Enhanced Modulation Depth Research Articles

A broadband modulator based on graphene/black phosphorus heterostructure with enhanced modulation depth

We theoretically present a broadband modulator based on graphene/black phosphorus heterostructure which can work over a large waveband from visible (VIS) to mid-infrared (MIR) regions. By utilizing the angle dependence of black phosphorus, surface plasmon polaritons (SPP) modulation can be achieved in VIS regime, while the wavelength is tuned within the near-infrared (NIR) or MIR regions, the enhanced modulation depth can be achieved by few-layer graphene films. Results show that the proposed plasmonic modulator exhibits a broad waveband from 400 nm to 3 μm. In addition, this proposed modulator features high modulation depth (MD), low insertion loss (IL), large 3-dB modulation bandwidth and small power consumption from VIS to MIR regions. Our work may extend the operation waveband of opto-electro devices based on the hybridized 2D materials and would promote their potential future applications.

Ultrafast THz spectroscopy of carbon nanotube-graphene composites

Mixed nanomaterial composites can combine the excellent properties of well-known low-dimensional nanomaterials. Here we highlight the potential of one-dimensional single-walled carbon nanotubes interfaced with two-dimensional graphene by exploring the composite’s ac conductivity and photoconductivity, and the influence of HAuCl4 doping. In the composite, the equilibrium terahertz conductivity from free carrier motion was boosted, while the localised plasmon peak shifted towards higher frequencies, which we attribute to shorter conductivity pathways in the composite. A negative terahertz photoconductivity was observed for all samples under 410 nm optical excitation and was reproduced by a simple model, where the Drude spectral weight and the momentum scattering rate were both lowered under photoexcitation. The composite had an enhanced modulation depth in comparison to reference carbon nanotube films, while retaining their characteristically fast (picosecond) response time. The results show that carbon nanotube-graphene composites offer new opportunities in devices by controlling charge carrier transport and tuning their optoelectronic properties.

Broadband All-Optical THz Modulator Based on Bi2Te3/Si Heterostructure Driven by UV-Visible Light.

All-optical terahertz (THz) modulators have received tremendous attention due to their significant role in developing future sixth-generation technology and all-optical networks. Herein, the THz modulation performance of the Bi2Te3/Si heterostructure is investigated via THz time-domain spectroscopy under the control of continuous wave lasers at 532 nm and 405 nm. Broadband-sensitive modulation is observed at 532 nm and 405 nm within the experimental frequency range from 0.8 to 2.4 THz. The modulation depth reaches 80% under the 532 nm laser illumination with a maximum power of 250 mW and 96% under 405 nm illumination with a high power of 550 mW. The mechanism of the largely enhanced modulation depth is attributed to the construction of a type-II Bi2Te3/Si heterostructure, which could promote photogenerated electron and hole separation and increase carrier density dramatically. This work proves that a high photon energy laser can also achieve high-efficiency modulation based on the Bi2Te3/Si heterostructure, and the UV-Visible control laser may be more suitable for designing advanced all-optical THz modulators with micro-level sizes.

SiO2 Passivated Graphene Saturable Absorber Mirrors for Ultrashort Pulse Generation.

Owing to its broadband absorption, ultrafast recovery time, and excellent saturable absorption feature, graphene has been recognized as one of the best candidates as a high-performance saturable absorber (SA). However, the low absorption efficiency and reduced modulation depth severely limit the application of graphene-based SA in ultrafast fiber lasers. In this paper, a single-layer graphene saturable absorber mirror (SG-SAM) was coated by a quarter-wave SiO2 passivated layer, and a significantly enhanced modulation depth and reduced saturation intensity were obtained simultaneously compared to the SG-SAM without the SiO2 coating layer. In addition, long-term operational stability was found in the device due to the excellent isolation and protection of the graphene absorption layer from the external environment by the SiO2 layer. The high performance of the SAM was further confirmed by the construction of a ring-cavity EDF laser generating mode-locked pulses with a central wavelength of 1563.7 nm, a repetition rate of 34.17 MHz, and a pulse width of 830 fs.

Embedded gold nanoparticles in TiO2 crystal for photonic applications

• Plasmonic Au nanoparticles are synthesized in TiO 2 crystal by ion implantation. • Third-order optical nonlinearity tailoring is achieved through charge transfer between Au nanoparticles and TiO 2 . • Enhanced modulation depth and optimized saturation intensity suggest significant potential photonic applications. Gold nanoparticles (Au NPs) are synthesized inside TiO 2 crystal by Au + ion implantation. The incorporation of Au + ions in TiO 2 brings out the modification of optical features of the bulk due to the localized surface plasmon resonance (LSPR) effect. The linear absorption spectra of Au NPs dispersed in TiO 2 crystal possesses an absorption peak at 638 nm. The third order optical nonlinearities of TiO 2 with Au NPs are modulated with reduced saturation intensity and increased modulation depth, compared with pure TiO 2 bulk. Our results pave a way to use TiO 2 with embedded Au NPs for nonlinear photonic applications.

Enhanced performance of a fast GaAs-based terahertz modulator via surface passivation

Surface-modified semiconductors show enormous potential for opto-terahertz (THz) spatial modulation due to their enhanced modulation depth (MD) along with their inherent broad bandwidth. Taking full advantage of the surface modification, a performance-enhanced, all-optical, fast switchable THz modulator was achieved here based on the surface-passivated GaAs wafer. With a decreased surface recombination rate and prolonged carrier lifetime induced by passivation, S-passivated GaAs was demonstrated as a viable candidate to enhance THz modulation performance in MD, especially at low photodoping levels. Despite a degraded modulation rate owing to the longer carrier lifetime, this passivated GaAs modulator simultaneously realizes a fast modulation at a 69-MHz speed and as high an MD as ∼ 94 % in a spectral wideband of 0.2–1.2 THz. The results demonstrated a new strategy to alleviate the tradeoff between high MD and speed in contrast to bare surfaces or heterogeneous films/unusual geometry on semiconductors including Si, Ge, and GaAs.

All-fiber saturable absorber based on nonlinear multimode interference with enhanced modulation depth.

We experimentally demonstrate a passively mode-locked fiber ring laser using a small-core fiber-graded index multimode fiber-single-mode fiber (SCF-GIMMF-SMF) structure as an effective saturable absorber based on the nonlinear multimode interference (NL-MMI) effect. A small-core fiber is used as an input single-mode fiber to improve the coupling of power into higher-order modes in a multimode fiber and to enhance the modulation depth of the structure. Stable fundamental mode-locked operation is obtained at a pump threshold of 166 mW and the output soliton pulses have a 647 fs duration at 1557.96 nm with a 5.2 nm spectral width at a repetition rate of 19.37 MHz. This NL-MMI SA can be applied to high-power mode-locked fiber lasers owing to its inherent high damage threshold, compact size, wavelength independent operation, and fabrication simplicity.

Nanoscale All-Solid-State Plasmochromic Waveguide Nonresonant Modulator.

Plasmochromics, the interaction of plasmons with an electrochromic material, have spawned a new class of active plasmonic devices. By introducing electrochromic materials into the plasmon's dielectric environment, plasmons can be actively manipulated. We introduce inorganic WO3 and ion conducting LiNbO3 layers as the core materials in a solid-state plasmochromic waveguide (PCWG) to demonstrate light modulation in a nanoplasmonic waveguide. The PCWG takes advantage of the high plasmonic loss at the high field located at the WO3/Au interface, where the Li+ ions are intercalated into a thin WO3 plasmon modulating layer. Through careful PCWG design, the direction for ion diffusion and plasmon propagation are decoupled, leading to enhanced modulation depth and fast EC switching times. We show that at a bias voltage of 2.5 V, the fabricated PCWG modulator achieves modulation depths as high as 20 and 38 dB for 10 and 20 μm long devices, respectively.

All-optical high-speed modulation of THz transmission through silicon core optical fibers.

High speed optical modulation of THz radiation is of interest for information processing and communications applications. In this paper infrared femtosecond pulses are used to generate free carriers that reduce the THz transmission of silicon based waveguides over a broad spectral range. Up to 96% modulation is observed from 0.5 to 7 THz in an optical fiber with a 210 µm diameter gold-doped silicon core. The observed carrier recombination time of 2.0 ± 0.2 ns makes this material suitable for high speed all-optical signal processing. These results show both enhanced modulation depth and reduced carrier lifetime when compared to the performance of a high resistivity float zone silicon rectangular guide with comparable cross sectional area.

A method for the reconstruction of multifocal structured illumination microscopy data with high efficiency

We present and demonstrate an efficient method for the reconstruction of profiles acquired by multifocal structured illumination microscopy (MSIM) utilizing few raw images. Firstly, we propose a method to produce nine raw multifocal images with enhanced modulation depth to accomplish the uniform illumination of the sample. Then, combing with the parameter of the arrays, we perform the standard construct reconstruction procedure of structured illumination microscopy (SIM) row by row and column by column. Finally, we combine these restored images together to obtain the final image with enhanced resolution and good contrast. Based on theoretical analysis and numerical simulations, this method shows great potential in the field of the image reconstruction of MSIM data.

Comparison Study of Gold Nanorod and Nanoparticle Monolayer Enhanced Optical Terahertz Modulators

Nanostructure modified semiconductors show enormous potential for the optical terahertz (THz) spatial modulation due to their enhanced modulation depth along with their inherent broad bandwidth, small insertion loss, and fast modulation speed. As a result, it is necessary to explore the mechanism of the performance enhancement by the interface modification. Here, the self-assembled monolayer of gold nanorods and nanoparticles was coated on silicon to enhance the optical modulation depth of THz wave. Comparison experiment and the finite-difference time-domain simulation have been performed between the self-assembled monolayer of gold nanorods and nanoparticles to understand the mechanism for improving the light modulation of THz wave. We found that the modulation depth and the speed of THz wave were mainly determined by the light resonance frequency of gold nanoparticles/nanorods and the area with enhanced light intensity on silicon surface. Our results provide a guide for the light-controlled THz modulator based on the nanostructure modified semiconductor.

Flexible terahertz modulators based on graphene FET with organic high-k dielectric layer

Graphene field-effect-transistor (GFET) based terahertz (THz) modulators usually possess an unfulfilling modulation depth (MD) of 15% ∼ 20%. In this work we developed a flexible GFET based THz modulator, where the graphene monolayer is coated with an organic high-K dielectric as the screening layer and an ion-gel layer as the gate. With this exquisite composite modulating structure, the new device possesses a significantly enhanced modulation depth (MD) up to 70% over a broad frequency band, an extremely low insert loss (IL) of 1.3 dB, and unexpected good structural and properties stability. The large intrinsic MD, low IL, as well as its flexibility, render this performance enhanced modulator versatile in fabrication of novel THz devices, such as multi-level modulator, for nonplanar or wearable applications.

Tunable Terahertz Metamaterials Employing Layered 2-D Materials Beyond Graphene

In this study, we extend recent investigations on graphene/metal hybrid tunable terahertz metamaterials to other two-dimensional (2-D) materials beyond graphene. For the first time, use of a nongraphitic 2-D material, molybdenum disulfide (MoS2), is reported as the active medium on a terahertz metamaterial device. For this purpose, high-quality few atomic layer MoS2 films with controlled numbers of layers were deposited on host substrates by means of pulsed laser deposition methods. The terahertz conductivity swing in those films is studied under optical excitation. Although no-appreciable conductivity modulation is observed in single-layer MoS2 samples, a substantial conductivity swing, i.e., 0 to ∼0.6 mS, is seen in samples with ∼60 atomic layers. Therefore, although exhibiting much smaller maximum terahertz conductivity than that in graphene, which is a consequence of much smaller carrier mobility, MoS2 can still be employed for terahertz applications by means of utilizing multilayer films. With this in mind, we design and demonstrate optically actuated terahertz metamaterials that simultaneously exhibit a large modulation depth (i.e., >2× larger than the intrinsic modulation depth by a bare MoS2 film) and low insertion loss (i.e., <3 dB). The advantages of using a 2-D material with a bandgap, such as MoS2, rather than a gapless material, such as graphene, are: 1) a reduced insertion loss, which is owed to the possibility of achieving zero minimum conductivity, and 2) an enhanced modulation depth for a given maximum conductivity level, which is due to the possibility of placing the active material in a much closer proximity to the metallic frequency selective surface, thus allowing us to take full advantage of the near-field enhancement. These results indicate the promise of layered 2D materials beyond graphene for terahertz applications.

Optically Modulatable Blue Fluorescent Proteins

Blue fluorescent proteins (BFPs) offer visualization of protein location and behavior, but often suffer from high autofluorescent background and poor signal discrimination. Through dual-laser excitation of bright and photoinduced dark states, mutations to the residues surrounding the BFP chromophore enable long-wavelength optical modulation of BFP emission. Such dark state engineering enables violet-excited blue emission to be increased upon lower energy, green coillumination. Turning this green coillumination on and off at a specific frequency dynamically modulates collected blue fluorescence without generating additional background. Interpreted as transient photoconversion between neutral cis and anionic trans chromophoric forms, mutations tune photoisomerization and ground state tautomerizations to enable long-wavelength depopulation of the millisecond-lived, spectrally shifted dark states. Single mutations to the tyrosine-based blue fluorescent protein T203V/S205V exhibit enhanced modulation depth and varied frequency. Importantly, analogous single point mutations in the nonmodulatable BFP, mKalama1, creates a modulatable variant. Building modulatable BFPs offers opportunities for improved BFP signal discrimination vs background, greatly enhancing their utility.

Wide spectral bandwidth electro-absorption modulator using coupled micro-cavity with asymmetric tandem quantum well

For reliable three dimensional (3D) imaging system, it is necessary for the optical shutter to have a wide spectral bandwidth operation and enhanced modulation depth. We propose an electro-absorption modulator (EAM) based on coupled Fabry-Perot cavities with micro-cavity (CCMC) which uses asymmetric tandem quantum wells (ATQWs) to obtain improved spectral bandwidth and enhanced modulation depth. Several modulator designs are investigated to obtain improved modulation performance such as wider spectral bandwidth and enhanced modulation depth. It was found that among all the studied modulator geometries, CCMC structure with ATQWs provides the widest spectral bandwidth of 9.6nm and high modulation depth in excess of 50% at -24V, which is good agreement with theoretical calculations. These results suggest that EAM has excellent potential as optical shutter for 3D imaging application.

Pulse-train modulation in a picosecond self-mode-locked laser

Pulse-train modulation was observed in a picosecond self-mode-locked Ti:sapphire laser with pump-power dependence when it was operated around the degenerate cavity configuration. By increasing the optical pumping power, the envelope of the periodic amplitude modulation splits into two or three clusters with enhanced modulation depth, and the amplitude modulation eventually becomes disordered at higher pump power. The amplitude modulation may be supported by exciting two sets of non-degenerate longitudinally mode-locked supermodes due to spatially inhomogeneous gain modulation in the Ti:sapphire crystal.

Discrete electroabsorption modulators with enhanced modulation depth

In this paper, we present an investigation into factors that limited the median modulation depth of a batch of packaged discrete waveguide EA modulators to 23 dB at a wavelength of 1.55 /spl mu/m. Results from detailed measurements of the DC absorption and photocurrent spectra are used to show how stray parasitic light can perturb the absorption characteristic and reduce the modulation depth of these high-speed multiple-quantum-well modulators. A novel ridged deeply etched buried heterostructure EA modulator design is presented in which stray light is removed from the immediate vicinity of the guided mode. The key structural difference between these and the previous devices is that they employ a much thicker Fe-doped InP current blocking layer that was grown by atmospheric pressure MOVPE using PCl/sub 3/ for planarization. Detailed measurements of the DC absorption spectra of a packaged ridged deeply etched buried heterostructure device confirm that stray light causes only a minor perturbation on its absorption characteristics. Consequently the new batch of EA modulator modules have a higher median modulation depth of 40 dB, as well as lower fiber-to-fiber insertion losses and picosecond pulse generation capabilities that are very similar to the previous devices which were used in 40 Gb/s optically time division multiplexed experiments.

Neural encoding of single-formant stimuli in the cat. II. Responses of anteroventral cochlear nucleus units.

1. We have studied responses of anteroventral cochlear nucleus (AVCN) units to single-formant stimuli (SFS), in an effort to make quantitative comparisons with responses observed in auditory-nerve fibers (ANFs) to the same stimuli (Wang and Sachs 1993) and to reveal some of the signal processing mechanisms at the AVCN. Single-unit recordings and subsequent analyses were performed on each type of commonly recorded units, namely primarylike (Pri), primarylike with notch (PN), sustained chopper (ChS), transient chopper (ChT), and onset chopper (OnC), as well as a few onset (On) units, from the AVCN in anesthetized cats. The responses were obtained at a wide range of sound levels and at a frequency range of 1-10 kHz. Modulation in the envelopes of discharge patterns was quantified by a measure called modulation depth. 2. At moderate to high sound levels, most AVCN units were found to have enhanced modulation depth compared with that of ANFs, although the degree of enhancement varies among different types. All AVCN units, except Pri type, showed an enhancement in modulation depth over that of the highest of ANFs at moderate to high sound levels in the order of (from the highest to the lowest) On, OnC, ChT/PN, and ChS. Specifically, 1) modulation depth in Pri units was comparable to that of high spontaneous rate (SR) ANFs at low sound levels and to that of low/medium SR ANFs at high sound levels (in dB SPL). When sound level was normalized by unit threshold, Pri units, on average, exhibited only limited enhancement in envelope modulation at high sound levels (> 80 dB re threshold); 2) PN units showed substantially enhanced modulation depth over that of all SR groups of ANFs at moderate to high sound levels in dB SPL or dB re threshold scales; 3) significant enhancement in modulation depth was seen in both ChS and ChT units, with a slightly higher modulation depth in ChT type across sound levels (in dB SPL or dB re threshold); 4) modulation depth of OnC units was higher than those of primary-like (Pri and PN) and chopper (ChS and ChT) units at a wide range of sound levels; 5) responses from a limited sample of On units showed the highest modulation depth among all types of AVCN units. 3. Detailed analysis revealed that the enhanced modulation depth in the responses of AVCN units is the result of increased envelope peak height and decreased envelope minimum, relative to those of ANFs.(ABSTRACT TRUNCATED AT 400 WORDS)

Large aperture stark modulated retroreflector at 10.8 μm

We describe the analysis, design, and construction of a large aperture, wide-field-of-view modulator at 10.8 μm using the Stark effect in ammonia (NH3). The modulator configuration incorporates a longitudinal electric field produced between closely spaced, large-diameter germanium windows. The windows are antireflection coated for high transmission, and their relatively low resistivity allows their direct use as (transparent) Stark electrodes. Enhanced modulation depth and improved electrical breakdown performance are obtained by using multiple interaction regions. The modulator is mounted at the entrance aperture of a corner cube reflector, thus allowing two passes of the signal beam in a configuration which is retroreflecting and polarization insensitive. This completed device has an aperture of 5.5 cm, a field of view of 38°, and a measured modulation depth of 25% at 1.4 MHz.