Tunable Emitters Research Articles

Active modulation of a metasurface emitter based on phase-change material GST arrays

Tunable thermal emitters have attracted much attention due to their applications in communications, sensing, and control system. To date, the emission performance of many reported thermal emitters require continuous external energy for the purpose of continuous modulation. Here, a Ge2Sb2Te5 (GST) array-based metasurface thermal emitter is proposed and measured. The maximum emissivity (crystalline) is 95.3%, while the minimum emission (amorphous) amplitude is 15.1%. Based on the phase transition states (amorphous and crystalline) of the GST array, the emission amplitude and resonance wavelength of this metasurface thermal emitter are clearly switchable. Moreover, the thickness of the GST arrays is set to be 1.5μm, 2.0μm, and 2.5μm, which results in the emissivity peak shifting to longer resonant wavelengths. The thermal emitter reveals continuous tunability, broad wavelength selectivity, and switchability.

Lead-Free Potassium Alloyed Cs2AgBiBr6: Tunable Optoelectronic Properties and Carrier-Lattice Interaction

The role of the carrier-lattice interaction and associated self-trapped excitons (STEs) in broadband white-light emission in perovskite halides is one of the key aspects when aiming to develop next generation semiconductors for lighting applications. Herein, we provide microscopic insight on optoelectronic properties and electron–phonon interactions in potassium (K+) alloyed Cs2AgBiBr6, while evaluating its potential as a single material tunable light emitter. While K+ partially occupying the Cs+ site leads to blue-light emission at low K+ ion concentrations, a sudden photoluminescence broadening and white-light emission occur at relatively higher alloying concentrations where K+ starts to occupy Ag+ sites. Simulation reveals an anomalous increase in carrier lattice coupling strength (via the Fröhlich mechanism) when K+ replaces Ag+. This along with the increased density of STEs (aided by strong electron–phonon coupling and more octahedral distortion) and induced free exciton (FE) emission is proposed to play a major role behind the observed photoluminescence (PL) broadening and white-light emission. This study should stimulate further research on single material based white-light emitters for next generation solid-state lighting applications.

Tuning terahertz transitions in cyclo[n]carbon rings

We develop an analytic model for an ideal polyyne ring which describes the induced terahertz (THz) gap in the molecular spectrum due to the Stark effect. This simple model can also be used to describe an odd-dimered cyclocarbon which has undergone a spontaneous symmetry-breaking event (due to the Jahn-Teller effect) as an effective dipole across an ideal ring. We show that both the size of the gap and the strength of optical transitions across it can be modulated by varying the external electric field strength. A THz emission scheme based on optical excitation is proposed, thus paving the way for tunable THz emitters based on $\mathrm{cyclo}[n]\mathrm{carbon}$.

Thermochromic VO2 based sandwich structure Ag/Al2O3/VO2 with low solar absorption and tunable emittance for spacecraft

The smart radiator device (SRD) with low solar absorption (αs) and large infrared emittance modulation (Δɛ) is desirable to a spacecraft thermal control system. In this work, the SRD was fabricated through depositing the Ag/Al2O3/VO2 triple-layer film on the Si substrate by magnetron sputtering. The properties of the SRD devices were optimized by tuning the thickness of the VO2 layer, and a fantastic SRD device was acquired, which showed low αs, high Δɛ, and intense high-temperature infrared emittance (ɛHT). For the device with a VO2 layer of 50 nm thickness, αs was as low as 29.7% and Δɛ reached 0.53 with ɛHT up to 0.87. The triple-layer film device shows great potential for applications in the spacecraft thermal control system.

Multilayer planar structure for optimized passive thermal homeostasis [Invited

We optimize planar, passive thermal-regulation devices that use the phase-change properties of VO2. We calculate the tunable total emittance, defined as the difference in normalized radiated power in the insulator and metallic states of VO2 at the phase transition temperature. A single-layer VO2/ZnSe/Au device achieves a tunable total emittance of 0.574 in simulation. An optimized multilayer device using the same materials achieves a value of 0.69 in simulation, which outperforms all planar devices found in the literature. We present an analysis showing that an increase in tunable total emittance reduces the temperature fluctuations experienced by the device within a fluctuating environment.

Laser‐Induced Tuning and Spatial Control of the Emissivity of Phase‐Changing Ge2Sb2Te5 Emitter for Thermal Camouflage

AbstractThe tuning of a material's emissivity in the atmospheric window range of 8–13 µm is crucial for infrared (IR) adaptive applications such as thermal camouflage, IR stealth, and radiative cooling. A resonator structure comprising a phase‐changing Ge2Sb2Te5 (GST) film on a metal mirror is promising as a tunable thermal emitter because its long‐wavelength IR emissivity can be varied by adjusting the degree of crystallization of the GST film through annealing. However, the space‐selective tuning of the emissivity required for practical use remains a significant challenge. Herein, a hybrid method combining laser irradiation and heat treatment is presented for space‐selective and continuous tuning of the emissivity of a GST‐based resonator. The current study shows that a laser‐induced crystalline layer formed in the near‐surface region of a 400 nm thick amorphous GST film can act as heterogeneous nuclei for crystallization when the film is subsequently annealed, thereby increasing the crystallization rate of the GST film. This results in a large difference in emissivity between the irradiated and nonirradiated areas, and the emissivity difference can be tuned by changing the annealing temperature. Thermal camouflage is experimentally demonstrated by making high‐and low‐temperature regions with low and high emissivity, respectively.

VO2-based thermally tunable emitter and preliminary design of switching for mid-infrared atmospheric windows

An atmospheric window is greatly significant in the radiative cooling field. In this study, we preliminarily design a thermally tunable metamaterial emitter that can be used as an internal and external emission switch for a mid-infrared atmospheric window (3.5–5 and 8–14 μm). The proposed emitter has a multilayer metal–dielectric–metal micropyramid structure with thermochromic VO2 spacers. Its broadband emission and thermal tunability are due to the resonance of the hybridization of separate magnetic polaritons, surface plasmon polaritons, and the thermal phase-transition characteristics of VO2. The structure has a warming effect at the temperature below the phase-change temperature. On the contrary, it has a cooling effect at the temperature above the phase-change temperature. These phenomena show the emitter’s potential application in smart temperature control systems.

Tunable nonreciprocal thermal emitter based on metal grating and graphene

Various nonreciprocal thermal emitters, which provide opportunities for higher energy conversion efficiency, have been proposed to completely violate the traditional Kirchhoff's law. However, the tunable nonreciprocal thermal emitters remain barely investigated. In this paper, by sandwiching a graphene monolayer between a top metallic grating and a bottom magneto-optical film backed with a metallic mirror, we achieve tunable nonreciprocal radiation effect. It is shown that strong nonreciprocal radiation for graphene under initial state is realized at wavelength around 14.845 μm when the incident angle is 30° and the external magnetic field is 3 T. The physical origin behind such phenomenon is disclosed by investigating both the distribution of magnetic field at the resonant peak and the coupled-mode theory. Besides, the nonreciprocal radiation performance remains perfectly within certain range of structure parameters, which is benefit for practical fabrication and applications. More importantly, the strong nonreciprocal radiation of the proposed scheme can be flexibly tuned by the gate voltage of graphene without redesigning and refabricating the structure. We believe that this work will provide new approaches for the design of novel energy harvesting systems and nonreciprocal thermal emitters.

Tunable Narrowband Silicon-Based Thermal Emitter with Excellent High-Temperature Stability Fabricated by Lithography-Free Methods.

Thermal emitters with properties of wavelength-selective and narrowband have been highly sought after for a variety of potential applications due to their high energy efficiency in the mid-infrared spectral range. In this study, we theoretically and experimentally demonstrate the tunable narrowband thermal emitter based on fully planar Si-W-SiN/SiNO multilayer, which is realized by the excitation of Tamm plasmon polaritons between a W layer and a SiN/SiNO-distributed Bragg reflector. In conjunction with electromagnetic simulations by the FDTD method, the optimum structure design of the emitter is implemented by 2.5 periods of DBR structure, and the corresponding emitter exhibits the nearly perfect narrowband absorption performance at the resonance wavelength and suppressed absorption performance in long wave range. Additionally, the narrowband absorption peak is insensitive to polarization mode and has a considerable angular tolerance of incident light. Furthermore, the actual high-quality Si-W-SiN/SiNO emitters are fabricated through lithography-free methods including magnetron sputtering and PECVD technology. The experimental absorption spectra of optimized emitters are found to be in good agreement with the simulated absorption spectra, showing the tunable narrowband absorption with all peak values of over 95%. Remarkably, the fabricated Si-W-SiN/SiNO emitter presents excellent high-temperature stability for several heating/cooling cycles confirmed up to 1200 K in Ar ambient. This easy-to-fabricate and tunable narrowband refractory emitter paves the way for practical designs in various photonic and thermal applications, such as thermophotovoltaic and IR radiative heaters.

Highly tunable thermal emitter with vanadium dioxide metamaterials for radiative cooling.

An efficient thermal emitter for selective radiative cooling is realized with vanadium dioxide metamaterials. The novel structure consists of patterned VO2 metamaterials on the multilayer substrate and a composite layer on it. To obtain the enhanced emissivity, the influence of the top composite layer and external thermal stimuli are comprehensively optimized. The emissivity can reach up to 0.952 in the metallic phase of VO2 with a composite layer in the atmospheric window, which is due to strong localization of the electric field in the cavity. The influence on the emissivity with different incident angles and geometric parameters is investigated elaborately. Finally, the cooling power is calculated and achieves a high value of 710W/m2 at 383 K, which is significantly higher than that of previous works. Thus, our proposed tunable emitter with high performance will be beneficial to the dynamic radiative cooling system and may open a potential application in building cooling and intelligent windows.

Switching plasmonic resonance in multi-gap infrared metasurface absorber using vanadium dioxide patches

Reconfigurable metasurface absorbers enable collecting or emitting radiation within selected frequency bands. It is thus necessary to decipher such behavior for many applications, including plasmonic energy harvesting, radiative cooling and thermal emitters. In this article, we propose a compact reconfigurable vanadium dioxide (VO2)-based metasurface absorber/emitter to demonstrate switching between dual and single-band absorption modes in the mid-infrared regime. The unit cell of the design employs a four-split gold circular ring resonator with gaps filled with VO2 patches. The phase-transition property of VO2 between semiconductor and metallic states is used to control the mode of operation of the metasurface absorber. When VO2 is in the semiconductor state, a dual-band absorption at 6 μm and 10.6 μm is obtained. When it attains a metallic state, the metasurface exhibits a single-band absorption at 8.25 μm. To achieve the maximum absorption efficiency in both single and dual-band modes, adaptive wind-driven optimization was employed as a global optimization technique. The proposed absorber provides polarization-independent behavior for both Transverse Electric and Transverse Magnetic polarizations. Moreover, the proposed design shows above 80% absorptance for incidence angle up to 45° for the dual-band mode, and up to 35° for the single-band mode. When operating the absorber as a tunable emitter, a switching of 79% in emissivity is achieved at 8.25 μm. These favorable findings may facilitate the development of important devices for temperature regulation, smart windows, and thermal imaging.

Tunable Thermal Camouflage Based on GST Plasmonic Metamaterial.

Thermal radiation control has attracted increasing attention in a wide range of field, including infrared detection, radiative cooling, thermal management, and thermal camouflage. Previously reported thermal emitters for thermal camouflage presented disadvantages of lacking either tunability or thermal stability. In this paper, we propose a tunable thermal emitter consisting of metal-insulator-metal (MIM) plasmonic metamaterial based on phase-change material Ge2Sb2Te5 (GST) to realize tunable control of thermal radiation in wavelength ranges from 3 μm to 14 μm. Meanwhile, the proposed thermal emitter possesses near unity emissivity at the wavelength of 6.3 μm to increase radiation heat dissipation, maintaining the thermal stability of the system. The underlying mechanism relies on fundamental magnetic resonance and the interaction between the high-order magnetic resonance and anti-reflection resonance. When the environmental background is blackbody, the tunable emitter maintains signal reduction rates greater than 80% in middle-IR and longer-IR regions from 450 K to 800 K and from room temperature to 800 K, respectively. The dependences of thermal camouflage on crystallization fraction of GST, incident angles and polarization angles have been investigated in detail. In addition, the thermal emitter can continuously realize thermal camouflage for various background temperatures and environmental background in atmospheric window in the range of 3–5 μm.

Efficient realization of daytime radiative cooling with hollow zigzag SiO2 metamaterials* *Project supported by the Natural Science Foundation of Henan Educational Committee (Grant No. 21A140026).

A tunable selective emitter with hollow zigzag SiO2 metamaterials, which are deposited on Si3N4 and Ag film, is proposed and numerically investigated for achieving excellent radiative cooling effects. The average emissivity reaches a high value of 98.7% in the atmospheric window and possesses a high reflectivity of 92.0% in the solar spectrum. To reveal the enhanced absorptivity, the confined electric field distribution is investigated, and it can be well explained by moth eye effects. Moreover, tunable emissivity can also be initiated with different incident angles and it stays above 83% when the incident angle is less than 80°, embodying the excellent cooling performance in the atmospheric transparency window. Its net cooling power achieves 100.6 W⋅m−2, with a temperature drop of 13°, and the cooling behavior can persist in the presence of non-radiative heat exchange conditions. Therefore, high and tunable selective emitters based on our designed structure could provide a new route to realizing high-performance radiative cooling. This work is also of great significance for saving energy and environmental protection.

Ultrathin Monolayer Mn2+‐Alloyed 2D Perovskite Colloidal Quantum Wells

AbstractAtomically thin monolayer Mn2+‐alloyed 2D Ruddlesden–Popper (RP) perovskite colloidal quantum wells (CQWs) are demonstrated as efficiently tunable emitter with wide color gamut. Ultrathin character evidenced by the well‐controlled thickness of ≈1.8 nm, leads to alloying rather than doping mechanism reported previously, which facilitates uniform and large‐ratio substitution of Pb2+ by Mn2+ within the metal‐halide octahedra layer. Photoluminescence spectra reveal dual color emissions constituted by blue‐greenish intrinsic excitonic emission and reddish Mn2+ emission with a high photoluminescence quantum yield (PLQY) up to 32%. Two individual channels are created for precisely targeting the emission color in a wide gamut by finely controlling the excitonic emission via the chemical compositions of Br/I and the Mn2+ emission via the chemical compositions of Mn/Pb, respectively. Experimentally, high quality single‐component white emitter ultrathin CQWs are achieved with CIE coordinate value of (0.33, 0.32), 21% PLQY, and good film processibility. The results reveal great potential as applicant for next generation lighting and display technologies.

Single-Port Superluminescent-Diode Gain-Chip for Tunable Single-Wavelength and Dual-Wavelength Blue-Laser

Reconfigurable photonic technology combining multiple functionalities in a single gain-chip is a crucial light-based platform for enabling hybrid applications. Here, we report on achieving a widely tunable and reconfigurable blue laser source using a single-port gain medium based on an InGaN/GaN superluminescent diode (SLD). As compared to tunable laser-diode emitters based solely on stimulated emission from a laser diode used as the gain medium, our amplified spontaneous emission system is capable of fast switching between the stimulated and amplified spontaneous emission regimes for on-demand use of time-incoherent, or time-coherent light with single- or dual-wavelengths. The single- and dual-wavelength laser tunability is designed using various external cavity (EC) configurations for a continuous selection and smooth transition of the emission wavelength from 436 nm to 443 nm. Moreover, the InGaN SLD can amplify the optical power within the EC configuration by 6.5 dB. Thus, the prototype system opens up a viable route for designing reconfigurable light sources based on a single gain chip, and promises disruptive innovation for medical instrumentation, optical sensing technology, and optical communication.

Dynamic coherent perfect absorption in nonlinear metasurfaces.

In this Letter, we propose a tunable coherent perfect absorber based on ultrathin nonlinear metasurfaces. A nonlinear metasurface is made of plasmonic nanoantennas coupled to an epsilon-near-zero material with a large optical nonlinearity. The coherent perfect absorption is achieved by controlling the relative phases of the input beams. Here, we show that the optical response of the nonlinear metasurface can be tuned from a complete to a partial absorption by changing the intensity of the pump beam. The proposed nonlinear metasurface can be used to design optically tunable thermal emitters, modulators, and sensors.

Directed Energy Transfer from Monolayer WS2 to Near-Infrared Emitting PbS-CdS Quantum Dots.

Heterostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) and inorganic semiconducting zero-dimensional (0D) quantum dots (QDs) offer useful charge and energy transfer pathways, which could form the basis of future optoelectronic devices. To date, most have focused on charge transfer and energy transfer from QDs to TMDs, that is, from 0D to 2D. Here, we present a study of the energy transfer process from a 2D to 0D material, specifically exploring energy transfer from monolayer tungsten disulfide (WS2) to near-infrared emitting lead sulfide–cadmium sulfide (PbS–CdS) QDs. The high absorption cross section of WS2 in the visible region combined with the potentially high photoluminescence (PL) efficiency of PbS QD systems makes this an interesting donor–acceptor system that can effectively use the WS2 as an antenna and the QD as a tunable emitter, in this case, downshifting the emission energy over hundreds of millielectron volts. We study the energy transfer process using photoluminescence excitation and PL microscopy and show that 58% of the QD PL arises due to energy transfer from the WS2. Time-resolved photoluminescence microscopy studies show that the energy transfer process is faster than the intrinsic PL quenching by trap states in the WS2, thus allowing for efficient energy transfer. Our results establish that QDs could be used as tunable and high PL efficiency emitters to modify the emission properties of TMDs. Such TMD-QD heterostructures could have applications in light-emitting technologies or artificial light-harvesting systems or be used to read out the state of TMD devices optically in various logic and computing applications.

Color Tunable Self-Trapped Emissions from Lead-Free All Inorganic IA-IB Bimetallic Halides Cs-Ag-X (X = Cl, Br, I).

Multi-metallic halides of group IA and IB metals are emerged as a new class of color tunable emitters. While chalcogenides and perovskites are extensively studied, these families of materials are little explored. In comparison, herein, lead and cadmium free bimetallic Cs-Ag-X (X = Cl, Br, I) halides are reported where the larger ion Ag+ helped in incorporating all the halide ions which in turn tune their emission color in spanning from 397nm (violet) to 820nm (near infrared) as a function of their composition. The synthesis method adopted here is the solvent free ball milling of respective halides of Cs and Ag and took the record shortest time and in bulk scale. From decay lifetimes, emissions from these bimetallic halides are found as a result of fast recombination of self-trapped excitons, which exhibited not only reasonably high quantum yield in the range of 17-68% but also excellent stability to air and moisture under ambient conditions. These also show wide Stokes shift with relatively longer decay lifetimes ranging above the exciton and below the surface trap or dopant induced emissions of inorganic semiconductors, indicating a new class of materials having unique identity of their optical behaviors.

Multiple Tunable Hyperbolic Resonances in Broadband Infrared Carbon-Nanotube Metamaterials

Aligned, densely-packed carbon nanotube metamaterials prepared using vacuum filtration are an emerging infrared nanophotonic material. We report multiple hyperbolic plasmon resonances, together spanning the mid-infrared, in individual resonators made from aligned and densely-packed carbon nanotubes. In the first near-field scanning optical microscopy (NSOM) imaging study of nanotube metamaterial resonators, we observe distinct deeply-subwavelength field profiles at the fundamental and higher-order resonant frequencies. The wafer-scale area of the nanotube metamaterials allows us to combine this near-field imaging with a systematic far-field spectroscopic study of the scaling properties of many resonator arrays. Thorough theoretical modeling agrees with these measurements and identifies the resonances as higher-order Fabry-P\'erot (FP) resonances of hyperbolic waveguide modes. Nanotube resonator arrays show broadband extinction from 1.5-10 {\mu}m and reversibly switchable extinction in the 3-5 {\mu}m atmospheric transparency window through the coexistence of multiple modes in individual ribbons. Broadband carbon nanotube metamaterials supporting multiple resonant modes are a promising candidate for ultracompact absorbers, tunable thermal emitters, and broadband sensors in the mid-infrared.

Tunable Orbital Angular Momentum Converter Based on Integrated Multiplexers

A four-orbital angular momentum (OAM) modes converter consisting of the cascade of an OAM demultiplexer and an OAM multiplexer is implemented and characterized. The OAM multiplexer is realized with an integrated photonic circuit based on four tunable OAM emitters, i.e. the order of the emitted OAM mode can be selected by thermal tuning. It works on linearly polarized signals. The OAM demultiplexer, completely passive, consists of the cascade of two patterned refractive elements followed by a lens and can work independently of the polarization of the input signals. The proposed OAM converter can receive at its input a bundle of spatially multiplexed beams mapped on different OAM modes, each one carrying a set of wavelength division multiplexed (WDM) channels. Each WDM channel can be independently converted to a different OAM mode and all the OAM beams are multiplexed at the converter output. The performance of the proposed scheme is assessed as a function of the order of the OAM beam before and after the conversion, as well as of the wavelength of the input signal. The measurements show a power penalty on the bit error-rate (BER) curves of less than 1 dB in all the considered cases. The scheme is tested also with 100 Gb/s real data-traffic generated with commercial network cards, showing full operability. The cascading ability of the proposed scheme is also investigated in an experiment where the signal to be converted is generated by an OAM multiplexer, i.e. the output stage of the OAM converter, showing a penalty of less than 2 dB. Moreover, considerations on scalability, bandwidth and power consumption are also included along with a possible application of OAM conversion within add/drop nodes in an OAM-wavelength multiplexed transmission scenario.