Frequency-dependent Refractive Index Research Articles

Distinguishing octane grades in gasoline using terahertz metamaterials

Distinguishing octane numbers of commercial gasoline is experimentally demonstrated by use of single split-ring resonator metamaterials functioning at terahertz frequencies. The differences in frequency-dependent absorption coefficients and refractive indices of various grades of gasoline lead to a modification in the surrounding dielectric environment and consequently the resonance properties of the planar metamaterials. This consequently enables a distinct frequency shift in the inductive-capacitive electric dipolar resonances. This paper reveals that such metamaterial arrays, as highly sensitive chemical sensors, have promising potential in petroleum industrial applications.

THz conductivities of indium-tin-oxide nanowhiskers as a graded-refractive-index structure

Indium-tin-oxide (ITO) nanowhiskers with attractive electrical and anti-reflection properties were prepared by the glancing-angle electron-beam evaporation technique. Structural and crystalline properties of such nanostructures were examined by scanning transmission electron microscopy and X-ray diffraction. Their frequency-dependent complex conductivities, refractive indices and absorption coefficients have been characterized with terahertz time-domain spectroscopy (THz-TDS), in which the nanowhiskers were considered as a graded-refractive-index (GRIN) structure instead of the usual thin film model. The electrical properties of ITO GRIN structures are analyzed and fitted well with Drude-Smith model in the 0.2~2.0 THz band. Our results indicate that the ITO nanowhiskers and its bottom layer atop the substrate exhibit longer carrier scattering times than ITO thin films. This signifies that ITO nanowhiskers have an excellent crystallinity with large grain size, consistent with X-ray data. Besides, we show a strong backscattering effect and fully carrier localization in the ITO nanowhiskers.

COMPARISON OF RADIO-FREQUENCY INTERFERENCE MITIGATION STRATEGIES FOR DISPERSED PULSE DETECTION

Impulsive radio-frequency signals from astronomical sources are dispersed by the frequency dependent index of refraction of the interstellar media and so appear as chirped signals when they reach earth. Searches for dispersed impulses have been limited by false detections due to radio frequency interference (RFI) and, in some cases, artifacts of the instrumentation. Many authors have discussed techniques to excise or mitigate RFI in searches for fast transients, but comparisons between different approaches are lacking. This work develops RFI mitigation techniques for use in searches for dispersed pulses, employing data recorded in a "Fly's Eye" mode of the Allen Telescope Array as a test case. We gauge the performance of several RFI mitigation techniques by adding dispersed signals to data containing RFI and comparing false alarm rates at the observed signal-to-noise ratios of the added signals. We find that Huber filtering is most effective at removing broadband interferers, while frequency centering is most effective at removing narrow frequency interferers. Neither of these methods is effective over a broad range of interferers. A method that combines Huber filtering and adaptive interference cancellation provides the lowest number of false positives over the interferers considered here. The methods developed here have application to other searches for dispersed pulses in incoherent spectra, especially those involving multiple beam systems.

Combination of Kramers–Kronig transform and time-domain methods for the determination of optical constants in THz spectroscopy

Absorption and reflection spectra have successfully been used for substance identification, which is also applicable to the THz spectral range. For optical spectroscopy, the Kramers–Kronig transformation (KKT) is a powerful tool to determine the complex refractive index n˜ (with k, n absorption and refractive indices, respectively) from either an absorption or reflection measurement. By terahertz time-domain spectroscopy (THz-TDS) materials are probed with short pulses of radiation. The detection is sensitive to the sample on both the amplitude and the phase of the electrical field. Thus, THz-TDS spectroscopy can provide more information than conventional Fourier-transform spectroscopy, by which a power spectrum is measured. In the case of transmission measurements, for instance, formulae exist by which the frequency-dependent complex refractive index is directly calculated from the time-dependent electrical field (waveform) without the necessity of a KKT. In the case of reflection experiments, a comparable computation is possible for the frequency-dependent phase angle; here a KKT can help to recover essential parameters. We present a combination of KKT and TDS methods for the calculation of optical constants in the THz regime.

First principles results on TbAl3(BO3)4: A promising magneto-optical material

The electronic and optical properties of TbAl3(BO3)4 are determined using density functional theory. The calculated total magnetic moment of 5.96 μB is close to the expected moment of Tb3+. The frequency dependent dielectric function, refractive index, extinction coefficient, absorption, optical reflectivity, and energy loss function are explained in terms of the transitions between the valence and conduction bands. We find very high Kerr angles for ultraviolet light and potential for extension, even into the visible range by band structure design.

Design and analysis of an isotropic two-dimensional planar Composite Right/Left-Handed waveguide structure

Abstract. A two-dimensional isotropic Composite Right/Left-Handed (CRLH) waveguide structure is proposed which is designed for operation in X-band. The balanced structure possesses left-handed behaviour over a large bandwidth from 7.5 GHz up to its transition frequency at 10 GHz. Above this region, the unit cell behaves in a right-handed manner up to 13.5 GHz. Operating the structure within these bands yields a frequency dependent index of refraction ranging from −2.5 ≤ n ≤ 0.8. Isotropic characteristics are obtained between 8.5 GHz ≤ f ≤ 12 GHz resulting in −1.5 ≤ n ≤ 0.8. The planar CRLH structure is designed based on transmission line theory, implemented in microstrip technology and optimized using full-wave simulation software. An equivalent circuit model is determined describing the electromagnetic behaviour of the structure whose element values are obtained by even and odd mode analysis. The design of the unit cell requires an appropriate de-embedding process in order to enable an analysis in terms of dispersion characteristics and Bloch impedance, which are performed both.

Analyzing Morphology and Thermal History of Polybutylene Terephthalate by THz Time-domain Spectroscopy

We have measured the frequency-dependent dielectric function of semi-crystalline polybutylene terephthalate (PBT) in the terahertz region between 100 GHz and approximately 2.8 THz. A characteristic band is observed around 2.38 THz. The intensity of this band is a good indicator of the degree of crystallinity of the different samples. A potential assignment of this band is proposed, based on the comparison with spectroscopic data of the structurally very similar polyethylene terephtalate (PET). Furthermore, the frequency-dependent index of refraction of PBT reveals more insight about the morphology and different thermal history of the samples under investigation.

Linear and nonlinear optical response of MgxZn1−xO: A density functional study

Total and partial density of states, frequency dependent complex refractive index including extinction coefficient, optical conductivity and transmission of MgxZn1−xO (0≤x≤1) in rocksalt and wurtzite phases are calculated using full potential linearized augmented plane wave (FP-LAPW) method. The real part of refractive index decreases while the extinction coefficient, optical conductivity and transmission for rocksalt phase increases with the increase in Mg concentration. In wurtzite phase, ordinary and extraordinary indices decrease while extinction coefficient, optical conductivity and transmission increase in parallel as well as perpendicular to c-axis with the increase in the Mg concentration.

Enhanced lens byεandμnear-zero metamaterial boosted by extraordinary optical transmission

In this paper we report directivity enhancement by a short-focal-length plano-concave lens engineered by stacked subwavelength hole arrays (fishnet-like stack) with an effective negative index of refraction close to zero, $n$ \ensuremath{\rightarrow} 0, that arises from \ensuremath{\varepsilon} and \ensuremath{\mu} near-zero extreme values. The plano-concave lens frequency response shows two enhancement peaks, one at the wavelength corresponding to $n$ $=$ \ensuremath{-}1 and, prominent in this configuration, another unexpected peak when $n$ \ensuremath{\rightarrow} 0 that comes as a result of the similar low values of \ensuremath{\varepsilon} and \ensuremath{\mu}. The frequency-dependent negative refractive index and beam-forming properties of the lens are supported by finite-integration-frequency- and time-domain simulations and experimental results. This near-zero metamaterial lens can find applications in terahertz and even optics since the building block, stacked extraordinary optical transmission layers, has already been reported for those regimes under the name of fishnet structure.

Enhanced transmission of electromagnetic waves through 1D plasmonic crystals

Transmission of electromagnetic waves through thick perfect conducting slabs perforated by one-dimensional arrays of rectangular holes was studied experimentally in the microwave frequency range. The observed thickness-dependent transmission clearly exhibits the evanescent and propagating nature of the involved electromagnetic excitations on the considered structures, which are effective surface plasmons and localized waveguide resonances, respectively. The 1D crystals showing transmission based on localized resonances further manifests the frequency-dependent effective refractive index depending on the filling ratio of the holes and accompanies resonant guided wave propagation.

Accuracy and resolution of THz reflection spectroscopy for medical imaging

The use of THz radiation as a potential tool for medical imaging is of increasing interest. In this paper three methods of analysis of THz spectroscopic information for diagnosis of tissue pathologies at THz frequencies are presented. The frequency-dependent absorption coefficients, refractive indices and Debye relaxation times of pure water and pure lipids were measured and used as prior knowledge in the different theoretical methods for the determination of concentration. Three concentration analysis methods were investigated: (a) linear spectral decomposition, (b) spectrally averaged dielectric coefficient method and (c) the Debye relaxation coefficient method. These methods were validated on water and lipid emulsions by determining the concentrations of phantom chromophores and comparing to the known composition. The accuracy and resolution of each method were determined to assess the potential of each method as a tool for medical diagnosis at THz frequencies.

Terahertz imaging applied to cancer diagnosis

We report on terahertz (THz) time-domain spectroscopy imaging of 10 µm thick histological sections. The sections are prepared according to standard pathological procedures and deposited on a quartz window for measurements in reflection geometry. Simultaneous acquisition of visible images enables registration of THz images and thus the use of digital pathology tools to investigate the links between the underlying cellular structure and specific THz information. An analytic model taking into account the polarization of the THz beam, its incidence angle, the beam shift between the reference and sample pulses as well as multiple reflections within the sample is employed to determine the frequency-dependent complex refractive index. Spectral images are produced through segmentation of the extracted refractive index data using clustering methods. Comparisons of visible and THz images demonstrate spectral differences not only between tumor and healthy tissues but also within tumors. Further visualization using principal component analysis suggests different mechanisms as to the origin of image contrast.

A 2-D Artificial Dielectric With <formula formulatype="inline"> <tex Notation="TeX">${0 \leq n < 1}$</tex></formula> for the Terahertz Region

We demonstrate a 2-D artificial dielectric medium suitable for the terahertz region by exploiting the characteristic frequency dependence in the phase velocity of the lowest order transverse-electric (TE1) mode of the parallel-plate waveguide (PPWG). This artificial medium exhibits a plasma-like behavior having a frequency-dependent refractive index that varies between zero and unity. Using this medium, we demonstrate the optical phenomena of total internal reflection and Brewster's effect, and also demonstrate a convergent PPWG-lens.

Molecular properties of liquid crystals in the terahertz frequency range

We report on a first experimental study of the molecular properties of nematic liquid crystals in the terahertz range. In the beginning, we extract the frequency and temperature dependent refractive index and absorption coefficient of the cyanobiphenyls 5CB, 6CB and 7CB from terahertz time domain spectroscopy measurements and investigate the impact of the alkyl chain length on the macroscopic liquid crystal characteristics, focusing especially on the pronounced odd and even effect. Next, we deduce the principle polarizabilities and the order parameter S by applying Vuks' approximation and Haller's approach. On this basis, we calculate the main polarizabilities along the longitudinal and transverse axis and link the observed terahertz properties to the molecular structure of the liquid crystals.

Electronic and optical properties of the orthorhombic compounds PdPX (X=S and Se)

Abstract We have investigated the structural, electronic and optical properties of the orthorhombic layered compounds PdPX (X = S and Se). The calculations are based on the total-energy calculations within the full-potential augmented plane-wave plus local orbitals (FP-LAPW + lo) method. We have used the local density approximation for the exchange and correlation potential. The structural properties are determined through the total energy minimization and inter atomic forces calculations. The obtained results agree well with the measured ones. Our calculations show that these compounds are indirect-gap semiconductors. The calculated anisotropic frequency dependent dielectric functions, reflectivity, refractive index and absorption spectra are obtained and discussed.

Influence of external magnetic field on magnon–plasmon polaritons in negative-index antiferromagnet–semiconductor superlattices

The peculiarities of the negative refraction in periodic multilayered antiferromagnet–semiconductor nanostructures are investigated in the presence of an external magnetic field parallel to the plane of the layers. Effective material tensors are obtained using method of anisotropic homogeneous medium. Dispersion and energetic relations for the mixed magnon–plasmon polaritons are investigated in the case of the Voigt geometry. Frequency regions of anomalous dispersion are found and studied in various regimes of the applied magnetic field. The necessary conditions are found under which the structure behaves as a left-handed negative-index metamaterial. Analytical expressions for the frequency-dependent phase and group refractive indices are obtained.

Dependence of low frequency spectra on solute and solvent in solutions studied by terahertz time-domain spectroscopy

This paper presents the results of measuring the frequency-dependent refractive index, the extinction coefficient, and the absorption coefficient of polar solutes (nitrobenzene, benzonitrile, and chlorobenzene) in non-polar solvents by using terahertz time-domain spectroscopy (THz-TDS) at room temperature. The intensities of the difference absorption spectra of the solution and the pure solvent show a specific peak for each particular solution. This finding demonstrates the potential application of THz-TDS in studying the microscopic details of a particular solute in solution. The low frequency spectra were analysed by using a model function for the time correlation function of the total dipole moment in order to understand the dynamics and the interactions in the systems. The picosecond exponential component was found to be less sensitive to the viscosity of the bulk solvent, and it is proposed that this component can be regarded as the result of structural relaxation in the solution. It was also found that the spectra of the benzonitrile solutions receive a large contribution from the Gaussian component, which is attributed to the librational motion. This suggests that the formation of dimers or oligomers might take place in the case of the benzonitrile solution.

Millimeter-Wave Left-Handed Extraordinary Transmission Metamaterial Demultiplexer

In this work, by using an extraordinary transmission metamaterial prism and taking advantage of its inherent dispersive behavior, the feasibility of manufacturing a demultiplexer/multiplexer or even a spectrogram analyzer at millimeter-waves is shown. Depending on the incoming beam frequency, the outgoing beam suffers in a wedge-shaped configuration a different deflection (governed by Snell's law). Since the frequency dependent index of refraction of this metamaterial can go from negative values to almost zero for the first propagation band (cut-off regime) and also be positive for the second propagation mode (non-cut-off regime), different negative and positive outgoing angles can be obtained and then, diverse paths for each frequency, as the Fresnel zone experimental results confirm.

Terahertz transmission properties of transparent conducting molybdenum-doped ZnO films

With direct current reactive magnetron sputtering method，transparent conducting molybdenum-doped ZnO （ZMO） thin films with different carrier concentration on glass substrate were fabricated by monitoring oxygen partial pressure. The dielectric responses of the ZMO films are characterized with terahertz time-domain spectroscopy. Frequency dependent conductivity，power absorption，and refractive index are obtained，and the experimental results can be well reproduced with classic Drude model. Our results reveal that，by adjusting the carrier concentration of ZMO film，the conducting ZMO film can serve as broadband antireflection coatings for substrates and optics in terahertz frequency range.

Carrier concentration dependence of terahertz transmission on conducting ZnO films

With the dc reactive magnetron sputtering method, conducting ZnO thin films with different carrier concentrations on glass substrate were fabricated. The dielectric responses of the ZnO films are characterized with terahertz time-domain spectroscopy. Frequency-dependent conductivity, power absorption, and refractive index are obtained, and the experimental results can be well reproduced with the classic Drude model. Our results reveal that by adjusting the carrier concentration of the ZnO film, the conducting ZnO film can serve as broadband antireflection coatings for substrates and optics in the terahertz frequency range.