Reflectance Modulation Research Articles

All-Optical Modulation and Ultrafast Switching in MWIR with Sub-Wavelength Structured Silicon

We investigated and optimised the performance of the all-optical reflective modulation of the Mid-Wave Infrared (MWIR) signal by means of the optically-pumped sub-wavelength-structured optical membranes made of silicon. The membranes were optically pumped by a 60-femtosecond, 800-nm laser, while another laser operating in the MWIR ranging between 4 and 6 μ m was used to probe the optical response and modulation. We were able to achieve the conditions providing the modulation depth of 80% using the pump fluence of 3.8 mJ/cm 2 . To get a better insight into the performance and the modulation mechanism, we developed an optical model based on a combination of the Wentzel–Kramers–Brillouin approximation, Drude and Maxwell–Garnett theories. The model allowed us to estimate the values of the dielectric function, carrier concentration and scattering rate of the optically-excited membrane in the MWIR range. Using the model, we optimised the performance and found the conditions at which the reflective modulation can be operated with the ultrafast response of 0.55 ps and modulation contrast of 30%.

Development of a III-nitride electro-optical modulator for UV–vis

We present first experimental results of a novel electro-optical modulator for UV–vis based on III-nitrides. This device consists of a modulation layer which changes its complex dielectric constant influenced by the electric field and a field-dependent carrier distribution. Here, we fabricated and investigated different configurations for the modulation layer structure. The core of the structures was a p–i–n structure in which the i-layer was either GaN or AlGaN. Furthermore, a conventional undoped AlGaN/GaN heterostructure containing a two-dimensional electron gas was grown. Samples were characterized by high-resulution X-ray diffraction (2theta-omega scans and reciprocal space mappings), atomic force microscopy and capacitance–voltage measurements; the reflectivity modulation was determined using a UV–vis white-light reflectometer and a DC voltage source. The results show that reflectivity modulation is readily achieved, caused by the control of the charge carrier density and the electric field. We obtained a relative change in reflectivity of ±1.5% from −8 V to +2 V.

Augmenting Bragg Reflection with Polymer-sustained Conical Helix

There has been a recent surge of interest in smart materials and devices with stimuli-responsive properties for optical modulations. Cholesteric liquid crystals (CLCs) are a unique class of light-manipulating materials, and strongly interact with light and other electromagnetic (EM) waves. Because of their intricate helical structure, new properties of CLC have emerged revealing unique optical behavior that has resulted in rewriting Braggs’ law for how light interacts with soft materials. The aim of this work is to push the limits of spectral tuning with a new method of augmenting light-cholesteric interactions using a polymer-sustained conical helix (PSCH) structure. We experimentally explore the reversibility of reflective wavelength modulation and validate the mechanism enhanced by a polymer-sustained helicoidal structure via theoretical analyses. The conical helix structure of a CLC, formed by low-field-induced oblique orientation of cholesteric helices, is comprised of a chiral dopant, a conventional nematic, and bimesogenic and trimesogenic nematics. Polymerizing a small amount of a reactive mesogen in the CLC with an applied electric field produces a templated helical polymer network that enables three switched optical states, including light-scattering and transparent states as well as color reflection in response to an applied increasing or decreasing electric field. An electro-activated PSCH optical film covers a wide color space, which is appropriate for tunable color device applications. We envisage that this PSCH material will lead to new avenues for controlling EM waves in imaging and thermal control, smart windows and electronic papers.

Multi-slicing matrix method for calculating s-polarization reflectance modulation induced by picosecond acoustic pulses for stratified media

Ultra short pulse lasers with sub-picosecond pulse duration can generate ultrasonic acoustic pulses with wavelengths in the sub-micrometer range. The reflectance modulation induced by the acoustic pulses can be detected by a probe laser. Once a method of calculating the acoustic pulse propagation and the reflectance modulation is established, it will be possible to extract information about the internal structures of a medium from measurements of the time-sequential reflectance modulation by fitting the calculations to experimental results. Thus, a multi-slicing matrix method of calculating the reflectance modulation with an arbitrary incident angle of the probe laser for s-polarization is proposed here. The method based on the Abeles transfer matrix method applicable to stratified media is mathematically validated and is shown to reproduce the analytical formula derived using the Born approximation. The method only uses matrix multiplications, which makes the calculation algorithm simple and is easy to code with matrix manipulation software. The thickness of a stratified layer of an amorphous carbon stacked on a substrate of silicon is demonstrated to be measured with the method.

Ultrafast All-Optical Light Control with Tamm Plasmons in Photonic Nanostructures

Ultrafast all-optical modulators are crucial parts of prospective photonic devices. A number of plasmonic and dielectric nanostructures were nominated as candidates for integrated all-optical circuits. The key principle in the design of such devices is to engineer artificial optical resonances to increase the magnitude of modulation or to change the characteristic switching time. The major drawback is that the manufacturing becomes rather sophisticated. Here, we propose a method to tailor the ultrafast response of photonic crystal–metal nanostructures by employing a spectral shift of the Tamm-plasmon resonance. We show that for the absorbed pump fluence of 6 pJ reflectance of the sample at the near-infrared probe wavelength in the vicinity of the Tamm-plasmon resonance changes 25× stronger as compared with a bare metal film. Additionally, we show that by choosing a proper wavelength around the resonance a background-free reflectance modulation can be achieved. The characteristic pulse-limited switching time, in this case, is 150 fs.

Implementing a discipline-specific reflective critical thinking module for Literature in English in Malaysia

ABSTRACTThe importance of critical thinking is championed in education, but it is also acknowledged that developing critical thinking is challenging particularly in a context like Malaysia where being critical often comes with negative connotations. This article details an attempt to develop reflective critical thinking in the discipline-specific study of Literature in English for undergraduates and practicing English as a Second Language (ESL) educators in Malaysia. A total of 32 participants attended workshops based on a created-creator module developed by the researchers. Participants went through three dimensions: exploring personal experiences, exploring author intention and exploring personal response to texts. Analysed as a case study, the module proved successful in developing initial responses to a level where participants could choose to accept or challenge identified sources of influence. This article presents empirical evidence that operationalises discipline-specific knowledge and extends critical thinking in reader response for Literature as a subject.

Coherent optical and acoustic phonons generated at lattice-matched GaP/Si(0 0 1) heterointerfaces

Thin GaP films can be grown on an exact Si(0 0 1) substrate with nearly perfect lattice match. We perform all-optical pump–probe measurements to investigate the ultrafast electron–phonon coupling at the buried interface of GaP/Si. Above-bandgap excitation with a femtosecond laser pulse can induce coherent longitudinal optical (LO) phonons both in the GaP overlayer and in the Si substrate. The coupling of the GaP LO phonons with photoexcited plasma is reduced significantly with decreasing GaP layer thickness from 56 to 16 nm due to the quasi-two-dimensional confinement of the plasma. The same laser pulse can also generate coherent longitudinal acoustic phonons in the form of a strain pulse. The strain pulse induces not only a periodic modulation in the optical reflectivity as it propagates in the semiconductor, but also a sharp spike when it arrives at the GaP layer boundary. The acoustic pulse induced at the GaP/Si interface is remarkably stronger than that at the Si surface, suggesting a possible application of the GaP/Si heterostructure as an opto-acoustic transducer. The amplitude and the phase of the reflectivity modulation varies with the GaP layer thickness, which can be understood in terms of the interference caused by the multiple acoustic pulses generated at the top surface and at the buried interface.

Characteristics and control of fiber ring resonator

Optical resonators play an active role in fundamental research and applications in atomic fine spectra, laser generation, precision measurements, and quantum information processing because of their high-resolution spectra and strong optical field enhancement. The fiber ring resonators, as a derivative of the resonant resonators, have the advantages of simple structure, small size, stable performance and easy integration. The fiber ring resonators are widely used in fiber lasers, optical communication devices, optical fiber sensing, etc. In this paper, we demonstrate the characteristics of a fiber ring resonator based on a tunable fiber beam splitter experimentally. Control of under-coupling, critical coupling and over-coupling state of the fiber ring resonator can be achieved by adjusting the splitting ratio of the tunable fiber beam splitter. The relationship between the minimum resonator reflectance and resonator loss is given. The intrinsic decay rate of the fiber ring resonator is <inline-formula><tex-math id="M92">\begin{document}${\kappa _0}{\rm{ = }}2{\text{π}} \times \left( {1.60 \pm 0.03} \right)\;{\rm{ MHz}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M92.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M92.png"/></alternatives></inline-formula>, and the quality factor is <inline-formula><tex-math id="M93">\begin{document}$Q = \left( {1.10 \pm 0.02} \right) \times {10.8}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M93.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M93.png"/></alternatives></inline-formula>. The resonance frequency of the fiber ring resonator is controlled by stretching the fiber. The fiber resonator is kept straight and fixed on a self-made U-shaped holder by gluing two points. A piezoelectric transducer is used to change the distance between the two glued points. The fiber ring resonator length is changed and controlled when the fiber is stretched. The Pound-Drever-Hall technique is used to lock the resonator to resonance with the laser. The phase of the laser beam is modulated by using an electro-optical modulator, and two sidebands of the laser frequency are generated. Due to the phase sensitivity of the fiber resonator, the reflected light of the fiber resonator with an intensity modulation is observed when the fiber ring resonator is locked. The intensity modulation is caused by the interference between the resonance frequency and the sidebands of the fiber ring resonator. The reflected spectrum of the fiber ring resonator carries the same-frequency modulation as the phase modulation. This is a disadvantage for the usage of the fiber ring resonator. Thus, we reduce the phase modulation power to reduce the intensity modulation of the resonator reflectance. The linear relationship between the phase modulation power and the intensity modulation of the resonator reflectance caused by the phase modulation is obtained. The fiber ring resonator can be locked when the phase modulation power decreases to –9 dBm. The fiber ring resonator has laid a solid experimental foundation for experimental research on the interaction between the fiber ring resonator and quantum emitters such as atoms and color centers in diamond.

Continuously tuning the impedance matching at the broadband terahertz frequency range in VO2 thin film

We experimentally investigated the terahertz (THz) reflection modulation property of tungsten-(W-) doped vanadium dioxide (V1-xWxO2) thin film enabled by impedance matching at the interface, which behaves as a giant modulation within a broadband THz frequency range during insulator-to-metal transition. A systematical theoretical model for impedance matching is also established, which includes three factors: sheet conductivity of V1-xWxO2 thin film, incidence angle of THz wave, and refractive index of substrate. With the modeling, we demonstrated a method to continuously tune the impedance matching at the interface of substrate-V1-xWxO2 thin films-air. Four different conductivities of V1-xWxO2 thin films realized by tuning W-doping densities (x = 0, 0.35%, 0.51%, and 1.06%), three incident angles (35°, 45°, 60°), and three substrates (SiO2, Al2O3, and Si,) with refractive indices of 1.95, 2.7, 3.4 were used in our experiments to validate our proposed model. These results provide a promising route for continuously tuning broadband THz wave modulation with the impedance matching effect at the thin film interface.

Sub-20 fs All-Optical Switching in a Single Au-Clad Si Nanodisk.

Dielectric nanoantennas have recently emerged as promising elements for nonlinear and ultrafast nanophotonics due to their ability to concentrate light on the nanometer scale with low losses, while exhibiting large nonlinear susceptibilities. In this work, we demonstrate that single Si nanodisks covered with a thin 30 nm thick layer of Au can generate positive and negative sub-20 fs reflectivity modulations of ∼0.3% in the vicinity of the first-order anapole mode, when excited around the second-order anapole mode. The experimental results, characterized in the visible to near-infrared spectral range, suggest that the nonlinear optical Kerr effect is the responsible mechanism for the observed all-optical switching phenomena. These findings represent an important step toward nanoscale ultrafast all-optical signal processing.

An Ellipsometric Technique with an ATR Module and a Monochromatic Source of Radiation for Measurement of Optical Constants of Liquids in the Terahertz Range

Measuring the optical constants of liquids, especially water-containing solutions, is very difficult in the terahertz range. The well-known methods of measurement of the refractive index and absorption coefficient in this range include time-domain spectroscopy and Fourier-transform spectrometry. We have developed a highly sensitive ellipsometry method for measuring the optical constants of liquids using the tunable monochromatic radiation from the Novosibirsk free-electron laser. The ellipsometer is supplemented with an internal reflection module for the measurement of highly absorbing samples. The angle of incidence on the sample in the silicon prism of the module has been optimized for maximum sensitivity to parameters to measure. Measurements of the optical constants of various liquids have been performed, and a sensitivity of 0.01 has been demonstrated.

Organic photovoltaic integrator with three complementary absorption bands to enhance efficiency

The spectrum of 400 to 1100 nm sunlight can be divided into three bands, each absorbed by organic photovoltaic devices that are particularly efficient under the band in question, to achieve higher photoelectric conversion efficiency. The three bands are the absorption bands of fullerene (C70), chloroaluminum phthalocyanine (ClAlPc), and tin naphthalocyanine dichloride (SnNcCl2), which have peak values of 500, 731, and 863 nm, respectively. C70 is a well-known acceptor, whereas ClAlPc and SnNcCl2 serve as donors. In combination with another donor made of 4,4’-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC), which is almost transparent in 400 to 1100 nm, three devices were fabricated and had active layers of TAPC : C70, ClAlPc : C70, and SnNcCl2 : C70. After the doping proportions of these materials had been optimized, the maximum power conversion efficiency (PCE) values of the three devices were 4.52%, 4.3%, and 1.33%, respectively. The properties of donor material dominated the differences among these device behaviors. Subsequently, the overall PCE of a simulated multiple reflection module was calculated using these three devices, which, depending on the arranged sequence in which they were exposed to light and reflected the light to another, generated different absorption spectrum and thus influenced the overall PCE of the photovoltaic integrator. The highest overall simulated and experimental PCE of the photovoltaic integrator was 6.12% and 5.9%, respectively.

Acousto-Optic Double Side-Band Amplitude Modulation of a Fiber Bragg Grating in a Four-Holes Suspended-Core Fiber

A dynamic multiwavelength reflection filter is acoustically induced by the reflectivity modulation of a fiber Bragg grating in a suspended-core fiber. The fiber composed by four air holes is combined with a specially designed acousto-optic modulator to increase the modulation efficiency and to reduce significantly the device size. The effect of the two strongest modulator modes on the grating is evaluated at 374 and 474 kHz. The high-frequency acoustic waves are amplitude modulated by a low-frequency signal from 100 to 500 Hz, generating resonant wavelengths peaks in the grating spectrum. The modulated grating reflectivity is experimentally characterized in the spectral domain. The modulation depth, bandwidth, and number of the induced resonances are tuned by the voltage and frequency of an electrical signal. Up to 15 wavelength peaks with a spectral bandwidth of ∼5 pm and a modulation depth up to 74% are generated, indicating new possibilities for compact and fast all-fiber acousto-optic devices.

Intensity-dependent reflectance modulation of femtosecond laser pulses in GaAs nanocylinders with magnetic resonances

We experimentally demonstrate modulation of reflectance in periodic arrays of subwavelength gallium arsenide nanocylinders with Mie-type resonances due to absorption saturation and changes in the refractive index of the semiconductor material of metasurface. The intensity-dependent reflectance modulation of up to 30% in the vicinity of the magnetic dipole resonance at a low laser fluence below 200 μJ/cm2 is shown by I-scan measurements.

WASP-104b is Darker Than Charcoal

Abstract By analyzing the K2 short-cadence data from Campaign 14, we detect phase-curve modulation in the light curve of the hot-Jupiter host star WASP-104. The ellipsoidal modulation is detected with high significance and in agreement with theoretical expectations, while Doppler beaming and reflection modulations are detected tentatively. We show that the visual geometric albedo is lower than 0.03 at 95% confidence, making it one of the least-reflective planets found to date. The light curve also exhibits a rotational modulation, implying a stellar rotational period likely to be near 23 or 46 days. In addition, we refine the system parameters and place tight upper limits for transit timing and duration variations, starspot occultation events, and additional transiting planets.

A Reflective Inorganic All-Thin-Film Flexible Electrochromic Device with a Seven-Layer Structure**Supported by the Science and Technology on Vacuum Technology and Physics Laboratory for the Research Project under Grant No 6142207040104.

A reflective electrochromic device is fabricated on a 10 cm×10 cm flexible PI/Al substrate using magnetron sputtering. The device has a complementary all-thin-film structure and consists of seven layers. Indium tin oxide (ITO) acts as a transparent electrode deposited on the top, meanwhile, an aluminum (Al) film is adopted as an inter-counter bottom electrode and provides high reflectance. Tungsten oxide (WO3) is used as the main electrochromic layer and nickel oxide (NiO) acts as the complementary electrochromic layer. Lithium niobate (LiNbO3) is applied as a Li+ ion conductor layer. Especially, in the seven-layer structure, two tantalum oxides (Ta2O5) are added as transition layers to prevent Li+ escaping from LiNbO3 when the potential is not applied on the device. When the device is in an electrochromic process, both Ta2O5 provide excellent conductivity for Li+ ions and act as the dielectric of electrons. The complementary device with structure Al/NiO/Ta2O5/LiNbO3/Ta2O5/WO3/ITO exhibits good optical properties, and the reflectance modulation reaches up to 55% measured by a spectrophotometer in the range of 400–1600 nm. The cyclic stability of the electrochromic device is investigated. The results indicate that the charge density involved in the electrochromic process decreases and the electrochromic response time increases with the cycle number because of the Li+ insertion in WO3.

Crystallized V2O5 as Oxidized Phase for Unexpected Multicolor Electrochromism in V2O3 Thick Film

Our e-connected society is eager to develop devices with tunable colors. Electrochromic materials, able to modify their optical properties under an applied voltage, offer a smart solution. In the present study, we have successfully synthesized two vanadium oxide powders from a polyol-mediated synthesis, and powder suspensions were coated on glass/ITO substrates by doctor blading. The electrochemical and optical properties of the VxOy films are investigated by cyclic voltammetry (CV) coupled with in situ UV–visible spectroscopy. Both V2O5 and V2O3 films exhibit reasonably good cycling stability, significant reflectance modulation, high optical contrast, and good memory effects revealing the unknown EC properties of V2O3. The similar green ⇄ blue ⇄ orange reversible color changes for both vanadium oxides appear suitable for display application. Then, the evolution of the vanadium cation oxidation states and of the structure of V2O5 and V2O3 upon cycling are analyzed by ex situ XPS and ex situ XRD (at grazin...

Graphene based terahertz phase modulators

Electrical control of amplitude and phase of terahertz radiation (THz) is the key technological challenge for high resolution and noninvasive THz imaging. The lack of active materials and devices hinders the realization of these imaging systems. Here, we demonstrate an efficient terahertz phase and amplitude modulation using electrically tunable graphene devices. Our device structure consists of electrolyte-gated graphene placed at quarter wavelength distance from a reflecting metallic surface. In this geometry, graphene operates as a tunable impedance surface which yields electrically controlled reflection phase. Terahertz time domain reflection spectroscopy reveals the voltage controlled phase modulation of π and the reflection modulation of 50 dB. To show the promises of our approach, we demonstrate a multipixel phase modulator array which operates as a gradient impedance surface.

Dual-Gated Active Metasurface at 1550 nm with Wide (>300°) Phase Tunability.

Active metasurfaces composed of electrically reconfigurable nanoscale subwavelength antenna arrays can enable real-time control of scattered light amplitude and phase. Achievement of widely tunable phase and amplitude in chip-based active metasurfaces operating at or near 1550 nm wavelength has considerable potential for active beam steering, dynamic hologram rendition, and realization of flat optics with reconfigurable focal lengths. Previously, electrically tunable conducting oxide-based reflectarray metasurfaces have demonstrated dynamic phase control of reflected light with a maximum phase shift of 184° ( Nano Lett. 2016 , 16 , 5319 ). Here, we introduce a dual-gated reflectarray metasurface architecture that enables much wider (>300°) phase tunability. We explore light-matter interactions with dual-gated metasurface elements that incorporate two independent voltage-controlled MOS field effect channels connected in series to form a single metasurface element that enables wider phase tunability. Using indium tin oxide (ITO) as the active metasurface material and a composite hafnia/alumina gate dielectric, we demonstrate a prototype dual-gated metasurface with a continuous phase shift from 0 to 303° and a relative reflectance modulation of 89% under applied voltage bias of 6.5 V.

Near-infrared absorption of graphene-metal nanostructure based on magnetic polaritons

As a two-dimensional material with single-layer carbon atoms, the absorptivity of graphene is only about 2.3% in visible and near-infrared region, which restricts its applications in photoelectric detection, modulation and solar cells. A way to enhance the graphene absorption in this wavelength region is to combine graphene with grating nanostructure. The grating nanostructure can generate strong near-field localization by magnetic polaritons (MPs). However, the existing structures based on MPs are facing some problems, such as sensitivity to the polarization direction of the incoming wave and difficulty in processing the deep grating. Moreover, the modulation effect of the hybrid nanostructure based on MPs combining graphene with nano-grating has not been studied. In this work, a hybrid two-dimensional shallow grating nanostructure is proposed to modulate the absorptivity of graphene based on MPs. The finite element simulation is conducted to calculate the absorptive properties. The equivalent circuit model is used to predict the resonance conditions. The current and field distributions further confirm the excitation of magnetic resonance. The influences of structural parameters and the chemical potential on absorption property are studied. The results show that the magnetic polaritons derived from the hybrid two-dimensional shallow grating structure can obviously improve the absorption of graphene in the near-infrared region. Under the specific structure, the overall absorptivity of the structure is 85%, and the absorptivity of graphene in the structure is 55%, which is over 24 times higher than that of free-standing monolayer graphene. The absorption spectra of the hybrid grating nanostructure for different geometric parameters are calculated. The results show that the absorption peak presents an obvious blue-shift as the thickness of the dielectric layer, the grating period or the width of the silver nanoparticles decrease. Numerical simulation results show that by adjusting the chemical potential of graphene, the overall absorptivity of the structure can be tuned dynamically. The reflection modulation depths of hybrid two-dimensional nanostructure under different structural parameters are calculated. By controlling the chemical potential of graphene in a range from 0.1 eV to 1 eV, the reflection modulation depths of 54.8% (1040 nm), 50.3% (890 nm) and 46.8% (750 nm) are obtained, respectively. Compared with the existing structures based on MPs, the present structure is insensitive to the incidence and polarization direction of the incident electromagnetic wave due to the symmetry in two-dimensional directions. Considering the design of shallow silver grating, the structure is easier to implement in the process. The research results provide good theoretical reference for graphene-based photoelectric detection and modulation.