arXiv subjects

Christopher A. Dirdal

Publications and source records attributed to Christopher A. Dirdal.

9 recordsLinked to original sources

arXiv2026

Piezoelectric MEMS Phase Modulator for Silicon Nitride Platform in the Visible Spectrum

Active photonic integrated circuits (PICs) in the visible spectrum are essential for on-chip applications, requiring low-loss waveguides with broad transparency and efficient, low-power phase modulation. Here, we demonstrate a compact, ultra-low-power phase modulator based on a silicon nitride (Si$_3$N$_4$) waveguide integrated with thin-film lead zirconate titanate (PZT) that actuates a bridge-type MEMS. The suspended actuator exploits PZT's strong piezoelectric effect to induce mechanically driven phase shifts, enabling efficient modulation in a Mach--Zehnder interferometer. For 3~mm and 5~mm modulators, phase shifts of $1.45π$ and $2.5π$ are achieved at 10~V, corresponding to a scalability metric ($V_π\cdot L$) of 2.25~V$\cdot$cm at 635~nm. This represents an order-of-magnitude improvement in scalability over stress-optic PZT modulators. The devices also exhibit ultralow power consumption ($\sim 12\,\mathrm{nW}$), $\sim 5\,\mathrm{ms}$ rise time, and optical loss $< 0.75\,\mathrm{dB/cm}$. Furthermore, we demonstrate on-chip beam shaping.

physics.optics↗

arXiv2025

Metasurface Polarimeter for Structural Imaging and Tissue Diagnostics

Histopathology, the study and diagnosis of disease through analysis of tissue samples, is an indispensable part of modern medicine. However, the practice is time consuming and labor intensive, compelling efforts to improve the process and develop new approaches. One perspective technique involves mapping changes in the polarization state of light scattered by the tissue, but the conventional implementation requires bulky polarization optics and is slow. We report the design, fabrication and characterization of a compact metasurface polarimeter operating at 640 nm enabling simultaneous determination of Stokes parameters and degree of polarization with $\pm$2% accuracy. To validate its use for histopathology we map polarization state changes in a tissue phantom mimicking a biopsy with a cancerous inclusion, comparing it to a commercial polarimeter. The results indicate a great potential and suggest several improvements with which we believe metasurface polarimeter based devices will be ready for practical histopathology application in clinical environment.

physics.optics↗

arXiv2021

MEMS-tunable dielectric metasurface lens using thin-film PZT for large displacements at low voltages

Tunable focusing is a desired property in a wide range of optical imaging and sensing technologies but has tended to require bulky components which cannot be integrated on-chip and have slow actuation speeds. Recently, integration of metasurfaces into electrostatic MEMS architectures has shown potential to overcome these challenges, but has offered limited out of plane displacement range while requiring large voltages. We demonstrate for the first time a movable metasurface lens actuated by integrated thin-film PZT MEMS, which has the advantage of offering large displacements at low voltages. An out of plane displacement of a metasurface in the range of 7.2 um is demonstrated under a voltage application of 23V. This is roughly twice the displacement at a quarter of the voltage of state of the art electrostatic out of plane actuation of metasurfaces. Utilizing this tunability we demonstrate a varifocal lens doublet with a focal shift on the order of 250um at the wavelength 1.55um. Thin-film PZT MEMS-metasurfaces is a promising platform for miniaturized varifocal components.

physics.optics↗

arXiv2020

Towards High Throughput Large Area Metalens Fabrication using UV-Nanoimprint lithography and Bosch Deep Reactive Ion Etching

We demonstrate the fabrication of diffraction-limited dielectric metasurface lenses for NIR by use of standard industrial high throughput silicon processing techniques: UV Nano Imprint Lithography (UV-NIL) combined with continuous Reactive Ion Etching (RIE) and pulsed Bosch Deep Reactive Ion Etching (DRIE). As the research field of metasurfaces moves towards applications these techniques are relevant as potential replacements of commonly used cost-intensive fabrication methods utilizing Electron Beam Lithography. We show that washboard-type sidewall surface roughness arising from the Bosch DRIE process can be compensated for in the design of the metasurface, without deteriorating lens quality. Particular attention is given to fabrication challenges that must be overcome towards high throughput production of relevance to commercial applications. Lens efficiencies are measured to be 30% and 17% at wavelengths λ = 1.55$μ$m and λ = 1.31$μ$m, respectively. A number of routes towards process optimization are proposed in relation to encountered challenges.

physics.app-ph↗

arXiv2019

Four definitions of magnetic permeability for periodic metamaterials

We state and compare four different definitions of magnetic permeability for periodic, artificial media, or metamaterials. The connection between them, and properties in general, are discussed in detail, including causality, passivity, symmetry, asymptotic behavior, and origin dependence. The analysis is limited to metamaterials made from linear and nonmagnetic constituents.

physics.class-ph↗

arXiv2018

Higher order multipoles in metamaterial homogenization

The higher order multipoles above the electric quadrupole are commonly neglected in metamaterial homogenization. We show that they nevertheless can be significant when second order spatial dispersive effects, such as the magnetic response, are considered. In this respect, they can be equally important as the magnetization and quadrupole terms, and should not automatically be neglected.

physics.optics↗

arXiv2018

Diamagnetism and the dispersion of the magnetic permeability

It is well known that the usual Kramers--Kronig relations for the relative permeability function $μ(ω)$ are not compatible with diamagnetism ($μ(0)<1$) and a positive imaginary part ($\text{Im}\,μ(ω)>0$ for $ω>0$). We demonstrate that a certain physical meaning can be attributed to $μ$ for all frequencies, and that in the presence of spatial dispersion, $μ$ does not necessarily tend to 1 for high frequencies $ω$ and fixed wavenumber $\mathbf k$. Taking the asymptotic behavior into account, diamagnetism can be compatible with Kramers--Kronig relations even if the imaginary part of the permeability is positive. We provide several examples of diamagnetic media and metamaterials for which $μ(ω,\mathbf k)\not\to 1$ as $ω\to\infty$.

cond-mat.other↗

arXiv2015

Relaxed dispersion constraints and metamaterial effective parameters with physical meaning for all frequencies

Metamaterial effective parameters may exhibit freedom from typical dispersion constraints. For instance, the emergence of a magnetic response in arrays of split-ring resonators for long wavelengths cannot be attained in a passive continuous system obeying the Kramers-Kronig relations. We characterize such freedom by identifying the three possible asymptotes which effective parameters can approach when analytically continued. Apart from their dispersion freedom, we also demonstrate that the effective parameters may be redefined in such a way that they have a certain physical meaning for all frequencies. There exists several possible definitions for the effective permittivity and permeability whereby this is achieved, thereby giving several possible frequency variations for high frequencies, while nevertheless converging to the same dispersion for long wavelengths.

physics.optics↗

arXiv2013

Superpositions of Lorentzians as the class of causal functions

We prove that all functions obeying the Kramers-Kronig relations can be approximated as superpositions of Lorentzian functions, to any precision. As a result, the typical text-book analysis of dielectric dispersion response functions in terms of Lorentzians may be viewed as encompassing the whole class of causal functions. A further consequence is that Lorentzian resonances may be viewed as possible building blocks for engineering any desired metamaterial response, for example by use of split ring resonators of different parameters. Two example functions, far from typical Lorentzian resonance behavior, are expressed in terms of Lorentzian superpositions: A steep dispersion medium that achieves large negative susceptibility with arbitrarily low loss/gain, and an optimal realization of a perfect lens over a bandwidth. Error bounds are derived for the approximation.

physics.optics↗