Ultrahigh light transmission through a C-shaped nanoaperture

Optical resolution beyond the diffraction limit can be achieved by use of a metallic nanoaperture in a near-field optical system. Conventional nanoapertures have very low power throughput. Using a numerical finite-difference time domain method, we discovered a unique C-shaped aperture that provides approximately 3 orders of magnitude more power throughput than a conventional square aperture with a similar near-field spot size of approximately 0.1 lambda. Microwave experiments at 6 GHz quantitatively confirmed the simulated transmission enhancement. The high transmission of the C-aperture--or one of the related shapes--is linked to both a propagation mode in the aperture and local surface plasmons.     

Raman scattering and fluorescence emission in a single nanoaperture: Optimizing the local intensity enhancement

We investigate the potential of a single subwavelength aperture milled in an aluminium film to enhance the local electromagnetic field. We compare the Raman scattering of unadsorbed chlorobenzene molecules and the fluorescence emission of Cyanine-5 dyes, having the same excitation and collection setup for both experiments. For the optimal nanoaperture diameter, we report a clear enhancement factor of about 5 of the Raman scattering intensity per unit volume. Since Raman scattering probes the molecular vibrational levels and avoids the resonant pumping of a real excited state, the observed Raman enhancement is disconnected from the effects of the molecular energy levels alteration previously reported for fluorescent dyes. The observations are similar for both Raman and fluorescence experiments, and stand in good agreement with numerical electromagnetic computations of the excitation intensity inside the nanoaperture.     

Enhanced nonlinear optical conversion from a periodically nanostructured metal film

We demonstrate an approximately10(4) increase in conversion efficiency for optical second-harmonic generation (SHG) from a periodically nanostructured metal structure consisting of a single subwavelength aperture in a thin silver film surrounded by a set of concentric surface grooves. The forward-transmitted second-harmonic radiation from this structure is measured relative to that from an identical aperture with no surrounding surface periodicity. We explain the observed SHG enhancement quantitatively in terms of a measured 120x increase in the strength of the fundamental radiation in the vicinity of the aperture resulting from the periodic nanostructuring.     

Second-harmonic generation circular dichroism spectroscopy from tripod-like chiral molecular films

The second-harmonic generation (SHG) circular dichroism in the light of reflection from chiral films of tripod-like chiral molecules is investigated.The expressions of the second-harmonic generation circular dichroism are derived from our presented three-coupled-oscillator model for the tripod-like chiral molecules.Spectral dependence of the circular dichroism of SHG from film surface composed of tripod-like chiral molecules is simulated numerically and analysed.Influence of chiral parameters on the second-harmonic generation circular dichroism spectrum in chiral films is studied.The result shows that the second-harmonic generation circular dichroism is a sensitive method of detecting chirality compared with the ordinary circular dichroism in linear optics.All of our work indicates that the classical molecular models are very effective to explain the second-harmonic generation circular dichroism of chiral molecular system.The classical molecular model theory can give us a clear physical picture and brings us very instructive information about the link between the molecular configuration and the nonlinear processes.     

Enhancement effect of cumulative second-harmonic generation by closed propagation feature of circumferential guided waves

On the basis of second-order perturbation approximate and modal expansion approach,we investigate the enhancement effect of cumulative second-harmonic generation(SHG) of circumferential guided waves(CGWs) in a circular tube,which is inherently induced by the closed propagation feature of CGWs.An appropriate mode pair of primary-and doublefrequency CGWs satisfying the phase velocity matching and nonzero energy flux is selected to ensure that the second harmonic generated by primary CGW propagation can accumulate along the circumference.Using a coherent superposition of multi-waves,a model of unidirectional CGW propagation is established for analyzing the enhancement effect of cumulative SHG of primary CGW mode selected.The theoretical analyses and numerical simulations performed directly demonstrate that the second harmonic generated does have a cumulative effect along the circumferential direction and the closed propagation feature of CGWs does enhance the magnitude of cumulative second harmonic generated.Potential applications of the enhancement effect of cumulative SHG of CGWs are considered and discussed.The theoretical analysis and numerical simulation perspective presented here yield an insight previously unavailable into the physical mechanism of the enhancement effect of cumulative SHG by closed propagation feature of CGWs in a circular tube.     

Second-harmonic generation in resonant waveguide gratings incorporating ionic self-assembled monolayer polymer films

Experimental results on resonantly excited second-harmonic generation (SHG) in a periodic ionically self-assembled monolayer (ISAM) film are reported. A double-layer guided-mode resonance filter (GMRF) structure is coated with 40 bilayers of pyrlium-based chi(2) ISAM thin film and excited with the fundamental of a Nd:YAG laser. Enhanced second-harmonic conversion in the ISAM film is achieved because of the local field enhancement associated with the fundamental resonating leaky mode. This method of SHG is particularly promising, as the ISAM films under investigation exhibit anomalous dispersion that may be applied for phase matching to improve nonlinear conversion efficiency.     

Corrugated-enhanced second harmonic generation in metal–insulator–metal plasmonic waveguides

Nonlinear effects such as second-harmonic generation (SHG) are important for applications such as switching and wavelength conversion. In this study, the generation of second harmonic in metal–insulator–metal (MIM) plasmonic waveguides was investigated for both symmetric and asymmetric structures. Symmetric means that the metals at the top and bottom of the dielectric layer are the same and asymmetric means that the metals at the top and bottom of the dielectric layer are different. Two different structures are considered here as plasmonic waveguide for generation of second harmonic and analyzed using finite-difference time domain method. Besides the structure has grating on both sides for more coupling between photons and plasmons. The wavelength duration of grating per length unit (number of grooves) will be optimized to reach the highest second harmonic generation. To perform this optimization, the wavelength of operation λ = 458 nm is considered. It was shown that field enhancement in symmetric MIM waveguides can result in enhancement of SHG magnitude compared to the literature values and asymmetric device results in more than two orders of magnitude enhancement in SHG compared to symmetric structure. It is also shown that the electric field of second harmonic depends on the thickness of crystal (insulator). So, its thickness is optimized to achieve the highest electric field.     

A review of progress on nano-aperture VCSELInvited Paper

This paper reviews the progress on nano-aperture vertical-cavity surface-emitting lasers (VCSELs). The design, fabrication, and polarization control of nano-aperture VCSELs are reviewed. With the nanoaperture evolving from conventional circular and square aperture to unique C-shaped, H-shaped, I-shaped, and bowtie-shaped aperture, both the near-field intensity and near-field beam confinement from nanoaperture VCSELs are significantly improved. As a high-intensity compact light source with sub-100-nm spot size, nano-aperture VCSELs are promising to realize many new near-field optical systems and applications.     

Thin noble metal films on Si (111) investigated by optical second-harmonic generation and photoemission

Thin noble metal films (Ag, Au and Cu) on Si (111) have been investigated by optical second-harmonic generation (SHG) in combination with synchrotron radiation photoemission spectroscopy. The valence band spectra of Ag films show a quantization of the sp-band in the 4-eV energy range from the Fermi level down to the onset of the d-bands. For Cu and Au the corresponding energy range is much narrower and quantization effects are less visible. Quantization effects in SHG are observed as oscillations in the signal as a function of film thickness. The oscillations are strongest for Ag and less pronounced for Cu, in agreement with valence band photoemission spectra. In the case of Au, a reacted layer floating on top of the Au film masks the observation of quantum well levels by photoemission. However, SHG shows a well-developed quantization of levels in the Au film below the reacted layer. For Ag films, the relation between film thickness and photon energy of the SHG resonances indicates different types of resonances, some of which involve both quantum well and substrate states. Received: 16 October 2001 / Revised version: 14 March 2002 / Published online: 29 May 2002     

Dielectric Chiral Metasurfaces for Second-Harmonic Generation with Strong Circular Dichroism

Dielectric chiral metasurfaces can generate harmonic waves with high efficiency and strong circular dichroism (CD), when they are supported by metallic substrates. Numerical results show that the second-harmonic generation (SHG) efficiency of about 10⁻³% for a peak pump intensity of about 5 GW cm⁻² can be achieved in the blue-UV, and the SHG CD reaches up to about 1.8, from a metasurface of Z-shaped lithium niobate nanoantenna array supported by gold substrate. Highly efficient and strong circular dichroic nonlinear responses are attributed to the plasmon-assisted local field enhancement in the dielectric chiral nanoantennae adjacent to the metallic substrate.     

Second harmonic generation by cold-deposited silver films

The second harmonic generation (SHG) of light diffusely scattered by cold-deposited silver films is negligible with respect to the specular intensities. Therefore, a cold-deposited silver film is well approximated by a homogeneous effective medium. The intensities of SHG at 1.06 μm and the optical absorption at 0.53 μm depend on both the square of the internal effective field at 0.53 μm and a change in the same way when annealing the film. This can be understood by the assumption that SHG is also mainly following the electric field within the metal phase and surface contributions to SHG interfere destructively. This is corroborated by the observation that the SHG intensity does not show the ‘chemical first layer effects’ seen by surface-enhanced Raman scattering (SERS).     

Sensitivity‐enhancement methods for surface plasmon sensors

Surface plasmon resonance (SPR) sensors have been a mature technology for more than two decades now, however, recent investigations show continuous enhancement of their sensitivity and their lower detection limit. Together with the recent investigations in localized SPR phenomena, extraordinary optical transmission through nanoapertures in metals, and surface‐enhanced spectroscopies, drastic developments are expected to revolutionize the field of optical biosensing. Sensitivity‐enhancement (SE) techniques are reviewed focusing both on the physical transduction mechanisms and the system performance. In the majority of cases the SE is associated with the enhancement of the electromagnetic field overlap integral describing the interaction energy within the analyte. Other important mechanisms are the interaction between plasmons and excitons and between the analyte molecules and the metal surface. The lower detection limit can be reduced significantly if systems with high signal‐to‐noise ratio are used such as common‐path interferometry, ellipsometry or polarimetry systems.     

酞菁铜/金属薄膜界面电位与光二次谐波特性分析

采用Kalvin探针和光二次谐波 (SecondHarmonicGeneration ,SHG)方法研究了铜酞菁衍生物 (Coppertetra tert butylPhthalocyanine ,CuttbPc )LB(Langmuir Blodgentt)膜与金属 (Al、Au)界面的空间电荷现象与非线性光学效应 .检测到空间电荷感应电场 (SpaceChargeInducedElectricField ,SCIEF)形成的表面电位与金属功函数有关 ,并随膜厚变化趋于饱和值 .尽管酞菁分子为中心对称体系 ,但仍有SHG效应 ,并观察到CuttbPc/Al样品在 12 6 0nm附近有异常增强的SH信号 ,而CuttbPc/Au样品未见该峰 .根据样品结构的物理模型 ,运用电磁场理论分析了界面电介质的非线性极化特性和光学效应产生机制 ,初步认为CuttbPc/Al的SH增强峰源于SCIEF形成的较强表面电位 ,说明SH信号的产生与界面静电现象有密切关系 .     

Use of a single-mode fiber coupler for second-harmonic-generation microscopy

We present a compact second-harmonic-generation (SHG) microscope based on a three-port single-mode fiber coupler. The fiber coupler is used to deliver a near-infrared ultrashort-pulsed laser beam as well as to collect the SHG signal in the visible wavelength range. The SHG microscope exhibits an axial resolution of 1.8 microm, which shows a slight enhancement of the optical sectioning effect compared with that for two-photon excitation at the same excitation wavelength. It is also demonstrated that SHG and two-photon fluorescence images under parallel and perpendicular laser excitation polarization can be simultaneously obtained.     

Enhanced light transmission through a single subwavelength aperture

The optical transmission through a subwavelength aperture in a metal film is strongly enhanced when the incident light is resonant with surface plasmons at the corrugated metal surface surrounding the aperture. Conversely, the aperture acts as a novel probe of the surface plasmons, yielding useful insights for optimizing the transmission enhancement. For the optimal corrugation geometry, a set of concentric circular grooves, three times more light is transmitted through the central subwavelength aperture than directly impinges upon it. This effect is useful in the fabrication of near-field optical devices with extremely high optical throughput.     

The CO + NH3 coadsorption over Re(0001) studied by optical second harmonic generation

The coadsorption of CO and NH₃ on Re(0001) was investigated utilizing the optical second harmonic generation (SHG) technique. The results were compared to temperature programmed desorption (TPD) and work function change measurements (Δ). It was found that the enhancement of the SHG due to ammonia is quenched very efficiently in the presence of small CO coverages. A quenching cross section by CO of the second harmonic response of the metal substrate due to ammonia adsorbates, is defined to be 100 ± 10 Ų. This extreme sensitivity of the SHG signal to the presence of CO adsorbed with NH₃ is in sharp contrast to both Δ and TPD measurements. These results present only a minor effect of CO on the Δ and TPD of ammonia on Re(0001). It is concluded that unlike the local contribution to the SHG signal generally assumed for separate adsorbates, the quenching by CO is a nonlocal effect. Moreover, the electronic interactions which govern the SHG from the NH₃-rhenium system, are very different from the interactions which lead to the TPD and Δ data. At ammonia preadsorption on the surface, the effect of the same amount of coadsorbed CO is largely reduced. This observation is discussed in terms of possible islanding mechanisms driven by the postadsorbed CO.     

Localization of Surface Plasmon Waves in Hybrid Photodetectors with Subwavelength Metallic Arrays

The spectral characteristics of the hole photocurrent in plasmon photodetectors based on Ge/Si heterostructures with Ge quantum dots combined with regular arrays of subwavelength apertures of various shapes in a gold film on the semiconductor surface are investigated. Dispersion relations characterizing the propagation of surface plasmon waves along the metal–semiconductor interface are determined from the dependences of the photocurrent on the angle of incidence of light. It is established that the plasmonic enhancement of the photocurrent in rectangular aperture array is suppressed as compared to that in circular and square aperture arrays. It is found that, in hybrid structures with rectangular apertures, there exists a range of wave vectors where the energy of surface plasmons is independent of the wave vector of incident radiation. The results are explained by the excitation of dipole modes localized at rectangular apertures with a large aspect ratio by light waves.     

Efficient second-harmonic generation in nonlinear plasmonic waveguide

We theoretically studied a nonlinear optical process in a hybrid plasmonic waveguide composed of a nonlinear dielectric waveguide and a metal film with a separation of a thin air gap. Owing to the hybridization effect of guided mode and surface plasmon polariton mode, this particular waveguide is able to confine the optical-field in a deep subwavelength scale together with low propagation loss. Based on this, efficient second-harmonic generations (SHG) were revealed at the fundamental wavelength of λ=1.55?μm with good field confinement. The SHG efficiency, as well as the coupling coefficient and mode area, were analyzed and discussed in detail with respect to the structural parameters.     

Enhanced infrared ATR spectra of surface layers using metal films

Infrared ATR spectra of solid samples were measured with a thin metal (Ag, Ni, Pd or Pt) film, which was placed between a KRS-5 hemicylinder and the sample. Polymer films which made good optical contact with the KRS-5 or the metal film were used as samples. Enhanced spectra were obtained with the metal films, compared with the ordinary ATR spectra of the same samples. The enhancement factor was especially large for the bands ascribed to the vibrations of the surface layer or surface species. The observed spectra with the Ni film were stronger than those with the Ag film. The dependences of the intensity on the thickness of the metal film, on the angle of incidence and on the polarization of light were studied. The mechanism of the enhancement and the application of the technique to studies of surface species are discussed.     

Enhancement of the second harmonic generation from monolayer WS2 coupled with a silica microsphere

Monolayer transition metal dichalcogenides (TMDs) are widely used for integrated optical and photoelectric devices. Owing to their broken inversion symmetry, monolayer TMDs have a large second-order optical nonlinearity. However, the optical second-order nonlinear conversion efficiency of monolayer TMDs is still limited by the interaction length. In this work, we theoretically study the second harmonic generation (SHG) from monolayer tungsten sulfide (WS₂) enhanced by a silica microsphere cavity. By tuning the position, size, and crystal orientation of the material, second-order nonlinear coupling can occur between the fundamental pump mode and different second harmonic cavity modes, and we obtain an optimal SHG conversion efficiency with orders of magnitude enhancement. Our work demonstrates that the microsphere cavity can significantly enhance SHG from monolayer 2D materials under flexible conditions.