Spectral Tuning Research Articles

Light tunable plasmonic metasurfaces.

Self-assembled plasmonic metasurfaces are promising optical platforms to achieve accessible flat optics, due to their strong light-matter interaction, nanometer length scale precision, large area, light weight, and high-throughput fabrication. Here, using photothermal continuous wave laser lithography, we show the spectral and spatial tuning of metasurfaces comprised of a monolayer of ligand capped hexagonally packed gold nanospheres. To tune the spectral response of the metasurfaces, we show that by controlling the intensity of a laser focused onto the metasurface that the absorption peak can be reconfigured from the visible to near-infrared wavelength. The irreversible spectral tuning mechanism is attributed to photothermal modification of the surface morphology. Combining self-assembled metasurfaces with laser lithography, we demonstrate an optically thin (λ/42), spectrally selective plasmonic Fresnel zone plate. This work establishes a new pathway for creating flat, large area, frequency selective optical elements using self-assembled plasmonic metasurfaces and laser lithography.

Tuning of Classical Electromagnetically Induced Reflectance in Babinet Chalcogenide Metamaterials.

SummaryMetamaterials analog of electromagnetically induced reflectance (EIR) has attracted intense attentions since they can provide various applications for novel photonic devices such as optical detectors with a high sensitivity and slow-light devices with a low loss. The development of dynamic photonic devices desires a tunable EIR feature in metamaterials. However, most metamaterials-induced EIR is not spectrally controllable particularly for the near-infrared (NIR) region. Herein, a tuning of EIR is illustrated in Babinet chalcogenide metamaterials in the NIR region. The EIR response is created by weak hybridization of two dipolar (bright) modes of the paired Au slots. Such a mode interference can be engineered through non-volatile phase transition to the refractive index of the Ge2Sb2Te5 (GST), resulting in an active controlling of the reflection window. A 15% spectral tuning of the reflectance peak is observed experimentally in the NIR region as switching the GST state between amorphous and crystalline.

Temperature Stability and Spectral Tuning of Long Period Fiber Gratings Fabricated by Femtosecond Laser Direct Writing.

Long period fiber gratings (LPFGs) were fabricated in a standard single mode fiber (SMF-28e) through femtosecond (fs) laser direct writing. LPFGs with longer and shorter periods were fabricated, which allows coupling from the fundamental core mode to lower and higher order asymmetric cladding modes (LP1,6 and LP1,12, respectively). For the grating periods of 182.7 and 192.5 µm, it was verified that the LP1,12 mode exhibits a TAP at approximately 1380 and 1448 nm in air and water, respectively. Characterization of the LPFGs subjected to high-temperature thermal treatment was accomplished. Fine-tuning of the resonance band’s position and thermal stability up to 600 °C was shown. The temperature sensitivity was characterized for the gratings with different periods and for different temperature ranges. A maximum sensitivity of −180.73, and 179.29 pm/°C was obtained for the two resonances of the 182.7 µm TAP LPFG, in the range between 250 and 600 °C.

The Origin of Ultrafast Multiphasic Dynamics in Photoisomerization of Bacteriophytochrome.

Red-light bacteriophytochromes regulate many physiological functions through photoisomerization of a linear tetrapyrrole chromophore. In this work, we mapped out femtosecond-resolved fluorescence spectra of the excited Pr state and observed unique active-site relaxations on the picosecond time scale with unusual spectral tuning of rises on the blue side and decays on the red side of the emission. We also observed initial wavepacket dynamics in femtoseconds with two low-frequency modes of 38 and 181 cm-1 as well as the intermediate product formation after isomerization in hundreds of picoseconds. With critical mutations at the active site, we observed similar dynamic patterns with different times for both relaxation and isomerization, consistent with the structural and chemical changes induced by the mutations. The observed multiphasic dynamics clearly represents the active-site relaxation, not different intermediate reactions or excitation of heterogeneous ground states. The active-site relaxation must be considered in understanding overall isomerization reactions in phytochromes, and such a molecular mechanism should be general.

Evolution-inspired design of multicolored photoswitches from a single cyanobacteriochrome scaffold

Cyanobacteriochromes (CBCRs) are small, bistable linear tetrapyrrole (bilin)-binding light sensors which are typically found as modular components in multidomain cyanobacterial signaling proteins. The CBCR family has been categorized into many lineages that roughly correlate with their spectral diversity, but CBCRs possessing a conserved DXCF motif are found in multiple lineages. DXCF CBCRs typically possess two conserved Cys residues: a first Cys that remains ligated to the bilin chromophore and a second Cys found in the DXCF motif. The second Cys often forms a second thioether linkage, providing a mechanism to sense blue and violet light. DXCF CBCRs have been described with blue/green, blue/orange, blue/teal, and green/teal photocycles, and the molecular basis for some of this spectral diversity has been well established. We here characterize AM1_1499g1, an atypical DXCF CBCR that lacks the second cysteine residue and exhibits an orange/green photocycle. Based on prior studies of CBCR spectral tuning, we have successfully engineered seven AM1_1499g1 variants that exhibit robust yellow/teal, green/teal, blue/teal, orange/yellow, yellow/green, green/green, and blue/green photocycles. The remarkable spectral diversity generated by modification of a single CBCR provides a good template for multiplexing synthetic photobiology systems within the same cellular context, thereby bypassing the time-consuming empirical optimization process needed for multiple probes with different protein scaffolds.

Radio-frequency stress-induced modulation of CdTe/ZnTe quantum dots

We demonstrate radio-frequency tuning of the energy of individual CdTe/ZnTe quantum dots (QDs) by Surface Acoustic Waves (SAWs). Despite the very weak piezoelectric coefficient of ZnTe, SAW in the GHz range can be launched on a ZnTe surface using interdigitated transducers deposited on a c-axis oriented ZnO layer grown on ZnTe containing CdTe QDs. The photoluminescence (PL) of individual QDs is used as a nanometer-scale sensor of the acoustic strain field. The energy of QDs is modulated by SAW in the GHz range and leads to the characteristic broadening of time-integrated PL spectra. The dynamic modulation of the QD PL energy can also be detected in the time domain using phase-locked time domain spectroscopy. This technique is, in particular, used for monitoring complex local acoustic fields resulting from the superposition of two or more SAW pulses in a cavity. Under a magnetic field, the dynamic spectral tuning of a single QD by SAW can be used to generate single photons with alternating circular polarization controlled in the GHz range.

Silicon-based microhole arrays for spectral tuning of white light emitted from InGaN nanocolumns grown by molecular-beam epitaxy

In this paper, the molecular beam epitaxy of InGaN nanocolumns on silicon-based microhole arrays has been investigated experimentally. A low-cost method has been designed, which contains the processes of photolithography and wet etching. Tuning uniform microhole arrays show the ability in modulating the luminescence spectra of InGaN nanocolumns, which could lead to the modulation of tristimulus coordinates and correlated color temperatures. Due to the shadow effect, it is found that the size of microholes and the area of flat Si surface have more obvious effects, while the configuration of microhole arrays has limited effect on the modulation of peak wavelength.

Tunable Resonance and Phase Vortices in Kirigami Fano‐Resonant Metamaterials

AbstractEmerging internet‐of‐things technologies require widespread antennas and sensors. They should be cheap enough to be widely deployed, but still should be highly functional and tunable. Here, a thorough, in‐depth study of spectral shifts and phase vortices in kirigami Fano‐resonant metamaterials is presented. Microwave metamaterials are printed on paper using metal inks. Then, the printed metamaterials are cut line‐by‐line and folded, so that a step height between neighboring unit cells can be created. By varying step heights, significant spectral tuning as well as resonance switching are obtained. The large spectral shift of the highly asymmetric Fano lineshape also enables significant control of radiation direction. The observations are explained based on the interactions between neighboring unit cells. Moreover, phase singularities are found at the zero‐amplitude position of the Fano resonance spectrum, and a pair of phase vortices appear in parameter space. The kirigami metamaterials allow an easy parameter scan, and therefore significant control of the spectral phase becomes possible. Such drastic phase changes around singularity points could be very useful for various applications, including optical sensing and wavefront manipulation. The kirigami metamaterials enable easily fabricated but highly functional and tunable elements, which could be useful for various antennas and sensors.

Visual pigment evolution in Characiformes: The dynamic interplay of teleost whole‐genome duplication, surviving opsins and spectral tuning

Vision represents an excellent model for studying adaptation, given the genotype-to-phenotype map that has been characterized in a number of taxa. Fish possess a diverse range of visual sensitivities and adaptations to underwater light, making them an excellent group to study visual system evolution. In particular, some speciose but understudied lineages can provide a unique opportunity to better understand aspects of visual system evolution such as opsin gene duplication and neofunctionalization. In this study, we showcase the visual system evolution of neotropical Characiformes and the spectral tuning mechanisms they exhibit to modulate their visual sensitivities. Such mechanisms include gene duplications and losses, gene conversion, opsin amino acid sequence and expression variation, and A1 /A2 -chromophore shifts. The Characiforms we studied utilize three cone opsin classes (SWS2, RH2, LWS) and a rod opsin (RH1). However, the characiform's entire opsin gene repertoire is a product of dynamic evolution by opsin gene loss (SWS1, RH2) and duplication (LWS, RH1). The LWS- and RH1-duplicates originated from a teleost specific whole-genome duplication as well as characiform-specific duplication events. Both LWS-opsins exhibit gene conversion and, through substitutions in key tuning sites, one of the LWS-paralogues has acquired spectral sensitivity to green light. These sequence changes suggest reversion and parallel evolution of key tuning sites. Furthermore, characiforms' colour vision is based on the expression of both LWS-paralogues and SWS2. Finally, we found interspecific and intraspecific variation in A1 /A2 -chromophores proportions, correlating with the light environment. These multiple mechanisms may be a result of the diverse visual environments where Characiformes have evolved.

Real-time en-face Gabor optical coherence tomographic angiography on human skin using CUDA GPU.

We recently proposed an optical coherence tomographic angiography (OCTA) algorithm, Gabor optical coherence tomographic angiography (GOCTA), which can extract microvascular signals from a spectral domain directly with lower computational complexity compared to other algorithms. In this manuscript, we combine a programmable swept source, an OCT complex signal detecting unit, and graphics process units (GPU) to achieve a real-time en-face GOCTA system for human skin microvascular imaging. The programmable swept source can balance the A-scan rate and the spectral tuning range; the polarization-modulation based complex signal detecting unit can double the imaging depth range, and the GPU can accelerate data processing. C++ and CUDA are used as the programming platform where five parallel threads are created for galvo-driving signal generation, data acquisition, data transfer, data processing, and image display, respectively. Two queues (for the raw data and en-face images, respectively) are used to improve the data exchange efficiency among different devices. In this study, the data acquisition time and data processing time for each 3D complex volume (256×304×608 pixels,) are 405.3 and 173.7 milliseconds respectively. To the best of our knowledge, this is the first time to show en-face microvascular images covering 3×3 mm2 at a refresh rate of 2.5 Hz.

Site-selective occupation of Eu2+ activators toward full-visible-spectrum emission in well-designed borophosphate phosphors

Efficient modulation of the site-selection of activators is of great significance for designing high-performance phosphors. Herein, we report that site-selective occupation of Eu2+ activators in mixed anion-group realizes full visible spectrum emission in borophosphate K2BaCa(PO4)2−x(BO3)x:0.03Eu2+ phosphors, which are designed by using K2BaCa(PO4)2:0.03Eu2+ cyan-emitting phosphor. In the pristine sample, there are four crystallographic sites (Ca, Ba, K1 and K2), yet, Eu2+ ions only occupy the K1 and K2 sites. By anion-group cosubstituting (PO4)3− with (BO3)3− units, the most strikingly discoveries here are that a Eu2+-substitution new pathway is opened up, rendering partial Eu2+ ions enter Ca crystallographic site that could never be occupied before, yielding an intense Eu2+ red emission band at 660 nm, and consequently realizing spectral tuning from original cyan-emitting to white-light emission. The structural analysis and DFT calculations support the concept that Eu2+ ion preferably occupy the Ca site, and revealing impressive insights of site-selective occupation of Eu2+ in the borophosphate white-light phosphors. Importantly, using only this full-visible-spectrum phosphor (x = 0.4), as-prepared WLED shows excellent color quality (Ra = 90.4 and R9 = 93.8). This work not only presents an in-depth investigation of site-selection of Eu2+, but also opens a new way to design full-visible-spectrum phosphors.

Polymeric Planar Microcavities Doped with a Europium Complex

Organo-metallic europium complex tetrakis (dibenzoyl methide) triethylammonium (EuD4TEA) shows a sharp emission spectrum, which makes it interesting for photonic applications. In this work, we embedded it into all-polymeric planar microcavities and investigated the effect of the photonic environment on its emission spectrum. To this end, submicron-sized EuD4TEA crystals were loaded into a blend of polystyrene and carboxylic terminated polystyrene matrix, which served to stabilize the emitter in the polymer and to make the composite processable. The new composite was then casted by spin-coating as a defect layer in a polymeric planar microcavity. Spectroscopic studies demonstrate that fine spectral tuning of the cavity mode on the sharp organometal luminescence is possible and produces spectral redistribution of the fluorophore emission, along with a remarkable cavity quality factor.

CW Operation of a Tunable 1550-nm VCSEL Integrating Liquid-Crystal Microcells

An InP-based Vertical-Cavity Surface-Emitting Laser (VCSEL) with a liquid crystal (LC) microcell monolithically integrated on its surface for spectral tuning is investigated. Unlike tunable VCSELs integrating a movable membrane, here the physical length of the cavity remains unchanged and only the voltage applied on the LC ensures a refractive index modification for a particular polarization emitted by the VCSEL. This tunable VCSEL operates in CW at room temperature and exhibits more than 23 nm wavelength tuning around $1.55~\mu \text{m}$ at a maximum applied voltage of 20 V. The measured laser threshold around 6.5 mW is still comparable to VCSEL without LC microcell, a clear indication that the optical losses related the LC are very low. On the other hand, for this first optically pumped device, the lasing characteristics suggest that the LC birefringence is lower than expected. To assess this hypothesis, thermo-optical simulations have been conducted.

A calibrated optogenetic toolbox of stable zebrafish opsin lines.

Optogenetic actuators with diverse spectral tuning, ion selectivity and kinetics are constantly being engineered providing powerful tools for controlling neural activity with subcellular resolution and millisecond precision. Achieving reliable and interpretable in vivo optogenetic manipulations requires reproducible actuator expression and calibration of photocurrents in target neurons. Here, we developed nine transgenic zebrafish lines for stable opsin expression and calibrated their efficacy in vivo. We first used high-throughput behavioural assays to compare opsin ability to elicit or silence neural activity. Next, we performed in vivo whole-cell electrophysiological recordings to quantify the amplitude and kinetics of photocurrents and test opsin ability to precisely control spiking. We observed substantial variation in efficacy, associated with differences in both opsin expression level and photocurrent characteristics, and identified conditions for optimal use of the most efficient opsins. Overall, our calibrated optogenetic toolkit will facilitate the design of controlled optogenetic circuit manipulations.

Spectral tuning of light-harvesting complex II in the siphonous alga Bryopsis corticulans and its effect on energy transfer dynamics

Light-harvesting complex II (LHCII) from the marine green macroalga Bryopsis corticulans is spectroscopically characterized to understand the structural and functional changes resulting from adaptation to intertidal environment. LHCII is homologous to its counterpart in land plants but has a different carotenoid and chlorophyll (Chl) composition. This is reflected in the steady-state absorption, fluorescence, linear dichroism, circular dichroism and anisotropic circular dichroism spectra. Time-resolved fluorescence and two-dimensional electronic spectroscopy were used to investigate the consequences of this adaptive change in the pigment composition on the excited-state dynamics. The complex contains additional Chl b spectral forms – absorbing at around 650 nm and 658 nm – and lacks the red-most Chl a forms compared with higher-plant LHCII. Similar to plant LHCII, energy transfer between Chls occurs on timescales from under hundred fs (mainly from Chl b to Chl a) to several picoseconds (mainly between Chl a pools). However, the presence of long-lived, weakly coupled Chl b and Chl a states leads to slower exciton equilibration in LHCII from B. corticulans. The finding demonstrates a trade-off between the enhanced absorption of blue-green light and the excitation migration time. However, the adaptive change does not result in a significant drop in the overall photochemical efficiency of Photosystem II. These results show that LHCII is a robust adaptable system whose spectral properties can be tuned to the environment for optimal light harvesting.

Enhanced quantum efficiency and thermal stability in tunable yellow-emitting Sr Ca1-AlSiN3:Ce3+ phosphor

Abstract Ce3+ doped CaAlSiN3 is a promising broadband orange phosphor that can be applied in white light-emitting diodes (wLEDs) solid state lighting for general illumination. However, its emission color and quantum efficiency are not good enough to achieve high color rendering properties and luminous efficacy of wLEDs. In this work, we proposed to realize the spectral tuning and property enhancement of CaAlSiN3:Ce3+ by bandgap engineering via the cationic substitution of Ca by Sr. The relationship between photoluminescence and local coordination structure as well as electronic structure were revealed. The substitution led to a blue-shift in emission maximum by 13 nm (from 595 to 582 nm) and a highest external quantum efficiency of 55% (enhanced by 10%). This broadband yellow phosphor enabled to produce wLEDs with a color rendering index of 83.3, luminous efficacy of 114.6 l m/W and a color temperature of 4604 K.

Phase change induced active metasurface devices for dynamic wavefront control

Plasmonic metasurfaces act as appealing platforms for flexible and accurate wave regulation, but their functionalities in wavefront or spectral tuning are settled upon manufactured. So the ability to dynamically tailor the amplitude, phase or polarization is highly desirable for many scenarios of micro & nano photonics and the integrated optoelectronics. Here, one type of plasmonic metasurface devices for active wavefront control is demonstrated in the mid-infrared regime. Fundamentally, the switchable and tunable lensing is exemplified by phase change of refractive index and the generalized Snell’s law as well as the Pancharatnam-Berry phase. As a proof of concept, the metal-insulator (MI) and metal-insulator-metal (MIM) architectures are employed with the phase change medium and an optimized energy conversion efficiency of cross polarization. In a wide spectral range between 2.8 μm and 3.8 μm, the reflective beam focuses with a cross-polarization efficiency of 80% in the amorphous state, but defocuses in the crystalline state with zero cross-polarization, i.e. the ‘on’ and ‘off’ states of the metalens. Further, both device architectures show tunable focal length from 4.4 µm to 4.8 µm (MI) and 3.4 µm and 3.6 µm (MIM) respectively upon switching between the two states. As a result, our scheme potentially paves a way to flexibly construct the active photonic devices for the reconfigurable wavefront engineering in integrated optoelectronics, i.e. beam switch, tunable steering and lensing, etc.

Bistable active spectral tuning of one-dimensional nanophotonic crystal by phase change.

Active spectral tuning of nanophotonic devices offers many fascinating prospects for the realization of novel optical function. Here, switchable spectral response is enabled by the architecture of one-dimensional (1D) photonic crystal (PC) integrated with phase change material of the germanium antimony telluride (GST). Active and precise tuning of the bistable passband and central resonant frequency is demonstrated in the 1D PC composed of alternate SiN and GST nanofilms. An analytical model is derived to specify the tunable spectral features, including the band gap and resonant frequencies. Both the measured and calculated results show distinct red shifts of passband and the resonant minima (or maxima), well confirming theoretical predictions. This work demonstrates a route to construct active photonic devices with the electrically or thermally tunable spectra via 1D PC and potentially extends diverse applications based on the PC platform.

Stimulus dependent transformations between synaptic and spiking receptive fields in auditory cortex

Auditory cortex neurons nonlinearly integrate synaptic inputs from the thalamus and cortex, and generate spiking outputs for simple and complex sounds. Directly comparing synaptic and spiking activity can determine whether this input-output transformation is stimulus-dependent. We employ in vivo whole-cell recordings in the mouse primary auditory cortex, using pure tones and broadband dynamic moving ripple stimuli, to examine properties of functional integration in tonal (TRFs) and spectrotemporal (STRFs) receptive fields. Spectral tuning in STRFs derived from synaptic, subthreshold and spiking responses proves to be substantially more selective than for TRFs. We describe diverse spectral and temporal modulation preferences and distinct nonlinearities, and their modifications between the input and output stages of neural processing. These results characterize specific processing differences at the level of synaptic convergence, integration and spike generation resulting in stimulus-dependent transformation patterns in the primary auditory cortex.

Type I phase matching in thin film of lithium niobate on insulator

Thin film of lithium niobate on insulator (LNOI) enables functional devices with compact sectional area and footprint, exhibiting high nonlinear optical conversion efficiencies and high performance electro-optical modulating. Here we report on the simulated results of type I phase matching of ridge waveguides on LNOI platform. We investigate the phase matching wavelength versus the width/height of the ridge waveguides. The phase matching relation (1550 nm–775 nm) between the width and height of ridge waveguides at different temperatures are also demonstrated. Our results show that wide spectral tuning with considerable conversion efficiency could be obtained in the present ridge waveguide configuration.