Efficient Light Modulation Research Articles

A high-efficiency programmable modulator for extreme ultraviolet light with nanometre feature size based on an electronic phase transition

AbstractThe absence of efficient light modulators for extreme ultraviolet (EUV) and X-ray photons considerably limits their real-life application, particularly when even slight complexity of the beam patterns is required. Here we report on a novel approach to reversible imprinting of a holographic mask in an electronic Wigner crystal material with a sub-90-nm feature size. The structure is imprinted on a sub-picosecond timescale using EUV laser pulses, and acts as a high-efficiency diffraction grating that deflects EUV or soft X-ray light. The imprinted nanostructure is stable after the removal of the exciting beams at low temperatures, but can be easily erased by a single heating beam. Modelling shows that the efficiency of the device can exceed 1%, approaching state-of-the-art etched gratings, but with the benefit of being programmable and tunable over a large range of wavelengths. The observed effect is based on the rapid change of lattice constant upon transition between metastable electronically ordered phases in a layered transition metal dichalcogenide. The proposed approach is potentially useful for creating tunable light modulators in the EUV and soft X-ray spectral ranges.

Sagnac interferometry and self-Kerr nonlinearity dependent photon drag

We present a theoretical investigation of the photon drag by incorporating self-Kerr nonlinearity (SKN) in a Sagnac interferometer featuring a four-level atomic system. By introducing SKN in the system, we demonstrate significant improvements in the photon drag angles, ranging from ±10 micro-radians to ±1 centi-radians. Additionally, the presence of SKN leads to electromagnetically induced transparency (EIT), reduced group velocities, and a positive group index enhancement within the system. Conversely, in the absence of SKN, a negative group index is observed, accompanied by larger group velocities and minimal photon drag angles. The results obtained may have potential applications in various fields, such as slow-light detection, controlled image coding/design, efficient light modulators, and phase-matching in Brillouin scattering.

Nonlocal electro-optic metasurfaces for free-space light modulation

Abstract Dynamic optical metasurfaces with ultrafast temporal response, i.e., spatiotemporal optical metasurfaces, provide attractive solutions and open fascinating perspectives for modern highly integrated optics and photonics. In this work, electro-optically controlled optical metasurfaces operating in reflection and utilizing resonant waveguide mode excitation are demonstrated from the viewpoint of free-space propagating light modulation. The modulation of reflected light power with superior characteristics in comparison with prior research is achieved by identifying a suitable low-loss waveguide mode and exploiting its resonant excitation. The electro-optic Pockels effect in a 300 nm-thick lithium niobate (LN) film sandwiched between a continuous thick gold film and an array of gold nanostripes, serving also as control electrodes, is exploited to realize fast and efficient light modulation. The fabricated compact (active area <1000 µm2) modulators operate in the wavelength range of 850–950 nm, featuring a maximum intensity modulation depth of 42 % at the driving voltage of ±10 V within the bandwidth of 13.5 MHz (with the potential bandwidth of 6.5 GHz). The introduced nonlocal electro-optic metasurface configuration opens new avenues towards the realization of ultrafast, efficient, and robust free-space light modulators based on an LN flat optics approach.

Switchable Plasmonic Chirality for Light Modulation: From Near-Field to Far-Field Coupling.

This paper describes a quasi-planar chiral metamaterial of metal-insulator-metal (MIM) tetramer arrays that support multiplasmon modes from a hybridization scheme to achieve significant chiroptical responses with the largest circular dichroism (CD) value of 42%. The chiroptical responses can be actively switched on and off by tuning the field coupling regime from near field to far field through the insulator (or spacer) thickness. Numerical calculations demonstrate that near-field coupling of the hybridized plasmons on the stacked metallic tetramers governs the chiroptical responses at small insulator thickness (tSiO2 < 160 nm). In contrast, far-field coupling of the plasmon radiations dominates at large spacing (tSiO2 > 160 nm) as phase retardation plays a crucial role. The quasi-planar chiral metamaterial with tunable plasmonic chirality enables efficient light modulation for polarization conversion: from circular to elliptical/linear polarization.

Electrical modulation of high-Q guided-mode resonances using graphene

Subwavelength dielectric resonators have been demonstrated to provide an authentic platform to excite high-Q resonances in flat optics. Here, we present a strategy to modulate the high-Q resonances electrically using graphene. In the proposed strategy, the high-Q resonance is invoked through the use of a lateral guided-mode resonance that is spectrally aligned with a vertical thin film interference, while graphene is employed to engineer the overall absorption rate. This configuration allows for near unity modulations in both the transmission and the reflection simultaneously. Theoretical analysis shows that the excitation of the high-Q guided-mode resonance is dictated by the electrically tunable interband absorption in graphene while the radiative coupling channel remains intact. In particular, it is shown that the electrical modulation of the high-Q guided-mode resonance facilitates remarkably efficient light modulation without being limited by the graphene quality.

Linear optical switch metasurface composed of cross-shaped nano-block and [formula omitted] film

The all-dielectric metasurface composed of nanostructured materials with a large dielectric constant is the latest research trend of metasurfaces, which overcomes the ohmic loss of metal metasurfaces at optical frequencies. However, once the metasurface is manufactured, its function is usually fixed, which has certain limitations for system integration and practical applications. Therefore, the all-dielectric metasurfaces that can be dynamically reconfigured and controlled have attracted more attention. Here, we present a design based on an optical switch metasurface consisting of a Ge2Sb2Te5 (GST-225) film with phase transition properties. Changing the crystalline state of GST-225, the maximum transmittance difference of meta-atoms is 0.7. Then, we demonstrate the deflective optical switch based on the dynamic control of the metasurface. By numerical simulation results, the proposed metasurface can modulate 1550 nm linearly polarized light, the average transmission efficiency difference can be up to 50.4%. Besides, a two-dimensional meta-lens based on the designed meta-atoms with sensitive switching performance is presented. We believe that the design and application in this work are significant for the design of reconfigurable and efficient metasurface light modulators.

Efficient Acousto-Optical Light Modulation at the Mid-Infrared Spectral Range by Planar Semiconductor Structures Supporting Guided Modes

Acousto-optical devices, such as modulators, filters or deflectors, implement a simple and effective way of light modulation and signal processing techniques. However, their operation wavelengths are restricted to visible and near-infrared frequency region due to a quadratic decrease of the efficiency of acousto-optical interaction with the wavelength increase. At the same time, almost all materials with high value of acousto-optic figure of merit are non-transparent at wavelengths larger than 5 {\mu}m, while the transparent materials possess significantly lower acousto-optic figure of merit. Here we propose and demonstrate by calculations how these limitations could be overcome using specially designed planar semiconductor structures supporting electromagnetic modes strongly coupled to the incident light in the Otto configuration. Such approach could be used for a novel efficient acousto-optical device operating in middle infrared range of 8-14 {\mu}m. Acoustic wave excited by a piezoelectric transducer in a semi-conductor prism is utilized to modulate the coupling coefficient of the incident light to the semiconductor structure which results in up to 100% modulation of the transmitted light at the spatial scale less than the ultrasound wavelength. It allows to utilize acoustic waves with short decay distance and therefore, it provides a unique possibility to achieve an efficient acousto-optical modulation at frequencies over several gigahertz, which are unreachable for traditional acousto-optics.

Efficient visible light modulation based on electrically tunable all dielectric metasurfaces embedded in thin-layer nematic liquid crystals

All-dielectric metasurfaces have attracted attention for highly efficient visible light manipulation. So far, however, they are mostly passive devices, while those allowing dynamic control remain a challenge. A highly efficient tuning mechanism is immersing the metasurface in a birefringent liquid crystal (LC), whose refractive index can be electrically controlled. Here, an all-dielectric tunable metasurface is demonstrated based on this concept, operating at visible frequencies and based on TiO2 nanodisks embedded in a thin LC layer. Small driving voltages from 3~5 V are sufficient to tune the metasurface resonances, with an associated transmission modulation of more than 65%. The metasurface optical responses, including the observed electric and magnetic dipole resonance shifts as well as the interfacial anchoring effect of the LC induced by the presence of the nanostructures, are systematically discussed. The dynamic tuning observed in the transmission spectra can pave the way to dynamically tunable metasurface devices for efficient visible light modulation applications.

In Pursuit of 2D Materials for Maximum Optical Response.

Despite being only a few atoms thick, single-layer two-dimensional (2D) materials display strong electron-photon interactions that could be utilized in efficient light modulators on extreme subwavelength scales. In various applications involving light modulation and manipulation, materials with strong optical response at different wavelengths are required. Using qualitative analytical modeling and first-principles calculations, we determine the theoretical limit of the maximum optical response such as absorbance ( A) and reflectance ( R) in 2D materials and also conduct a computational survey to seek out those with best A and R in various frequency ranges, from mid-infrared to deep-ultraviolet. We find that 2D boron has broadband reflectance R > 99% for >100 layers, surpassing conventional thin films of bulk metals such as silver. Moreover, we identify 2D monolayer semiconductors with maximum response, for which we obtain quantitative estimates by calculating quasiparticle energies and accounting for excitonic effects by solving the Bethe-Salpeter equation. We found several monolayer semiconductors with absorbances ≳30% in different optical ranges, which are more than half of the maximum possible value, Alim = 1/2, for a freestanding 2D material. Our study predicts 2D materials which can potentially be used in ultrathin reflectors and absorbers for optoelectronic application in various frequency ranges.

Rotational dynamics of colloidal hexaferrite nanoplates

Here, we report an experimental study on the rotational dynamics of hard magnetic hexaferrite nanoparticles in water. A stable aqueous colloid of SrFe12O19 was synthesized by the borate glass-ceramic dissolution technique and studied by TEM, small-angle X-ray scattering (SAXS), magnetometry, and optical transmission methods in applied DC and AC magnetic fields. The particles represent nanoplates with a mean diameter of 50 nm and a mean thickness of 5 nm having a coercive force of 4700 Oe and a saturation magnetization of 51.5 emu/g. According to magnetic field dependent SAXS data, a probability orientation function was suggested considering the colloidal particle rotation in the applied field as an activation-free process. The magnetization dynamics of the colloidal rotators was described by an interplay of magnetic torque and drag force in the frame of a non-interacting particle model. At frequencies below 100 Hz, the particles are able to fully rotate after the magnetic field. At higher frequencies, the complete following becomes impossible due to the energy dissipation and the particle movement changes to oscillations around randomly oriented axes. These vibrational axes can be aligned by a strong external permanent magnetic field, causing the coherent particle oscillations and correspondingly the rise of the high-frequency response of the colloid optical transmission. As a result, the efficient magneto-optical light modulation has been achieved at frequencies exceeding 5 kHz, revealing the fastest response rates among known colloidal magneto-optical media.

Hybrid graphene metasurfaces for high-speed mid-infrared light modulation and single-pixel imaging

During the past decades, major advances have been made in both the generation and detection of infrared light; however, its efficient wavefront manipulation and information processing still encounter great challenges. Efficient and fast optoelectronic modulators and spatial light modulators are required for mid-infrared imaging, sensing, security screening, communication and navigation, to name a few. However, their development remains elusive, and prevailing methods reported so far have suffered from drawbacks that significantly limit their practical applications. In this study, by leveraging graphene and metasurfaces, we demonstrate a high-performance free-space mid-infrared modulator operating at gigahertz speeds, low gate voltage and room temperature. We further pixelate the hybrid graphene metasurface to form a prototype spatial light modulator for high frame rate single-pixel imaging, suggesting orders of magnitude improvement over conventional liquid crystal or micromirror-based spatial light modulators. This work opens up the possibility of exploring wavefront engineering for infrared technologies for which fast temporal and spatial modulations are indispensable.

Paper No P11: Nanoparticles‐ and Ionic Liquid‐Modified Liquid Crystal Systems for Spatial Light Modulators

AbstractWe present some approaches for modulation of light by hybrid liquid crystal materials. Mixtures of liquid crystals with chiral ionic liquids are used for modulating the incident light amplitude by alternative electric field. Hybrid cells of liquid crystals with inner surfaces, being modified by metallic and semiconductor nanoparticles, are used for phase or amplitude modulation of light. Those groups of the materials used for modification of liquid crystal cells are promising for construction of efficient light switches and modulators.

Light Modulation Based on Fiber Cladding Mode Coupling Between Concatenated Long-Period Gratings

In-fiber efficient light modulation using two concatenated long-period gratings (TCLPG), written in single-mode fiber, is reported. The device is compressed in the middle of both gratings against a corrugated surface, with a piezoelectric, producing a transfer of energy between cladding modes which results in modulated light transmission. The TCLPG operates at 1542 nm, with a bandwidth of 29 nm and an insertion loss of 0.3 dB. The modulation depth is 20 dB and the rise time is 7.5 ms at a repetition rate of kilohertz.

High-Extinction Ratio and Low-Driving-Voltage Spatial Light Modulator by Use of Ultrahigh-Purity GaAs

Highly efficient surface normal spatial light modulators (SLMs) using ultrahigh-purity GaAs layers (30 µm thick) grown on (100)-oriented n+-GaAs substrate by liquid phase epitaxy (LPE) method have been realized. Clear exciton absorption has been observed even at room temperature. The extinction ratio of 25 dB has been demonstrated with a low-driving voltage (32 V) at 895 nm, based on electroabsorption (EA) effect. Very large depletion length over 150 µm is confirmed by analyzing the extinction ratio and capacitance–voltage (C–V) measurements. The calculated value of impurity concentration is very low (≈1012 cm-3), which indicates that the concentration of donor and acceptor in the device are highly compensated.

Acousto-optical multiple interference devices

We present a new concept for waveguide acousto-optical devices based on coupled Mach–Zehnder interferometers driven by acoustic waves. These acousto-optical multiple interference devices use the periodic refractive index modulation induced by the acoustic wave to realize functionalities such as ON/OFF switching for an arbitrary time interval, as well as for efficient light modulation at high harmonics of the acoustic frequency and pulse shaping for, e.g., integrated Q-switches. We also discuss application of the concepts to light modulation by very high acoustic frequencies, where the acoustic wavelengths become much shorter than the optical ones.

Diffraction of light by ultrasound in paratellurite crystals under Fresnel reflection conditions

We show that layers of acousto-optic gyrotropic paratellurite crystals perturbed by ultrasound can be used as efficient light modulators in transmission and reflection mode. We have established that the relative intensities of the diffracted waves are determined by the ratio of the refractive indices of the bounding media, the intensity of the ultrasound, and the thickness of the modulated layer. We show that for an asymmetric diffraction structure, efficient acousto-optic conversion is possible in reflection mode, and the weak effect of the gyrotropy is due to its suppression by Fresnel reflection at the boundaries of the layer.

Diffraction of light by ultrasound under the conditions of Fresnel reflection

The features of the Bragg diffraction of light by ultrasound under the conditions of Fresnel reflection from the boundaries of a perturbed layer have been investigated. It has been shown that layers based on acousto-optical gyrotropic crystals perturbed by ultrasound can be used as efficient light modulators. The relative intensities of diffracted waves have been found to be determined by the relation of the refractive indices of adjacent media and by the ultrasound intensity. With an increase in the thickness of the modulated layer, the reflection and transmission coefficients of the first-order diffracted waves achieve their maxima at a lower ultrasound intensity.

Saturable absorber intensity modulator

We propose and demonstrate a reflection-type optical modulator, with surface-normal architecture that exploits saturation of absorption in semiconductor quantum wells pumped optically. The modulation section, composed of quantum wells placed within a Fabry-Perot cavity, is exposed to an intensity-modulated beam generated by an in-plane laser integrated monolithically on the same wafer. Since the modulation section and the in-plane laser share the same medium, an efficient coupling between the control beam and the signal beam is achieved. Design guidelines and device performance are presented. We demonstrate that the modulator provides an efficient light modulation mechanism that is sufficient to actively mode-lock an erbium-doped fiber laser.

Efficient all-optical light modulation in a piezoelectric heterostructure at room temperature

We demonstrate a novel piezoelectric-barrier heterostructure designed for efficient all-optical light modulation. The modulation mechanism relies upon drastic photocarrier separation by the piezoelectric field in the barrier layers. We present room temperature results showing that an optical “control” power of 70 W/cm2 creates in the heart of the structure a space-charge field of about 30 kV/cm, which induces large spectral shifts (≈100 nm) in the photoluminescence spectra of a CdHgTe quantum well in the 1.5 μm range.

Novel Piezoelectric Heterostructure for all-Optical Infrared Light Modulation

AbstractWe present an innovative-design heterostructure based on the exploitation of in-the-barrier piezoelectric field for all-optical light modulation. The novel layout allows an efficient light modulation with low power densities (few tens of W/cm2), easily attainable with standard laser diodes. The modulation mechanism relies upon drastic photocarrier separation by the piezoelectric field in the barrier layers. We present room temperature results showing that an optical “control” power of 70 W/cm2 creates in the heart of the structure a space-charge field of about 30 kV/cm, inducing large spectral shifts (∼100 nm) in the photoluminescence spectra of a CdHgTe quantum well in the 1.5 μm range.