Journal Papers

[26] Azadeh Bader, Sophie Pelton, and Nasim Mohammadi Estakhri “Inverse-designed integrated biosensors,”  Optical Materials Express, Vol. 14, No. 7, pp. 1710 (11 pages), June 2024. [Link]

Abstract: We propose a refractive index sensor in a silicon-on-insulator (SOI) platform inspired by the operation of multimode fiber sensors. The sensor utilizes an optimized SOI resonator through adjoint-based inverse design topology optimization. The device’s refractive index distribution is calculated with a suitable figure of merit tailored for telecommunication band operation (1450 nm to 1650 nm) and is compatible with the standard fabrication processes. The flexibility of design offered through topology optimization and the localized interactions of the wave around the metastructure can be tailored to achieve maximum sensitivity. Our results may find interesting applications in wearable technologies, biosensing, and environmental monitoring.

[25] Azadeh Bader, Nooshin M. Estakhri, and Nasim Mohammadi Estakhri “Adaptive Plasmonic Metasurfaces for Radiative Cooling and Passive Thermoregulation,”  Frontiers in Photonics, Editors' Showcase: Frontiers in Photonics, Vol. 4, No. 1193479 (8 pages), June 2023. [Link]

Abstract: In this work, we investigate a class of planar photonic structures operating as passive thermoregulators. The radiative cooling process is adjusted through the incorporation of a phase change material (Vanadium Dioxide, VO2) in conjunction with a layer of transparent conductive oxide (Aluminum-doped Zinc Oxide, AZO). VO2 is known to undergo a phase transition from the “dielectric” phase to the “plasmonic” or “metallic” phase at a critical temperature close to 68°C. In addition, AZO shows plasmonic properties at the long-wave infrared spectrum, which, combined with VO2, provides a rich platform to achieve low reflections across the atmospheric transparency window, as demanded in radiative cooling applications, while also maintaining a compact size. Using numerical analysis, we study two classes of patterned and non-patterned compact multilayer metal-dielectric-metal metasurfaces, aiming to maximize the overall absorption in the first atmospheric transparency window (8–13 µm) while maintaining a high reflection across the solar spectrum (0.3–2.5 µm). Surfaces are initially designed based on a round of coarse optimization and further improved through analyzing the impact of geometric parameters such as size and periodicity of the metasurface elements. Our findings are relevant to applications in thermal regulation systems and passive radiative cooling of high-temperature devices, such as electronic elements.

[24] Alex Vallone, Nooshin M. Estakhri, and Nasim Mohammadi Estakhri, “Region-specified inverse design of absorption and scattering in nanoparticles by using machine learning,” Journal of Physics: Photonics, focus on Photonics Machine Learning, Vol. 5, No. 024002 (10 pages), April 21, 2023. [Link]. 

Abstract: Machine learning provides a promising platform for both forward modeling and the inverse design of photonic structures. Relying on a data-driven approach, machine learning is especially appealing for situations when it is not feasible to derive an analytical solution for a complex problem. There has been a great amount of recent interest in constructing machine learning models suitable for different electromagnetic problems. In this work, we adapt a region-specified design approach for the inverse design of multilayered nanoparticles. Given the high computational cost of dataset generation for electromagnetic problems, we specifically investigate the case of a small training dataset, enhanced via random region specification in an inverse convolutional neural network. The trained model is used to design nanoparticles with high absorption levels and different ratios of absorption over scattering. The central design wavelength is shifted across 350–700 nm without re-training. We discuss the implications of wavelength, particle size, and the training dataset size on the performance of the model. Our approach may find interesting applications in the design of multilayer nanoparticles for biological, chemical, and optical applications as well as the design of low-scattering absorbers and antennas.

[23] Nooshin M. Estakhri, Nasim Mohammadi Estakhri, and T. B. Norris, “Emergence of coherent backscattering from sparse and finite disordered media,”  Scientific Reports, Vol. 12, No. 22256 (10 pages), December 23, 2022. [Link]

Abstract: Coherent backscattering (CBS) arises from complex interactions of a coherent beam with randomly positioned particles, which has been typically studied in media with large numbers of scatterers and high opacity. We develop a first-principles scattering model for scalar waves to study the CBS cone formation in finite-sized and sparse random media with specific geometries. The current study provides insights into the effects of density, volume size, and other relevant parameters on the angular characteristics of the CBS cone emerging from sparse and bounded random media for various types of illumination, with results consistent with well-known CBS studies which are typically based on samples with much larger number of scatterers and higher opacity. The enhancements are observed in scattering medium with dimensions between 10× and 40× wavelength and the number of particles as few as 370. This work also highlights some of the potentials and limitations of employing the CBS phenomenon to characterize disordered configurations. The method developed here provides a foundation for studies of complex electromagnetic fields beyond simple incident classical beams in randomized geometries, including structured wavefronts in illumination and quantized fields for investigating the effects of the quantum nature of light in multiple scattering, with no further numerical complications.

[22] N. Mohammadi Estakhri and N. Engheta, “Tunable metasurface-based waveplates- A proposal using inverse design,” Comptes Rendus Physique-French Academy of Science, Vol. 21, No. 7-8, 625-639, January 19, 2021. [Link]

Abstract: An approach to achieve tunable free-space waveplate operation based on a two-layer cascaded metastructure is proposed. Phase retardation is varied through changing the axial distance between the two layers. Full control on the ellipticity of the output wave is attained with wavelength-scale variations in the axial distance. The theoretically desired characteristics of the metastructures are presented and multiple physical implementations are suggested based on inverse design topology optimization.

[21] N. Mohammadi Estakhri, N. Engheta, and R. Kastner “Electromagnetic Funnel: Reflectionless Transmission and Guiding of Wave through Subwavelength Apertures,” Physical Review Letters, Vol. 124, No. 3, 033901, January 22, 2020. [Link]

Abstract: Confining and controlling electromagnetic energy typically involves a highly resonant phenomenon, especially when subwavelength confinement is desired. Here, we present a class of nonresonant, self-dual planar metastructures capable of protected energy transmission from one side to the other, through arbitrarily narrow apertures. It is shown that the transmission is in the form of matched propagating modes and is independent of the thickness and specific composition of the surface. We analytically prove that the self-dual condition is sufficient to guarantee 100% transmission that is robust to the presence of discontinuities along the propagation path. The results are confirmed numerically through study of various scenarios. The operation is broadband and subject only to the bandwidth of the constituent materials. The polarization of the internal field can also be independently controlled with respect to the incident one. Our structures are promising for applications in sensing, particle trapping, near-field imaging, and wide scan antenna arrays.

[20] N. Mohammadi Estakhri, Brian Edwards, and N. Engheta, “Solving Equations with Metamaterials,” Optics and Photonics News, Year in Optics, Vol. 30, No. 12, p. 50, December 1, 2019. [Link]

[19] N. Mohammadi Estakhri*, Brian Edwards*, and N. Engheta, “Inverse-designed metastructures that solve equations,” Science, Vol. 363, No. 6433, pp. 1333-1338, March 22, 2019. [Link]

Abstract: Metastructures hold the potential to bring a new twist to the field of spatial-domain optical analog computing: migrating from free-space and bulky systems into conceptually wavelength-sized elements. We introduce a metamaterial platform capable of solving integral equations using monochromatic electromagnetic fields. For an arbitrary wave as the input function to an equation associated with a prescribed integral operator, the solution of such an equation is generated as a complex-valued output electromagnetic field. Our approach is experimentally demonstrated at microwave frequencies through solving a generic integral equation and using a set of waveguides as the input and output to the designed metastructures. By exploiting subwavelength-scale light-matter interactions in a metamaterial platform, our wave-based, material-based analog computer may provide a route to achieve chip-scale, fast, and integrable computing elements.

[18] M. Sakhdari, N. Mohammadi Estakhri, H. Bagci, and P. Y. Chen, “Low-Threshold Lasing and Coherent Perfect Absorption in the Generalized PT-Symmetric Optical Structures,” Physical Review Applied, Vol. 10, No. 2 (8 Pages) 024030, August 21, 2018. [Link]

Abstract: Achieving exact balance between spatially separated gain and loss is generally regarded as a necessary condition for parity-time- (PT-)symmetric optical systems. We introduce generalized PT- (GPT-)symmetric optical structures, which have an asymmetric and unbalanced gain-loss profile, while exhibiting similar scattering properties and phase transitions as their traditional PT-symmetric counterparts. In particular, we show that the concept of GPT symmetry may help to reduce the threshold gain in achieving newly discovered PT-enabled applications, such as the coherent perfect absorber laser and exceptional points. The concept proposed herein will facilitate the practice of PT-symmetric optical and photonic devices by offering greater design flexibility to tailor gain-loss profiles and their thresholds.

[17] X. Xu, H. Kwon, B. Gawlik, N. Mohammadi Estakhri, A. Alù, S. V. Sreenivasan, and A. Dodabalapur, “Enhanced Photoresponse in Metasurface-Integrated Organic Photodetectors,” Nano Letters, Vol. 18, No. 6, pp. 3362-3367, April 30, 2018. [Link]

Abstract: In this work, we experimentally demonstrate metasurface-enhanced photoresponse in organic photodetectors. We have designed and integrated a metasurface with broadband functionality into an organic photodetector, with the goal of significantly increasing the absorption of light and generated photocurrent from 560 up to 690 nm. We discuss how the metasurface can be integrated with the fabrication of an organic photodiode. Our results show large gains in responsivity from 1.5× to 2× between 560 and 690 nm.

[16] J. Shi, M. H. Lin, Y. T. Chen, N. Mohammadi Estakhri, X. Q. Zhang, Y. Wang, H. Y. Chen, C. A. Chen, C. K. Shih, A. Alù, X. Li, Y. H. Lee, and S. Gwo, “Cascaded Exciton Energy Transfer in a Monolayer Semiconductor Lateral Heterostructure Assisted by Surface Plasmon Polariton,” Nature Communications, Vol. 8, No. 35 (7 pages), June 26, 2017. [Link]

Abstract: Atomically thin lateral heterostructures based on transition metal dichalcogenides have recently been demonstrated. In monolayer transition metal dichalcogenides, exciton energy transfer is typically limited to a short range (~1 μm), and additional losses may be incurred at the interfacial regions of a lateral heterostructure. To overcome these challenges, here we experimentally implement a planar metal-oxide-semiconductor structure by placing a WS2/MoS2 monolayer heterostructure on top of an Al2O3-capped Ag single-crystalline plate. We find that the exciton energy transfer range can be extended to tens of microns in the hybrid structure mediated by an exciton-surface plasmon polariton–exciton conversion mechanism, allowing cascaded exciton energy transfer from one transition metal dichalcogenides region supporting high-energy exciton resonance to a different transition metal dichalcogenides region in the lateral heterostructure with low-energy exciton resonance. The realized planar hybrid structure combines two-dimensional light-emitting materials with planar plasmonic waveguides and offers great potential for developing integrated photonic and plasmonic devices.

[15] N. Mohammadi Estakhri, V. Neder, M. Knight, A. Polman, and A. Alù, “Visible Light, Wide-Angle Graded Metasurface for Back Reflection,” ACS Photonics, Vol. 4, No. 2, pp. 228-235, January 23, 2017. [Link]

Abstract: Metasurfaces, or phase-engineered quasi-2D interfaces, enable a large degree of control over the reflection, refraction, and transmission of light. Here we demonstrate the design and realization of a visible light gradient metasurface tailored for highly efficient back reflection based on the Huygens–Fresnel principle. The metasurface emulates the functionality of a Littrow grating, capable of efficiently channeling light into the first negative Floquet order over a broad angular range and bandwidth at visible frequencies. Our theoretical results predict unitary efficiency for extremely low profiles and an optical response that is robust against discretization and design modifications. The experimentally realized metasurface is comprised of high-index TiOx nanowires over a protected Ag mirror, enabling back reflection with efficiency above 85% in the visible range, close to the reflectivity of the bare silver mirror. The presented analytical design methodology and the resulting low-profile device are advantageous compared to conventional gratings, while offering broadband efficiencies over a range of incidence angles.

[14] B. Orazbayev, N. Mohammadi Estakhri, A. Alù, and  M. Beruete , “Experimental Demonstration of Metasurface-Based Ultrathin Carpet Cloak for Millimetre Waves,” Advanced Optical Materials, Vol. 5, no. 1, January 2017. [Link] (featured in the 2019 "Metasurfaces" virtual issue highlighting recent cutting-edge research in this area. )

Abstract: A metasurface carpet cloak for millimeter‐wave range with polarization‐independent performance is experimentally demonstrated. It is shown that the cloak is able to mimic the ground plane by fully restoring the amplitude and phase distributions for both transverse electric and transverse magnetic polarizations, with a relatively wide frequency and angular widths response.

 [13] N. Mohammadi Estakhri and A. Alù , “Wavefront Transformation with Gradient Metasurfaces,” Physical Review X, Vol. 6, No. 4, 041008 (17 pages), October 14, 2016. [Link]  

Abstract: Relying on abrupt phase discontinuities, metasurfaces characterized by a transversely inhomogeneous surface impedance profile have been recently explored as an ultrathin platform to generate arbitrary wave fronts over subwavelength thicknesses. Here, we outline fundamental limitations of passive gradient metasurfaces in molding the impinging wave and show that local phase compensation is essentially insufficient to realize arbitrary wave manipulation, but full-wave designs should be considered. These findings represent a critical step towards realistic and highly efficient conformal wave manipulation beyond the scope of ray optics, enabling unprecedented nanoscale light molding.

[12] N. Mohammadi Estakhri and A. Alù, “Recent Progress in Gradient Metasurfaces,” Journal of the Optical Society of America B, Special Feature Issue on Electromagnetic Metasurfaces, Vol. 33, No. 2, pp. A21-A30, December 16, 2015, (invited paper). [Link]

Abstract: Recent advances in metasurfaces, i.e., artificial arrays of engineered inclusions assembled over a thin surface, have opened promising venues to control electromagnetic waves in unique and unprecedented ways, by means of locally engineering their near-field wave–matter interactions. Gradient or locally nonperiodic metasurfaces are one of the most exciting recent advances in nano-optics, due to the promise of enabling ultimate light molding, in both the near-field and the far-field, with large efficiency and a minimal footprint. These artificial surfaces are characterized by a transverse variation of their surface properties and lack of local periodicity, distinguishing them from conventional frequency selective surfaces and optical gratings. In this paper, we review recent work in the area of gradient metasurfaces, aimed at arbitrary wave shaping. The significant recent progress and novel applications achieved through optical metasurfaces, including ultrathin invisibility cloaks and polarization-dependent light splitting, are discussed, outlining the typical challenges and their outstanding prospects in integrated nanophotonic devices. Following our discussion on metasurface design approaches, we then revisit the problem of controlling the distribution of energy between multiple diffraction orders by means of gradient metasurfaces. Our discussions reveal that Huygens-based designs hold the promise of overcoming the low conversion efficiency issues associated with other techniques.

[11] N. Mohammadi Estakhri, C. Argyropoulos, and A. Alù, “Graded Metascreens to Enable a New Degree of Nanoscale Light Management,” Philosophical Transactions A, Special Issue on Spatial Transformations: from Fundamentals to Applications, Vol. 373, No. 2049, 20140351 (15 pages), July 27, 2015, (invited paper). [Link]

Abstract: Optical metasurfaces, typically referred to as two-dimensional metamaterials, are arrays of engineered subwavelength inclusions suitably designed to tailor the light properties, including amplitude, phase and polarization state, over deeply subwavelength scales. By exploiting anomalous localized interactions of surface elements with optical waves, metasurfaces can go beyond the functionalities offered by conventional diffractive optical gratings. The innate simplicity of implementation and the distinct underlying physics of their wave–matter interaction distinguish metasurfaces from three-dimensional metamaterials and provide a valuable means of moulding optical waves in the desired manner. Here, we introduce a general approach based on the electromagnetic equivalence principle to develop and synthesize graded, non-periodic metasurfaces to generate arbitrarily prescribed distributions of electromagnetic waves. Graded metasurfaces are realized with a single layer of spatially modulated, electrically polarizable nanoparticles, tailoring the scattering response of the surface with nanoscale resolutions. We discuss promising applications based on the proposed local wave management technique, including the design of ultrathin optical carpet cloaks, alignment-free polarization beam splitters and a novel approach to enable broadband light absorption enhancement in thin-film solar cells. This concept opens up a practical route towards efficient planarized optical structures with potential impact on the integrated nanophotonic technology.

[10] B. Orazbayev, N. Mohammadi Estakhri, M. Beruete, and A. Alù, “Terahertz Carpet Cloak Based on a Ring Resonator Metasurface,” Physical Review B, Vol. 91, No. 19, 195444 (5 pages), May 29, 2015. [Link]

Abstract: In this work we present the concept and design of an ultrathin (λ/22) terahertz (THz) unidirectional carpet cloak based on the local phase compensation approach enabled by gradient metasurfaces. A triangular surface bump with center height of 4.1 mm (1.1λ) and tilt angle of 20° is covered with a metasurface composed of an array of suitably designed closed ring resonators with a transverse gradient of surface impedance. The ring resonators provide a wide range of control for the reflection phase with small absorption losses, enabling efficient phase manipulation along the edge of the bump. Our numerical results demonstrate a good performance of the designed cloak in both near field and far field, and the cloaked object mimics a flat ground plane within a broad range of incidence angles, over 35° angular spectrum centered at 45°. The presented cloak design can be applied in radar and antenna systems as a thin, lightweight, and easy to fabricate solution for radio and THz frequencies.

[9] Y. L. Wang, N. Mohammadi Estakhri, A. Johnson, H. Y. Li, L. X. Xu, L. Y. Sun, Z. Zhang,  A. Alù, Q. Q. Wang, C. K. Shih, “Tailoring Plasmonic Enhanced Upconversion in Single NaYF4:Yb3+/Er3+ Nanocrystals,” Scientific Reports, Vol. 5, No. 10196 (7 pages), May 15, 2015. [Link]

Abstract: By using silver nanoplatelets with a widely tunable localized surface plasmon resonance (LSPR), and their corresponding local field enhancement, here we show large manipulation of plasmonic enhanced upconversion in NaYF4:Yb3+/Er3+ nanocrystals at the single particle level. In particular, we show that when the plasmonic resonance of silver nanolplatelets is tuned to 656 nm, matching the emission wavelength, an upconversion enhancement factor ~5 is obtained. However, when the plasmonic resonance is tuned to 980 nm, matching the nanocrystal absorption wavelength, we achieve an enhancement factor of ~22 folds. The precise geometric arrangement between fluorescent nanoparticles and silver nanoplatelets allows us to make, for the first time, a comparative analysis between experimental results and numerical simulations, yielding a quantitative agreement at the single particle level. Such a comparison lays the foundations for a rational design of hybrid metal-fluorescent nanocrystals to harness the upconversion enhancement for biosensing and light harvesting applications.

[8] N. Mohammad Estakhri and A. Alù, “Ultrathin Carpet Cloaking and Wavefront Reconstruction with Graded Metasurfaces,” IEEE Antennas and Wireless Propagation Letters, Special Cluster on Transformation Electromagnetics, Vol. 13, pp. 1775-1778, January 28, 2015, (invited paper). [Link]

Abstract: Using a suitably designed, ultra-thin graded metasurface, we demonstrate the possibility of hiding an arbitrarily shaped/sized object from an impinging plane wave. The metasurface is tailored to provide an abrupt, inhomogeneous discontinuity to the electromagnetic field that compensates for the unwanted scattering created by the object. The desired field distribution is generated based on the equivalence principle through reconstruction of the electric/magnetic fields at the metasurface location, resembling a flat conducting surface for an external observer. We apply this concept to hide electrically large, cylindrical (two-dimensional, 2-D) and spherical (3-D) domes at optical frequencies and discuss practical cloaking designs for microwave and terahertz regimes. The presented graded metasurface-based cloaks may find interesting applications as low-profile, tunable covers for low observability and noise reduction in wireless commutation systems.

[7] Y. Wu, C. Zhang, N. Mohammadi Estakhri, Y. Zhao, J. Kim, M. Zhang, X. X. Liu, G. K. Pribi, A. Alù, C. K. Shih, and X. Li, “Intrinsic Optical Properties and Enhanced Plasmonic Response of Epitaxial Silver,” Advanced Materials, Vol. 26, No. 35, pp. 6106-6110, erratum at pp. 6054-6055, September 17, 2014. [Link]

Abstract: Using atomically smooth epitaxial silver films, new optical permittivity highlighting significant loss reduction in the visible frequency range is extracted. Largely enhanced propagation distances of surface plasmon polaritons are measured, confirming the low intrinsic loss in silver. The new permittivity is free of extrinsic spectral features associated with grain boundaries and localized plasmons inevitably present in thermally deposited films. 

[6] N. Mohammadi Estakhri and A. Alù, “Manipulating Optical Reflections Using Engineered Nanoscale Metasurfaces,” Physical Review B, Vol. 89, No. 23, 235419 (8 pages), June 16, 2014. [Link]

Abstract: We propose a comprehensive scheme for the efficient design of graded optical metasurfaces capable of rerouting the impinging energy at will in a flexible way. We show that carefully designed conjoined optical nanoelements may be used as basic building blocks to arbitrarily shape the reflected phase front, while providing large controllability and low loss over deeply subwavelength thicknesses. The metasurface elements are designed using transmission line concepts combined with nanocircuit theory, and they can be realized with conventional lithographic techniques. Based on these concepts, we put forward a fast and accurate analytical model to design the optical analog of reflectarrays for light bending, steering, and focusing. The proposed designs show large efficiency over a broad angular spectrum, accompanied by broad bandwidths of operation. Our technique may lead to significant advances in the field of planarized nanophotonics and light manipulation over a surface, with potential applications in light trapping and efficient photonic couplers.

[5] C. Argyropoulos, F. Monticone, N. Mohammadi Estakhri, and A. Alù, “Tunable Plasmonic and Hyperbolic Metamaterials,” International Journal of Antennas and Propagation, Special Issue on ‘Reconfigurable Electromagnetics through Metamaterials’, Vol. 2014, 532634 (11 pages), April 6, 2014, (invited paper). [Link]

Abstract: We present here tunable and reconfigurable designs of linear and nonlinear plasmonic and hyperbolic metamaterials. Rich scattering features of multilayered composite nanoparticles are demonstrated, which include complex and exotic scattering signatures combining multiple dipolar Fano resonances and electromagnetic induced transparency (EIT) features. These dipole-dipole multi-Fano scattering responses can be further tuned through altering the plasmonic properties of the concentric layers or the permittivity of the core, for instance, by the presence of nonlinearities. Strong third-order nonlinear effects, such as optical bistability, may also be induced in the scattering response of nonlinear nanoparticles due to the highly enhanced and confined fields inside their core. Nonlinear hyperbolic metamaterial designs are also explored, which can realize tunable positive-to-negative refraction at the same frequency, as a function of the input intensity. Negative Goos-Hänchen shift is demonstrated based only on the hyperbolic dispersion properties of these layered metamaterials without the usual need of negative index metamaterials. The Goos-Hänchen shift may be tuned from positive-to-negative values, when the structure is illuminated with different frequencies. A plethora of applications are envisioned based on the proposed tunable metamaterials, such as ultrafast reconfigurable imaging devices, tunable sensors, novel nanotag designs, and efficient all-optical switches and memories.

[4] N. Mohammadi Estakhri and A. Alù, “Minimum-scattering Superabsorbers,” Physical Review B, Rapid Communications, Vol. 89, No. 12, 121416(R) (5 pages), March 31, 2014. [Link]

Abstract: Absorption and scattering are inherently related, as it is not possible to absorb power without creating a far-field shadow. We show, however, that properly overlapped resonant modes in a suitably designed system may in principle lead to arbitrarily large absorption levels, while at the same time minimizing the total scattering. We discuss the fundamental limits on scattering and absorption of an arbitrary receiving system and envision a composite nanoparticle that demonstrates the concept of a minimum-scattering superabsorber, with potential applications in energy harvesting, sensing, and imaging.

[3] C. Argyropoulos, N. Mohammadi Estakhri, F. Monticone, and A. Alù, “Negative Refraction, Gain and Nonlinear Effects in Hyperbolic Metamaterials,” Optics Express, Focus Issue on Hyperbolic Metamaterials: Fundamentals and Applications, Vol. 21, No. 12, pp. 15037 (11 pages), June 17, 2013, (invited paper). [Link]

Abstract: The negative refraction and evanescent-wave canalization effects supported by a layered metamaterial structure obtained by alternating dielectric and plasmonic layers is theoretically analyzed. By using a transmission-line analysis, we formulate a way to rapidly analyze the negative refraction operation for given available materials over a broad range of frequencies and design parameters, and we apply it to broaden the bandwidth of negative refraction. Our analytical model is also applied to explore the possibility of employing active layers for loss compensation. Nonlinear dielectrics can also be considered within this approach, and they are explored in order to add tunability to the optical response, realizing positive-to-zero-to-negative refraction at the same frequency, as a function of the input intensity. Our findings may lead to a better physical understanding and improvement of the performance of negative refraction and subwavelength imaging in layered metamaterials, paving the way towards the design of gain-assisted hyperlenses and tunable nonlinear imaging devices.

[2] F. Monticone, N. Mohammadi Estakhri, and A. Alù, “Full Control of Nanoscale Optical Transmission with a Composite Metascreen,” Physical Review Letters, Vol. 110, No. 20, 203903 (5 pages), May 14, 2013. [This paper has been selected as PRL Editor’s suggestion]. [Link]

Abstract: By applying the optical nanocircuit concepts to metasurfaces, we propose an effective route to locally control light transmission over a deeply subwavelength scale. This concept realizes the optical equivalent of a transmit-array, whose use is demonstrated for light bending and focusing with unprecedented efficiency over a subwavelength distance, with crucial benefits for nano-optics applications. These findings may lead to large improvements in the manipulation of optical transmission and processing of nanoscale optical signals over conformal and Si-compatible substrates.

[1] N. Mohammadi Estakhri and A. Alù, “Physics of Unbounded, Broadband Absorption/Gain Efficiency in Plasmonic Nanoparticles,” Physical Review B, Vol. 87, No. 20, 205418 (9 pages), May 10, 2013. [Link]

Abstract: Anomalous resonances in properly shaped plasmonic nanostructures can in principle lead to infinite absorption/gain efficiencies over broad bandwidths of operation. By developing a closed-form analytical solution for the fields scattered by conjoined hemicylinders, we outline the fundamental physics behind these phenomena, associated with broadband adiabatic focusing of surface plasmons at the nanoscale. Over a continuous frequency range, our proposed composite nanostructure shows finite amount of absorption/amplification even in the limit of infinitesimally small intrinsic material loss/gain. Detailed physical insights are provided to justify the nature of this apparent paradox, and its counterintuitive behavior is discussed for potential applications in nonlinear optics, spasing, sensing, and energy-harvesting devices.