Measured Transmission Spectra Research Articles

Generation of the radial higher-order orbital angular momentum mode in helically twisted elliptic fiber.

The generation mechanism and regulation method of the radial higher-order orbital angular momentum (OAM) mode based on helically twisted elliptic fiber (HTEF) are investigated. This work aims to reveal the quantitative relation between the twisted rate (α) and the target radial higher-order OAM mode. From theoretical simulations, it is concluded that α is the key to regulating the radial higher-order OAM mode. In the experiment, OAM2n modes with radial higher-order n of 2, 3, 4, and 5 are successfully generated at 1550 nm by regulating α of the HTEF. Finally, the transmission spectrum and purity measurements are performed for the OAM24 mode, and it can be concluded that the fabricated experimental samples have more than 90% conversion efficiency and close to 94% purity under the appropriate α and twisted length.

Observation of zero-transmission dip in a single-layer electric metamaterial with finite non-radiative loss

We propose a theory for realizing a zero-transmission dip in the transmission spectrum of a reflectionless single-layer metamaterial designed based on the Brewster effect by variably controlling the radiative loss of the metamaterial in response to the non-radiative loss. The radiative loss can be controlled while maintaining broadband zero reflection by varying the relationship between the orientation of the constituent meta-atoms and the incident electromagnetic fields. As a verification of the proposed theory, we design a reflectionless metamaterial by arranging meta-atoms that exhibit a simple electric dipole resonance in a two-dimensional lattice. The numerically calculated and experimentally measured transmission spectra of this metamaterial demonstrate that the radiative loss can be controlled by changing the arrangement of the meta-atoms without altering their structure, and that a zero-transmission dip can be observed for a certain arrangement of the meta-atoms. This study could lead to the development of material sensing, especially for lossy materials based on resonant metamaterials.

Quantitative Complex Refractive Index Changes in Thin Films: A Pump-Probe Spectroscopy Analysis Approach.

Photoexcitation induces intricate changes in both the real and imaginary components of the complex refractive index of thin film materials, which is essential for interpreting transient spectral features. Here, we employ a Kramers-Kronig-based analytical approach to elucidate light-induced changes in the complex refractive index from transient transmission spectra of thin films. Using gold-perovskite films as model systems, we conduct experimental measurements of transient transmission spectra for both individual gold and perovskite films, as well as for the bilayer heterostructure. Our analysis reveals significant changes in the refractive index and absorption for these systems. Notably, we observe negligible photocarrier transfer between the gold and perovskite layers based on transient spectroscopic analysis. These findings have implications for the design and optimization of bilayer heterostructures in optoelectronic applications. This work highlights the importance of spectroscopic techniques in studying the photophysical properties of heterostructure films.

New polymeric nanocomposite thin films for transparent optoelectronics: Impact of In(acac)3 on the structural, optical and photoelectrical characteristics of PVA films

Concerning the rapid progress in elastic and transparent optoelectronics, the need for cost-effective materials for transparent optoelectronic devices has become a mainstream research trend. Therefore, this report aimed to explore the modifications of the PVA characteristics due to In(acac)3 doping. New PVA:In(acac)3 nanocomposite thin films were fabricated using a cost-effective and facile spin-coating technique with varying the In(acac)3 loading concentration. The structural and molecular feature variations of PVA have been tracked using XRD and FTIR techniques, respectively. Furthermore, by varying In(acac)3′s concentration, the fabricated films' linear/nonlinear optical properties have been investigated symmetrically using the UV–visible-NIR spectrophotometric technique. The measured transmittance spectra of the prepared films showed the tunability of the energetic position of the fundamental edge while conserving the high transparency. The energy gap and Urbach energy of the prepared thin films have been calculated as a function of doping concentration. The nonlinear refractive index and optical susceptibilities were estimated for all the fabricated films. On the other hand, the photodetection response of the fabricated Schottky barrier devices was tested. The fabricated devices showed pronounced UV response, where the photocurrent density was significantly enhanced under UV illumination up to 0.1 mA/cm2 for the 20 wt.% In(acac)3 doped films. The photodetection performance of the manufactured nanocomposite thin films under repeatable On/Off switching cycles was tested. The optimum doping concertation of In(acac)3 was determined in terms of the analyzed optical and photodetection characteristics to be 20 wt.%. The obtained results support the careful reconsideration of the possibility of exploiting some cheap polymers as important components in advanced optoelectronic applications.

Experimental investigation of acoustic moiré effect controlled by twisted bilayer gratings

Recently, the moiré pattern has attracted lots of attention by superimposing two planar structures of regular geometries, such as two sets of metasurfaces or gratings. Here, we show the experimental investigation of acoustic moiré effect by using twisted bilayer gratings (i.e., one grating twisted with respect to the other). We observed the guided resonance that occurred when the incident ultrasound beam was coupled with the guiding modes in a meta-grating, significantly influencing the reflection and transmission. Tunable guided resonances from the moiré effect with complete ultrasound reflection at different frequencies were further demonstrated in experiments. Combining the measurements of transmission spectra and the Fast Fourier Transform analyses, we reveal the guided resonance frequencies of moiré ultrasonic metasurface can be effectively controlled by adjusting the twisting angle of the bilayer gratings. Our results can be explained in a simplified model based on the band folding theory, providing a reliable prediction on the precise control of ultrasound reflection via the twisting angle adjustment. Our work extends the moiré metasurface from optics into acoustics, which shows more possibilities for the ultrasound beam engineering from the moiré effect and enables the exploration of functional acoustic devices for ultrasound imaging, treatment and diagnosis.

Low thermal crosstalk silicon MZI optical switch with high speed and low power consumption

Abstract We developed a compact thermo-optic Mach–Zehnder interferometer switch with a direct heating heater using multimode interference and achieved a sufficiently low thermal crosstalk performance. Large-scale switch systems, such as optical neural networks, require thermo-optical switches with low power consumption, fast switching speed, compact size, and low thermal crosstalk. This switch is equipped with a heater that directly heats the Si core waveguide, which is a structure that connects non-doped Si wires between phase shifters and a heatsink. As a result, a significant miniaturization with a phase shifter length of approximately 7 μm, low π-phase shift power consumption of less than 20 mW, and fast switching in sub-microseconds were achieved. The improved phase shifter showed a very small figure of merit of 8.89 mW∙μs. Simultaneously, transmission spectrum measurements of nearby ring resonators show that the thermal crosstalk is significantly reduced even at a distance of only 30 μm. This device can contribute to the overall circuit performance and footprint reduction in large-scale optical integrated circuits and optical neural network configurations.

Influence of Congored on the Optical, Dielectric and Thermal Behavior of Polymethyl Methacrylate and Polyvinyl Chloride

Polymethyl methacrylate (PMMA)/Congo Red (CR) and polyvinyl chloride (PVC)/CR composite films were prepared on optical glass by the drop method for optical measurements and also prepared as tablets under pressure for dielectrical measurements. The optical transmission spectra measurements for PMMA and PVC film doped with CR showed a highly absorption in the ultraviolet-visible (UV-Vis) optical range (at 353 nm and 518 nm) and considerably blocking in broad visible range (190 nm–640 nm). PMMA/CR and PVC/CR composite films containing congo red at a concentration of 1 wt% showed maximum absorption bands at 290 nm and 416 nm, but their transmittance percentages were reasonably reduced compared to those of pure PMMA and PVC. Both of composites indicated high transmittances between 640 nm and 800 nm. The optical constants of the PMMA/CR and PVC/CR composite films such as optical band energy gaps (Eg) estimated using Tauc’s model, refractive index and reflectance (%) were calculated. UV–Vis, Fourier transform infrared spectroscopy (FTIR) and Thermogravimetry Analysis (TGA) measurements were used to confirm the breakdown of the -N = N- azo group bonds attached to the aromatic ring as a result of thermal decomposition of CR at 500 °C, and it was suggested that N2 gas was eliminated from the CR above 360 °C. The dielectric plots showed that the dielectric constant increased when the CR is doped up to 70 wt% in the polymer matrices. A further increase in the doping concentration of CR resulted in an increase of the dielectric constant. The dielectric constant decreased with an increase in the frequency for all of the samples. The ac (alternating current) conductivity values of PMMA and PVC showed an increase with increasing CR dopant in the polymer composites, while the ac conductivity values were found to be values between those of the pure polymers and CR.

Optical and morphological properties of DCM thin films co-doped of Znq2 by PVD: Theoretical and experimental investigations

In this article, new experimental results of the morphological and optical properties of thin films of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran co-doped by 8-hydroxyquinoline zinc (Znq2) for different concentrations were presented. The physical vapor deposition in a high vacuum was used to create the thin films on glass substrates with a thickness of 100 nm. The morphological properties of the samples were identified using the Atomic Force Microscopy (AFM) apparatus in tapping mode. TEM images show amorphous structures. The phase composition of the samples was assessed using FTIR methods. The Tauc plot approach was used to examine the measured transmittance spectra in order to estimate the energy gap. Analysis of the results showed that the co-doping of DCM thin film by Znq2 influenced the structural and optical properties of the samples. To evaluate the experimental results found, theoretical calculations based on the maximum one-photon absorption (OPA) wavelengths of DCM and Znq2 have been first measured through the UV–Vis spectral technique. Then, the experimental conclusions on values for DCM and Znq2 have been compared with their corresponding simulation data procured from the time-dependent self-consistent-field (TD-SCF) computation utilizing the density functional theory (DFT) at B3LYP/6-311G(d,p) level. Besides, we have explored the first and second frontier molecular orbitals (MOs) for title compounds and their energy gaps via the DFT procedure. The calculations of maximum OPA wavelengths and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) contributions on both molecules have also been crosschecked with the results of already published data in the literature.

Inverse-designed flexural wave metamaterial beams with thermally induced tunability

Elastic metamaterials (EMMs) have been developed for more than two decades, but their practical applications have been scarcely reported due to their narrow bandgaps, relatively high working frequency ranges, poor load-bearing capabilities, and other implementation-related issues. As an attempt to solve some of these problems, in this work, a novel kind of tunable metamaterial beams is proposed to mitigate or isolate low-frequency and broadband flexural waves, where the integrated tunable elements made of a 3D-printed thermally sensitive material are inversely designed. The tunability is further achieved through the thermal-driving on the tunable elements. The optimized tunable elements are fabricated independently and have pretty good load-bearing capabilities under quasi-static uniaxial compression tests. They are directly glued on the host beams to form simple structured metamaterial beams without destruct the host beams. The experimentally measured transmission spectra under different thermal loadings demonstrate that the third pass-bands of the optimized three metamaterial beams at room temperature (RT) are changed to bandgaps or stop-bands as the temperature increases to about 60 °C, and vice versa. In addition, this temperature change induced bandgap is concatenated to the optimization obtained bandgap to form a wider bandgap. The good thermal reversibility is also validated by experimental tests under several thermal-loading cycles. Our present work suggests a dual-technique combined method to design low-frequency and broadband EMMs.

Experimental study of modified Tavis-Cummings model with directly-coupled superconducting artificial atoms.

The Tavis-Cummings model is intensively investigated in quantum optics and has important applications in generation of multi-atom entanglement. Here, we employ a superconducting circuit quantum electrodynamic system to study a modified Tavis-Cummings model with directly-coupled atoms. In our device, three superconducting artificial atoms are arranged in a chain with direct coupling through fixed capacitors and strongly coupled to a transmission line resonator. By performing transmission spectrum measurements, we observe different anticrossing structures when one or two qubits are resonantly coupled to the resonator. In the case of the two-qubit Tavis-Cummings model without qubit-qubit interaction, we observe two dips at the resonance point of the anticrossing. The splitting of these dips is determined by Δ λ=2g12+g32, where g1 and g3 are the coupling strengths between Qubit 1 and the resonator, and Qubit 3 and the resonator, respectively. The direct coupling J12 between the two qubits results in three dressed states in the two-qubit Tavis-Cummings model at the frequency resonance point, leading to three dips in the transmission spectrum. In this case, the distance between the two farthest and asymmetrical dips, arising from the energy level splitting, is larger than in the previous case. The frequency interval between these two dips is determined by the difference in eigenvalues (Δ λ=ε 1+-ε 1-), obtained through numerical calculations. What we believe as novel and intriguing experimental results may potentially advance quantum optics experiments, providing valuable insights for future research.

A simple new method for retrieving spectral changes of the refractive index of thin films from transmission spectra

A new method for calculating the refractive index spectral behaviour of thin films is presented. The method is based on introducing the idea of the order of appearance technique to accurately find the order of interference and Cauchy fitting parameters of the interference peaks observed in the transmission spectra of the thin film under investigation. A very simple mathematical relationship was found that relates the order of appearance and wavenumber of the interference peaks observed in the transmission spectrum. Fitting the data extracted from the measured transmission spectra to the newly found simple mathematical relation, we can extract the order of interference and Cauchy fitting parameters of the thin film under investigation. In addition, the thickness of the film was extracted and found to agree well with the thickness measured using the spectroscopic ellipsometery technique. The proposed method of calculation was successfully applied to five binary semiconductor samples, which are ZnO, ZnTe, ZnS, ZnSe, and SiO2. A very good agreement was found between the proposed method of calculation and the well-known method presented by Swanepoel. Finally, the effect of surface roughness on the FECO peak wavelength was discussed.

Electron spin resonance study of an annealed Cr,Mg:YAG epitaxial saturable absorber

We have demonstrated the use of electron spin resonance (ESR) spectroscopy to observe the influence of high-temperature annealing in an oxidizing atmosphere on Cr3+ ion concentration changes in an epitaxial Cr,Mg:YAG saturable absorber. For this purpose, a set of samples of epitaxial Cr,Mg:YAG/YAG structures differing in both the concentration of Cr and Mg ions and the concentrations ratio of these ions have been synthesized using liquid phase epitaxy. The X-ray diffraction analysis and optical transmission spectra measurements were performed to characterize the obtained layers. The usefulness of the ESR technique for monitoring certain high-temperature induced changes in epitaxial Cr,Mg:YAG absorber has been verified.

Experimental study of the elastic enhancement factor in a three-dimensional wave-chaotic microwave resonator exhibiting strongly overlapping resonances.

We study the elastic enhancement factor and the two-point correlation function of the scattering matrix obtained from measurements of reflection and transmission spectra of a three-dimensional (3D) wave-chaotic microwave cavity in regions of moderate and large absorption. They are used to identify the degree of chaoticity of the system in the presence of strongly overlapping resonances, where other measures such as short- and long-range level correlations cannot be applied. The average value of the experimentally determined elastic enhancement factor for two scattering channels agrees well with random-matrix theory predictions for quantum chaotic systems, thus corroborating that the 3D microwave cavity exhibits the features of a fully chaotic system with preserved time-reversal invariance. To confirm this finding we analyzed spectral properties in the frequency range of lowest achievable absorption using missing-level statistics.

Application of polymeric materials in free space optics for communications

Design of free space polymeric optics for communications is presented. Selection of polymers is accomplished on base of their measured transmission spectra, refractive and dispersive properties. The proposed telescopic system of the transmitter consists of a doublet objective composed of styrenic and acrylic materials and an eye-piece made of carbonate and acrylic polymers to minimise the spherical aberrations. The receiver optics is more precise and is realised as a high aperture Cassegrain objective to collect as much as possible of the incoming light and to couple it to the optical fiber.

Sandwich Plate Structure Periodically Attached by S-Shaped Oscillators for Low Frequency Ship Vibration Isolation.

Locally resonant phononic crystals are a kind of artificial periodic composite material/structure with an elastic wave band gap that show attractive application potential in low-frequency vibration control. For low-frequency vibration control problems of ship power systems, this paper proposes a phononic crystal board structure, and based on the Bloch theorem of periodic structure, it uses a finite element method to calculate the band structure and the displacement fields corresponding to the characteristic mode and vibration transmission curve of the corresponding finite periodic sandwich plate structure, and the band gap characteristics are studied. The mechanism of band gap formation is mainly attributed to the mode coupling of the phononic crystal plate structure. Numerical results show that the sandwich plate structure has a double periodicity, so it has a multi-stage elastic wave band gap, which can fully inhibit the transmission of flexural waves and isolate the low-frequency flexural vibration. The experimental measurements of flexural vibration transmission spectra were conducted to validate the accuracy and reliability of the numerical calculation method. On this basis, the potential application of the proposed vibration isolation method in a marine power system is discussed. A vibration isolation platform mounted on a steel plate is studied by numerical simulation, which can isolate low-frequency vibration to protect electronic equipment and precision instruments.

Consideration of spectroscopic measurements with broadband Fourier domain mode-locked laser with two semiconductor optical amplifiers at ∼1550 nm

In this study, an experimental system was developed using a broadband wavelength-swept laser for fast and broadband spectroscopic measurements at ∼1550 nm. The broadband wavelength-swept laser employed Fourier domain mode-locking (FDML) to realize a high-speed sweep rate of 50.7 kHz. FDML lasers at ∼1550 nm experience a problem of the sweep bandwidth being limited by the amplification wavelength range of the semiconductor optical amplifier (SOA). To realize a sweep bandwidth >100 nm, the proposed broadband FDML laser incorporated SOAs with different amplification wavelength ranges in parallel. Consequently, it expanded the sweep bandwidth to 120 nm at a center wavelength of 1544 nm. The experimental system employed the broadband FDML laser and introduced reference and compensation optics. These optics can compensate for the effects of fluctuations in optical output intensity and wavelength shift in the laser to improve measurement stability. Moreover, the experimental system demonstrated fast transmission spectrum measurements with a wavelength range of 1500 to 1580 nm.

Photon echoes using atomic frequency combs in Pr:YSO - experiment and semiclassical theory.

Photon echoes in rare-earth-doped crystals are studied to understand the challenges of making broadband quantum memories using the atomic frequency comb (AFC) protocol in systems with hyperfine structure. The hyperfine structure of Pr3+ poses an obstacle to this goal because frequencies associated with the hyperfine transitions change the simple picture of modulation at an externally imposed frequency. The current work focuses on the intermediate case where the hyperfine spacing is comparable to the comb spacing, a challenging regime that has recently been considered. Operating in this regime may facilitate storing quantum information over a larger spectral range in such systems. In this work, we prepare broadband AFCs using optical combs with tooth spacings ranging from 1 MHz to 16 MHz in fine steps, and measure transmission spectra and photon echoes for each. We predict the spectra and echoes theoretically using the optical combs as input to either a rate equation code or a density matrix code, which calculates the redistribution of populations. We then use the redistributed populations as input to a semiclassical theory using the frequency-dependent dielectric function. The two sets of predictions each give a good, but different account of the photon echoes.

Experimental demonstration of a silicon-slot quasi-bound state in the continuum in near-infrared all-dielectric metasurfaces

We theoretically and experimentally investigate a metasurface supporting a silicon-slot quasi-bound state in the continuum (qBIC) mode resonating in the near-infrared spectrum. The metasurface is composed of circular slots etched in a silicon layer on a sapphire substrate. The symmetry of the metasurface unit cell is reduced in order to provide access to the symmetry-protected mode, whose properties are investigated by finite-element full-wave and eigenfrequency analysis. The measured transmittance spectra verify the excitation of the investigated qBIC mode with experimental quality factors exceeding 700. The near-field distribution of the resonant qBIC mode shows strong field confinement in the slots, leading to high sensitivity values for refractometry.

Appreciably optimization of PVA/PVP nanocomposite blend for enhanced optoelectronics properties and multifunctional applications

The optical and electrical properties of PVA/PVP were manipulated by optimizing the blend ratio between those polymers. The (40/60) wt.% of PVA/PVP achieved the lowest energy gap and the highest electrical conductivity. After that, the optical and electrical properties of blend films were engineered using Ni(OAc)2 for enhanced optoelectronic properties. The doping impact on the semicrystalline and molecular structures of the prepared blend was investigated. The optical properties of the doped blend films were examined using optical transmission and absorption spectra measurements. The nonlinear optical parameters such as nonlinear refractive index and third-order optical susceptibility were estimated. The prepared films showed a pronounced laser cut-off performance. The electrical properties of the fabricated films showed a tunable behavior with the variation of doping level. The engineered doped polymer blend films exhibited good sensitivity to visible light with promising values of responsivity and specific detectivity ∼5.89 mA/W, and 4.31 × 109 Jones, respectively.

Ultra-High Performance All-Silicon TM Polarizer Covering O-U Optical Communication Bands

Polarizers are the essential components to purify the polarization in the system demanding for the single polarization propagation. Polarizers with broad working bandwidth, low excess loss (EL) and large polarization extinction ratio (PER) are usually preferred. TE polarizers with ultrahigh performance have been demonstrated. However, the bandwidth of the previously reported transverse magnetic (TM) integrated polarizers is still limited to be < ∼160 nm when the EL < 1 dB and PER > 20 dB are simultaneously satisfied. In this paper, an all-silicon integrated TM polarizer with > 415 nm working bandwidth from 1260 nm to 1675 nm is experimentally demonstrated. The device consists of a double slot structure with 180° Euler bending on silicon waveguides. The effective refractive index (ERI) for TE polarization in the double slot structure is lower than that of the silica buffer/cladding while ERI for TM polarization is much larger than that of the silica buffer/cladding. Thus, TE polarization will leak into the substrate while TM polarization can propagate through the device with low loss. The measured transmission spectra for the fabricated polarizer show low EL < 0.9 dB and large PER > 25 dB over an ultrabroad band beyond the O-, E-, S-, C-, L-and U- bands.