Yun-Ching Chang

Giant electro-optic effect in nanodisordered KTN crystals

The electro-optic (EO) effect in nanodisordered potassium tantalate niobate (KTN) crystal is quantitatively investigated. It is found out that the EO coefficient of nanodisordered KTN crystal depends not only on the cooling temperature but also on the cooling rate. A larger EO coefficient can be obtained by employing a faster cooling rate. A Kerr EO efficient (s(11) - s(12) = 6.94 × 10(-14) m(2)/V(2)) is obtained at a cooling rate of 0.45 °C/s. The enhanced EO efficient by employing a faster cooling rate will be greatly beneficial for a variety of applications such as laser Q switches, laser pulse shaping, high-speed optical shutters, and modulating retroreflectors.

Kovacs effect enhanced broadband large field of view electro-optic modulators in nanodisordered KTN crystals

The unique physical effect-Kovacs effect is explored to enhance the performance of EO modulators by employing the non-thermal equilibrium state nanodisordered KTN crystals created by super-cooling process, which can have a significant 3.5 fold increase in quadratic electro-optic coefficient. This enables to reduce the switching half wave voltage (almost by half) so that a broadband (~GHz range) and large field of view (+/-30 deg) electro-optic modulator can be realized with much lowered driving power, which can be very useful for a variety of applications: laser Q-switches, laser pulse shaping, high speed optical shutters and modulating retro reflectors.

Surface enhanced Raman spectroscopy by interfered femtosecond laser created nanostructures

A type of surface enhanced Raman spectroscopy (SERS) by interfered femtosecond laser created nanostructures on Cu metal is presented. It is found out that finer and more uniform nanostructures (with an average feature size 100 nm or smaller) can be created on Cu metal by interfered femtosecond illumination with a phase mask. Significantly enhanced Raman signal (with an enhancement factor around 863) can be realized by using the nanostructured Cu substrate created by the interfered femtosecond laser illumination. The experimentally measured enhancement factor agrees relatively well with the theoretical analyses. Since the nanostructures can be inscribed in real time and at remote locations by the femtosecond laser inscription, the proposed SERS can be particularly useful for the standoff detection of chemicals.

Analysis of terahertz generation via nanostructure enhanced plasmonic excitations

In this paper, we conduct a quantitative study on the physical mechanism of electrons dynamics near the nanostructured metal film surfaces, as well as the efficiency of generated terahertz radiation associated with different types of nanostructures. The simulation results show that although the oscillating motion of emitted electrons outside the metal surface may affect the terahertz generation efficiency to some extent, this efficiency is predominantly determined by the electric field magnitude inside the metal film associated with nanostructure enhanced plasmonic excitations. Due to the field enhancement effect of the nanostructure, an appropriately designed nanostructured surface could greatly enhance the strength of generated terahertz signal via the increased nonlinear interactions between the light and the nanostructures.

Field induced dynamic waveguides based on potassium tantalate niobate crystals

In this paper, a new type of optical waveguide based on potassium tantalate niobate (KTN) electro-optic crystal is presented. The guiding property of the optical waveguide can be quickly (on the order of nanosecond) tuned and controlled by the applied external electric field, which can be useful for many applications such as broadband ultrafast optical modulators, variable optical attenuators, and dynamic gain equalizers.

Super broadband ultrafast waveguide switches based on dynamic waveguiding effect

In this paper, a new type of waveguide switch-field induced dynamic optical waveguide switch is presented. The switching mechanism is based on electric-field induced dynamic waveguiding effect in nanodisordered potassium tantalate niobate (KTN) crystals. By applying an electric field at different locations, different waveguide paths are created, which result in different output locations. The major advantages of this unique optical switch are broad bandwidth, covering the entire 1300 nm – 1600 nm fiber optic communication window, and ultrafast switching speed (on the order of nanosecond), which can be very useful for next generation optical networks such as the one used in data center networks.

Electrically and mechanically tunable photonic metamaterials

In recent years, much of effort has been devoted in the field of optical switches, including electro-optics (EO), magnetooptics (MO), acousto-optics (AO), liquid crystal (LC), and microelectromechanical systems (MEMS). However, issues which involve switching speed, aperture size, and extinction ratio cannot be simultaneously settled by the present approaches. The paper proposes a novel optical switch based on tunable photonic metamaterial. By the controllable external electrical or magnetic field, the nano-structure is forced to vary its optical properties to be an optical switch. The theoretical studies suggest that the device could offer the merit features of ultra-fast speed, large aperture, and high extinction ratio. In the future, we will not only thoroughly model the proposed devices, but investigate kinds of possible fabrication process to implement the design. To be a next-generation optical switch, the tunable photonic metamaterial has large potential in several civilian applications, including mobile high-speed display, free-space optical communication, solar concentration, and the optical printing.

Broadband large field of view electro-optic modulators using potassium tantalate niobate (KTN) crystals

By taking advantage of the recent advances in high quality sizable KTN crystal growth, a broadband large field of view (FOV) EO modulator is developed and presented in this paper. The experimental and theoretical studies indicate that the built EO modulator not only has a broad bandwidth (~1 GHz) but also has a large FOV (+/- 30 deg) because cubic phase KTN crystals do not introduce any intrinsic birefringence without applying the external electric field.

Dynamic and tunable optical waveguide based on KTN electro-optic crystals

The study reveals an innovative design of the optical waveguide based on potassium tantalate niobate (KTN) crystal. The device’s guiding properties, which benefits from the large electro-optic coefficients of KTN crystal, can be dynamically controlled by an external electric field in a relatively simple waveguide structure. Finite Element Method (FEM) and Beam Propagation Method (FEM) are used in the theoretical part of this work which shows that the KTN based dynamic waveguide has good potential on applications of several kinds of optical switches and modulators.

Nanostructures created by interfered femtosecond laser

The method by applying the interfered femtosecond laser to create nanostructured copper (Cu) surface has been studied. The nanostructure created by direct laser irradiation is also realized for comparison. Results show that more uniform and finer nanostructures with sphere shape and feature size around 100 nm can be induced by the interfered laser illumination comparing with the direct laser illumination. This offers an alternative fabrication approach that the feature size and the shape of the laser induced metallic nanostructures can be highly controlled, which can extremely improve its performance in related application such as the colorized metal, catalyst, SERS substrate, and etc.