Wenjian Cai

Polarization selective computer-generated holograms realized in glass by femtosecond laser induced nanogratings

We demonstrate polarization selective computer-generated holograms (PSCGH) for visible light operation fabricated in glass by a femtosecond laser. For this purpose we create arrays of tailored microwaveplates by controlling the laser formation of nanogratings embedded in fused silica. A birefringent cell-oriented encoding method adapted to the characteristics of the physical writing process is proposed and implemented. According to this method, each cell contains a micro-waveplate with controlled phase retardation and orientation. A detour of each microwaveplate, combined with the orientation of its principal optical axis, simultaneously realizes a different phase function for each polarization. PSCGHs are attractive for integration with other free-space and guided-wave devices embedded in glass.

Patterning of silica microsphere monolayers with focused femtosecond laser pulses

We demonstrate the patterning of monolayer silica microsphere lattices with tightly focused femtosecond laser pulses. We selectively removed microspheres from a lattice and characterized the effect on the lattice and the substrate. The proposed physical mechanism for the patterning process is laser-induced breakdown followed by ablation of material. We show that a microsphere focuses radiation in its interior and in the near field. This effect plays an important role in the patterning process by enhancing resolution and accuracy and by reducing the pulse energy threshold for damage. Microsphere patterning could create controlled defects within self-assembled opal photonic crystals.

Computer-generated volume holograms fabricated by femtosecond laser micromachining

We define computer-generated volume holograms (CGVHs) as arbitrary 3D refractive index modulations designed to perform optical functions based on diffraction, scattering, and interference phenomena. CGVHs can differ dramatically from classical volume holograms in terms of coding possibilities, and from thin computer-generated holograms in terms of efficiency and selectivity. We propose an encoding technique for designing such holograms and demonstrate the concept by scanning focused femtosecond laser pulses to produce localized refractive index modifications in glass. These CGVHs show a significant increase in efficiency with thickness. Consequently, they are attractive for photonic integration with free-space and guided-wave devices, as well as for encoding spatial and temporal information.

Computer generated holograms embedded in glass fabricated with a femtosecond laser

We report the design of three-dimensional computer-generated holograms. We fabricated holograms embedded in glass using femtosecond laser induced breakdown and showed three-dimensional effects to improve efficiency.

Low pulse-energy micromachining in bulk glass with a short-cavity femtosecond oscillator

We report the first experiments showing structural changes in borosilicate glass using just a short-cavity Ti:sapphire oscillator at 800 nm and 90 MHz. Micromachining is achieved with infrared femtosecond pulses having record-low energies. We show 3-D patterning capability, including waveguides and 3-D gratings embedded in the volume of a glass slab.

Novel functionality of three-dimensional nanostructured devices fabricated with femtosecond pulse laser

The optical properties of modulated three- dimensional periodic structures called computer generated volume holograms are studied and different encoding techniques are proposed. The structures are fabricated using femtosecond laser pulses to modify the refractive index in the volume of dielectric materials.

Polarization selective computer-generated holograms created by femtosecond pulse induced birefringence

We demonstrate polarization selective birefringent computer-generated holograms fabricated in glass with femtosecond lasers. We present a coding technique adapted to the writing process. The experimental reconstruction shows excellent polarization selectivity at the design wavelength.

Micropatterning of Microsphere-Based Thin Films with a Femtosecond Laser

We demonstrate the possibility of micropatterning arrays of silica microspheres using femtosecond laser pulses. Selective removal of individual spheres is observed and the interaction of the pulse with the microsphere is characterized.

Record low pulse-energy micromachining in bulk glass with an ultrafast laser

In this paper, we report the machining of various types of glasses by femtosecond pulses generated directly from a Ti:sapphire oscillator with less than half of that pulse energy. In our experiments we used a Ti:Sapphire laser pumped by a double-frequency Nd:YVO/sub 4/ laser. Our strategy is to tightly focus sub-30fs pulses with a high numerical aperture objective lens while optimizing the bandwidth and pulse temporal and spatial shape to achieve minimum threshold energy for permanent structural damage.