Localized Plasmon Resonance Research Articles

Multipole Excitation of Localized Plasmon Resonance in Asymmetrically Coated Core–Shell Nanoparticles Using Optical Vortices

AbstractPlasmonic interactions between an asymmetrically coated core–shell (ACCS) nanoparticle and an optical vortex produce a novel engagement of the spin angular momentum (SAM) and the orbital angular momentum (OAM) of the input. Simulations based on a discrete dipole approximation (DDA) indicate that the SAM and the OAM of the incident beam determine the modal order of resonance, correctly identifying the peak wavelength, and both the direction and magnitude of optical torque exerted upon the excited, localized plasmon resonance in the ACCS particle. These simulations also indicate higher‐order resonances, including hexapole and octupole modes, and a zero‐order resonance (expressible as a monopole mode), can be excited by judicious selection of the SAM and OAM. A detailed symmetry analysis shows how the multipoles associated with eigenmode excitations connect to the radiation multipoles at the heart of the multipole expansion. It is also shown how additional, distorted resonance modes due to the asymmetricity of the structure are also exhibited. These specific plasmonic characteristics, which cannot be realized by plane wave excitation, become possible through the ACCS asymmetry engaging with the distinct optical vortex nature of the excitation.

Construction Strategy for Flexible and Breathable SiO2/Al/NFs/PET Composite Fabrics with Dual Shielding against Microwave and Infrared-Thermal Radiations for Wearable Protective Clothing.

Microwave and infrared-thermal radiation-compatible shielding fabrics represent an important direction in the development of wearable protective fabrics. Nevertheless, effectively and conveniently integrating compatible shielding functions into fabrics while maintaining breathability and moisture permeability remains a significant challenge. Here, we take hydrophilic PVA-co-PE nanofibrous film-coated PET fabric (NFs/PET) as a flexible substrate and deposit a dielectric/conductive (SiO2/Al) bilayer film via magnetron sputtering. This strategy endows the fabric surface with high electrical conductivity, nanoscale roughness comparable to visible and infrared waves, and a dielectric-metal contact interface possessing localized plasmon resonance and Mie scattering effects. The results demonstrate that the optimized SiO2/Al/NFs/PET composite conductive fabric (referred to as S4-1) possesses favorable X-band electromagnetic interference (EMI) shielding effectiveness (50 dB) as well as excellent long-wave infrared (LWIR) shielding or IR stealth performance (IR emissivity of 0.60). Notably, the S4-1 fabric has a cooling effect of about 12.4 °C for a heat source (80 °C) and an insulating effect of about 17.2 °C for a cold source (-20 °C), showing excellent shielding capability for heat conduction and heat radiations. Moreover, the moisture permeability of the S4-1 fabric is about 300 g/(m2·h), which is better than the requirement concerning moisture permeability for wearable fabrics (≥2500-5000 g/(m2·24 h)), indicating excellent heat and moisture comfort. In short, our fabrics have lightweight, thin, moisture-permeable and excellent shielding performance, which provides novel ideas for the development of wearable multi-band shielding fabrics applied to complex electromagnetic environments.

A micro random laser of dye solution-filled tube system based on electrospun fibers

Random lasers (RLs) have good application prospects in speckle-free imaging, micro sensing and micro-nano photonic devices due to their properties such as cavityless, miniaturization and low-spatial coherence. The minimization, emission mechanism and lasing property control of RLs still need to be explored. In this paper, we present a novel micro RL system in a dye solution-filled tube, where the coherent random lasing action is facilitated by the integration of electrospun fibers and gold nanoparticles (Au NPs). The electrospun fibers serve as the scattering medium, while the Au NPs produce the local plasmon resonance effect. The interaction effect improves the photon scattering rate, makes the photons form a ring resonator in the system, and realizes the coherent RL in the tube system. The microtube RL has a very small size, a convenient preparation method and stable emission properties. It can withstand more than 2000 times pumping while maintaining a stable emission intensity. The electrospun fibers are stable and uniform in morphology, which makes it easy to achieve mass production of microtube RL. The speckle-free imaging experiment shows the RL has low spatial coherence, which indicates that it’s a good choice for micro speckle-free imaging. This work provides a convenient way to realize micro coherent RLs by electrospun fibers and could pave a way for applications in photonic devices.

Superhydrophobic silicone microcup array on periodic microswelling structure of silicone rubber formed by 193 nm ArF excimer laser-induced photodissociation

Many reports have been made on the study of imparting high functionality to materials surface by micro/nanofabrication of three-dimensional structures. In various three-dimensional structural geometries, the nanocups enable high-sensitivity analysis by combining with localized plasmon resonance. On the other hand, the formation of microcups and their applications have not been found much. In this paper, we found a new formation method of the silicone microcups and suggested its application. Single layer of silica glass microspheres was formed on silicone rubber, then a 193 nm ArF excimer laser irradiated the surface. Main chains of SiOSi bonds of the silicone rubber underneath the microspheres could be photodissociated to lower the molecular weight, resulting in the photochemical formation of periodic microswelling structure of silicone rubber. Concurrently, a hemispherical microcup was also formed on each the microswelling structure. The optimal laser irradiation conditions for their formation as well as the chemical etching time of microspheres were examined. It was also confirmed that the microcups are consisted of silicone. Moreover, the microcups exhibit superhydrophobicity. The results make it possible to confine the microcups in an air gap underwater, and it is expected to be applied as a micropower storage device that functions in seawater.

Spectral Tuning of Plasmonic Activity in 3D Nanostructures via High‐Precision Nano‐Printing

AbstractPlasmonic nanoparticles reveal unique optical properties and are increasingly incorporated into commercial products and technologies, ranging from photovoltaics to biological and chemical sensors. Shifting and tuning their plasmonic response according to the targeted application strongly depends on the ability to control the geometry in every detail and has not been reliably demonstrated for complex 3D nano‐architectures yet. Following that motivation, it herein presents how Focused Electron Beam Induced Deposition (FEBID), a highly flexible additive 3D direct‐write technology with spatial nano‐scale precision, is used for the controlled and tunable fabrication of plasmonically active 3D nanostructures that exhibit highly concentrated, well defined and predictable local plasmonic resonances. As model systems, planar Au nanowires and 3D nano‐tips of various geometries are prepared via FEBID and plasmonically characterized via scanning transmission electron microscopy based electron energy loss spectroscopy (STEM‐EELS) mapping measurements. The findings are complemented with corresponding plasmon simulations, revealing very good agreement with experimental findings. This way, on‐demand spectral tuning of the plasmonic response becomes accessible via upfront modeling and design of suitable 3D nanostructures, to achieve customized plasmonic responses, therefore paving the way for yet unrealized plasmonic applications in 3D space.

Simulation of Fe2O3/Au@MoS2 plasmonic core/shell@shell nanocomposites: Optical properties and photothermal application

The plasmonic and magneto-plasmonic nanocomposites are multi-functional structures that are applicable to medical applications such as tumor therapy, diagnosis, and biosensors. Here, the simulation of the optical and photothermal properties of plasmonic nanocomposites has been done. The spherical core@shell and core/shell@shell nanocomposites have been modeled using the generalized Mie theory. Au and Fe2O3 have been used as plasmonic and magnetic structures because of their bio-compatibility and then the role of MoS2 shell has been examined. The results show that these structures are very effective in photothermal therapy since their localized surfaced plasmon resonance is situated in the near-infrared region of electromagnetic waves. Among the simulated structures, Fe2O3@Au shows the best plasmonic behavior and absorption efficiency enhanced about 7 when the core radius is 20 nm, and the shell thickness and wavelength of incident light are between 5 and 10 nm and 700–800 nm, respectively. Maxwell-Garnett's effective medium theory has been used to calculate the effective permittivity of nanocomposites as well as the simulation of the temperature increase under laser radiation. The temperature enhancement of single nanocomposites shows very promising results. The single Fe2O3/Au@MoS2core/shell@shell nanocomposite illustrates the highest performance in temperature increase with 10−3K.

Scattering Amplitude of Surface Plasmon Polariton Excited by a Finite Grating.

Unusual optical properties of laser-ablated metal surfaces arise from the excitation of local plasmon resonances in nano- and microstructures produced by laser-processing and from the mutual interaction of those structures through surface plasmon polariton (SPP) waves. This interaction provides a synergistic effect, which can make the optical properties of the composite nanostructure drastically different from the properties of its elements. At the same time, the prediction and analysis of these properties are hampered by the complexity of the analytical solution to the problem of SPP excitation by surface objects of arbitrary configuration. Such a problem can be reduced to a simpler one if one considers the geometry of a structured surface as a superposition of harmonic Fourier components. Therefore, the analytical solution to the problem of surface plasmon polariton excitation through the scattering of light by a sinusoidally perturbed plasmonic metal/vacuum boundary becomes very important. In this work, we show that this problem can be solved using a well-known method for calculating guided-mode amplitudes in the presence of current sources, which is used widely in the waveguide theory. The calculations are carried out for the simplest 2D cases of (1) a sinusoidal current of finite length and (2) a finite-length sinusoidal corrugation on a plasmonic metal surface illuminated by a normally incident plane wave. The analytical solution is compared with the results of numerical simulations. It is shown that, in the first case, the analytical and numerical solutions agree almost perfectly. In the second case, the analytical solution correctly predicts the optimum height of the corrugation xopt, providing the maximum SPP excitation efficiency. At the same time, the analytical and numerical values of the SPP amplitude agree very well when the corrugation height x turns out to be x≪xopt or x≫xopt (at least up to 3xopt); at x=xopt, the mismatch of those does not exceed 25%. The limitations of the analytical model leading to such a mismatch are discussed. We believe that the presented approach is useful for modeling various phenomena associated with SPP excitation.

Dynamically Adjusting Borophene-Based Plasmon-Induced Transparency in a Polymer-Separated Hybrid System for Broadband-Tunable Sensing.

Borophene, an emerging two-dimensional (2D) material platform, is capable of supporting highly confined plasmonic modes in the visible and near-infrared wavebands. This provides a novel building block for light manipulation at the deep subwavelength scale, thus making it well-suited for designing ultracompact optical devices. Here, we theoretically explore a borophene-based plasmonic hybrid system comprising a continuous borophene monolayer (CBM) and sodium nanostrip gratings (SNGs), separated by a polymer spacer layer. In such a structure, a dynamically tunable plasmon-induced transparency (PIT) effect can be achieved by strongly coupling dark and bright plasmonic modes, while actively controlling borophene. Here, the bright mode is generated through the localized plasmon resonance of SNGs when directly excited by TM-polarized incident light. Meanwhile, the dark mode corresponds to a propagating borophene surface plasmon (BSP) mode in the CBM waveguide, which cannot be directly excited, but requires phase matching with the assistance of SNGs. The thickness of the polymer layer has a significant impact on the coupling strength of the two modes. Owing to the BSP mode, highly sensitive to variations in the ambient refractive index (RI), this borophene-based hybrid system exhibits a good RI-sensing performance (643.8 nm/RIU) associated with a wide range of dynamically adjustable wavebands (1420-2150 nm) by tuning the electron density of borophene. This work offers a novel concept for designing active plasmonic sensors dependent on electrically gating borophene, which has promising applications in next-generation point-of-care (PoC) biomedical diagnostic techniques.

Inkjet Patterning of Plasmonic Metal Nanoparticles in Silicone Elastomers

AbstractThe incorporation of the plasmonic metal nanoparticles (NPs) in microfluidic systems is highly desirable since their unique characteristic such as localized plasmonic resonance can be largely beneficial to many biomedical applications; however, the typical incorporation process requires a long immersion time and totally lacks patterning capability. This work develops a method for embedding patterned plasmonic AuNPs and AgNPs in silicone elastomer by inkjet printing metal ion solution into uncured silicone elastomer. This work finds that both energetics and kinetics of the solvent's mixing behaviors are critical to successful patterning, both experimentally and theoretically. Based on these results, patterned metal NPs are literally “embedded” in silicon elastomers efficiently, not formed just on surfaces. This work demonstrates the patterning capability by reproducing pixel arts with metal NPs in silicone elastomer. In addition, an AuNPs‐based photothermal microfluidic pump that responds to light at a specific wavelength is fabricated, demonstrating the potential utility of the inkjet patterned metal NPs in silicone elastomer.

Nanoplasmonic biosensors for precision medicine.

Nanoplasmonic biosensors have a huge boost for precision medicine, which allows doctors to better understand diseases at the molecular level and to improve the earlier diagnosis and develop treatment programs. Unlike traditional biosensors, nanoplasmonic biosensors meet the global health industry's need for low-cost, rapid and portable aspects, while offering multiplexing, high sensitivity and real-time detection. In this review, we describe the common detection schemes used based on localized plasmon resonance (LSPR) and highlight three sensing classes based on LSPR. Then, we present the recent applications of nanoplasmonic in other sensing methods such as isothermal amplification, CRISPR/Cas systems, lab on a chip and enzyme-linked immunosorbent assay. The advantages of nanoplasmonic-based integrated sensing for multiple methods are discussed. Finally, we review the current applications of nanoplasmonic biosensors in precision medicine, such as DNA mutation, vaccine evaluation and drug delivery. The obstacles faced by nanoplasmonic biosensors and the current countermeasures are discussed.

Optical coupling of bio-inspired mustard protein-based bimetallic nanohybrids with propagating surface plasmon polaritons for femtomolar nitrite ion sensing: Cellphone-based portable detection device

Different classes of biologically and environmentally relevant molecules and ions are quantitatively and qualitatively analyzed using the synergistic approach wherein the properties of both the plasmonic nanomaterials and the fluorescent moieties are explored. In this study, such hybrid systems, called plasmophores (or plasmon coupled fluorophores), are investigated using the surface plasmon-coupled emission (SPCE) platform with silver, gold and silver-gold hybrid nano-structures synthesized via sustainable bio-inspired routes. A simple, rapid, user-friendly and green chemistry approach has been adopted using biocompatible mustard protein as both the reducing and capping agent during nanosynthesis. The morphologically and functionally tunable plasmonic nanomaterials were synthesized by simply exposing a mixture of the respective metal ions and protein to UV-light irradiation. Comprehensive analysis of the out-coupled spontaneous emission in the SPCE framework using spacer, cavity and extended cavity nanointerfaces yielded intriguing results, presenting optimum prerequisites for realizing high SPCE enhancements. More than a 950-fold improvement in the fluorescence signal intensity of rhodamine B was demonstrated utilizing the plasmonic cavity hotspots rendered by the hybrid coupling of the propagating surface plasmon polaritons of Ag with localized plasmon resonances of the bimetallic AgAu nanohybrids. The sharply directional and highly polarized SPCE enhancements, realized without increasing the illumination intensity, were utilized for quantitative detection of biologically relevant nitrite ions in drinking water samples at trace concentrations (limit of detection: 0.1 femtomolar). The applicability and relevance of the demonstrated method for sustainable use of plasmonic nanomaterials in photo-plasmonic research is presented by accomplishing the sensing using a cost-effective smartphone camera with a good correlation with the data obtained from a conventional detector system. Hence, the bio-nano-inspired strategy described here reveals several potential opportunities for the development of environmentally benign portable chemosensing devices for utility in academia and industry, especially for low-and-middle income countries.

Encoding Mie, plasmonic, and diffractive structural colors in the same pixel

Abstract We present a 1D reflective multi-level structural color design that incorporates Mie, plasmonic, and diffractive mechanisms in the same pixel. Comprised of a metallodielectric grating made of TiOx nanowires sandwiched between Ag thin film and Ag substrate, the design can exhibit either a Mie resonance or a localized plasmonic resonance depending on the polarization of incident light, resulting in dramatically different color states. Due to the periodicity, the grating also diffracts light, providing an additional color state. Since diffraction can be turned on or off by the degree of coherence of the incoming light, both Mie and plasmonic colors can be modulated using objective lenses with different numerical apertures. Exploiting the different color generating modes, we encode four layers of information in a pixel array, where each layer is unveiled using a different combination of excitation and imaging settings. These results introduce new possibilities for data encryption, anticounterfeiting, and data storage.

A Simple Method to Fabricate the Highly Sensitive SERS Substrate by Femtosecond Laser-Based 3D Printer

Surface-enhanced Raman spectroscopy (SERS) is a potent technique for analyzing and detecting various targets, including toxic ions, pesticides, and biomarkers, at the single-molecule level. The efficiency of SERS techniques relies heavily on the underlying SERS substrate, which is primarily responsible for the strong induction of localized plasmon resonance on nanostructures. Noble metals such as gold and silver were commonly used to fabricate SERS substrates, leveraging the electromagnetic mechanism (EM) to enhance the Raman signal. However, chemically synthesized nanoparticle-based SERS substrates suffer from low uniformity and reproducibility. Furthermore, the high cost associated with noble metals makes most SERS substrates expensive to produce. In this study, we present a straightforward method for fabricating a highly uniform and reproducible SERS substrate using a femtosecond laser-based 3D printer. Notably, our method offers good cost competitiveness since it requires only a minimal amount of gold coating for the SERS signal. Moreover, the proposed method exhibits exceptional versatility in SERS analysis and detection, catering to numerous targets in the field.

Plasmon-Controlled Selective Emission Enhancement of Eu3+ with (Au Core)@(Y(V,P)O4:Eu) Nanostructures.

Plasmonic nanostructures have a capability to control the photoluminescence (PL) emission properties of optical species and thus can dramatically enhance the performances of diverse optical systems and devices. Lanthanide ions typically exhibit multiple PL emission lines. Systematic studies on the plasmon-enabled selective enhancement for the different emission lines of lanthanide ions are still highly desired in order to achieve the fine manipulation on the spectral profile and luminescence intensity ratio (LIR). Herein we report on the synthesis and PL emission properties of monodisperse spherical (Au core)@(Y(V,P)O4:Eu) nanostructures, which integrate the plasmonic and luminescent units into an individual core@shell structure. The localized surface plasmon resonance adjusted through control of the size of the Au nanosphere core enables the systematic modulation of the selective emission enhancement of Eu3+. As revealed by single-particle scattering and PL measurements, the five luminescence emission lines of Eu3+ originating from the 5D0,1 excitation states are affected by the localized plasmon resonance to different extents, which are dependent on both the dipole transition nature and the intrinsic quantum yield of the emission line. Based on the plasmon-enabled tunable LIR, high-level anticounterfeiting and optical temperature measurements for photothermal conversion are further demonstrated. Our architecture design and PL emission tuning results offer many possibilities for constructing multifunctional optical materials by integrating plasmonic and luminescent building blocks into hybrid nanostructures with different configurations.

Single-mode lasing by tailoring the excitation of localized surface plasmon resonances to whispering gallery modes in a microring laser.

Cavity mode manipulation in lasers is urgent for the stable single-mode operation of a microring laser. Here, we propose and experimentally demonstrate the plasmonic whispering gallery mode microring laser for strong coupling between local plasmonic resonances and whispering gallery modes (WGM) on the microring cavity to achieve pure single-mode lasing. The proposed structure is fabricated based on integrated photonics circuits consisting of gold nanoparticles deposited on a single microring. Additionally, our numerical simulation provides deep insight into the interaction between the gold nanoparticles and WGM modes. The manufacture of microlasers for the advancement of lab-on-a-chip devices and all-optical detection of ultra-low analysts may benefit from our findings.

Exploring the resonance absorption of subwavelength-patterned epitaxial-grown group-IV semiconductor composite structures.

We experimentally and theoretically demonstrate a mid-infrared perfect absorber with all group-IV epitaxial layered composite structures. The multispectral narrowband strong absorption (>98%) is attributed to the combined effects of the asymmetric Fabry-Perot (FP) interference and the plasmonic resonance in the subwavelength-patterned metal-dielectric-metal (MDM) stack. The spectral position and intensity of the absorption resonance were analyzed by reflection and transmission. While a localized plasmon resonance in the dual-metal region was found to be modulated by both the horizontal (ribbon width) and vertical (spacer layer thickness) profile, the asymmetric FP modes were modulated merely by the vertical geometric parameters. Semi-empirical calculations show strong coupling between modes with a large Rabi-splitting energy reaching 46% of the mean energy of the plasmonic mode under proper horizontal profile. A wavelength-adjustable all-group-IV-semiconductor plasmonic perfect absorber has potential for photonic-electronic integration.

Bandwidth-tunable absorption enhancement of visible and near-infrared light in monolayer graphene by localized plasmon resonances and their diffraction coupling

Bandwidth-tunable light absorption enhancement in monolayer graphene is practically important for graphene-based photoelectric devices. Especially, it is still a huge challenge to achieve high-efficiency graphene absorption with extremely narrow sub-nanometer bandwidth much smaller than one nanometer. In this work, both broadband and narrowband absorption peaks of monolayer graphene are numerically achieved in visible and near-infrared wavelength ranges. The broadband absorption peak is ascribed to localized dipolar plasmon resonances in individual silver nanodisks, and the narrowband absorption peak arises from collective first-order diffraction coupling effect in periodic array of silver nanodisks. By changing the array period, the full width at half maximum (FWHM) of the broadband absorption peak can vary from 100 nm to 50 nm. Correspondingly, the FWHM of the narrowband absorption peak is tuned from about 6.4 nm to only 0.25 nm, thus realizing an ultra-narrow sub-nanometer bandwidth.

Interaction of plasmonic bound states in the continuum

Bound states in continuum (BICs) are believed to have the ability to achieve high quality factor (Q factor) resonances, which is very important for plasmonics. However, the study of plasmonic BICs is not sufficient. Herein, we design and fabricate a metal−insulator−metal (MIM) metasurface and demonstrate a one-dimensional plasmonic BIC experimentally. The even-order localized surface plasmon resonance (LSPR) modes have even parity at normal incidence. The symmetry-protected BIC can be achieved at Γ point. The band structure can be tuned by strong coupling between the localized plasmonic resonance and plasmonic lattice mode. Interestingly, two of the hybrid modes are also BICs. Although BICs cannot interact with the far field, we successfully demonstrate BIC splitting through far-field excitation. By further tuning the pitch of the MIM grating, the Friedrich–Wintgen BIC is also observed. Finally, we propose and preliminarily demonstrate an ultrathin bandpass spatial filter. These findings provide a new platform to study optical multipole BICs and can have applications in fields such as nano lasers, ultrasensitive sensors, filters, nonlinearity enhancement, and quantum optics.

One-Shot Laser-Pulse Modification of Bare and Silica-Coated Gold Nanoparticles of Various Morphologies.

Gold nanoparticles are widely used in laser biomedical applications due to their favorable properties, mainly localized plasmon resonance. However, laser radiation can cause a change in the shape and size of plasmonic nanoparticles, thus resulting in an unwanted reduction of their photothermal and photodynamic efficiency due to a drastic alteration of optical properties. Most previously reported experiments were carried out with bulk colloids where different particles were irradiated by different numbers of laser pulses, thus making it difficult to accurately evaluate the laser power photomodification (PM) threshold. Here, we examine the one-shot nanosecond laser-pulse PM of bare and silica-coated gold nanoparticles moving in a capillary flow. Four types of gold nanoparticles, including nanostars, nanoantennas, nanorods, and SiO2@Au nanoshells, were fabricated for PM experiments. To evaluate the changes in the particle morphology under laser irradiation, we combine measurements of extinction spectra with electron microscopy. A quantitative spectral approach is developed to characterize the laser power PM threshold in terms of normalized extinction parameters. The experimentally determined PM threshold increases in series were as follows: nanorods, nanoantennas, nanoshells, and nanostars. An important observation is that even a thin silica shell significantly increases the photostability of gold nanorods. The developed methods and reported findings can be useful for the optimal design of plasmonic particles and laser irradiation parameters in various biomedical applications of functionalized hybrid nanostructures.

Unveiling light collection and pump enhancement from quantum wells with plasmonic metasurfaces using power dependent measurements

Low light extraction efficiency (LEE) is the greatest limiting factor for the brightness of reduced-size light-emitting diodes (LEDs) as it limits their emission intensity. In addition, LEDs have a Lambertian emission, which requires secondary optics to control the emission directionality. Plasmonic metasurfaces can introduce a way of manipulating the generated light from LEDs to enhance their LEE and steer the emitted light by reshaping the far-field emission. Here, we fabricate resonant plasmonic metasurfaces on top of a typical blue emitting wafer consisting of InGaN/gallium nitride quantum wells developed for commercial LED devices. The metasurface is separated from the InGaN quantum wells by p-GaN and indium-tin-oxide (ITO) layers with a cumulative thickness of 110 nm. Since this distance value is close to the emission wavelength in the corresponding medium, enhanced near-fields of localized plasmonic resonances do not reach the active region. Despite this, we observe a strong influence of the metasurfaces on the far-field photoluminescence emission from the quantum wells as demonstrated by Fourier imaging. Power-dependent excitation measurements of the samples allow us to retrieve the pump and light collection enhancement factors provided by the plasmonic metasurfaces. We demonstrate that the plasmonic metasurfaces can provide a pump enhancement factor of up to 4.1 and a collection enhancement factor of up to 3.2. We have also performed simulations based on the reciprocity principle and achieved a good qualitative agreement with the experimental results.