Jia-Han Li

Functional waveguide mode transformers

Field transformers having irregular scattering elements in a rectangular waveguide are introduced as a means to achieve frequency-dependent mode conversion, mode-selective reflection, multiple mode conversion, and phase shifting. A stepwise wall profile provides for a large number of degrees of freedom and also induces large evanescent field content. A multiresolution optimization strategy allows good convergence during synthesis. The resulting structures are compact, with dimension of a few wavelengths, and can have functionality not achievable through periodic means. In some cases, more than one solution can be found that have different spectral characteristics, a result of the large number of degrees of freedom and the constraint set. This structure class may prove useful in microwave sources where multiple or higher order modes are excited, in waveguide transitions, in frequency-dependent switching, and as phase-shifting elements.

Design of near-field irregular diffractive optical elements by use of a multiresolution direct binary search method

A multiresolution direct binary search iterative procedure is used to design small dielectric irregular diffractive optical elements that have subwavelength features and achieve near-field focusing below the diffraction limit. Designs with a single focus or with two foci, depending on wavelength or polarization, illustrate the possible functionalities available from the large number of degrees of freedom. These examples suggest that the concept of such elements may find applications in near-field lithography, wavelength-division multiplexing, spectral analysis, and polarization beam splitters.

Functional field transformation with irregular waveguide structures

We demonstrate functional field transformation in irregular waveguide structures having strong scatter from a stepped conducting wall geometry. The resulting degrees of freedom and significant evanescent field content make it possible to accomplish virtually perfect transformation in a distance of several wavelengths. A microwave experiment verifies frequency-dependent transformation, suggesting that wavelength division multiplexing based on this principle is possible. A mode-selective transformation, which exhibits disparate reflectivities for different modes at the same frequency, may find application in multimode laser cavities.

Resonant waveguide field enhancement in dimers

It is shown that a waveguide mode, which exists between metal dimer particles and does not cut off, produces an enhanced electric field and hence a mechanism for surface-enhanced Raman scattering. Structures can thus be designed that provide large average field enhancement over significant length scales.

Surface-enhanced Raman scattering from a resonant waveguide mode

Resonance in a metal-insulator-metal waveguide mode is shown to lead to significant field enhancement and hence surface-enhanced Raman scattering. The results from the propagating waveguide model are consistent with experimental and numerical results.

Irregular Field Transformation Structures

Irregular waveguides and diffractive elements with strong scatter provide degrees of freedom which allow new functionalities and compact devices necessary for solid state processing. Simulations and also experiments illustrate their efficacy.

Field transformation and phase control in ?m-scale metallic waveguides

The transformation of optical fields and the control of phase in metallic waveguides within a scale of several wavelengths is achieved. Optical devices are being fabricated using electron-beam lithography and a lift-off process.

Near-field functional field transformation structures

Irregular wavelength-scale transformation structures offer potential for wavelength division multiplexing and laser mode control. A microwave waveguide experiment demonstrates for the first time wavelength-dependent mode conversion. Optical realization using silicon processing is facilitated by lift-off.

Wavelength-scale functional waveguide mode conversion

A new type of field transformation mechanism is introduced that employs compact synthesized irregular waveguide structures. Results from a microwave experiment are given. Fabrication for optical applications using a lift-off process is proposed.