Kris Ohlinger

A tunable three layer phase mask for single laser exposure 3D photonic crystal generations: bandgap simulation and holographic fabrication

Through the use of a multi-layer phase mask to produce five-beam interference, three-dimensional photonic crystals can be formed through single exposure to a photoresist. In these holographically formed structures, the interconnectivity is controlled by the relative phase difference among contributing beams. Photonic band gaps are calculated and the simulation shows a maximum bandgap of 18% of the middle gap frequency when the phase difference is optimized. A three-layer phase mask is fabricated by placing a spacer layer between two orthogonally-orientated gratings. The phase difference is controlled by thermal-tuning of the spacer thickness. Photonic crystal templates are holographically fabricated in a photosensitive polymer using the phase mask.

Spatially addressable design of gradient index structures through spatial light modulator based holographic lithography

In this paper, we present an achievable gradient refractive index in bi-continuous holographic structures that are formed through five-beam interference. We further present a theoretic approach for the realization of gradient index devices by engineering the phases of the interfering beams with a pixelated spatial light modulator. As an example, the design concept of a gradient index Luneburg lens is verified through full-wave electromagnetic simulations. These five beams with desired phases can be generated through programming gray level super-cells in a diffractive spatial light modulator. As a proof-of-concept, gradient index structures are demonstrated using synthesized and gradient phase patterns displayed in the spatial light modulator.

Holographic fabrication of 3D photonic crystals using silicon based reflective optics element

We present a silicon based single optical element that is able to automatically generate desired laser beam polarizations and intensities for the holographic fabrication of woodpile-type photonic crystal templates. A polydimethylsiloxane (PDMS) mold based reflective optics element is fabricated for the generation of five-beam interferences where four beams are arranged four-fold symmetrically around a central beam. Silicon chips in the inner surfaces of the mold are used to reflect the circularly or elliptically polarized beam into four side beams that are linearly polarized with electric fields normal to the incident plane, and reduce their laser intensities. Photonic crystal templates are holographically fabricated in a photosensitive polymer through this silicon-on-PDMS based single optical element and single beam based configuration.

Nanoimprinting lithography of a two-layer phase mask for three-dimensional photonic structure holographic fabrications via single exposure

We report a combined holographic and nanoimprinting lithography technique to produce three-dimensional woodpile photonic crystal templates through only one single exposure. The interference lithography process uses an integratable diffractive optical element for large throughout 3D pattern manufacturing. The diffractive optical element consists of two layers of phase grating separated by an intermediate layer, fabricated by repeated nanoimprinting lithography, followed by an SU8 photoresist bonding technique. Grating periods, relative orientation, diffraction angle, and efficiency, as well as layer to layer phase delay, are well designed during manufacturing. By thermally optimizing the thickness of the intermediate layer, this paper demonstrates the fabrication of interconnected 3D photonic structures with arbitrary symmetry through a single laser exposure. The two-layer phase mask approach enables a CMOS-compatible monolithic integration of 3D photonic structures with other integrated optical elements and waveguides.

Transformation optics designed general optical Luneburg lens with flattened shapes

It is well-known that the conventional lens design suffers from the aberration, which will lead to imperfect imaging. One way to solve this problem is to use gradient index (GRIN) lenses such as Luneburg lens. However, the spherical geometry of Luneburg lens imposes difficulty for manufacturing. Also, it is desired to design the Luneburg lens with arbitrary focal length. To address these issues, in this paper, we propose to apply the transformation optics techniques to the general Luneburg lens design. In this way, the spherical lens surface will be transformed to flattened shapes, which can be practically fabricated on a flat substrate. Specifically, three-dimensional (3D) Luneburg lenses with different focal lengths will be studied. Moreover, discussion on the fabrications of proposed lens has been included. It is desired to ensure that the modified design lies within the available material properties of various polymer photoresists.

Undistorted 3D microstructures in SU8 formed through two-photon polymerization

This paper presents the wavelength dependence of two-photon polymerization in SU-8 between 720-780 nm. The study is performed by microstructuring SU-8 through a single-shot exposure of SU-8 to 140 fs tunable laser pulses with 80 MHz repetition rate, or by laser direct writing. Two-photon absorption is closely related to one-photon absorption in pristine SU-8. By careful design of the neighboring micro-structures, or by varying wet-processing parameters during development, undistorted and unbended 3D micro-structures have been fabricated through direct laser writing.

Holographic Fabrication of Three-Dimensional Woodpile-Type Photonic Crystal Templates Using Phase Mask Technique

The telecommunication and computing industries are currently facing increasing challenges to transfer data at a faster rate. Researchers believe that it might be possible to engineer a device operate at optical frequencies. Photonic technology using photon instead of electron as a vehicle for information transfer paves the way for a new technological revolution in this field. Photons used for communication has several advantages over electrons which are currently being used in electronic circuits. For example, photonic devices made of a specific material can provide a greater bandwidth than the conventional electronic devices and can also carry large amount of information per second without interference. Photonic crystals are such kind of material. They are periodic structures that allow us to control the flow of photons. (John, 1987; Yablonovitch, 1987) To some extent it is analogous to the way in which semiconductors control the flow of electrons: Electrons transport in a piece of silicon (periodic arrangement of Si atoms in diamond-lattice), and interact with the nuclei through the Coulomb force. Consequently they see a periodic potential which brings forth allowed and forbidden electronic energy bands. The careful control of this electronic band allowed the realization of the first transistor. Now, we change our perspective from atom scale to wavelength scale and imagine a slab of dielectric material in which periodic arrays of air cylinders are placed. Photons propagating in this material will see a periodic change in the index of refraction. To a photon this looks like a periodic potential analogous to the way it did to an electron. The difference of the refractive index between the cylinders and the background material can be adjusted such that it confines light and therefore, allowed and forbidden regions for photon energies are formed. (Joannopoulos et al., 1995) Nowadays, extensive theoretical and experimental studies have revealed many unique properties of photonic crystals useful in optical communication. Intrigued by their vast potential in photonics engineering, tremendous efforts have been invested into the fabrication of three dimensional (3D) photonic crystal structures. However, the fabrication of those photonic crystals with a complete photonic bandgap, i.e. can exhibit bandgaps for the incident lights from all directions, still proves to be a challenge. Considerable efforts have been dedicated to develop fabrication techniques to produce large area defect-free 3D

Holographic fabrication of woodpile-type photonic crystal templates using silicon based single reflective optical element

In this work, we present the holographic fabrication of woodpile-type photonic crystal templates in photosensitive polymer using a silicon-on-PDMS based reflective optical element. The reflective optical element is fabricated from four silicon chips placed inside a polydimethylsiloxane (PDMS) mold, which reflects a circularly or elliptically polarized beam into four linearly polarized side beams, arranged four-fold symmetrically about a central beam, with electric fields normal to the incident plane, and also reduces the laser intensity of the side beams. With a single beam and a single reflective optical element, we can generate the desired laser beam intensities and polarization of each beam, thereby creating woodpile-type photonic crystal templates, and improving the contrast of 3D structures.

Negative refraction in the third photonic band of a two-dimensional elliptical rod photonic crystal in a centered rectangular lattice

Study of structures that demonstrate negative refraction is important in the search for metamaterials suitable for imaging capabilities below the diffraction limit. In this work, we study negative refraction behavior for the third photonic band of two dimensional elliptical rod photonic crystals in a centered rectangular lattice in air background using analysis of the equifrequency contours of this band combined with FDTD simulations. Hyperbolic equifrequency contours on the third photonic band indicate both negative and positive refraction at different angles. FDTD simulations are used to verify negative and positive refraction in the third band and search for potential imaging capabilities. If these behaviors are found, this photonic crystal design could potentially find use in sub-diffraction limit imaging applications.

Simulation of photonic bandgaps in real holographically formed 3D photonic crystals and holographic fabrication

This paper presents a photonic bandgap simulation for real holographic 3D photonic crystals instead of optimal photonic crystal structures. The holographic photonic crystals are formed through five-beam interference generated by multi-layer phase mask. The photonic bandgap depends on the relative phase difference among the interfering beams. A maximum bandgap of 20% of the middle bandgap can exist in these structures which can be formed through single beam, single phase mask, and single laser exposure process. We also fabricate the multi-layer phase mask by placing a spacer layer between gratings. Using the multi-layer phase mask, photonic crystal templates are holographically fabricated in a photosensitive polymer.