Showing papers in "Laser & Photonics Reviews in 2015"
TL;DR: In this paper, a uniform array of silicon nanodisks can exhibit close-to-unity transmission at resonance in the visible spectrum, and a single-layer gradient metasurface utilizing this concept is shown to achieve around 45% transmission into the desired order.
Abstract: Recently, metasurfaces have received increasing attention due to their ability to locally manipulate the amplitude, phase and polarization of light with high spatial resolution. Transmissive metasurfaces based on high-index dielectric materials are particularly interesting due to the low intrinsic losses and compatibility with standard industrial processes. Here, it is demonstrated numerically and experimentally that a uniform array of silicon nanodisks can exhibit close-to-unity transmission at resonance in the visible spectrum. A single-layer gradient metasurface utilizing this concept is shown to achieve around 45% transmission into the desired order. These values represent an improvement over existing state-of-the-art, and are the result of simultaneous excitation and mutual interference of magnetic and electric-dipole resonances in the nanodisks, which enables directional forward scattering with a broad bandwidth. Due to CMOS compatibility and the relative ease of fabrication, this approach is promising for creation of novel flat optical devices.
599 citations
TL;DR: Sub-wavelength structures with a subwavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves as discussed by the authors, and their applications include anti-reflective coatings, polarization rotators, high-efficiency fiber-chip cou-plers, spectrometers, highreflectivity mirrors, athermal waveg- uides, multimode interference couplers.
Abstract: Periodic structures with a sub-wavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves. While the use of these structures in waveguide optics was proposed in the 1990s, it has been with the more recent developments of silicon photonics and high-precision lithography techniques that sub-wavelength structures have found widespread application in the field of pho- tonics. This review first provides an introduction to the physics of sub-wavelength structures. An overview of the applications of sub-wavelength structures is then given including: anti-reflective coatings, polarization rotators, high-efficiency fiber-chip cou- plers, spectrometers, high-reflectivity mirrors, athermal waveg- uides, multimode interference couplers, and dispersion engi- neered, ultra-broadband waveguide couplers among others. Particular attention is paid to providing insight into the design strategies for these devices. The concluding remarks provide an outlook on the future development of sub-wavelength structures and their impact in photonics.
496 citations
TL;DR: In this article, the most interesting properties of photonic metasurfaces are reviewed, demonstrating their useful functionalities such as frequency selectivity, wavefront shaping, polarization control, etc.
Abstract: Optical metasurfaces are thin-layer subwavelength-patterned structures that interact strongly with light. Metasurfaces have become the subject of several rapidly growing areas of research, being a logical extension of the field of metamaterials towards their practical applications. Metasurfaces demonstrate many useful properties of metadevices with engineered resonant electric and magnetic optical responses combined with low losses of thin-layer structures. Here we introduce the basic concepts of this rapidly growing research field that stem from earlier studies of frequency-selective surfaces in radiophysics, being enriched by the recent development of metamaterials and subwavelength nanophotonics. We review the most interesting properties of photonic metasurfaces, demonstrating their useful functionalities such as frequency selectivity, wavefront shaping, polarization control, etc. We discuss the ways to achieve tunability of metasurfaces and also demonstrate that nonlinear effects can be enhanced with the help of metasurface engineering.
444 citations
TL;DR: In this article, a special plasmonic waveguide composed of two ultrathin corrugated metallic strips on top and bottom surfaces of a dielectric substrate with mirror symmetry is presented, which is easy to integrate with the amplifier.
Abstract: Efficient amplification of spoof surface plasmon polaritons (SPPs) is proposed at microwave frequencies by using a subwavelength-scale amplifier For this purpose, a special plasmonic waveguide composed of two ultrathin corrugated metallic strips on top and bottom surfaces of a dielectric substrate with mirror symmetry is presented, which is easy to integrate with the amplifier It is shown that spoof SPPs are able to propagate on the plasmonic waveguide in broadband with low loss and strong subwavelength effect By loading a low-noise amplifier chip produced by the semiconductor technology, the first experiment is demonstrated to amplify spoof SPPs at microwave frequencies (from 6 to 20GHz) with high gain (around 20dB), which can be directly used as a SPP amplifier device The features of strong field confinement, high efficiency, broadband operation, and significant amplification of the spoof SPPs may advance a big step towards other active SPP components and integrated circuits
209 citations
TL;DR: The femtosecond laser direct-writing of waveguide circuits in glasses is among the few integrated optical platforms to have produced individually every component required for scalable quantum computation.
Abstract: The femtosecond laser direct-writing (FLDW) of waveguide circuits in glasses has seen interest from a number of fields over the previous 20 years. It has evolved from a curiosity to a viable platform for the rapid prototyping of small scale circuits. The field of quantum information science has exploited this capability and in the process advanced the fabrication technique. In this review the technological aspects of the laser inscription method relevant to quantum information science will be discussed. A range of demonstrations which have been enabled by laser written circuits will be outlined; these include novel circuits, simulations, photon sources and detection. This places the FLDW technique among the few integrated optical platforms to have produced individually every component required for scalable quantum computation.
195 citations
TL;DR: In this paper, an ultrabroadband superoscillatory lens (UBSOL) is proposed and realized by utilizing the metasurface-assisted law of refraction and reflection in arrayed nanorectangular apertures with variant orientations.
Abstract: Conventional optics is diffraction limited due to the cutoff of spatial frequency components, and evanescent waves allow subdiffraction optics at the cost of complex near-field manipulation. Recently, optical superoscillatory phenomena were employed to realize superresolution lenses in the far field, but suffering from very narrow working wavelength band due to the fragility of the superoscillatory light field. Here, an ultrabroadband superoscillatory lens (UBSOL) is proposed and realized by utilizing the metasurface-assisted law of refraction and reflection in arrayed nanorectangular apertures with variant orientations. The ultrabroadband feature mainly arises from the nearly dispersionless phase profile of transmitted light through the UBSOL for opposite circulation polarization with respect to the incident light. It is demonstrated in experiments that subdiffraction light focusing behavior holds well with nearly unchanged focal patterns for wavelengths spanning across visible and near-infrared light. This method is believed to find promising applications in superresolution microscopes or telescopes, high-density optical data storage, etc.
192 citations
TL;DR: In this article, the authors discuss challenges, opportunities, and early demonstrations of nanophotonic EOMs attempting to address the weak light-matter interaction (LMI) challenge, and the early benchmarks suggest that densely integrated high-performance photonic integrated circuits.
Abstract: As electronic device feature sizes scale-down, the power consumed due to onchip communications as compared to computations will increase dramatically; likewise, the available bandwidth per computational operation will continue to decrease. Integrated photonics can offer savings in power and potential increase in bandwidth for onchip networks. Classical diffraction-limited photonics currently utilized in photonic integrated circuits (PIC) is characterized by bulky and inefficient devices compared to their electronic counterparts due to weak light–matter interactions (LMI). Performance critical for the PIC is electro-optic modulators (EOM), whose performances depend inherently on enhancing LMIs. Current EOMs based on diffraction-limited optical modes often deploy ring resonators and are consequently bulky, photon-lifetime modulation limited, and power inefficient due to large electrical capacitances and thermal tuning requirements. In contrast, wavelength-scale EOMs are potentially able to surpass fundamental restrictions set by classical (i.e. diffraction-limited) devices via (a) high-index modulating materials, (b) nonresonant field and density-of-states enhancements such as found in metal optics, and (c) synergistic onchip integration schemes. This manuscript discusses challenges, opportunities, and early demonstrations of nanophotonic EOMs attempting to address this LMI challenge, and early benchmarks suggest that nanophotonic building blocks allow for densely integrated high-performance photonic integrated circuits.
190 citations
TL;DR: In this article, the authors discuss the conceptual schemes and experimental tools that can be employed for the generation, amplification, control, and combination of separate light pulses, and the main techniques for the spectrotemporal characterization of the synthesized fields are also described.
Abstract: The generation of sub-optical-cycle, carrier–envelope phase-stable light pulses is one of the frontiers of ultrafast optics. The two key ingredients for sub-cycle pulse generation are bandwidths substantially exceeding one octave and accurate control of the spectral phase. These requirements are very challenging to satisfy with a single laser beam, and thus intense research activity is currently devoted to the coherent synthesis of pulses generated by separate sources. In this review we discuss the conceptual schemes and experimental tools that can be employed for the generation, amplification, control, and combination of separate light pulses. The main techniques for the spectrotemporal characterization of the synthesized fields are also described. We discuss recent implementations of coherent waveform synthesis: from the first demonstration of a single-cycle optical pulse by the addition of two pulse trains derived from a fiber laser, to the coherent combination of the outputs from optical parametric chirped-pulse amplifiers.
179 citations
TL;DR: The state of the art in numerical methods for nanophotonics is reviewed and which method is the best suited for specific problems is described, such as those involving light propagation, localization, scattering, or multiphysical systems.
Abstract: Nanoscale photonic systems involve a broad variety of light-matter interaction regimes beyond the diffraction limit and have opened the path for a variety of application opportunities in sensing, solid-state lighting, light harvesting, and optical signal processing. The need for numerical modeling is central for the understanding, control, and design of plasmonic and photonic nanostructures. Recently, the increasing sophistication of nanophotonic systems and processes, ranging from simple plasmonic nanostructures to multiscale and complex photonic devices, has been calling for highly efficient numerical simulation tools. This article reviews the state of the art in numerical methods for nanophotonics and describes which method is the best suited for specific problems. The widespread approaches derived from classical electrodynamics such as finite differences in time domain, finite elements, surface integral, volume integral, and hybrid methods are reviewed and illustrated by application examples. Their potential for efficient simulation of nanophotonic systems, such as those involving light propagation, localization, scattering, or multiphysical systems is assessed. The numerical modeling of complex systems including nonlinearity, nonlocal and quantum effects as well [GRAPHICS] as new materials such as graphene is discussed in the perspective of actual and future challenges for computational nanophotonics.
141 citations
TL;DR: In this paper, high refractive-index dielectric nanoparticles support not only electric but also strong magnetic resonances, which are accessible under plane-wave illumination and can be selectively addressed and studied while not permitting excitation of other particle modes.
Abstract: High refractive‐index dielectric nanoparticles support not only electric but also strong magnetic resonances, which are accessible under plane‐wave illumination. Here, it is shown that by tailoring the excitation field, individual multipole resonances can be selectively addressed and studied while not permitting excitation of other particle modes.
136 citations
TL;DR: In this paper, a light confinement mechanism is proposed based on the photonic bound state in the continuum (BIC), which can induce strong coupling between photons and the nitrogen-vacancy center in diamond.
Abstract: Light is usually confined in photonic structures with a band gap or relatively high refractive index for broad scientific and technical applications. Here, a light confinement mechanism is proposed based on the photonic bound state in the continuum (BIC). In a low-refractive-index waveguide on a high-refractive-index thin membrane, optical dissipation is forbidden because of the destructive interference of various leakage channels. The BIC-based low-mode-area waveguide and high-Q microresonator can be used to enhance light–matter interaction for laser, nonlinear optical and quantum optical applications. For example, a polymer structure on a diamond membrane shows excellent optical performance that can be achieved with large fabrication tolerance. It can induce strong coupling between photons and the nitrogen–vacancy center in diamond for scalable quantum information processors and networks.
TL;DR: In this article, a simple and ultracompact photonic-crystal structure, where a waveguide is coupled to a single nanocavity, is proposed and experimentally demonstrated, showing very efficient optical diode functionality.
Abstract: Nanostructures that feature nonreciprocal light transmission are highly desirable building blocks for realizing photonic integrated circuits. Here, a simple and ultracompact photonic-crystal structure, where a waveguide is coupled to a single nanocavity, is proposed and experimentally demonstrated, showing very efficient optical diode functionality. The key novelty of the structure is the use of cavity-enhanced material nonlinearities in combination with spatial symmetry breaking and a Fano resonance to realize nonreciprocal propagation effects at ultralow power and with good wavelength tunability. The nonlinearity of the device relies on ultrafast carrier dynamics, rather than the thermal effects usually considered, allowing the demonstration of nonreciprocal operation at a bit-rate of 10 Gbit s−1 with a low energy consumption of 4.5 fJ bit−1.
TL;DR: In this paper, a 64-channel hybrid demultiplexer based on silicon-on-insulator nanowires is proposed and demonstrated experimentally to enable wavelength-division-multiplexing and mode-division multiplexing simultaneously in order to realize an ultra-large capacity on-chip optical-interconnect link.
Abstract: O R IG IN A L P A P ER Abstract A compact 64-channel hybrid demultiplexer based on silicon-on-insulator nanowires is proposed and demonstrated experimentally to enable wavelength-division-multiplexing and mode-division-multiplexing simultaneously in order to realize an ultra-large capacity on-chip optical-interconnect link. The present hybrid demultiplexer consists of a 4-channel mode multiplexer constructed with cascaded asymmetrical directionalcouplers and two bi-directional 17 × 17 arrayed-waveguide gratings (AWGs) with 16 channels. Here each bi-directional AWG is equivalent as two identical 1 × 16 AWGs. The measured excess loss and the crosstalk for the monolithically integrated 64-channel hybrid demultiplexer are about -5 dB and -14 dB, respectively. Better performance can be achieved by minimizing the imperfections (particularly in AWGs) during the fabrication processes.
TL;DR: In this article, the degree of conversion and refractive index of the microstructures created by two-photon polymerization (TPP) were analyzed via Raman microspectroscopy and measured with the interferometric technique employing a Michelson interferometer.
Abstract: We report a study of the determination of polymer cross-linking, namely the degree of conversion and refractive index of the microstructures created by two-photon polymerization (TPP). The influence of TPP processing parameters such as laser intensity and scanning velocity is investigated. The degree of conversion is analyzed via Raman microspectroscopy and the refractive index is measured with the interferometric technique employing a Michelson interferometer. Moreover, the relationship between these two properties is revealed and details are discussed. The largest refractive index change that we have obtained is of the order of 10−2. Finally, we propose and demonstrate experimentally the realization of the gradient-index (GRIN) structure, resulting from a laser-induced local refractive index modification due to monomer cross-linking, i.e. degree of conversion. This work implies that the TPP technique is a valuable tool for the fabrication of GRIN microoptics for (in)homogeneous molding of light flow at the micrometer scale.
TL;DR: Subwavelength features in conjunction with light-guiding structures have gained significant interest in recent decades due to their wide range of applications to particle and atom trapping as discussed by the authors, and the focus of particle trapping has shifted from the microscale to the nanoscale.
Abstract: Subwavelength features in conjunction with light-guiding structures have gained significant interest in recent decades due to their wide range of applications to particle and atom trapping. Lately, the focus of particle trapping has shifted from the microscale to the nanoscale. This few orders of magnitude change is driven, in part, by the needs of life scientists who wish to better manipulate smaller biological samples. Devices with subwavelength features are excellent platforms for shaping local electric fields for this purpose. A major factor that inhibits the manipulation of submicrometer particles is the diffraction-limited spot size of free-space laser beams. As a result, technologies that can circumvent this limit are highly desirable. This review covers some of the more significant advances in the field, from the earliest attempts at trapping using focused Gaussian beams, to more sophisticated hybrid plasmonic/metamaterial structures. In particular, examples of emerging optical trapping configurations are presented.
TL;DR: In this paper, a nonlinear optical platform based on silicon-rich nitride, deposited at a low temperature of 250°C compatible with back-end CMOS processing, is presented.
Abstract: CMOS-compatible nonlinear optics platforms with negligible nonlinear losses and high nonlinearity are of great merit. Silicon, silicon nitride and Hydex glass have made significant headway in nonlinear optical signal processing, though none of these platforms possesses the highly sought after combination of high nonlinearity and negligible nonlinear losses. In this manuscript, we present a nonlinear optics platform based on silicon-rich nitride, deposited at a low temperature of 250°C compatible with back-end CMOS processing. The silicon-rich nitride is designed and engineered in composition to have a bandgap of 2.05 eV, such that the two-photon absorption edge is well below 1.55 μm. The designed and developed waveguides have a nonlinear parameter of 550 W−1/m, 500 times larger than that in silicon nitride waveguides, while at the same time not possessing two-photon and free-carrier losses. Using 500-fs pulses, we generate supercontinuum exceeding 0.6 of an octave.
TL;DR: In this article, strong two-photon absorption (TPA) in monolayer MoS2 is demonstrated in contrast to saturable absorption (SA) under the excitation of femtosecond laser pulses in the near infrared region.
Abstract: Strong two-photon absorption (TPA) in monolayer MoS2 is demonstrated in contrast to saturable absorption (SA) in multilayer MoS2 under the excitation of femtosecond laser pulses in the near-infrared region. MoS2 in the forms of monolayer single crystal and multilayer triangular islands are grown on either quartz or SiO2/Si by employing the seeding method through chemical vapor deposition. The nonlinear transmission measurements reveal that monolayer MoS2 possesses a nonsaturation TPA coefficient as high as ∼(7.62 ±0.15) ×103 cm/GW, larger than that of conventional semiconductors by a factor of 103. As a result of TPA, two-photon pumped frequency upconverted luminescence is observed directly in the monolayer MoS2. For the multilayer MoS2, the SA response is demonstrated with the ratio of the excited-state absorption cross section to ground-state cross section of ∼0.18. In addition, the laser damage threshold of the monolayer MoS2 is ∼97 GW/cm2, larger than that of the multilayer MoS2 of ∼78 GW/cm2.
TL;DR: Simulations indicate that PIAs could reach data rate x reach products in excess of 14,000 Tb/s × km in realistic wavelength-division multiplexed long-haul networks, and technical challenges remaining to be addressed in order for fiber OPAs to become useful for long- haul communication networks are discussed.
Abstract: The prospects for using fiber optical parametric amplifiers (OPAs) in optical communication systems are reviewed. Phase-insensitive amplifiers (PIAs) and phase-sensitive amplifiers (PSAs) are considered. Low-penalty amplification at/or near 1 Tb/s has been achieved, for both wavelength- and time-division multiplexed formats. High-quality mid-span spectral inversion has been demonstrated at 0.64 Tb/s, avoiding electronic dispersion compensation. All-optical amplitude regeneration of amplitude-modulated signals has been performed, while PSAs have been used to demonstrate phase regeneration of phase-modulated signals. A PSA with 1.1-dB noise figure has been demonstrated, and preliminary wavelength-division multiplexing experiments have been performed with PSAs. 512 Gb/s have been transmitted over 6,000 km by periodic phase conjugation. Simulations indicate that PIAs could reach data rate x reach products in excess of 14,000 Tb/s × km in realistic wavelength-division multiplexed long-haul networks. Technical challenges remaining to be addressed in order for fiber OPAs to become useful for long-haul communication networks are discussed.
TL;DR: In this article, the broadband enhancement of single-photon emission from nitrogen-vacancy centers in nanodiamonds coupled to a planar multilayer metamaterial with hyperbolic dispersion is studied experimentally.
Abstract: The broadband enhancement of single-photon emission from nitrogen-vacancy centers in nanodiamonds coupled to a planar multilayer metamaterial with hyperbolic dispersion is studied experimentally. The metamaterial is fabricated as an epitaxial metal/dielectric superlattice consisting of CMOS-compatible ceramics: titanium nitride (TiN) and aluminum scandium nitride (AlxSc1-xN). It is demonstrated that employing the metamaterial results in significant enhancement of collected single-photon emission and reduction of the excited-state lifetime. Our results could have an impact on future CMOS-compatible integrated quantum sources.
TL;DR: In this article, a scheme incorporating dual coupled microresonators through which mode interactions are intentionally introduced and controlled for Kerr frequency comb (microcomb) generation in the normal dispersion region is presented.
Abstract: We demonstrate a scheme incorporating dual coupled microresonators through which mode interactions are intentionally introduced and controlled for Kerr frequency comb (microcomb) generation in the normal dispersion region. Microcomb generation, repetition rate selection, and mode locking are achieved with coupled silicon nitride microrings controlled via an on-chip microheater. Our results show for the first time that mode interactions can be programmably tuned to facilitate broadband normal-dispersion microcombs. The proposed scheme increases freedom in microresonator design and may make it possible to generate microcombs in an extended wavelength range (e.g., in the visible) where normal material dispersion is likely to dominate.
TL;DR: In this paper, the authors investigated the Mie scattering of spherical nanoparticles through a complete multipole expansion method which has incorporated radiating toroidal moments and showed that toroidal dipoles, which are negligible under long-wavelength approximations, can be excited within high permittivity dielectric particles and influence effectively the dipolar scattering in the optical regime.
Abstract: We investigate the Mie scattering of spherical nanoparticles through a complete multipole expansion method which has incorporated radiating toroidal moments. It is shown that toroidal dipoles, which are though negligible under long-wavelength approximations, can be excited within high permittivity dielectric particles and influence effectively the dipolar scattering in the optical regime. We further reveal that the scattering transparencies of core-shell plasmonic nanoparticles can be classified into two categories: the trivial transparency with no effective mode excitation within the particle, and the non-trivial one induced by the destructive interferences of electric and toroidal moments excited. The incorporation of toroidal moments offers new insights into the study on nanoparticle scattering in both the near and far fields, which may shed new light to many related applications, such as biosensing, nanoantennas, photovoltaic devices and so on. PACS numbers: 78.67.-n, 42.25.Fx, 73.20.Mf,
TL;DR: In this paper, the authors present a review of optical micro-electro-mechanical systems (MEMS) realized on a silicon chip that is enabling accurate control of the etching depth, the aspect ratio, the verticality and the curvature of the etched surfaces.
Abstract: The integration of microactuators within a silicon photonic chip gave rise to the field of optical micro-electro-mechanical systems (MEMS) that was originally driven by the telecommunication market. Following the latter's bubble collapse in the beginning of the third millennium, new directions of research with considerable momentum appeared focusing on the realization and applications of miniaturized instrumentation in biology, chemistry, physics and materials science. At the heart of these applications light interferometry is a key optical phenomenon, in which miniaturized scanning interferometers are the manipulating optical devices. Monolithic free-space optical interferometers realized on a silicon chip take advantage of the recent progress in the microfabrication technology that is enabling accurate control of the etching depth, the aspect ratio, the verticality and the curvature of the etched surfaces. The fabrication technology, the library of micro-optical and mechanical components, the realized architectures and their characterization are described in detail in this review, followed by a discussion of the foreseen challenges.
TL;DR: In this article, a topologically protected optical mode was successfully demonstrated in a plasmonic waveguide array with a kinked interface that exhibits a robust nonspreading feature.
Abstract: P A P ER Abstract Recent realization of nontrivial topological phases in photonic systems has provided unprecedented opportunities in steering light flow in novel manners. Based on the Su– Schriffer–Heeger (SSH) model, a topologically protected optical mode was successfully demonstrated in a plasmonic waveguide array with a kinked interface that exhibits a robust nonspreading feature. However, under the same excitation conditions, another antikinked structure seemingly cannot support such a topological interface mode, which appears to be inconsistent with the SSH model. Theoretical calculations are carried out based on the coupled-mode theory, in which the mode properties, excitation conditions, and the robustness are studied in detail. It is revealed that under the exact eigenstate excitations, both kinked and antikinked structures do support such robust topological interface modes; however, for a realistic single-waveguide input only the kinked structure does so. It is concluded that the symmetry of interface eigenmodes plays a crucial role, and the odd eigenmode in a kinked structure offers the capacity to excite the nonspreading interface mode in the realistic excitation of a one-waveguide input. Our finding deepens the understanding of mode excitation and propagation in coupled waveguide systems, and could open a new avenue in optical simulations and photonic designs.
TL;DR: In this paper, the waveguiding properties and mode structure of planar anisotropic metamaterial waveguides are characterized experimentally and theoretically, while ordinary modes are the typical guided modes of the highly aisotropic waveguide, extraordinary modes, below the effective plasma frequency, exist in the form of bulk plasmon-polaritons.
Abstract: Hyperbolic metamaterials comprised of an array of plasmonic nanorods provide a unique platform for designing optical sensors and integrating nonlinear and active nanophotonic functionalities. In this work, the waveguiding properties and mode structure of planar anisotropic metamaterial waveguides are characterized experimentally and theoretically. While ordinary modes are the typical guided modes of the highly anisotropic waveguides, extraordinary modes, below the effective plasma frequency, exist in a hyperbolic metamaterial slab in the form of bulk plasmon-polaritons, in analogy to planar-cavity exciton-polaritons in semiconductors. They may have very low or negative group velocity with high effective refractive indices (up to 10) and have an unusual cut-off from the high-frequency side, providing deep-subwavelength (λ0/6–λ0/8 waveguide thickness) single-mode guiding. These properties, dictated by the hyperbolic anisotropy of the metamaterial, may be tuned by altering the geometrical parameters of the nanorod composite.
TL;DR: In this article, a model that includes the multiple-scattering process of the probing tip and the nano-antenna is developed, with use of the reciprocity theorem of electromagnetism.
Abstract: Near-field optical microscopy techniques provide information on the amplitude and phase of local fields in samples of interest in nanooptics. However, the information on the near field is typically obtained by converting it into propagating far fields where the signal is detected. This is the case, for instance, in polarization-resolved scattering-type scanning near-field optical microscopy (s-SNOM), where a sharp dielectric tip scatters the local near field off the antenna to the far field. Up to now, basic models have interpreted S- and P-polarized maps obtained in s-SNOM as directly proportional to the in-plane (Ex or Ey) and out-of-plane (Ez) near-field components of the antenna, respectively, at the position of the probing tip. Here, a novel model that includes the multiple-scattering process of the probing tip and the nanoantenna is developed, with use of the reciprocity theorem of electromagnetism. This novel theoretical framework provides new insights into the interpretation of s-SNOM near-field maps: the model reveals that the fields detected by polarization-resolved interferometric s-SNOM do not correlate with a single component of the local near field, but rather with a complex combination of the different local near-field components at each point (Ex, Ey and Ez). Furthermore, depending on the detection scheme (S- or P-polarization), a different scaling of the scattered fields as a function of the local near-field enhancement is obtained. The theoretical findings are corroborated by s-SNOM experiments which map the near field of linear and gap plasmonic antennas. This new interpretation of nanoantenna s-SNOM maps as a complex-valued combination of vectorial local near fields is crucial to correctly understand scattering-type near-field microscopy measurements as well as to interpret the signals obtained in field-enhanced spectroscopy.
TL;DR: In this article, the fingerprint emissions of molecules in strong laser fields are discussed based on the understanding of strong-field-molecule interactions in atmospheric as well as in vacuum environments.
Abstract: Recent studies have demonstrated that femtosecond laser pulses have high potential in application to environmental science. Because of the properties of ultrafast, broadband and high power, the propagation of femtosecond laser pulses in air can lead to the generation of a strong field of 1013–1014 W/cm2 with a large distance range from meter to kilometers. The strong laser field induces ionization and fragmentation of molecules in the laser propagation path, resulting in characteristic fingerprint emissions. This paper mainly focuses on recent research advances in environmental sensing by using femtosecond laser pulses through strong-field-induced ionization and fragmentation of molecules. The fingerprint emissions of molecules in strong laser fields are discussed based on the understanding of strong-field–molecule interactions in atmospheric as well as in vacuum environments. This is followed by a comprehensive review of several recently developed optical methods for coherent control of fingerprint emissions of molecules. Lastly, both current challenges and a future perspective of this dynamic field are discussed.
TL;DR: In this article, an ultra-fast source of heralded single photons emitted at the wavelength of 1540 nm is reported, where the single photons are heralded at a rate as high as 2.1 MHz with a heralding efficiency of 42%.
Abstract: The realization of an ultra-fast source of heralded single photons emitted at the wavelength of 1540 nm is reported. The presented strategy is based on state-of-the-art telecom technology, combined with off-the-shelf fiber components and waveguide non-linear stages pumped by a 10 GHz repetition rate laser. The single photons are heralded at a rate as high as 2.1 MHz with a heralding efficiency of 42%. Single photon character of the source is inferred by measuring the second-order autocorrelation function. For the highest heralding rate, a value as low as 0.023 is found. This not only proves negligible multi-photon contributions but also represents the best measured value reported to date for heralding rates in the MHz regime. These prime performances, associated with a device-like configuration, are key ingredients for both fast and secure quantum communication protocols.
TL;DR: In this paper, a novel concept of differential absorption lidar based on the Scheimpflug principle is demonstrated by measuring the range-resolved atmospheric oxygen concentration with a tunable continuous-wave narrow-band laser diode emitting around 761 nm over a path of one kilometer during night time.
Abstract: This work proves the feasibility of a novel concept of differential absorption lidar based on the Scheimpflug principle. The range-resolved atmospheric backscattering signal of a laser beam is retrieved by employing a tilted linear sensor with a Newtonian telescope, satisfying the Scheimpflug condition. Infinite focus depth is achieved despite employing a large optical aperture. The concept is demonstrated by measuring the range-resolved atmospheric oxygen concentration with a tunable continuous-wave narrow-band laser diode emitting around 761 nm over a path of one kilometer during night time. Laser power requirements for daytime operation are also investigated and validated with single-band atmospheric aerosol measurements by employing a broad-band 3.2-W laser diode. The results presented in this work show the potential of employing the continuous-wave differential absorption lidar (CW-DIAL) technique for remote profiling of atmospheric gases in daytime if high-power [GRAPHICS] narrow-band continuous-wave light sources were to be employed.