Spectral Modulation Research Articles

Dynamic tuning of optical absorbance and structural color of VO2-based metasurface

Abstract Vanadium dioxide (VO2) is an attractive thermal-control material exhibiting low thermal hysteresis and excellent temperature cycling performance. However, the deficiencies including weak spectral shift and narrow-band absorption during insulating-metallic transitions hinder its application in optoelectronics. The transition metal dichalcogenides (TMDs) can provide a promising solution with their high dielectric properties and robust optical coupling. Here, we report a MoS2/VO2/Au/Si metasurface and investigate the dynamic tunability of its optical absorbance and structural color upon heating via spectroscopic ellipsometry measurements and numerical simulations. The first-principles calculations reveal that the dielectric absorptions of metallic and insulating VO2 oppositely response to temperature, closely related to the difference in the transitions of O-2p states. Finite-element simulations reveal that the introduction of MoS2 nanostructure induces more absorption peaks by 2∼3 and achieves strong absorption in the full wavelength range of visible light. The Fabry–Perot (F–P) resonance is the critical factor for the optimized optical absorption. The structural color is sensitive to environmental perturbations at high-ε state of VO2, lower oblique incidence angles, and heights of MoS2. This work seeks to facilitate the spectral modulation of phase change metamaterials and can be extended to photoelectric detection and temperature sensing applications.

Intelligent optical management for energy-efficient windows driven by mechano-thermochromism

The thermochromic windows based on vanadium dioxide (VO2) can intelligently modulate the transmittance of indoor solar radiation, which is significant for reducing building energy consumption. However, conventional thermochromic windows only have the ability of single spectral modulation, continuously switching the solar spectral transmittance in response to varying ambient temperature on a window is challenging. In this work, a SiO2-VO2-PDMS-driven by mechano-thermochromism intelligent optical management system was proposed for automatic switching and continuous adjustment of the solar spectral transmittance. Compared with the single-layer VO2 film, the SiO2-VO2 double-layer thermochromic window provides a gradual refractive index change from the top with nair = 1 to the middle with around 1.5, and then to the bottom with a higher refractive index, which suppresses reflections over a wider wavelength range. The results show that the SiO2-VO2 double-layer coating with a refractive index buffer layer exhibits improved optical performance with a solar modulation ability (ΔTsol) of 18.4% and an average luminous transmittance (Tlum,avg) of 49.1%. Furthermore, the design can achieve increases in the stretch-related solar modulation ability (ΔTsol,stretch) and the stretch-related luminous transmittance (Tlum,stretch) from 18.4% to 43.8% and 49.1% to 60.5%, respectively, which is realized by the deformation of polydimethylsiloxane (PDMS). Combining elastic stretching with thermochromic windows can dynamically modulate the solar spectral transmittance to improve thermal comfort. This work provides a promising approach to improve the industrial application of VO2 thermochromic windows, contributing to the advancement of research in green buildings, window design, and automotive energy efficiency.

Full-Stokes polarization multispectral images of various stereoscopic objects

Polarization multispectral imaging (PMI) has been applied widely with the ability of characterizing physicochemical properties of objects. However, traditional PMI relies on scanning each domain, which is time-consuming and occupies vast storage resources. Therefore, it is imperative to develop advanced PMI methods to facilitate real-time and cost-effective applications. In addition, PMI development is inseparable from preliminary simulations based on full-Stokes polarization multispectral images (FSPMI). Whereas, FSPMI measurements are always necessary due to the lack of relevant databases, which is extremely complex and severely limits PMI development. In this paper, we therefore publicize abundant FSPMI with 512 × 512 spatial pixels measured by an established system for 67 stereoscopic objects. In the system, a quarter-wave plate and a linear polarizer are rotated to modulate polarization information, while bandpass filters are switched to modulate spectral information. The required FSPMI are finally calculated from designed 5 polarization modulation and 18 spectral modulation. The publicly available FSPMI database may have the potential to greatly promote PMI development and application.

Synergistic photothermal-thermoelectric-photovoltaic energy generation via a transparent spectral modulating solar module

The efficiencies of photovoltaic (PV) and thermoelectric (TE) have been limited by the intrinsic materials properties. In solar applications, PV utilizes the shorter wavelength end of the solar spectrum while its longer portion (IR) can be photothermally (PT) converted to heat by special nano structures for TE electricity generation. We have developed spectral modulating nano systems made of Fe3O4@Cu2-XS nanoparticles and chlorophyllin capable of synergistic PT-TE-PV solar harvesting and energy generation. In this system, solar light is harvested through the transparent photothermal spectral modulator (TPSM) and segregated into different wavelengths: the IR is absorbed to photothermally heat up the TE hot end for the required thermoelectric temperature span; the UV/visible is directed to PV with reduced IR for significantly reduced heating, thus enhanced power conversion efficiency (PCE). Upon removal of IR, the PV surface temperature can be lowered from 86.9 ℃ to 65 ℃ after 120 min solar irradiation, resulting in PCE increase from 16.3 % (dropped from 25.1 % at 0 min) to 19. 4 %. A PT − TE − PV solar energy module (PTPSEM) is designed based on TPSM to synergistically generate energy by separately utilizing the full spectrum of solar light. Using the same solar light source, while generating electricity via PV, an additional 2.4 % is gained by a series of commercial thermoelectric generators (TEG), adding up to a total of 21. 8 % system efficiency which significantly exceeds the average efficiency of the commercial silicon PV panel used in this study. The spectral modulation mechanism through TPSM is identified based on the optical behaviors of chlorophyllin and Fe3O4@Cu2-XS. Also discussed is the design concept of PTPSEM.

Generation of Tunable Terahertz Waves from Tailored Versatile Spintronic Meta-Antenna Arrays.

Broadband spintronic terahertz (THz) radiation can be efficiently generated by spin-to-charge current conversion in a ferromagnetic/nonmagnetic heterostructure. There had been many studies on realizing the enhancement or the modulation of spintronic terahertz waves. However, reported devices so far focus on implementing certain specific modulation methods, either related to the spintronic stacks or related to the metamaterial structures. In this study, a set of femtosecond laser-driven versatile spintronic terahertz devices are proposed by integrating meta-antenna structures with W/CoFeB/Pt nanolayer stacks. These monolithic integrated devices exhibit spintronic terahertz wave emission, spectral modulation, and polarization manipulation simultaneously. The terahertz pulses are generated within the ferromagnetic heterostructure interfaces and transmitted along the metallic structures, leading to the modulation of the spintronic terahertz waves. Results have shown that the center-frequency shift is up to 140 GHz and the value of ellipticity can reach 0.6, demonstrating a set of integrated and efficient spintronic terahertz devices to modulate the emitted wave. In addition, compared with the slotline antenna, the maximum peak value of the bandpass band is enhanced up to 1.63 times by carefully designing the metamaterial structure. The spintronic meta-antenna array proposed here paves an integrated way for the manipulation of spintronic terahertz optoelectronic devices.

Aggregation Induced Transition by Modulation of Substituent Groups on Rofecoxib Analogues and Multi‐Stimulus Response Properties

AbstractAggregation‐induced emission luminogens (AIEgens) have been widely investigated due to their promising applications in data storage, organic light‐emitting diodes, and deep tissue bioimaging as compared to conventional fluorophores. In this work, we have designed and synthesized two novel rofecoxib analogues functionalized with different terminal groups (−NH2, −Br) on the benzene ring B. Interestingly, the compound 2 a‐3 with −NH2 substituent shows aggregation‐caused quenching effect whereas the compound 1 b‐3 with −Br group displays aggregation‐induced emission (AIE) property which may be caused by the twisted conformation and restricted – stacking of 1 b‐3 according to the X‐ray single crystal diffraction analysis. These opposite results indicated that the terminal groups could have great influence on the photophysical properties of targets. Moreover, the compound 1 b‐3 exhibited multi‐stimuli response luminescence behaviors under various external stimuli including solvatochromism, mechanochromism and acidochromism. Furthermore, lipid droplets imaging investigation proved the compound 1 b‐3 was capable of achieving specific lipid droplets bioimaging. This work not only presents a better understanding of the structure‐fluorescent property relationship based on the scaffold of rofecoxib, but also provides a new avenue for spectral modulation and the development of new MSR materials.

Metal-Oxide Heterojunction Optoelectronic Synapse and Multilevel Memory Devices Enabled by Broad Spectral Photocarrier Modulation.

Broad spectral response and high photoelectric conversion efficiency are key milestones for realizing multifunctional, low-power optoelectronic devices such as artificial synapse and reconfigurable memory devices. Nevertheless, the wide bandgap and narrow spectral response of metal-oxide semiconductors are problematic for efficient metal-oxide optoelectronic devices such as photonic synapse and optical memory devices. Here, a simple titania (TiO2 )/indium-gallium-zinc-oxide (IGZO) heterojunction structure is proposed for efficient multifunctional optoelectronic devices, enabling widen spectral response range and high photoresponsivity. By overlaying a TiO2 film on IGZO, the light absorption range extends to red light, along with enhanced photoresponsivity in the full visible light region. By implementing the TiO2 /IGZO heterojunction structure, various synaptic behaviors are successfully emulated such as short-term memory/long-term memory and paired pulse facilitation. Also, the TiO2 /IGZO synaptic transistor exhibits a recognition rate up to 90.3% in recognizing handwritten digit images. Moreover, by regulating the photocarrier dynamics and retention behavior using gate-bias modulation, a reconfigurable multilevel (≥8 states) memory is demonstrated using visible light.

Simulation of spatially varying ground motion of urban buildings based on wavelet packet neural network

AbstractSite‐structure cluster interaction (SSCI) has a significant effect on the seismic response of densely distributed buildings, and rationally and effectively quantifying its effect when modelling the urban seismic damage can provide more optimal decisions to mitigate earthquake disasters. This paper focuses on the input motion of the structures, by extracting the main influential parameters of SSCI and adopting the loosely typed wavelet packet neural network to rapidly simulate the spatially varying ground motion in the urban environment. In the proposed framework, the wavelet packet energy ratio is presented to describe the variation of ground motion characteristics and used as the sole output to carry out the multi‐resolution spectral modulation, and the training samples were accumulated by a validated finite element simulation method. The developed surrogate model considers the effects of a series of factors, including the earthquake intensity, site condition, configuration of structure cluster, structural dynamic characteristics and spacing, and is superior to the one using conventional artificial neural network. It is verified by a virtual test that the waveshape and spectral features of the predicted ground motion agree well with the target result with an error of peak acceleration being only 1.23%. The suggested approach has the advantages of better modulation precision and lower sample size requirement. Moreover, it is almost zero cost to use the developed surrogate model to correct the ground motion of urban buildings and to consider the influence of SSCI, and the structural seismic response can be more factually displayed in the time and space domains. These specialties make it a promising technique in the rapid assessment of urban seismic damage.

Selecting resonances in molecular scattering by anti-Zeno effect.

Utilizing the anti-Zeno effect, we demonstrate that the resonances of ultracold molecular interactions can be selectively controlled by modulating the energy levels of molecules with a dynamic magnetic field. We show numerically that the inelastic scattering cross section of the selected isotopic molecules in the mixed isotopic molecular gas can be boosted for 2-3 orders of magnitude by modulation of Zeeman splittings. The mechanism of the resonant anti-Zeno effect in the ultracold scattering is based on matching the spectral modulation function of the magnetic field with the Floquet-engineered resonance of the molecular collision. The resulting insight provides a recipe to implement resonant anti-Zeno effect in control of molecular interactions, such as the selection of reaction channels between molecules involving shape and Feshbach resonances, and external field-assisted separation of isotopes.

Enable thermal insulation improvement of Al2O3 aerogels doped with graded oriented extinction phase-change fiber at large temperature differences

Phase change materials (PCMs) play essential roles in extending transient thermal insulation in the aerospace field; however, the extinction properties variations during the phase change process have seldom been investigated. The present study presents an extinction phase-change fiber (EPCF) design, i.e., composed of extinction material shell as well as the PCM core and dispersed in Al2O3 aerogel. The extinction performance (especially after the phase transition) and the insulation performances at steady and transient states according to specific infrared radiation wavelength modulation are numerically investigated. This study reveals that the extinction characteristic of VO2@SiO2 is superior to that of SiO2 and TiO2 fiber. The extinction performance is improved by ∼8.37% after the phase transition of VO2. Besides, the time constant of VO2@SiO2 increased by ∼2.17 times the pure aerogel at the insulation aspect. Similar conclusions could be self-evidently found in some other EPCFs; furthermore, multi-layer EPCFs design is studied for spectral modulation principle combined with the transient heat transfer characteristic. Our developmental work could inspire some novel ideas on insulation material design in the future and improve the understanding of phase change insulation.

Attosecond ionization dynamics of modulated, few-cycle XUV pulses

Few-cycle, attosecond extreme ultraviolet (XUV) pulses in the strong field regime are becoming experimentally feasible, prompting theoretical investigating of the ionization dynamics induced by such pulses. Here, we provide a systematic study of the atomic ionization dynamics beyond the regime of the slowly varying envelope approximation. We discuss the properties of such XUV pulses and report on temporal and spectral modulations unique to the attosecond nature of the pulse. By employing different levels of theory, namely the numerical solution to the time-dependent Schrödinger equation, perturbation theory and a semi-analytical approach, we investigate the ionization of atoms by modulated, few-cycle XUV pulses and distinguish first and higher order effects. In particular, we study attosecond ionization in different intensity regimes aided by a general wave function splitting algorithm. Our results show that polarization and interference effects in the continuum prominently drive ionization in the few-cycle regime and report on carrier-envelope phase (CEP)- and intensity-dependent asymmetries in the photoelectron spectra. The use of spectrally modulated attosecond pulses allows us to distinguish between temporal effects causing asymmetries and dynamic interference, and spectral effects inducing a redshift of the photoelectron spectrum.

An improved spectral amplitude modulation method for rolling element bearing fault diagnosis

An improved spectral amplitude modulation method for rolling element bearing fault diagnosis

Actively Tunable Spectral Filter for Compact Hyperspectral Camera Using Angle‐Sensitive Plasmonic Structures

AbstractHyperspectral imaging provides enhanced classification and identification of veiled features for diverse biomedical applications such as label‐free cancer detection or non‐invasive vascular disease diagnostics. However, hyperspectral cameras still have technical limitations in miniaturization due to the inherently complex and bulky configurations of conventional tunable filters. Herein, a compact hyperspectral camera using an active plasmonic tunable filter (APTF) with electrothermally driven spectral modulation for feature‐augmented imaging is reported. APTF consists of angle‐sensitive plasmonic structures (APS) over an electrothermal MEMS (Microelectromechanical systems) actuator, fabricated by combining nanoimprint lithography, and MEMS fabrication ona 6‐inch wafer. APS have a complementary configuration of Au nanohole and nanodisk arrays supported on asilicon nitride membrane. APTF shows a large angular motion at operational voltages of 5–9 VDC for continuous spectral modulation between 820 and 1000 nm (45 nm/V). The compact hyperspectral camera was fully packaged with a linear polarizer, APTF and amonochromatic camera, exhibiting asize of 16 mm (ϕ) × 9.5 mm (h). Feature‐augmented images of subcutaneous vein and a fresh fruit have been successfully demonstrated after the hyperspectral reconstruction and spectral feature extraction. This functional camera provides a new compact platform for point‐of‐care or in vivo hyperspectral imaging in biomedical applications.

Enhanced Silicon Photovoltaic Efficiency by Solar Light Spectral Modulation via Photonically Tuned Porphyrin–Iron Oxide Hybrid Thin Films

The temperature dependency of photovoltaic power conversion efficiency (PCE) has been a key challenge to solar applications due to intrinsic processes. Herein, an alternative strategy is developed by modulating the solar light spectrum with a series of photonic hybrids. Transparent thin films are synthesized with the solutions of porphyrin compounds and iron oxides which exhibit strong absorptions in the UV and IR regions. These spectral modulating thin films are photonically tuned via compositional optimization to absorb photons near 400 nm and above 1127 nm from solar spectrum to reduce thermalization and sub‐bandgap absorption. These spectral modulators are applied in a particular configuration above a commercial silicon panel to partially filter the simulated solar light. The PCE of the silicon panel suffers a significant decrease due to temperature increase from 22.9 to 92.9 °C after 60 min solar irradiation, resulting in a PCE decrease from 25.1% to 16.3%. With the transparent spectral modulators, upon solar irradiation for 60 min, the maximum PCE has maintained at 20.5%. The mechanisms of PCE enhancement are identified based on reduced thermalization and sub‐bandgap absorption.

High-Power Heterogeneously Integrated III-V/Silicon Superluminescent Diode

We demonstrate heterogeneously integrated O-band superluminescent diodes (SLDs) emitting 10 mW of continuous-wave output power at a device temperature of up to 65 °C. At 10 mW output power, the SLD has a wall-plug efficiency of 3.1% at 25 °C and 1.3% at 65 °C; a 3-dB optical bandwidth of 20-30 nm; and spectral modulation (ripple amplitude) of 0.13 (1.1 dB) at 25 °C and 0.09 (0.8 dB) at 65 °C. This is the highest reported output power and efficiency for a SLD integrated with silicon by more than one order of magnitude, which opens up new applications for cheaper and more compact SLD-based photonics integrated circuits.

Deep Learning for Waveform Level Receiver Design With Natural Redundancy

High-speed information transmission system demands more powerful communication receivers. In this paper, we integrate deep learning algorithms into constructing waveform level receiver, which is capable of harnessing natural redundancy (NR) in raw message sources. Owing to end-to-end learning paradigm of neural network (NN), the proposed NN receiver can accomplish joint symbol timing synchronization and demodulation based on oversampled waveform sequences directly. The prevalent bidirectional LSTM (BiLSTM) and Transformer structures, suitable for excavating sequential correlation of complex sources and waveform sequences, are considered as the backbone of NN receivers. Under the cases of clean label and noisy label, we conceive two novel training algorithms and interpret their principles in theory. The clean label bits are accessible from original source encoding schemes, while noisy label bits are available from the decision results of canonical receivers with independently noised waveform sequences. When noisy labels possess sufficiently small error rates, optimized NN receiver is competitive with that adopting the accurate labels. As for Markov sources and natural language text sources, simulation results show that the proposed NN receivers’ performances are superior to the classical receivers without a priori knowledge, especially when high spectral efficiency modulation schemes are encountered and random noise is significant.

Characteristics of spatial heterodyne spectroscopy for polarization measurement.

In this paper, a spatial static polarization modulation interference spectrum technique is proposed, which combines polarimetric spectral intensity modulation (PSIM) technology and spatial heterodyne spectroscopy (SHS), and can obtain the total Stokes parameters of the target light simultaneously. Moreover, there are no moving parts or electronically controlled modulation parts. In this paper, the mathematical model of the modulation process and demodulation process of spatial static polarization modulation interference spectroscopy is deduced, a computer simulation is carried out, the principle prototype is developed, and a verification experiment is carried out. Simulation and experimental results show that the combination of PSIM and SHS can achieve high-precision static synchronous measurement of high spectral resolution, high time resolution, and continuous band complete polarization information.

A computational model to simulate spectral modulation and speech perception experiments of cochlear implant users.

Speech understanding in cochlear implant (CI) users presents large intersubject variability that may be related to different aspects of the peripheral auditory system, such as the electrode-nerve interface and neural health conditions. This variability makes it more challenging to proof differences in performance between different CI sound coding strategies in regular clinical studies, nevertheless, computational models can be helpful to assess the speech performance of CI users in an environment where all these physiological aspects can be controlled. In this study, differences in performance between three variants of the HiRes Fidelity 120 (F120) sound coding strategy are studied with a computational model. The computational model consists of (i) a processing stage with the sound coding strategy, (ii) a three-dimensional electrode-nerve interface that accounts for auditory nerve fiber (ANF) degeneration, (iii) a population of phenomenological ANF models, and (iv) a feature extractor algorithm to obtain the internal representation (IR) of the neural activity. As the back-end, the simulation framework for auditory discrimination experiments (FADE) was chosen. Two experiments relevant to speech understanding were performed: one related to spectral modulation threshold (SMT), and the other one related to speech reception threshold (SRT). These experiments included three different neural health conditions (healthy ANFs, and moderate and severe ANF degeneration). The F120 was configured to use sequential stimulation (F120-S), and simultaneous stimulation with two (F120-P) and three (F120-T) simultaneously active channels. Simultaneous stimulation causes electric interaction that smears the spectrotemporal information transmitted to the ANFs, and it has been hypothesized to lead to even worse information transmission in poor neural health conditions. In general, worse neural health conditions led to worse predicted performance; nevertheless, the detriment was small compared to clinical data. Results in SRT experiments indicated that performance with simultaneous stimulation, especially F120-T, were more affected by neural degeneration than with sequential stimulation. Results in SMT experiments showed no significant difference in performance. Although the proposed model in its current state is able to perform SMT and SRT experiments, it is not reliable to predict real CI users' performance yet. Nevertheless, improvements related to the ANF model, feature extraction, and predictor algorithm are discussed.

Portable automated rapid testing: Validation of automated testing on listeners from India

Portable Automated Rapid Testing (PART) is an iPad application (https://braingamecenter.ucr.edu/games/p-a-r-t/) capable of measuring various psychoacoustic thresholds (speech on speech masking, temporal, spectral, and spectrotemporal modulation detection, temporal gap detection, frequency modulation tasks, and pure tone detection in noise) in an automated and rapid format using commercially available headphones. The app has been validated previously in native speakers of English and an adapted version of the app has been validated in young normal hearing listeners from Mexico. Here, we present psychoacoustic threshold data for a large cohort of younger college going listeners from India whose native language is not English but are proficient in English. The thresholds obtained from this group will be compared against previously published thresholds based on native speakers of English. It is hypothesized that there will not be any significant differences in thresholds between listeners from India and the thresholds published in the literature. These results will give us the required evidence to start using this freely available app to measure reliable thresholds of various central auditory processing measures in non-native speakers of English who are proficient in the language as well.

In2S3 nanoflakes-based saturable absorber for multi-state soliton generation

Attributing to their excellent photoelectric properties, metal sulfides are potential alternatives for femtosecond photonics. Here, indium sulfide nanoflakes are employed as saturable absorbers. The morphology and chemical composition of In2S3 nanoflakes are characterized. An In2S3 nanoflakes-based SA is fabricated by the method of evanescent light deposition with modulation depth of 2.7 % and saturable intensity of 5 MW/cm2. Stable mode locking with central wavelength of 1530 nm, 3 dB bandwidth of 4.9 nm, pulse duration of 733 fs and fundamental repetition rate of 8.18 MHz is obtained. By carefully adjusting of polarization controller and pumping power, harmonic mode locking with repetition rate of 538 MHz could be achieved. By changing the length of single mode fiber and altering the angle of PC, harmonic soliton molecules centered at 1560 nm are generated with spectral modulation period of 1.3 nm and pulse interval of 4.96 ps. The multi-state soliton generation of the laser confirms the excellent nonlinear absorption characteristics of In2S3 in the communication band, which could be expected to enable more practical instrumentations for spectroscopy, optical sensing and optoelectronics, etc.