Finite Spectral Range Research Articles

All‐Dielectric Meta‐Waveguides for Flexible Polarization Control of Guided Light

AbstractGuided waves are the perfect carriers of electromagnetic signals in planar miniaturized devices due to their high localization and controlled propagation direction. However, it is still a challenge to control the polarization of propagating guided waves. In this work, the broadband polarization TE‐TM degeneracy of highly localized guided waves propagating along an all‐dielectric metasurface and a subwavelength chain of dielectric high‐index cylinders is discovered, both theoretically and experimentally. Using the discovered near‐field polarization degree of freedom, the polarization transformation for guided waves propagating along a subwavelength chain of dielectric cylinders at any frequency within the finite spectral range is demonstrated experimentally. Namely, the simplest planar near‐field polarization device – the quarter‐wave‐retardation (linear‐to‐circular) polarization transformer of guided waves is implemented. The results obtained discover the polarization degree of freedom for guided waves paving the way toward numerous applications in flat optics and planar photonics.

Improved peak-to-peak method for cavity length measurement of a Fabry-Perot etalon using a mode-locked femtosecond laser.

Differing from the conventional peak-to-peak method using two neighboring spectral peaks in the frequency-domain fringe spectrum of the spectral response of a Fabry-Perot etalon to a femtosecond laser, which contains N spectral peaks equally spaced with a spacing of the etalon free spectral range (FSR), the proposed method employs a pair of spectral peaks with a spacing of an integer multiple k (k ≫ 1) of FSR for measurement of the etalon cavity length d with a reduced measurement error. Under the constrain of the total N spectral peaks obtainable in the finite spectral range of the femtosecond laser, the optimized k is identified to be N∕2 in consideration of an averaging operation using N - k samples of d to achieve the minimum measurement error. The feasibility of the proposed method is demonstrated by experimental results with an uncertainty analysis based on "Guides to the Expression of Uncertainty in Measurement".

Unusually strong, tunable, and nonreciprocal light trapping and absorption in helically structured magnetoactive media

We investigated the absorption properties of photonic structures with both large optical activity and magneto-optical activity parameters. The photonic structure is a periodic helical 1D medium in an external magnetic field. The magneto-optical activity parameter g change effects on the absorption nonreciprocity spectra for both linear and circular polarizations and those for unpolarized (natural) light were investigated. We showed that in certain cases, the external magnetic field leads to a strong increase in light trapping, absorption, and absorption nonreciprocity. This system can operate as a powerful light source and asymmetric and tunable wide range high absorber as well. We also investigated the dependence of Q (total energy absorbed in a finite spectral range) and its density on the layer thickness for different values of magneto-optical activity parameter g. With a change in the parameter g, the value of Q changes over a wide range (increasing or decreasing over 40 or more times), i.e., we have a broadly tunable and nonreciprocal integral absorption, again, for both polarized and unpolarized light. We showed that there are values of the parameter of magneto-optical activity at which the integral absorption does not depend on the polarization of the incident light.

Verification of the Kramers-Kronig relations between ultrasonic attenuation and phase velocity in a finite spectral range for CFRP composites

The aim of this work is to verify applicability of the Kramers-Kronig relations between the attenuation coefficient and phase velocity of longitudinal acoustic waves in carbon fiber reinforced plastic (CFRP) composites in a finite megahertz frequency range. To measure these characteristics, the method of broadband acoustic spectroscopy with a laser source of ultrasound is used. We have experimentally shown that absolute attenuation in CFRPs is determined by both absorption in a polymer matrix and scattering on carbon fibers and gas pores of several tenths of microns in size. We have also found that the increasing composite porosity leads to the decrease in the absolute value of the ultrasonic velocity and to the increase in its relative dispersion in the entire studied frequency range of 1–10 MHz. The experimental results have shown that for the studied CFRPs in this range, features of the mechanism causing attenuation and dispersion of longitudinal acoustic waves by propagation in composites do not influence on applicability of the local Kramers-Kronig relations.

Interpretation of runaway electron synchrotron and bremsstrahlung images

The crescent spot shape observed in DIII-D runaway electron synchrotron radiation images is shown to result from the high degree of anisotropy in the emitted radiation, the finite spectral range of the camera and the distribution of runaways. The finite spectral camera range is found to be particularly important, as the radiation from the high-field side can be stronger by a factor 106 than the radiation from the low-field side in DIII-D. By combining a kinetic model of the runaway dynamics with a synthetic synchrotron diagnostic we see that physical processes not described by the kinetic model (such as radial transport) are likely to be limiting the energy of the runaways. We show that a population of runaways with lower dominant energies and larger pitch-angles than those predicted by the kinetic model provide a better match to the synchrotron measurements. Using a new synthetic bremsstrahlung diagnostic we also simulate the view of the gamma ray imager diagnostic used at DIII-D to resolve the spatial distribution of runaway-generated bremsstrahlung.

Scattering of Gaussian beams by disordered particulate media

A frequently observed characteristic of electromagnetic scattering by a disordered particulate medium is the absence of pronounced speckles in angular patterns of the scattered light. It is known that such diffuse speckle-free scattering patterns can be caused by averaging over randomly changing particle positions and/or over a finite spectral range. To get further insight into the possible physical causes of the absence of speckles, we use the numerically exact superposition T-matrix solver of the Maxwell equations and analyze the scattering of plane-wave and Gaussian beams by representative multi-sphere groups. We show that phase and amplitude variations across an incident Gaussian beam do not serve to extinguish the pronounced speckle pattern typical of plane-wave illumination of a fixed multi-particle group. Averaging over random particle positions and/or over a finite spectral range is still required to generate the classical diffuse speckle-free regime.

Application of the three-component bidirectional reflectance distribution function model to Monte Carlo calculation of spectral effective emissivities of nonisothermal blackbody cavities

We applied the bidirectional reflectance distribution function (BRDF) model consisting of diffuse, quasi-specular, and glossy components to the Monte Carlo modeling of spectral effective emissivities for nonisothermal cavities. A method for extension of a monochromatic three-component (3C) BRDF model to a continuous spectral range is proposed. The initial data for this method are the BRDFs measured in the plane of incidence at a single wavelength and several incidence angles and directional-hemispherical reflectance measured at one incidence angle within a finite spectral range. We proposed the Monte Carlo algorithm for calculation of spectral effective emissivities for nonisothermal cavities whose internal surface is described by the wavelength-dependent 3C BRDF model. The results obtained for a cylindroconical nonisothermal cavity are discussed and compared with results obtained using the conventional specular-diffuse model.

Assessment of wavelength dependent complex refractive index of strongly light absorbing liquids

A practical measurement procedure for the determination of the complex refractive index of strongly absorbing liquids within a finite spectral range was developed. The method is based on separate measurements of reflectance and transmittance of the liquid sample, a property of dispersion and absorption, and exploitation of Fresnel's theory. The advantage of the method is that the knowledge of the layer thickness of the light absorbing medium, which is required typically in transmittance measurements, is not needed. In addition, both measurements, the transmittance and the reflectance, were accomplished with one spectrophotometer using a home-built reflectometer and without any sample dilution. The method is validated by numerical simulation using the Lorentz model for permittivity of an insulator, and also by experimental data obtained from three strongly absorbing offset inks, namely magenta, yellow and cyan.

Optimization-based design of surface textures for thin-film Si solar cells

We numerically investigate the light-absorption behavior of thin-film silicon for normal-incident light, using surface textures to enhance absorption. We consider a variety of texture designs, such as simple periodic gratings and commercial random textures, and examine arbitrary irregular periodic textures designed by multi-parameter optimization. Deep and high-index-contrast textures exhibit strong anisotropic scattering that is outside the regime of validity of the Lambertian models commonly used to describe texture-induced absorption enhancement for normal incidence. Over a 900-1100 nm wavelength range, our optimized surface texture in two dimensions (2D) enhances absorption by a factor of 2.7 πn, considerably larger than the original πn Lambertian result and exceeding by almost 50% a recent generalization of Lambertian model for periodic structures in finite spectral range. However, the πn Lambertian limit still applies for isotropic incident light, and our structure obeys this limit when averaged over all the angles. Therefore, our design can be thought of optimizing the angle/enhancement tradeoff for periodic textures.

Spectral analysis of a one-dimensional scattering medium with the differential multiply subtractive Kramers-Kronig method

The differential multiply subtractive Kramers-Kronig (DMSKK) method is utilized for deriving the optical spectral response of a 1D scattering medium. The technique is based on the multiply subtractive Kramers-Kronig technique, where the phase spectrum is reconstructed from the amplitude spectrum in a finite spectral range with the aid of one or more phase-anchoring values. We employ a new phase-anchoring technique in the DMSKK method to partially mitigate the finite-range effects. This method incorporates spectral ballistic imaging to anchor the phase difference (instead of the phase directly) at one or more reference wavelengths. The simplicity of phase derivative measurements and its utilization in the DMSKK method are emphasized by a simple experiment.

Comparison of subtractive Kramers-Kronig analysis and maximum entropy model in resolving phase from finite spectral range reflectance data

The maximum entropy model (MEM) and Kramers-Kronig (K-K) analysis were compared with the aim of phase retrieval from reflectance. The object was to test two different phase-retrieval methods when reflectance is known at a finite frequency range and data fitting is not performed beyond the finite frequency band. In addition, it was assumed that the phase is known only at one or two anchor points. As an example, we study the terahertz reflection spectrum related to a semiconductor and an optical spectrum of potassium chloride. It is shown that the MEM resolves the complex refractive index of a medium, in the vicinity of initial and final points of the spectra, better than singly and doubly subtractive K-K relations. Both methods give only satisfactory results in the event of one anchor point, but in the case of two anchor points, the MEM is better than doubly subtractive K-K. It is proposed that the MEM should be used instead of K-K analysis, for a priori information of phase at two anchor points, for the purpose of resolving the complex refractive index of a medium from reflectance with high accuracy.

Left-handed properties of erbium-doped crystals

We show that lanthanide-doped crystals can be made left handed for a finite spectral range. The electronic transitions in the dopant ions are both dipolar electric and dipolar magnetic allowed. This enables tuning the permittivity and the permeability at the same frequency. The analysis focuses on erbium-doped crystals where left-handed properties are predicted inside an experimentally reachable domain of parameters.

Electromagnetically Induced Left Handedness in Optically Excited Four-Level Atomic Media

We show that left-handed properties can be electromagnetically induced in a general four-level atomic medium for a finite spectral range. We use an electric (magnetic) atomic transition as an electric (magnetic) resonator to modify the permittivity (permeability), both at the same frequency. The implementation of the four-level model is carried out in atomic hydrogen and neon. In each case the existence of left-handed properties is predicted inside an experimentally reachable domain of parameters.

Multiply subtractive Kramers–Krönig relations for arbitrary-order harmonic generation susceptibilities

Kramers–Krönig (K–K) analysis of harmonic generation optical data is usually greatly limited by the technical inability to measure data over a wide spectral range. Data inversion for real and imaginary part of χ n ( nω; ω,…, ω) can be more efficiently performed if the knowledge of one of the two parts of the susceptibility in a finite spectral range is supplemented with a single measurement of the other part for a given frequency. Then it is possible to perform data inversion using only measured data and subtractive K–K relations. In this paper multiply subtractive K–K relations are, for the first time, presented for the nonlinear harmonic generation susceptibilities. The applicability of the singly subtractive K–K relations are shown using data for third-order harmonic generation susceptibility of polysilane.

Feasibility of 5 Gbit/s wavelength division multiplexing using quantum dot lasers

The dynamics of single-mode quantum dot lasers is modeled theoretically. It is predicted that, assuming reasonable material properties, eye-patterns remain open for 5 Gbit/s large signal modulation within a finite spectral range (>50 nm), corresponding to 64 wavelength division multiplexing channels with 0.8 nm separation.

Effective absorption coefficient of a selective medium within a finite spectral range with regard for scattering

The effect of scattering on the lineshape of outgoing radiation for the regular absorption band model is investigated for the model of a homogeneous plane-parallel radiating, absorbing, and scattering layer with transparent boundaries. The investigation is based on numerical solution of the integrodifferential equation of radiation transport. Based on an analysis of results obtained, methods more accurate than the known ones are proposed for calculation of the absorption coefficient by a selective component of the medium averaged over a finite spectral range.

On the theory of planar spectrographs

The theory of planar spectrographs is presented. It is proven that by no means all aberrations up to the order of x/sup 4/ can be corrected over a finite spectral range. A general procedure to construct gratings with two stigmatic points of (nearly) arbitrary wavelength and location is proposed. Rowland-type and flatfield spectrographs are discussed as numerical examples. >

A method for determination of the refractive index in a region of absorption. Anomalous dispersion of CS 2 in the UV range

A method is described for determining the refractive index of, e.g., a liquid solution, within an absorption band by studying the linear dichroism generated by different reflections at inclined optical incidence. The method requires a sensitive differential photometric technique, such as that accomplished in a circular dichroism spectrometer. The refractive index of CS 2 is reported in the region 260–450 mm. and the result compared to that obtained from a Kramers-Kronig integration on the 320 nm absorption band. Though this band is fairly isolated the experimentally gained result was found more reliable. This was due to difficulties in obtaining an accurate absorption coefficient of liquid CS 2 as well as to the fact that the integration had to be restricted to a finite spectral range.

A macroscopic theory of interference and diffraction of light from finite sources, I. Fields with a narrow spectral range

A macroscopic theory of interference and diffraction of light in stationary fields produced by finite sources which emit light within a finite spectral range is formulated. It is shown that a generalized Huygens principle may be obtained for such fields, which involves only observable quantities. The generalized Huygens principle expresses theintensityat a typical point of the field in terms of an integral taken twice independently over an arbitrary surface, the integral involving the intensity distribution over the surface and the values of a certain correlation factor, which is found to be the ‘degree of coherence’ previously introduced by Zemike. Next it is shown that under fairly general conditions, this correlation factor is essentially the normalized integral over the source of the Fourier (frequency) transform of the spectral intensity function of the source, and that it may be determined from simple interference experiments. Further, it is shown that in regions where geometrical optics is a valid approximation, the coherence factor itself then obeys a simple geometrical law of propagation. Several results on partially coherent fields, established previously by Van Cittert, Zernike, Hopkins and Rogers, follow as special cases from these theorems. The results have a bearing on many optical problems and can also be applied in in ­ vestigations concerned with other types of radiation.