Modeling vanadium dioxide phase transition due to continuous-wave optical signals.

Vanadium dioxide (VO(2)) is a material that undergoes thermal phase transition resulting in drastic changes in its material properties. The phase change can also be brought on by optical pumping. Several experimental results have been presented in the literature dealing with such phase transitions brought on by optical pumping. In this manuscript we present a theoretical framework, which addresses this problem by self consistently solving the electromagnetic problem and the thermodynamic problem using a multiphysics approach when such transitions are thermally mediated, as is the case with continuous-wave optical pumps. Such an analysis provides us with insights into the transition process and also helps explain the conditions under which some of the observed experimental results like bistability takes place. Such optically induced phase transition materials also present the intriguing possibility of ultrahigh nonlinearity where the input optical signal essentially converts a dielectric into a plasmonic material. These materials can find significant applications in nonlinear metatronics.

Hotspots from nonreciprocal surface waves.

We present theoretical and numerical results for a new method of obtaining hotspots that relies on nonreciprocal "one-way" propagation as opposed to the well-known case of resonance-based hotspots. The nonreciprocal propagation is achieved by the breaking of time-reversal symmetry through the use of magnetically biased medium. The location and existence of the hotspots depends on the magnetic bias that results in the breaking of the time-reversal symmetry. This results in the intriguing possibility of switching and spatial control of the hotspots through the magnetic bias.

Plasmonic enhancement of nanophosphor upconversion luminescence in Au nanohole arrays.

Arrays of subwavelength holes (nanoholes) in Au films were computationally designed, fabricated, and used as templates to localize and enhance the luminescence of upconversion nanophosphors (UCNPs)--hexagonal phase NaYF4 doped with Yb(3+) and Er(3+). The dimensions of nanohole Au arrays were designed to accept only a single UCNP upon particle filling and with a periodicity to be resonant with the excitation wavelength of the upconversion. Frequency-dependent luminescence enhancements of up to 35-fold and a concomitant shortening of the UCNP luminescence rise time were observed, consistent with simulations of plasmonic enhancement of the UCNP absorption.

Internal homogenization: effective permittivity of a coated sphere.

The concept of internal homogenization is introduced as a complementary approach to the conventional homogenization schemes, which could be termed as external homogenization. The theory for the internal homogenization of the permittivity of subwavelength coated spheres is presented. The effective permittivity derived from the internal homogenization of coreshells is discussed for plasmonic and dielectric constituent materials. The effective model provided by the homogenization is a useful design tool in constructing coated particles with desired resonant properties.

Metal-enhanced upconversion luminescence tunable through metal nanoparticle-nanophosphor separation.

We have demonstrated amplification of luminescence in upconversion nanophosphors (UCNPs) of hexagonal phase NaYF(4) (ß-NaYF(4)) doped with the lanthanide dopants Yb(3+), Er(3+) or Yb(3+), Tm(3+) by close proximity to metal nanoparticles (NPs). We present a configuration in which close-packed monolayers of UCNPs are separated from a dense multilayer of metal NPs (Au or Ag) by a nanometer-scale oxide grown by atomic layer deposition. Luminescence enhancements were found to be dependent on the thickness of the oxide spacer layer and the type of metal NP with enhancements of up to 5.2-fold proximal to Au NPs and of up to 45-fold proximal to Ag NPs. Concomitant shortening of the UCNP luminescence decay time and rise time is indicative of the enhancement of the UCNP luminescence induced by resonant plasmonic coupling and nonresonant near-field enhancement from the metal NP layer, respectively.

Enhancement of radiation from dielectric waveguides using resonant plasmonic coreshells.

Here, we present parametric studies of a method for enhancing radiation from a dielectric waveguide through the use of resonant coreshells. These coreshells act as a compact impedance matching element between the guided modes of the waveguide and radiation modes in free space. Furthermore, we also show that we can sense the distance between the waveguide end and the coreshell by monitoring the reflectance of the waveguide mode. Coreshell decoupled radiation from dielectric waveguides could hence find use for highly integrated optical coupling elements or nanometric distance sensors.

Loss-free and active optical negative-index metamaterials.

The recently emerged fields of metamaterials and transformation optics promise a family of exciting applications such as invisibility, optical imaging with deeply subwavelength resolution and nanophotonics with the potential for much faster information processing. The possibility of creating optical negative-index metamaterials (NIMs) using nanostructured metal-dielectric composites has triggered intense basic and applied research over the past several years. However, the performance of all NIM applications is significantly limited by the inherent and strong energy dissipation in metals, especially in the near-infrared and visible wavelength ranges. Generally the losses are orders of magnitude too large for the proposed applications, and the reduction of losses with optimized designs seems to be out of reach. One way of addressing this issue is to incorporate gain media into NIM designs. However, whether NIMs with low loss can be achieved has been the subject of theoretical debate. Here we experimentally demonstrate that the incorporation of gain material in the high-local-field areas of a metamaterial makes it possible to fabricate an extremely low-loss and active optical NIM. The original loss-limited negative refractive index and the figure of merit (FOM) of the device have been drastically improved with loss compensation in the visible wavelength range between 722 and 738 nm. In this range, the NIM becomes active such that the sum of the light intensities in transmission and reflection exceeds the intensity of the incident beam. At a wavelength of 737 nm, the negative refractive index improves from -0.66 to -1.017 and the FOM increases from 1 to 26. At 738 nm, the FOM is expected to become macroscopically large, of the order of 10(6). This study demonstrates the possibility of fabricating an optical negative-index metamaterial that is not limited by the inherent loss in its metal constituent.

Yellow-light negative-index metamaterials.

A well-established, silver fishnet design has been further miniaturized to function as a negative-index material at the shortest wavelength to date (to our knowledge). By studying the transmittance, reflectance, and corresponding numerical simulations of the sample, we report in this Letter a negative refractive index of -0.25 at the yellow-light wavelength of 580 nm.

Frequency-domain simulations of a negative-index material with embedded gain.

We solve the equations governing light propagation in a negative-index material with embedded nonlinearly saturable gain material using a frequency-domain model. We show that available gain materials can lead to complete loss compensation only if they are located in the regions where the field enhancement is maximal. We study the increased enhancement of the fields in the gain composite as well as in the metal inclusions and show analytically that the effective gain is determined by the average near-field enhancement.

The Ag dielectric function in plasmonic metamaterials.

Ag permittivity (dielectric function) in coupled strips is different from bulk and has been studied for strips of various dimensions and surface roughness. Arrays of such paired strips exhibit the properties of metamagnetics. The surface roughness does not affect the Ag dielectric function, although it does increase the loss at the plasmon resonances of the coupled strips. The size effect in the imaginary part of the dielectric function is significant for both polarizations of light, parallel and perpendicular to the strips with relatively large A-parameter.

Designs for optical cloaking with high-order transformations.

Recent advances in metamaterial research have provided us a blueprint for realistic cloaking capabilities, and it is crucial to develop practical designs to convert concepts into real-life devices. We present two structures for optical cloaking based on high-order transformations for TM and TE polarizations respectively. These designs are possible for visible and infrared wavelengths. This critical development builds upon our previous work on nonmagnetic cloak designs and high-order transformations.

Dual-band negative index metamaterial: double negative at 813 nm and single negative at 772 nm.

This work is concerned with the experimental demonstration of a dual-band negative index metamaterial. The sample is double negative (showing both a negative effective permeability and a negative effective permittivity) for linearly polarized light with a wavelength between 799 and 818 nm, and the real part of its refractive index is approximately -1.0 at 813 nm. The ratio of -Re(n)/Im(n) is close to 1.3 at 813 nm. For an orthogonal polarization, the same sample also exhibits a negative refractive index in the visible (at 772 nm). The spectroscopic measurements of the material are in good agreement with the results obtained from a finite-element electromagnetic solver for the actual geometry of the fabricated sample at both polarizations.

Comment on "Negative refractive index in artificial metamaterials".

We dispute Grigorenko's statement [Opt. Lett. 31, 2483 (2006)] that measuring only the reflection intensity spectrum is sufficient for determining the effective refractive index. In addition, our simulations do not confirm his conclusions regarding the negative refractive index and the negative permeability of the nanopillar sample in the visible range.

A negative permeability material at red light.

A negative permeability in a periodic array of pairs of thin silver strips is demonstrated experimentally for two distinct samples. The effect of the strip surface roughness on negative permeability is evaluated. The first sample, Sample A, is fabricated of thinner strips with a root mean square roughness of 7 nm, while Sample B is made of thicker strips with 3-nm roughness. The real part of permeability, mu', is -1 at a wavelength of 770 nm in Sample A and -1.7 at 725 nm in Sample B. Relative to prototypes simulated with ideal strips, larger strip roughness acts to decrease |mu| by a factor of 7.8 in Sample A versus a factor of 2.4 decrease for Sample B.

Metamagnetics with rainbow colors.

A family of coupled nanostrips with varying dimensions is demonstrated exhibiting optical magnetic responses across the whole visible spectrum, from red to blue. We refer to such a phenomenon as rainbow magnetism. The experimental and analytical studies of such structures provide us with a universal building block and a general recipe for producing controllable optical magnetism for various practical implementations.

Negative index metamaterial combining magnetic resonators with metal films.

We present simulation results of a design for negative index materials that uses magnetic resonators to provide negative permeability and metal films for negative permittivity. We also discuss the possibility of using semicontinuous metal films to achieve better manufacturability and enhanced impedance matching.

Negative index of refraction in optical metamaterials.

A double-periodic array of pairs of parallel gold nanorods is shown to have a negative refractive index in the optical range. Such behavior results from the plasmon resonance in the pairs of nanorods for both the electric and the magnetic components of light. The refractive index is retrieved from direct phase and amplitude measurements for transmission and reflection, which are all in excellent agreement with simulations. Both experiments and simulations demonstrate that a negative refractive index n' approximately -0.3 is achieved at the optical communication wavelength of 1.5 microm using the array of nanorods. The retrieved refractive index critically depends on the phase of the transmitted wave, which emphasizes the importance of phase measurements in finding n'.