (>40% for a binary phase level element). The experimental evaluation was in good agreement with the design and modeling predictions.

Analysis of enhanced second-harmonic generation in periodic nanostructures using modified rigorous coupled-wave analysis in the undepleted-pump approximation

We present an extension of the rigorous coupled-wave analysis technique to analyze second-harmonic generation (SHG) in periodic optical nanostructures in the undepleted-pump approximation. We apply this method to analyze SHG in two example nanostructures for which we predict enhanced nonlinearity due to transverse near-field localization of the fundamental optical field in the nonlinear material. First, we examine a periodic nanostructure that yields up to twice the transmitted SHG intensity output compared with the bulk nonlinear material but only for small nanostructure depths because of mismatch of the fundamental and second-harmonic mode phase velocities. Second, we develop and analyze a modified nanostructure and find that this nanostructure concurrently achieves transverse localization and phase matching for SHG. In principle, this permits an arbitrary coherent interaction length, and for several specific nanostructure depths we predict a transmitted SHG intensity output more than two orders of magnitude greater than that of the bulk material.

Ultrashort pulse propagation in near-field periodic diffractive structures by use of rigorous coupled-wave analysis

We present a method for near-field analysis of ultrashort optical pulse propagation in periodic structures-including subwavelength and resonant grating structures-based on the integration of Fourier spectrum decomposition and rigorous coupled-wave analysis (RCWA). We discuss the spectral decomposition, including considerations for computational efficiency, the application of the RCWA method to compute the internal and external fields of the structure, and the synthesis of the resulting fields to obtain the time-domain solution. We apply this tool to the analysis of two photonic structures: (1) a nanostructured polarization-selective mirror that exhibits the desired broadband performance characteristics when operated at the design wavelength but yields strongly diminished polarization selectivity and modulation of the pulse envelope at an offset wave-length and (2) a two-mode coupled waveguide structure that produces from one incident pulse two transmitted pulses whose temporal separation depends on the waveguide geometry. In both examples, we apply our new modeling tool to investigate the near fields and find that near-field effects are critical in determining the performance characteristics of nanostructured devices. Furthermore, detailed observation and understanding of near-field phenomena in nanostructures may be applied to the design of novel photonic devices.

Single-substrate birefringent computer-generated holograms.

A polarization-selective computer-generated hologram fabricated upon a single substrate of birefringent YVO(4) crystal is demonstrated. Rigorous couple-wave analysis was used to model the element. The experimentally measured first diffraction order showed a close-to-theoretically predicted diffraction efficiency of 39%. The polarization contrast ratio was measured to be 33:1.

Near-field localization of ultrashort optical pulses in transverse 1-D periodic nanostructures.

We present a transverse 1-D periodic nanostructure which exhibits lateral internal Thorneld localization for normally incident ultrashort pulses, and which may be applied to the enhancement of nonlinear optical phenomena. The peak intensity of an optical pulse propagating in the nanostructure is approximately 12 times that of an identical incident pulse propagating in a bulk material of the same refractive index. For second harmonic generation, an overall enhancement factor of approximately 10.8 is predicted. Modeling of pulse propagation is performed using Fourier spectrum decomposition and Rigorous Coupled-Wave Analysis (RCWA).

Analysis of near-field effects in artificial dielectric structures using rigorous coupled-wave analysis

Artificial dielectric (AD) materials, where the optical properties of a material are altered through the introduction of subwavelength periodic structuring, have extensive applications in optoelectronic and photonic components to implement properties such as antireflectivity, birefringence, or tuning of the refractive index. However, as the sophistication of AD structures grows, a detailed understanding of the near-field effects in the structure becomes critical. For dense device integration or multifunctional device designs, near-field effects may result in performance degradation. A tool capable of accurately modeling the near fields in an AD is crucial for the study of these effects and for advanced device design. In order to achieve these goals, we have developed a new method which extends the capabilities of rigorous coupled-wave analysis (RCWA) to compute the internal (in addition to the external) fields of an AD with quasimonochromatic or ultrashort pulse illumination. We then apply this technique to investigate one important application of near-field effects: field concentration within an AD. For example, in the study of nonlinear optical interactions with matter, ultrashort pulse lasers are commonplace, as the nonlinear effects are greatly enhanced by the extremely high peak power resulting from the temporal concentration of the field. By using an appropriate AD structure, field localization in the transverse direction can also be achieved. To demonstrate this idea, we design two AD structures that rely on transverse field concentration: (i) a device which exhibits field confinement in the transverse direction when illuminated by an ultrashort optical pulse, thereby allowing enhancement of nonlinear effects; and (ii) a multifunctional device combining an effective l-D photonic crystal (PC) color filter with an AD for field localization, allowing for enhanced detection efficiency.