Subwavelength Source Research Articles

Negative Transient Flux in the Near Field of a Subwavelength Source

The emission of waves by a source is often analyzed in the Fourier domain, which hampers the discovery of transient wave phenomena. Based on a fully temporal analysis, the authors observe in acoustic experiments that the energy can flow backward for short durations in the near field of a deep-subwavelength source in a homogenous medium. Through an impedance analysis, this negative transient flux phenomenon is ascribed to the geometry of the outgoing field. This finding is generic and may find applications in emission and scattering scenarios and could be useful for designing time-varying media.

Design of spaser-like nanoparticle systems for nonclassical optical states generation

The quantum features of photon generation in multiparticle spaser systems consisting of metal nanoparticles and semiconductor quantum dots are investigated. We found the conditions for stationary regime of single-photon generation in spaser systems. In particular, in order to achieve a balance between gain and loss in the system, we determined the chemical and geometric characteristics of nanoparticles and quantum dots by using the optimization algorithm. The strong dipole-dipole interaction between nanoparticles in the spaser system is studied. We show that these conditions can lead to the violation of Cauchy-Schwartz inequality and appearance of nonclassical correlations in the system. The generation of entangled photons in a three-particle spaser system with nonlinear plasmon-exciton interaction is theoretically demonstrated. The introduction of an additional degree of freedom in the form of an external magnetic field leads to a change in the energy levels of quantum dots and provides the possibility to control the quantum properties of generated photons. We proposed the model of an effective subwavelength source of entangled photons controlled by an external magnetic field.

Formation of Nonclassical Optical States in Spaser Systems Controlled by an External Magnetic Field

The quantum statistical and correlation properties of photons generated in a multiparticle spaser are investigated. A model of an effective subwavelength source of entangled photons controlled by an external magnetic field is proposed.

Time-reversing a monochromatic subwavelength optical focus by optical phase conjugation of multiply-scattered light

Due to its time-reversal nature, optical phase conjugation generates a monochromatic light wave which retraces its propagation paths. Here, we demonstrate the regeneration of a subwavelength optical focus by phase conjugation. Monochromatic light from a subwavelength source is scattered by random nanoparticles, and the scattered light is phase conjugated at the far-field region by coupling its wavefront into a single-mode optical reflector using a spatial light modulator. Then the conjugated beam retraces its propagation paths and forms a refocus on the source at the subwavelength scale. This is the first direct experimental realisation of subwavelength focusing beyond the diffraction limit with far-field time reversal in the optical domain.

Near-Infrared Spectroscopic Cathodoluminescence Imaging Polarimetry on Silicon Photonic Crystal Waveguides

We measure polarization- and wavelength-resolved spectra and spatial emission intensity distributions from silicon photonic crystal waveguides in the near-infrared spectral range using spectroscopic cathodoluminescence imaging polarimetry. A 30 keV electron beam, incident along the surface normal of the sample, acts as an ultrabroadband and deeply subwavelength excitation source. For photonic crystal waveguides with a broad range of design parameters, we observe a dominant emission intensity distribution that is strongly confined to the waveguide. For a period of 420 nm and a hole radius of 120 nm, this occurs at a wavelength of 1425 nm. The polarization-resolved measurements demonstrate that this feature is fully linearly polarized along the waveguide axis. Comparing the modal pattern and polarization to calculations of the electric field profiles confirms that we measure the odd TE waveguide mode of the system. This result demonstrates that the electron beam can couple to modes dominated by in-plane field components in addition to the more commonly observed modes dominated by out-of-plane field components. From the emission directionality, we conclude that we sample a leaky portion of the odd waveguide mode.

Acoustic beam control in biomimetic projector via velocity gradient

A biomimetic projector (BioP) based on computerized tomography of pygmy sperm whale's biosonar system has been designed using gradient-index (GRIN) material. The directivity of this BioP device was investigated as function of frequency and the velocity gradient of the GRIN material. A strong beam control over a broad bandwidth at the subwavelength scale has been achieved. Compared with a bare subwavelength source, the main lobe pressure of the BioP is about five times as high and the angular resolution is one order of magnitude better. Our results indicate that this BioP has excellent application potential in miniaturized underwater sonars.

Coherence properties of the electric field generated by an incoherent source of currents distributed on the surface of a sphere

We derive analytical expressions of the cross-spectral density of the electric field arising from an incoherent source whose current density is located on the surface of a sphere. Our approach is based on the series expansion in terms of vector spherical harmonics of the electric field generated by the aforementioned current distribution. We analyze in detail the spectrum, the degree of coherence, and the degree of polarization of the electric field for all regions in space (from the near field to the far field). The relationship of the high-order harmonics to the coherence properties is discussed. The spectrum turns out to be isotropic and it is different from that of the source. We found that the degree of coherence and degree of polarization are strongly influenced by the size of the source. We show the appearance of special features: a zone with a high degree of coherence in the near field for a subwavelength source, the radial degree of coherence is nearly constant in an extended region where two radial points belong to the far field, and a particular radial distance for which the degree of polarization vanishes (3D unpolarized light).

Far-field subwavelength imaging with near-field resonant metalens scanning at microwave frequencies.

A method for far-field subwavelength imaging at microwave frequencies using near-field resonant metalens scanning is proposed. The resonant metalens is composed of switchable split-ring resonators (SRRs). The on-SRR has a strong magnetic coupling ability and can convert evanescent waves into propagating waves using the localized resonant modes. In contrast, the off-SRR cannot achieve an effective conversion. By changing the switch status of each cell, we can obtain position information regarding the subwavelength source targets from the far field. Because the spatial response and Green’s function do not need to be measured and evaluated and only a narrow frequency band is required for the entire imaging process, this method is convenient and adaptable to various environment. This method can be used for many applications, such as subwavelength imaging, detection, and electromagnetic monitoring, in both free space and complex environments.

A biomimetic projector with high subwavelength directivity based on dolphin biosonar

Based on computed tomography of a Yangtze finless porpoise's biosonar system, a biomimetic structure was designed to include air cavity, gradient-index material, and steel outer-structure mimicking air sacs, melon, and skull, respectively. The mainlobe pressure was about three times higher, the angular resolution was one order of magnitude higher, and the effective source size was orders of magnitude larger than those of the subwavelength source without the biomimetic structure. The superior subwavelength directivity over a broad bandwidth suggests potential applications of this biomimetic projector in underwater sonar, medical ultrasonography, and other related applications.

Evanescent wave amplification and subwavelength imaging by ultrathin uniaxial μ-near-zero material

We demonstrate strong evanescent wave amplification by a thin slab of uniaxial μ-near-zero (UMNZ) material. It is found that while retaining the same amplification effect, the slab can be made arbitrarily thin when the negative permeability along the axis of anisotropy approaches zero. Numerical results show that using a single layer of split-ring resonators (SRRs) with its thickness equal three thousandth of the incident wavelength (λ/3000), a subwavelength source distribution with λ/4 resolution can be transferred to a distance of λ/3.

A Sub-Wavelength RF Source Tracking System for GPS-Denied Environments

A sub-wavelength source tracking system utilizing highly miniaturized antennas in the HF range for applications in GPS-denied environments including indoor and urban scenarios is proposed. A technique that combines a high resolution direction finding and radio triangulation utilizing a compact transmit (Tx) and receive (Rx) antenna system is pursued. Numerical models are used to investigate wave propagation and scattering in complex indoor scenarios as a function of frequency. We choose HF band to minimize attenuation through walls and multipath in indoor environments. In order to achieve a compact system, a low-profile and highly miniaturized antenna ( λ/300 height and λ/100 lateral dimensions at 20 MHz) having omnidirectional, vertically polarized field is designed. At such low frequencies, accurate measurement of the phase difference between the signals at the Rx antennas having very small baseline is challenging. To address this issue, a biomimetic circuit that mimics the hearing mechanism of a fly (Ormia Ochracea) is utilized. With this circuit, very small phase differences are amplified to measurable values. The numerical simulations are used to analyze direction of arrival retrieval and source localization in highly cluttered environments. A compact system prototype is also realized and source tracking in complex indoor scenarios is successfully demonstrated.

Exact Reconstruction of THz Sub-$\lambda$ Source Features in Knife-Edge Measurements

The spatial features of a subwavelength terahertz source are not accessible using time-integrated knife-edge (KE) techniques due to the nonseparable space-time nature of the radiated field and to systematic modifications induced by the blade itself. We show that by combining KE with a time-resolved electro-optical sampling, the space-time coupling can be addressed and the source field profile can be exactly reconstructed.

Investigation of image magnification properties of hyperlenses formed by a tapered array of metallic wires using a spatially dispersive finite-difference time-domain method in cylindrical coordinates

In this paper, the spatially dispersive finite-difference time-domain (FDTD) method in cylindrical coordinates is applied to model the hyperlenses formed by a tapered array of metallic wires. The hyperlens device is modeled using the effective medium theory as a frequency and spatially dispersive dielectric. The gradual increase in spacing between wires along the radial direction of the physical structure is represented by the reduction of plasma frequency of the effective medium. Simulation results show that the image transfer and magnification capability of the hyperlens is insensitive to its transverse dimension. A seven-fold magnification of a subwavelength source distribution transferred to a distance of three wavelengths is demonstrated using a hyperlens with high plasma frequency at its front interface.

Chiral Surface Plasmon Polaritons on Metallic Nanowires

Chiral surface plasmon polaritons (SPPs) can be generated by linearly polarized light incident at the end of a nanowire, exciting a coherent superposition of three specific nanowire waveguide modes. Images of chiral SPPs on individual nanowires obtained from quantum dot fluorescence excited by the SPP evanescent field reveal the chirality predicted in our theoretical model. The handedness and spatial extent of the helical periods of the chiral SPPs depend on the input polarization angle and nanowire diameter as well as the dielectric environment. Chirality is preserved in the free-space output wave, making a metallic nanowire a broad bandwidth subwavelength source of circular polarized photons.

Comprehensive modeling of THz microscope with a sub-wavelength source

The sub-wavelength THz emission point on a nonlinear electrooptical crystal, used in broadband THz near-field emission microscopy, is computationally modeled as a radiating aperture of Gaussian intensity profile. This paper comprehensively studies the Gaussian aperture model in the THz near-field regime and validates the findings with dual-axis knife-edge experiments. Based on realistic parameter values, the model allows for THz beam characterisation in the near-field region for potential microscopy applications. An application example is demonstrated by scanning over a cyclic-olefin copolymer sample containing grooves placed sub-wavelengths apart. The nature of THz microscopy in the near-field is highly complex and traditionally based on experiments. The proposed validated numerical model therefore aids in the quantitative understanding of the performance parameters. Whilst in this paper we demonstrate the model on broadband electro-optical THz near-field emission microscopy, the model may apply without a loss of generality to other types of THz near-field focused beam techniques.

THz Generation by Optical Rectification and Competition with Other Nonlinear Processes

We present a study of the competition between tera-hertz (THz) generation by optical rectification in (110) ZnTe crystals, two-photon absorption, second harmonic generation and free-carrier absorption. The two-photon nonlinear absorption coefficient, second harmonic generation efficiency and free-carrier absorption coefficient in the THz range are measured independently The incident pump field is shown to be depleted by two-photon absorption and the THz radiation is shown to be reduced, upon focusing, by free-carrier absorption. The reduction of the generated THz radiation upon tight focusing is explained, provided that one also takes into account diffraction effects from the sub-wavelength THz source.

THz emission Microscopy with sub-wavelength broadband source

A versatile THz/IR near field microscope is demonstrated. Collecting the scattered light from a THz in-situ subwavelength source, this microscope provides images with resolution better than lambda/10. The physical origin of the contrast is explained by a Mie scattering diffraction model. Owing to the classical nature of this microscope working in the near field, resolution of THz/IR images is improved using deconvolution process.

Negative refraction and focusing of ultrasound in two-dimensional phononic crystals

We present experimental demonstrations of negative refraction and focusing of ultrasonic waves in two-dimensional phononic crystals made of stainless steel rods assembled in a triangular lattice and immersed in a liquid. Negative refraction is achieved for the range of frequencies in the second band, where the directions of the wave vector and group velocity are antiparallel to each other due to circular equifrequency contours. Negative refraction is unambiguously observed using a prism-shaped crystal. By exploiting the circular equifrequency contours in the second band, focusing of the ultrasonic field emitted by a pointlike source was demonstrated using a flat phononic crystal filled with and immersed in water. During these experiments, the importance of imaging in the regime of all angle negative refraction (AANR) was established for obtaining high-quality images. The regime of AANR was achieved in a similar flat crystal, in which the liquid inside the crystal (methanol) was different from the outside medium (water). This design resulted in matching circular equifrequency contours at the frequency of $0.55\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, implying that a flat ultrasonic lens with an effective refractive index of $\ensuremath{-}1$ was realized. By imaging a subwavelength line source with this crystal, a resolution of $0.55\ensuremath{\lambda}$ was observed, which is just above the diffraction limit.

Two-dimensional dopant profiling of silicon with submicron resolution using near field optics on silicon/electrolyte contacts

We demonstrate the possibility to use near field optics to perform two-dimensional dopant profiling on silicon surface, with deep submicron spatial resolution. The sample surface is contacted by an aqueous electrolyte giving a reverse biased junction that is illuminated by a subwavelength optical source, in near filed conditions. A staircase calibration structure was used with several boron-doped layers with either 4 μm or 0.4 μm thickness and doping between 10 17 and 10 20 at/cm 3. Measurements were performed on the sample cross section. It is shown that photocurrent surface mapping shows up the doped areas with a lateral resolution better than 100 nm.

Localized terahertz generation via optical rectification in ZnTe

We report the measurement of terahertz (THz) radiation, generated via optical rectification in a 20 µm thick ZnTe crystal, as a function of the size of optical excitation. The result shows that, before the onset of significant higher-order nonlinear processes, the THz emission obtained with a fixed excitation power is largely size independent for excitation sizes smaller than the THz wavelength. This experimental finding is well described by a theoretical model including the generation of THz radiation through optical rectification from a subwavelength source and its propagation into the far field. The characteristic size dependence of the radiation from a subwavelength THz source is advantageous for use in apertureless near-field microscopy.