Optical resonators play an essential role in many areas of modern photonics, from quantum plasmonics, optical metamaterials, integrated photonic circuits, to optical sensors. This predominant role is the outcome of the recent progress of bottom-up and top-down technologies that have allowed a proliferation of totally new resonator constructs with very distinct properties and dimensions, such as high-Q dielectric microcavities, plasmonic nanoantennas or associations of them. Comparatively, the resonator modelling has seen much less progress, and we enter an era where optical resonator technologies are limited by the lack of effective design tools rather than by creativity and fabrication.

Here we elaborate a theoretical and numerical formalism that has the potential to bring a real difference in resonator design. Developed for the most general case of 3D plasmonic resonators in inhomogeneous backgrounds, the formalism differs markedly from classical methods. It is the equivalent of waveguide-mode-theory for resonators, and offers similar strengths: an intuitive modelling that sticks to the physics of the resonance and a high-performance with computational speeds that are much faster than those presently available with classical methods.

Related recent publications by the group:

1. Yang, H. Giessen, P. Lalanne, Nano Lett. 15, 3439 (2015)

"Simple Analytical Expression for the Peak-Frequency Shifts of Plasmonic Resonances for Sensing"

1. Yang, M. Perrin, P. Lalanne, Phys. Rev. X 5, 021008 (2015)

"Analytical formalism for the interaction of 2-level quantum systems with metal nanoresonators"

1. Faggiani, A. Losquin, J. Yang, E. Mårsell, A. Mikkelsen, P. Lalanne, ACS Photonics (submitted).

"Modal analysis of the ultrafast dynamics of optical nanoresonators"

1. Fauché, S.G. Kosionis and P. Lalanne, (submitted)

"Collective scattering in hybrid nanostructures with many atomic oscillators coupled to an electromagnetic resonance"

Metasurfaces are comprised of two-dimensional (2D) subwavelength metal/dielectric structures produced in a planer fashion to be able to introduce abrupt changes in optical properties, enabling users’ independence of propagation effect in contrast to the heavy dependence of conventional bulky optical elements. Based on optimization algorithm, metasurfaces can be rigorously engineered to tailor their the amplitude and phase responses to achieve a variety of manipulations, such as wavefronts control of incident beams, beam focusing and steering, polarization conversion and angular momentum manipulation. It opens a new avenue in the context of photonic miniaturization to scale traditional bulky optical items down to ultrathin and ultracompact components, which has been a rapidly growing research fields in recent years.       

Typically metasurfaces can be realized by two different strategies from the materials perspectives. It can be made of plasmonic materials being of a negative permittivity such as metals, doped semiconductors, transition metal oxides and nitrides. However, their efficiency is greatly limited due to intrinsic absorption losses across a wide spectrum from THz to visible wavelengths. The other way is to employ high refractive index and low loss dielectrics to achieve high efficiency. Recently, relatively high aspect-ratio nano-pillars without resonances of unit cells have been demonstrated to achieve high efficiency of 80%-90% in the transmission mode. Silicon (amorphous, poly- & single crystalline) and titanium dioxide have been overwhelmingly utilized for the spectrum spanning the infrared to visible regime. Unfortunately, these materials exhibit relatively high loss in the ultraviolet (UV) range while high efficiency UV light metasurfaces hasn’t been demonstrated yet for exploitation of this essential optical domain. To address it, in this talk, I will introduce our latest achievement in the development of high aspect ratio nanostructured dielectric metasurfaces with high efficiency in the UV range and metasurfaces devices have been fabricated to demonstrate focusing and hologram. This work facilitates metasurfaces to find applications in optical nanolithography and anticounterfeiting and extend the operating wavelength of high efficiency metasurfaces from infrared, visible to the UV regime.  

By designing a two-dimension acoustic complementary medium of water, we demonstrate the possibility of realizing a sound-impenetrable hole that can block acoustic waves in water. The complementary medium is composed of core-shell rubber cylinders in a square lattice, and possesses the exact negative values of water in both the effective density and bulk modulus at a working frequency. The effects of negative refraction as well as the sound-impenetrable hole are verified by numerical simulations. Interestingly, by introducing a small amount of loss, we find that the functionality of such a sound-impenetrable hole becomes robust and works in a broad frequency range.

Plasmons in metal surfaces and clusters have been extensively studied due to their potential applications in sensing, imaging, light harvesting and optical metamaterials. Graphene is a semimetal with tunable conductivity and hence can support plasmons as well. In addition to the tunability, graphene plasmons have relatively weak damping due to the high carrier mobility. In this talk, I’ll present our recent progress on the studies of plasmon excitations in graphene micro- and nano-structures and their behavior in an external magnetic field. The collective motion of Dirac fermions, which are relativistic with zero rest mass, shows peculiar properties with a tunable “plasmon mass”. We showed strong light-matter interaction in the terahertz frequency regime and demonstrated graphene plasmonic terahertz filters and polarizers with graphene/insulator stacks. Localized plasmons in graphene nano-structures go beyond terahertz frequencies. As an atomically thin plasmonic material which is sensitive to the surrounding environment, strong coupling between plasmons in graphene and substrate surface polar phonons has been unambiguously identified in the mid-infrared regime. A new plasmon damping channel through the emission of graphene optical phonons has been revealed for high frequency plasmons. Our study paves the way for graphene applications in photonics, optoelectronics and metamaterials, especially in the terahertz frequency regime.

Holography is of great interests to both scientific research and industry application, but has always concerned with the sensitivity to wavelength and polarization of the incident light. Photon nanosieve is a nano-hole or -void structure formed in a thin film that can affect the light field property in the far field. Having revisited the Huygens-Fresnel principle, we propose to use photon nano-sieves to mimic the forward radiation field of the point sources to ease some of the concerns from conventional and metasurface based holograms. With the help of analytical model and elaborate selection of the discrete point sources, ultrahigh capacity photon nano-sieves are designed and showed uniform optical hologram with high diffraction efficiency throughout the visible to near IR wavelength range, polarization independence and large viewing angle. This robust hologram offers a large angle-of-view and possesses a unique lensing effect, which can work as a high-resolution and lens-less micro-projector. This might open an avenue to high-tolerance holographic technique for electromagnetic and acoustic waves.

It is known that humans are faced with a global energy crisis, namely, an increasing shortage of nonrenewable energy resources, such as coal, petroleum, and natural gas. However, much of the energy generated from nonrenewable energy resources is changed into thermal energy, which is hard for humans to re-use freely. Therefore, it is meaningful and challenging to manipulate the flow of heat (thermal energy). Here, by establishing temperature-dependent transformation thermotics for treating materials whose conductivity depends on temperature, we show evidence for switchable thermal cloaking and a macroscopic thermal diode based on the cloaking. Meanwhile, we also establish a theory of temperature trapping, and then propose and fabricate a new thermostat for maintaining constant temperatures (within a temperature gradient in the environment) without the need of consuming additional energy. These results suggest that our theory could be adopted for achieving novel macroscopic heat management by using thermal metamaterials, and it could provide new guidance for energy saving.

Metamaterials are array of subwavelength structures that can be engineered to achieve exotic material properties that does not occur in nature. Active control of metamaterial response is highly critical for the realization of practical metamaterial applications. Microelectromechanical systems (MEMS) is the enabling technology to achieve tunable metamaterials. MEMS provides a wide palette of micro- and nano- scale actuators that can be integrated into the subwavelength structures to structural deformation and hence active control of metamaterial response. In our group, we explored the out-of-plane deformable, stress engineered microbeam as the active metamaterial element. The MEMS metamaterials based on CMOS compatible fabrication process allow for active control of wide range of electromagnetic properties in the terahertz region. We have proposed and demonstrated the enhanced controllability at unit cell level. The experimental demonstration of advanced control of slow light behavior, anisotropy and multifunctional metamaterials is reported.

In order to easily achieve multi-resonant metamaterials with a stacked structure, we have proposed a simple fabrication method. In this presentation, we demonstrate the simple fabrication method and fabrication results of the multi-resonant metamaterials. In the fabrication, a silver nanoparticle inkjet printing technique was applied, which enables us to achieve a simple and low cost metamaterials. Moreover, our multi-resonant metamaterials have been achieved on a paper substrate.

We designed split-ring resonator (SRR) arrays as the metamaterials in THz region, which have different resonant frequencies. The resonant frequencies of the fabricated SRRs are evaluated by using THz time-domain spectroscopy (TDS).

As a result, we have achieved good agreement between the simulated and the measured values for x- and y-polarized incident waves. Furthermore, we could successfully obtain the multi-resonant frequencies for x-polarized incident wave. We also discuss the influence of misalignment between SRR arrays on the resonant frequencies.

Finally, we also introduce recent research results regarding metasurfaces consisted of complementary metal checkerboard patterns.

Transparent media are the foundation of almost all optical systems. However, due to general reflection caused by impedance mismatch, transparency has never been perfect in natural materials such as dielectrics. Here, we propose to utilize the spatial dispersive effective media to realize perfectly transparent media with omnidirectional impedance matching, which allows near 100% transmission of light at all incident angles. The equal frequency contours of the effective media are designed to be elliptical and shifted in k space, and thus contain strong spatial dispersions. The realization of such effective media is based on pure dielectric photonic crystals. By designing the nonlocal effective parameters of photonic crystals to special values, we demonstrate the ultratransparency effect [1] of photonic crystals, which can enhance the transparency of photonic crystals to an extreme level beyond any existing solid materials on earth. In an optimized example, near 100% transmission rate have been realized for almost any incident angle within (-90, 90) degrees. A proof-of-principle microwave experiment has been performed to demonstrate near 100% transmission within (-60, 60) degrees. Moreover, broadband, wide-angle, and polarization-insensitive impedance matching effect is obtained in one-dimensional dielectric photonic crystals [2], paving the way to broadband and polarization-independent ultratransparent media.

More interestingly, we demonstrate that such ultratransparent media provide an excellent platform for realizing transformation optics [3, 4] devices at optical frequencies. The impedance and refractive behavior of the dielectric photonic crystals can be flexibly tuned to satisfy the requirement of transformation optics. Interestingly, the shift of the dispersion in k space provides additional freedom beyond the original framework of transformation optics based on local media [1, 5].

ACKNOWLEDGEMENTS – Work in collaboration with Prof. Z. H. Hang and Prof. C.T. Chan.

REFERENCES:

[1] Luo, Y. Yang, Z. Yao, W. Lu, B. Hou, Z. H. Hang, C. T. Chan, and Y. Lai, “Ultratransparent media and transformation optics with shifted spatial dispersions,” Phys. Rev. Lett. 2016, 117, 223901.

[2] Yao, J. Luo, and Y. Lai, “Photonic crystals with broadband, wide-angle, and polarization-insensitive transparency,” Opt. Lett. 2016, 41, 5106-5109.

[3] B. Pendry, D. Schurig, and D. R. Smith, “Controlling electromagnetic fields,” Science 2006, 312, 1780-1782.

[4] Leonhardt, “Optical conformal mapping,” Science 2006, 312, 1777-1780.

[5] Yao, J. Luo, and Y. Lai, To be submitted.

Electromagnetic (EM) absorption plays a foremost important role in the area of energy harvesting, stealth technology, interference shielding and biological imaging etc. Perfect metamaterial/metasurface absorber, which obtains near-unity EM absorption through subwavelength artificial structure, usually suffers from narrow bandwidth. Here, for the first time, we demonstrate a curved water-based metasurface which functions as an active ultra-broadband absorbing material working across the entire Ku, K and Ka bands. Distinct from conventional metallic metamaterial/metasurface, the proposed water-based metasurface acquires broadband absorption property from the dielectric Mie resonance and periodic grating effect, which exhibits an experimental absorptivity of ∼99% and an absorption bandwidth (absorptivity higher than 90%) that covers 71.4% of the central frequency. Furthermore, near-unity absorption is maintained when the soft metasurface is bent into different curvatures, promoting its potential applications on non-planar circumstances.