Optical Frequency Region Research Articles

Femtosecond-comb based 10 MHz-to-optical frequency link with uncertainty at the 10−18 level

The International System of Units (SI) definition of the ‘second’ is likely to be redefined in the optical frequency region in the near future. Therefore, uncertainty reduction in the frequency link between 10 MHz, which is used in the frequency link with an artificial satellite, and the ultrastable laser for observing clock transitions in optical clocks, is an important issue for numerical determination in the new definition and for International Atomic Time operation after the redefinition. We have evaluated all the possible uncertainty factors in the frequency link with a femtosecond comb between 10 MHz and an optical frequency and obtained the lowest uncertainty yet reported of 3.9 × 10−18 by temperature-stabilizing RF multipliers and by using temperature insensitive coaxial cables. Furthermore, we have validated the uncertainty of the frequency link by simultaneously measuring an optical frequency of an identical laser using two femtosecond combs referenced to a common 10 MHz source and confirmed to be consistent with the evaluated uncertainty.

Ultra-precise determination of thicknesses and refractive indices of optically thick dispersive materials by dual-comb spectroscopy.

Precise measurements of the geometrical thickness of a sample and its refractive index are important for materials science, engineering, and medical diagnosis. Among the possible non-contact evaluation methods, optical interferometric techniques possess the potential of providing superior resolution. However, in the optical frequency region, the ambiguity in the absolute phase-shift makes it difficult to measure these parameters of optically thick dispersive materials with sufficient resolution. Here, we demonstrate that dual frequency-comb spectroscopy can be used to precisely determine the absolute sample-induced phase-shift by analyzing the data smoothness. This method enables simultaneous determination of the geometrical thickness and the refractive index of a planar sample with a precision of five and a half digits. The thickness and the refractive index at 193.414 THz (λ = 1550 nm) of a silicon wafer determined by this method are 0.5204737(19) mm and 3.475625(58), respectively, without any prior knowledge of the refractive index.

Rare-Earth-Mediated Optomechanical System in the Reversed Dissipation Regime

Strain-mediated interaction between phonons and telecom photons is demonstrated using excited states of erbium ions embedded in a mechanical resonator. Owing to the extremely long-lived nature of rare-earth ions, the dissipation rate of the optical resonance falls below that of the mechanical one. Thus, a "reversed dissipation regime" is achieved in the optical frequency region. We experimentally demonstrate an optomechanical coupling rate g_{0}=2π×21.7 Hz, and numerically reveal that the interaction causes stimulated excitation of erbium ions. Numerical analyses further indicate the possibility of g_{0} exceeding the dissipation rates of erbium and mechanical systems, thereby leading to single-photon strong coupling. This strain-mediated interaction, moreover, involves the spin degree of freedom, and has a potential to be extended to highly coherent opto-electro-mechanical hybrid systems in the reversed dissipation regime.

Dual‐broadband highly efficient reflective multi‐polarisation converter based on multi‐order plasmon resonant metasurface

In this study, a dual-broadband reflective polarisation converter with high efficiency for both linear-to-linear and linear-to-circular polarisations based on a metasurface is proposed. Owing to the characteristics of strong anisotropy and multi-order plasmon resonances, the proposed polarisation converter can rotate an y / x polarised electromagnetic wave to its cross-polarised ( x / y ) direction in the lower frequency band of 7.74–14.44 GHz (a fractional bandwidth of 60.4%) with over 0.9 polarisation conversion ratio. Besides, the proposed structure can also convert a linearly polarised incident wave to a circularly polarised one after reflection in the higher frequency band of 14.95–17.35 GHz (a fractional bandwidth of 14.9%). The performance in the two bands can be controlled separately by altering the proper parameters of the structure. Numerical analysis is used to predict the polarisation states of the proposed polarisation converter. Moreover, the physical mechanism of multiple resonances is discussed based on surface current distributions. A prototype of the polarisation converter is fabricated and measured. A reasonable agreement between the experiments and simulations is obtained. The design has a simple and scalable geometry, and is a good candidate for polarisation control devices in microwave, terahertz and optical frequency regions.

“Super-efficient gradient estimation technique,” Recent advances in efficient adjoint sensitivity analysis and its application in metamaterial design

Computer-aided design (CAD) tools in electromagnetics allow accurate modeling of the preferred responses. We can adjust a desired response by conducting an optimization algorithm. This requisite the gradient estimation of structures with respect to potentially N number of optimizable parameters. In conventional gradient estimation methods, the number of required simulations scales linearly with N. Adjoint variable method (AVM) is an extremely efficient sensitivity analysis method that estimates the gradients with respect to the all N parameters by conducting only 2 simulations, regardless of N. We have developed AVM method for gradient analysis of anisotropic and dispersive anisotropic structures. Then, we applied it in wideband inversely invisibility cloak design for 2-D and 3-D structures at optical and microwave frequency regions. In those examples, our algorithm accelerates the gradient estimation over 1400 and 12 500 times per iteration, respectively, compared to the conventional methods. This advanced optimization based metamaterial cloak design to 3-D arbitrary shape objects and optical frequency region which was not feasible before. For the next step, we are planning to extend this efficient gradient estimation algorithm to the state of the art machine learning techniques and recently introduced continuous neural networks such as neural ordinary differential equations (ODE-Net).

Simultaneous control of polarization and amplitude over broad bandwidth using multi-layered anisotropic metasurfaces.

In this paper, the broadband transmissive modulation of polarization and amplitude is demonstrated with high efficiency and tunability using multi-layered aluminum metasurfaces. Broadband and nondispersive optical rotation in the optical frequency region is realized by using Fabry-Pérot-like cavity and phase compensation. Simultaneously, the transmission amplitude can be independently controlled by adjusting the twist angle of the anisotropic metasurfaces. The proposed polarization-amplitude modulators are numerically demonstrated to achieve large tunability with an amplitude modulation depth of 0.95 and maximum rotation angle of 180°.

Space-time focusing in a highly multimode fiber via optical pulse shaping.

Fields propagating through a highly scattering material will be distorted in both space (intensity speckles) and time (spectral and temporal speckles), inhibiting tasks such as imaging and communication in both the optical and radio frequency regions. In optics, research thus far has demonstrated spatial focusing, image transmission, and short pulse delivery through bulk scattering materials and multimode fibers by taking advantage of spatial wavefront-shaping techniques. Here, we exploit spectral phase shaping for reference-free characterization of spectral and temporal speckle, and space-time focusing of broadband ultrafast pulses distorted by modal dispersion in a multimode fiber. We show that temporal speckle fields at different multimode fiber output locations are uncorrelated and demonstrate the ability to focus a short pulse at a specific output spatial location, while keeping the field at other output locations noise-like, offering opportunities to expand multimode fiber imaging and communication capacity.

Multiple pulse coherent dynamics and wave packet control of the N2 a'' 1Σ dark state by attosecond four-wave mixing.

Nonlinear multidimensional spectroscopy is ubiquitous in the optical and radio frequency regions as a powerful tool to access structure and dynamics. The extension of this technique into the extreme ultraviolet (XUV) region with attosecond pulses holds promise for probing electronic dynamics and correlations with unprecedented time and spatial resolution. In this work, we use noncollinear four-wave mixing of a weak XUV attosecond pulse train (11-17 eV) and few-femtosecond NIR pulses (800 nm) to spectroscopically and dynamically probe the dipole-forbidden double-well potential of the a'' 1∑+g electronic state of nitrogen. The results demonstrate optical coupling of the inner and outer limits of the initial XUV-prepared vibrational wave packet in the valence character b' 1∑+u state to the inner and outer wells, respectively, of the a'' 1∑+g double well state by 800 nm light. Two four-wave mixing schemes with different pulse timing sequences and noncollinear beam geometries are used (one NIR pulse collinear and one NIR pulse noncollinear versus both NIR pulses noncollinear to the XUV beam) to measure the a'' dark state energetic structure and to control the dynamical preparation and motion of a dark state wave packet by selective population of either the inner Rydberg or outer valence-character potential well. Experimental measurements of the a'' 1∑+g outer well vibrational spacing and anharmonicity closely match the values theoretically predicted for this previously unobserved state.

Design of an Ultrabroadband and High-efficiency Reflective Linear Polarization Convertor at Optical Frequency

A simple design of an ultrabroadband and high-efficiency reflective linear polarization convertor is presented in the optical frequency region. This ultrabroadband convertor is formed from the planar anisotropic metasurface using a meander wire structure. The numerical simulation demonstrates that the polarization conversion ratio over 90% is achieved from 181 to 506 THz, which is equivalent to 94.6% relative bandwidth for y -polarized incident lights under normal incidence. Further simulation results show that the high polarization conversion efficiency can be sustained well when the incident angle increases to 45 ${}^{\circ}$ for both transverse electric (TE) and transverse magnetic (TM) modes. By taking advantage of surface current distribution, the physical mechanisms are elucidated in detail. The further theoretical calculation results based on the interference theory are good agreement with the simualtion. Our design provides a unique approach in realizing the polarization manipulation with high performances in the optical frequency region.

90° Polarization rotator and antireflector using meanderline chiral metamaterials: Analytical and numerical approach

We demonstrate theoretically and numerically a chiral slab that can exhibit optical activity, circular dichroism, and antireflection in detail. Copolar and crosspolar transmitted and reflected waves depending on permittivity, chirality and thickness are analyzed in order to display the characteristic features of the proposed chiral slab both for TE and TM incident waves. The suggested chiral slab is realized by a commercial full-wave EM solver to confirm theoretical results. From the results, the theoretical results are in good agreement with the numerical study. It can be seen that the suggested system can be used for the application to design efficient EM filters and polarization converter at microwave, millimeter wave and optical frequency regions.

Equivalent-nanocircuit-theory-based design to infrared broad band-stop filters.

We theoretically introduced a design paradigm and tool by extending the circuit functionalities from radio frequency to near infrared domain, and a broad band-stop filter, is successfully demonstrated by cascading triple layers of nano-square arrays. The feasibility is confirmed by its consistency with the rigorous FDTD calculation. Moreover, such a third-order Butterworth filter is not only insensitive to the incident angle and but also to input light's polarization. The new paradigm forms a theoretical foundation for designing optical devices and also enriches the classic circuit operations at the optical frequency region.

Conical effect in diffraction radiation

When light is scattered on a grating, the scattered waves are distributed over a cone surface, which is called “conical diffraction effect”. In the present work the similar effect in theory of diffraction radiation (DR) from a charged particle moving near a target at some angle to its edge is predicted and described for the first time. The equation describing the surface of maximal radiation, which is conical surface, is derived from the first principles. The effect is illustrated by spectral-angular distributions of DR from a thin target (screen) in X-ray, EUV, optical and lower frequency regions for two cases: (a) the particle moves in the plane perpendicular to the screen; (b) the particle moves in the plane parallel to the screen. The radiation for these two orientations of the target is shown to differ mainly in direction of mirror reflection to the trajectory of the particle. Due to similar nature of DR and transition radiation (TR) the same effects should arise for TR from targets of limited transversal size.

Frequency Division in the Optical Frequency Region and Generation of Phase-Locked High Harmonics

Frequency Division in the Optical Frequency Region and Generation of Phase-Locked High Harmonics

ELECTROMAGNETICALLY INDUCED ABSORPTION IN METAMATERIALS IN THE INFRARED FREQUENCY

In this paper, the author studies, through numerical simulation, the classical analog of the electromagnetically induced absorption/re∞ection (EIA) in a planar metamaterial structure in the near infrared spectral region. The structure is designed by transforming an electromagnetically induced transparency (EIT) structure into an EIA structure using Babinet's principle. The structure exhibits a coupling between a bright mode (a complementary ring resonator (CRR)) and a dark mode (pair of parallel straight slits) imprinted on a glass substrate. A narrow absorption window, induced in a wide transparent window, is achieved by the structure and the strength of coupling is tuned by the degree of breaking symmetry and relative displacement of the two mode elements. Electromagnetically induced transparency (EIT) is a quantum mechanical phenomenon in which an absorption is transformed into a transparency, and this is associated with strong dispersion that reduces the group velocity (1{3). This phenomena has many potential applications in signal processing, optical flltering and sensing technologies (4{7). The classical analog of the EIT has been achieved and reported by many researchers using a planar metamaterial structures that are generally composed of two metallic elements imprinted on a dielectric substrate. The two elements are metallic resonators that act either as two bright modes or a bright and dark mode. A bright mode element couples strongly with the incident excitation fleld while the dark element does not couple with the incident fleld but couples with the magnetic fleld induced by the bright element. Several EIT metamaterial structures have been employed and reported in the microwave, terahertz and optical frequency regions (8{14). In this paper, we propose a new EIT structure that achieves a more pronounced narrow transparency window sandwiched between two absorption bands compared with a similar structure proposed in (14). Also compared with (14), the structure proposed here is independent of the polarization orientation with respect to the bright mode element. Then we transform this structure into an EIA structure using Babinet's principle (15,16), where a narrow absorption/re∞ection window is sandwiched between two transparency bands. The structures proposed here may be used as obscurant particles that allow the passage of a speciflc window/windows of the electromagnetic spectrum and obscure the neighboring frequencies, for this reason the transmission spectrum will be discussed and addressed for the potential obscurant applications. Although the orientational average response of the structure is what matters, it is of good beneflt to get the behavior in a given orientation. The EIT structure is composed of a metallic ring resonator (RR) which acts as a bright mode and a pair of metallic rods that act as a dark mode. Here we overlap the RR wide antenna resonance and the narrow line width quadrupole rod resonance, which in turn produces the EIT-like efiect. The structure is transformed into an EIA structure by replacing the metallic RR and the pair of rods resonator with complementary ring resonator (CRR) and

Double Negative Optical Metamaterial with All-Dielectric Fishnet Structure

An all-dielectric fishnet metamaterial (DFM) composed of multilayer dielectric structure with periodically patterned holes is proposed to generate left-handed behavior in optical frequency region. Different from the metallic fishnet negative index metamaterial realized by coupling the neighbouring functional layers along the propagation direction, this DFM owes its simultaneous negative permeability and permittivity to the hybrid coupled resonances in the same layer. Based on this configration, a wide-band and polarization-independent negative refraction with maximum figure of merit as large as 13.85 is obtained by a silicon-based DFM at 450 nm wavelength. Simple and easy to fabricate, this concept is believed to provide an alternative route to realize high-performance negative index metamateirals at optical frequencies.

Investigations on Binary Mixtures of Propan-2-ol with Methyl Benzoate and Ethyl Benzoate

The molecular interactions between the polar systems propan-2-ol with alkyl benzoates (methyl benzoate and ethyl benzoate), for various mole fractions and different temperatures, were studied by determining the dielectric permittivity using a LF impedance analyzer and Abbe’s refractometer in the static and optical frequency regions, respectively. The effective Kirkwood correlation factor, corrective Kirkwood correlation factor, dipole moment, excess dipole moment, and excess Helmholtz free energy were calculated using the experimental data. Hamiltonian quantum mechanical calculations (ab initio and semiempirical) were performed using PC Spartan and Argus lab Modeling software for both pure and equimolar binary systems of propan-2-ol with alkyl benzoates.

Terahertz Frequency Metrology Based on Frequency Comb

Two techniques for terahertz (THz) frequency metrology based on frequency comb, namely, a THz-comb-referenced spectrum analyzer and a continuously tunable, single-frequency continuous-wave (CW)-THz generator, are reviewed. Since the frequency comb enables to coherently link the frequency among microwave, optical, and THz regions, it is possible to establish the THz frequency metrology traceable to time of the SI base units. Using a THz-comb-referenced spectrum analyzer based on a stable THz comb generated in a photoconductive antenna for THz detection, the absolute frequency of CW test sources in the sub-THz and THz regions was determined at a precision of 10-11. Furthermore, a continuously tunable, single-frequency CW-THz generator was demonstrated around 120 GHz by photomixing of an accurately tunable CW laser and a tightly fixed CW laser in the optical frequency region, phase locked to two independent optical combs. The combination of the CW-THz generator with the THz-comb-referenced spectrum analyzer will open the door for establishment of frequency metrology in the THz region.

Thermodynamic, Dielectric and Conformational Studies on Hydrogen Bonded Binary Mixtures of Propan-1-ol with Methyl Benzoate and Ethyl Benzoate

Dielectric relaxation studies of propan-1-ol with alkyl benzoates (methyl benzoate and ethyl benzoate) have been carried out, for various mole fractions, at different temperatures using a LF impedance analyzer, Plunger method, and an Abbe’s refractometer in the radio, microwave and optical frequency regions, respectively. Kirkwood’s effective correlation factor, the corrective Kirkwood correlation factor, Bruggeman parameter, relaxation time, excess inverse relaxation time and thermodynamic parameters were calculated using the experimental data. Conformational analysis of the formation of hydrogen bonds in the equi-molar binary mixtures of propan-1-ol with alkyl benzoates is supported by experimental and theoretical FT-IR values.

Experiments of Negative-Index Refraction in Optical Frequency Region

Recently, significant attention has been devoted to a new class of smart materials – so-called negative-index materials (NIM). There are some direct experimental demonstrations of NIM in the microwave frequency region, but not in optical frequency region. Here we report a kind of optical negative-index material which is 2D photonic crystal and, by measuring the refractive angle of the transmitted beam through a prism fabricated from this material, we determine the effective n. These experiments directly confirm this material is an optical negative-index one.

Continuously tunable, phase-locked, continuous-wave terahertz generator based on photomixing of two continuous-wave lasers locked to two independent optical combs

A continuously tunable, phase-locked, single-frequency, continuous-wave (cw) terahertz generator has been demonstrated around 120 GHz, corresponding to the spectral bandwidth of the F-band unitraveling-carrier photodiode (UTC-PD) used as a photomixer in this study. This cw-terahertz generator is based on photomixing of an accurately tunable cw laser and a tightly fixed cw laser in the optical frequency region, phase locked to two independent optical combs. The continuous tuning range of the presented method was three orders of magnitude around 0.1 THz and two orders around 1 THz broader than that of a previous photomixing method in which two cw lasers are phase locked to a single optical comb, and fully covered the available spectral bandwidth of the F-band UTC-PD. The spectral behavior of the tight locking and continuous tuning of 120 GHz cw-terahertz radiation was confirmed in real time by use of a terahertz-comb-referenced spectrum analyzer. This cw-terahertz generator shows promise as a terahertz clock and synthesizer for terahertz frequency metrology.