Wavelength Axis Research Articles

Linear and circular dichroisms of cholesteric liquid crystals in a longitudinal external magnetic field

ABSTRACT We investigated the properties of linear and circular dichroisms of cholesteric liquid crystal layer in external static magnetic field directed along helix axis. We showed that when the direction of light propagation and the direction of external magnetic field coincide, increasing the latter magnitude leads to a decrease in the linear dichroism, whereas when these directions are opposite, increasing the external magnetic field ( B e x t ) magnitude leads an increase in the linear dichroism. The presence of external magnetic field leads to the shift of circular dichroism curves both along wavelength axis and along dichroism axis. Then, we investigated the peculiarities of dichroism dependencies on B e x t for different wavelengths of incident light. We compared the dependence of circular dichroism on B e x t for a magnetically active cholesteric liquid crystal layer and an isotropic magnetically active layer and showed that while these dependencies are similar for wavelengths away from the photonic band gap, diffraction significantly affects these dependencies for wavelengths near and inside the photonic band gap. We investigated the evolution of spectra of linear and circular dichroisms at the change in B e x t , too. The results obtained can find application in the design of innovative polarised optoelectronic devices.

18‐4: Late‐News Paper: Lambda Capture method ”λ‐Capture” and the applications for quality control of Displays and the devices

This paper describes a new, fast, Wave‐Length of Center‐of‐gravity (WLC) detection method, named “λ‐Capture”. The paper presents and explains the principle of this new technology as a wavelength management technique for displays, utilizing a Linear Reflectance Gradient on the wavelength axis (LRG) dichroic mirror. It is then applied to a mini‐LED wafer: sample of actual display elements and demonstrated to have satisfactory performance. Furthermore, the possibility of new applications based on this principle is demonstrated.

Single- and Double-Comb Tilted Fibre Bragg Grating Refractive Index Demodulation Methods with Fourier Transform Pre-Processing.

The development of fibre optic sensors for measuring the refractive index is related to the creation of new periodic structures and demodulation algorithms for the measured spectrum. Recently, we proposed a double-comb Tilted fibre Bragg grating (DCTFBG) structure. In this article, we analyse such a structure for measuring the refractive index in comparison to a single classical structure. Increasing the number of modes causes a significant change in the Fourier spectrum of optical spectra. For the purpose of data pre-processing, we propose the Fourier Transform as a filtering method in the frequency domain. Then, we analyse separately the band-filtered optical spectra for several frequency ranges. For quantitative analysis, we use algorithms that use quantitative changes in the transmission, i.e., the method of the envelope and the length of the spectrum contour. We propose the use of the Hilbert transform as the envelope method. The second type of algorithms used are methods determining the shift of spectrum features along the wavelength axis. The method of determining the centre of gravity of the area bounded by the envelope and the maximum of the second derivative of the smoothed cumulative spectrum contour length is proposed here. Using the developed methods, the measurement resolution was achieved at the level of 2 × 10−5 refractive index unit.

Methods of Projecting Mode Amplitude Changes on the Wavelength Axis in Order to Determine the Bending Radius on the Basis of TFBG Grating Spectra.

Tilted fibre Bragg grating (TFBG) are used as sensors to determine many quantities such as refractive index, temperature, stress, rotation and bending. The TFBG spectrum contains a lot of information and various algorithms are used for its analysis. However, most of these algorithms are dedicated to the analysis of spectral changes under the influence of the refractive index. The most popular algorithm used for this purpose is to calculate the area occupied by cladding modes. Among the remaining algorithms, there are those that use the determination of the cut-off wavelength as a surrounding refractive index (SRI) indicator. Projection on the wavelength axis can also be used to calculate the bending radius of the fibre. However, this is a more difficult task than with SRI, because the mode decay in bending is not so easy to catch. In this article, we propose a multi-step algorithm that allows to determine the impact of bending on mode leakage. At the same time, the place on the wavelength from the side of the Bragg mode and the ghost mode is determined, which represents the cladding mode radiated from the cladding under the influence of bending. The developed algorithm consists of the following operations carried out on the transmission spectrum: Fourier filtering, calculation of the cumulative value of the spectral length, low-pass filtering of the cumulative curve or its corresponding polynomial approximation, determination of the first and second derivative of the approximated curve, and projection of the second derivative of the curve on the wavelength axis. The shift of the wavelength determined in this way indirectly indicates the bending radius of the optical fibre. Based on multiple measurements, we prove that the presented algorithm provides better results when determining the bending radius compared to other algorithms adopted for this purpose and proposed for SRI measurements. Additionally, we analyse the method of determining the shift of a fragment of the spectrum using the phase of the discrete Fourier transform.

Unmixing of the background components in an off-axis holographic-mirror-based imaging system using spectral image processing.

An imaging system employing a volume holographic optical element (vHOE) has a unique feature by which the image of an object placed in front of a transparent screen can be captured. The system obtains an image of the light diffracted by the vHOE; however, unwanted background components, including direct reflection or transmitted components, are present in the captured image. In this study, we propose a method to eliminate such background components from the captured images via multispectral image processing. The image components obtained from the diffraction by vHOE can be successfully extracted utilizing the facts that it diffracts light with a narrow spectral range and the reflection spectrum of a natural object is mostly smooth in the wavelength axis. Computer simulations and experiments confirm the effectiveness of the proposed method, along with video applications that demonstrate its real-time capability.

The Essence and Application of the Optical Absorption Method for Quantitative and Qualitative Analysis of Radiation Defects in Optical Crystals

Today ionic crystals are widely used in devices for various purposes. In X-ray spectral optics they are widely used as crystal monochromators; ionic crystals are used in optical devices where lenses and transparent optical media (light filters) are made of optically pure materials - ionic crystals. In general, the main positive feature of these materials is transparency regarding the transmission of radiation in the visible region of the spectrum (transmittance of about 0.9) and neutrality - that is, approximately the same reaction of the medium to different spectral ranges of radiation. Ionic crystals are also widely used in detectors (scintillators, ionizing radiation dosimeters) and lasers. They are also widely used in acousto-optics and electrical engineering (lines of electrical signals delay, which gain efficiency due to the relatively small absorption of ultrasonic waves, and, therefore, it is possible to work with a wide sequence of signals probing the crystal). It is known that when ionizing radiation passes through ionic crystals, color centers appear in them, which can change the spectral composition of radiation both in the UV region and in the visible range. For example, the simplest configurations of color centers (F-centers) lead to the appearance in optical materials of additional absorption bands localized on the wavelength axis with a maximum at the wavelength lmax = 248 нм , but more complex configurations of radiation damage in solids already lead to the appearance of absorption bands at wavelengths in the visible range. This already presents some difficulties for developers and designers of relevant equipment, as changes in the spectral composition of radiation passing through the optical system of the device can lead, for example, to loss of efficiency of the selected radiation receiver, the main characteristic of which is primarily spectral sensitivity. Taking into account possible changes in the spectral composition of radiation is an important and urgent task of modern optical instrumentation. The purpose of this work is the analysis and justification of a method that takes into account structural changes in externally irradiated ionic crystals.

An enhancement approach of fluorescence signatures in excitation emission matrixes for water contaminant analysis

Three-dimensional fluorescence spectroscopy has been widely used for the analysis of water contaminants. However, the problems of weak signals and overlapping fluorescence peaks remain unresolved. In this work, we studied the impact of absorption on the spectral shape of fluorescence and found that it is a major cause of overlapping peaks and weak signals. An approach is proposed to purify fluorescent signals and enhance fluorescence signatures based on the theory of fluorescence quantum yield. Using this theory, the problems of noise amplification and singularity points were identified, and an optimization algorithm was proposed related to Wiener filtering. For practical application to multiple compounds, three overlapping cases were discussed theoretically. The effectiveness of this procedure in subsequent parallel factor analysis was assessed and compared with original data by conducting experiments with six typical compounds and real water samples. The results indicate that overlapping along the excitation wavelength axis can be reduced despite the existence of multiple compounds, and the sensitivity of weak fluorescent signals can be significantly improved. The proposed method can enhance fluorescence signatures for the separation and analysis of fluorescent components in water contaminants.

Scanning Spectrometer-on-a-Chip Using Thermo-Optical Spike-Filters or Vernier-Comb Filters

This theoretical modeling and simulation paper presents designs and projected performance of an on-chip scanning spectrometer using thermo-optical (TO) spike- or Vernier-comb filters operating around 3600 nm for the germanium-on-nitride technological platform. Raman spectra and absorption spectra are targeted. The spike filter consists of a thermo-optically tunable waveguide Bragg grating resonator (WBGR). The length of the WBGR is chosen to realize a Lorentzian resonant spectral profile with a bandwidth changing between 0.1 and 1 nm. The Vernier-comb filter is obtained using the cascade of two multi-Sagnac loop Mach–Zehnder interferometers, in which through the spectrum of the second MZI is shifted along the wavelength axis by means of a low-power TO heater stripe atop each loop-connector. The sagnac loop reflectors length is designed according to the wavelength-channel spacing requirements in order to induce the Type-1 Vernier effect. Time-scanned discrete sampling of the unknown input spectrum is obtained by driving the TO heater stripe with a periodic ramp current. We examined the device performance in terms of signal reconstruction in the mid-infrared region. The investigation demonstrated that high quality spectrum reconstruction is obtained for arbitrary input signals with an observation wavelength range from 23.4 to 77.4 nm. We also show that spectrum reconstruction of arbitrary signals is feasible in the germanium on nitride chips even in presence of additive noise.

Integrated On-Chip Bragg Time-Delay System for Thermo-Optical Control of a Microwave Antenna

This theoretical modeling-and-simulation paper presents designs and projected performance of ∼1550-nm silicon-on-insulator beamsteering of a microwave phased array antenna by the use of true time delays. The tunable on-chip optical delay line is a cascade connection of waveguide Bragg grating resonators (WBGRs) separated by a piece of straight waveguide. The notch in the reflectivity spectrum is translated along the wavelength axis by means of a low-power TO heater stripe atop the grating, inducing a time delay that depends upon the line position of the WBGR affected by TO switching. The filter resonator is a new in-guide array of identical Bragg structures, each one comprising N closely coupled phase-shifted Bragg-grating resonators. The length of each grating cavity in an N group is chosen according to the Butterworth filter technique to provide one resonant spectral profile with 40 GHz optical bandwidth. Finally, we examined the performances of the beamformer system operating in the X and Ku bands, respectively. The investigation demonstrated that steering angles up to 48° are feasible by assuming a minimum steering angle of 8° and a minimum WBGR spacing of about 354 and 236 μ m for X and Ku bands, respectively.

Optical modeling of wide-bandgap perovskite and perovskite/silicon tandem solar cells using complex refractive indices for arbitrary-bandgap perovskite absorbers

Wide-bandgap perovskites are attractive top-cell materials for tandem photovoltaic applications. Comprehensive optical modeling is essential to minimize the optical losses of state-of-the-art perovskite/perovskite, perovskite/CIGS, and perovskite/silicon tandems. Such models require accurate optical constants of wide-bandgap perovskites. Here, we report optical constants determined with ellipsometry and spectrophotometry for two new wide-bandgap, cesium-formamidinium-based perovskites. We validate the optical constants by comparing simulated quantum efficiency and reflectance spectra with measured cell results for semi-transparent single-junction perovskite cells and find less than 0.3 mA/cm2 error in the short-circuit current densities. Such simulations further reveal that reflection and parasitic absorption in the front ITO layer and electron contact are responsible for the biggest optical losses. We also show that the complex refractive index of methylammonium lead triiodide, the most common perovskite absorber for solar cells, can be used to generate approximate optical constants for an arbitrary wide-bandgap perovskite by translating the data along the wavelength axis. Finally, these optical constants are used to map the short-circuit current density of a textured two-terminal perovskite/silicon tandem solar cell as a function of the perovskite thickness and bandgap, providing a guide to nearly 20 mA/cm2 matched current density with any perovskite bandgap between 1.56 and 1.68 eV.

Multiple-Sagnac-Loop Mach–Zehnder Interferometer for Wavelength Interleaving, Thermo-Optical Switching and Matched Filteri

This theoretical modeling and simulation paper presents design and projected performance of an on-chip thermo-optical multi Sagnac loop Mach–Zehnder interferometer (MZI), operating at C-band and 3300 nm for silicon-on-insulator and germanium-on-silicon technological platforms, respectively. The MZI arms are comprised of an N -cascaded connection of Sagnac loop reflectors, each pair separated by a waveguide whose length is designed according to the wavelength-channel spacing requirements. N is chosen according to the Butterworth filter technique to provide one spectral response having maximal flatness within the desired bandwidth. Switched, spatial routing of odd and even channels is achieved by shifting the Through and Drop MZI spectra along the wavelength axis by means of a low-power thermo-optical heater stripe atop each loop-connector that changes its effective refractive index appropriately. We examined the SOI device performance for wavelength-division multiplexed wavelength (de)interleving. The analysis predicted good performance in terms of bandwidth, insertion loss, and channel isolation for 200, 50, and 25 GHz channel spacing. For the Ge-on-Si Sagnac MZI operating in the mid infrared, we showed that a comb filter is feasible whose lines exactly match the spectral signature of methane for trace-gas sensing applications.

On-Chip Digital Fourier-Transform Spectrometer Using a Thermo-Optical Michelson Grating Interferometer

This theoretical modeling and simulation paper presents designs and projected performance of an on-chip digital Fourier transform spectrometer using a thermo-optical (TO) Michelson grating interferometer operating at ∼1550 and 2000 nm for silicon-on-insulator and for germanium-on-silicon technological platforms, respectively. The Michelson interferometer arms consist of two unbalanced tunable optical delay lines operating in the reflection mode. They are comprised of a cascade connection of waveguide Bragg grating resonators (WBGRs) separated by a piece of straight waveguide with lengths designed according to the spectrometer resolution requirements. The length of each WBGR is chosen according to the Butterworth filter technique to provide one resonant spectral profile with a bandwidth twice that of the spectrometer bandwidth. A selectable optical path difference (OPD) between the arms is obtained by shifting the notch in the reflectivity spectrum along the wavelength axis by means of a low-power TO heater stripe atop the WBGR, inducing an OPD that depends on the line position of the WBGR affected by TO switching. We examined the device performances in terms of signal recostruction in the radio-frequency (RF) spectrum analysis application at 1 GHz and at 1.5 GHz of spectrometer resolution. The investigation demonstrated that high-quality spectrum reconstruction is obtained for both Lorentzian and arbitrary input signals with a bandwidth up to 40 GHz. We also show that spectrum reconstruction of 100–200 GHz RF band input signals is feasible in the Ge-on-Si chips.

Reconfigurable optical-microwave filter banks using thermo-optically tuned Bragg Mach-Zehnder devices.

A new reconfigurable, tunable on-chip optical filter bank is proposed and analyzed for the silicon-on-insulator platform at the ~1550 nm wavelength. The waveguided bank is a cascade connection of 2 x 2 Mach-Zehnder interferometer (MZI) filters. An identical standing-wave resonator is situated in each MZI "arm." Using the thermo-optic (TO) effect to perturb this waveguide's index, the TO heater stripes provide continuous tuning of the filter by shifting the resonance smoothly along the wavelength axis. To reconfigure and program the cascade array, a broadband 2 x 2 MZI-related switch is inserted between adjacent filters. The novel TO switch, described here, can provide either single or double interconnection of 2 x 2 filters. The filter resonator is a new in-guide array of N closely coupled phase-shifted Bragg-grating resonators that provide one resonant spectral profile with 5 to 100 GHz bandwidth. The length of each grating cavity in the N group is chosen according to the Butterworth filter technique, and this gives high peak transmission for the composite. The predicted spectral profiles of a three-stage cascade show two-or-three peaks, or two-or-three notches with movable wavelength-locations as well as tunable wavelength-separations between those features. A tunable notch within a wider movable passband is also feasible. Potential applications include microwave photonics, wavelength-selective systems, optical spectroscopy and optical sensing.

Filter Selection for Optimizing the Spectral Sensitivity of Broadband Multispectral Cameras Based on Maximum Linear Independence.

Previous research has shown that the effectiveness of selecting filter sets from among a large set of commercial broadband filters by a vector analysis method based on maximum linear independence (MLI). However, the traditional MLI approach is suboptimal due to the need to predefine the first filter of the selected filter set to be the maximum ℓ2 norm among all available filters. An exhaustive imaging simulation with every single filter serving as the first filter is conducted to investigate the features of the most competent filter set. From the simulation, the characteristics of the most competent filter set are discovered. Besides minimization of the condition number, the geometric features of the best-performed filter set comprise a distinct transmittance peak along the wavelength axis of the first filter, a generally uniform distribution for the peaks of the filters and substantial overlaps of the transmittance curves of the adjacent filters. Therefore, the best-performed filter sets can be recognized intuitively by simple vector analysis and just a few experimental verifications. A practical two-step framework for selecting optimal filter set is recommended, which guarantees a significant enhancement of the performance of the systems. This work should be useful for optimizing the spectral sensitivity of broadband multispectral imaging sensors.

Application of JC1 for non-toxic isolation of cells with MDR transporter activity by flow cytometry.

The DNA intercalating dye Hoechst 33342 or its close analog DCV are actively removed from cells by the multidrug resistance transporter ABCG2, a protein overexpressed in metastatic cells and somatic stem cells. In bivariate blue-red flow cytometry fluorescent plots active Hoechst or DCV efflux combined with a concentration dependent bathochromic shifts of these nuclear dyes leads to the segregation of the transporter-rich cells into a distinct cell cohort tilted towards the shorter wavelength axis of the plot, the cohort is generically known as the side population (SP). This feature has facilitated the surface marker-independent isolation of live stem cells. A drawback, though, is the known toxicity of Hoechst dyes. In this study we show that JC1, a bathochromic mitochondrial membrane potential-sensitive dye applied at proper concentration, can yield flow cytometry fluorescent emission bivariate plots containing a low JC1 accumulation (JC1low) cohort. Using a combination of multiple cell lines, ABC-transporter inhibitors and viral vector-driven insertion of the ABCG2 gene or ABCG2 and ABCB1 shRNAs we demonstrate that JC1low can be generated by either of the two aforementioned multidrug resistance transporters. Complete wash out of mitochondrial bound JC1 required more than 24 h. In spite of this tight binding, the dye did not affect either the mitochondrial membrane potentials or the proliferation rate. In contrast, contemporaneous with its nuclear accumulation, Hoechst 33342 or DVC, caused changes in the fluorescent emission of mitochondrial membrane potential sensitive dyes resembling the effects caused by the mitochondrial uncoupler FCCP. In a number of cell lines exposure to Hoechst resulted in marked slow-down of proliferation and abolition of ABCG2 transport activity during the subsequent 2 days but in K562 cells the exposure induced cell extended death. Overall, its lack of toxicity vis. a vis. the toxicity and genotoxicity of the DNA intercalating dyes makes JC1 an ideal tool for isolating live cells expressing high multidrug resistance transport activity.

Self-reference line-dispersion interferometric microscopy

We have developed a line-scanning interferometric microscope capable of measuring the quantitative phase over a wide continuous spectrum. The sample is illuminated with a spatially coherent broadband light source shaped into a line beam that interferes with itself in a Michelson configuration, coupled with a grating-based spectrometer. The spatial modulation of the interference pattern in the direction orthogonal to the wavelength axis in the spectrograph image plane allows obtaining the spectral quantitative phase along the line illumination in a single measurement. We show the feasibility of the technique by quantifying the refractive index of polystyrene beads immersed in oil and the dispersive properties of live cells over the visible spectrum.

Detailed analysis of the online near-infrared spectra of pharmaceutical blend in a rotary tablet press feed frame

The present study is an in-depth analysis of the online near-infrared (NIR) spectra acquired with a spectrometer mounted on the rotary tablet press feed frame. A 3.5% active pharmaceutical ingredient (API) formulation is analyzed. An attempt is made to determine the API univariately via the 2nd derivative spectra due to favorable appearance of the API and placebo bands in the formulation. However, the signal at the univariate API peak is ambiguous and principal component analysis is hence applied to understand better the structure of the data. To eliminate effect of the bias along the wavelength axis which is found to dominate the data, the analysis is restricted only to the spectral region that covers the API band of interest. This leads to significantly better results in terms of the univariate profile of the API now heavily overlapping with the first principal component and the elimination of the bias. Having thus proven that the univariate analysis is a viable option, an attempt is made to calibrate the API response by using some previous runs that involved alternation of the placebo and the formulation. This analysis produces mixed results due to baseline differences in the two sets of spectra. The final univariate profile of the API is therefore compared with the assay of the tablets and the two are found to agree very well. It is, therefore, concluded that the NIR probe in the feed frame can rapidly detect moderate changes in API concentration in the blend and be a good predictor of tablet potency at the same time point.

Spectral and temporal characteristics of a ZnSe:Fe2+ laser pumped by a non-chain HF(DF) laser at room temperature

Spectral and temporal characteristics of a ZnSe:Fe2+ laser with a nonselective resonator pumped at room temperature by the radiation of a pulse-periodical electrodischarge HF(DF) laser are studied. It was established that the spectral distributions of the energy and peak power of ZnSe:Fe2+ laser generation depend on a spectral composition of the pumping radiation. The spectra exhibit a line structure with spectral intervals between neighboring lines δλ ≈ 6.8 ÷ 8.6 nm. The shape of the ZnSe:Fe2+ laser generation pulse is wavelength dependent. In a short-wavelength range, the pulse has the form of a peak with a duration of ~5 ns at half-maximum. At a longer wavelength, the peak is accompanied by a ‘tail’. The duration and amplitude of the tail increase with wavelength, in a long-wavelength spectrum range, the peak actually becomes unnoticeable on a background of the ‘tail’. The spectral dependence of the ZnSe:Fe2+ laser generation pulse’s shape affects the positions of the energy and peak power maxima on the wavelength axis. The dynamics of ZnSe:Fe2+ laser generation under the pumping by the pulsed HF(DF) laser is discussed.

Model-based, closed-loop control of PZT creep for cavity ring-down spectroscopy

Cavity ring-down spectrometers typically employ a PZT stack to modulate the cavity transmission spectrum. While PZTs ease instrument complexity and aid measurement sensitivity, PZT hysteresis hinders the implementation of cavity-length-stabilized, data-acquisition routines. Once the cavity length is stabilized, the cavity’s free spectral range imparts extreme linearity and precision to the measured spectrum’s wavelength axis. Methods such as frequency-stabilized cavity ring-down spectroscopy have successfully mitigated PZT hysteresis, but their complexity limits commercial applications. Described herein is a single-laser, model-based, closed-loop method for cavity length control.

Potential problems in collection and data processing of luminescence signals

Luminescence studies are central to a very wide range of disciplines, both as a primary experiment and in a minor role for additional discrimination between samples. Unfortunately when luminescence studies are not the central objective and expertise the data collection, instrumental corrections and data analysis are not always being totally, or correctly, employed. There are often historical reasons for this but with modern equipment one can readily make the requisite compensations. The problems are outlined with emphasis on spectral and polarisation response of spectrometers and detectors. Typical data processing errors are noted with demonstrations of their consequent effects on the signals. These include the fact that the peak in the wavelength presentation can significantly differ from the true energy centre of a Gaussian emission band. There can be failure to totally compensate for the spectral sensitivity of the detection system, as well as the incorrect use of band de-convolution on the wavelength representation, and energy plots where only the wavelength axis has been corrected; these mistakes all distort the true spectra. Not only are these analyses physically incorrect, but they are misleading, and introduce false features. This brief review indicates why such processing errors can generate spectral differences cited in the luminescence literature that are from measurement, rather than differences between source materials.