Large Modulation Depth Research Articles

Enhanced nonlinear optical response of Se-doped MoS2 nanosheets for passively Q-switched fiber laser application

An enhanced nonlinear optical (NLO) performance was observed in Se-doped MoS2 nanosheets synthesized through a facile annealing process. Se-doped MoS2 nanosheets with a large saturable intensity and high modulation depth generated stable passively Q-switched fiber laser pulses at 1559 nm. In comparison with the Q-switched fiber laser utilizing the pristine MoS2 nanosheets as a saturable absorber, the passive Q-switching operation based on Se-doped MoS2 nanosheets could be conducted at a lower threshold power of 50 mW, a wider range of repetition rates from 28.97 to 130 kHz, and a higher SNR of 56 dB. More importantly, the shortest pulse duration of 1.502 μs was realized and the output power and pulse energy reached 17.2 mW and 133.07 nJ, respectively. These results indicate that tailoring the chemical composition of optical nanomaterials by introducing a dopant is a feasible method of improving the NLO response of the MoS2 nanosheets and realizing excellent ultrafast pulse generation.

An Ultrafast Switchable Terahertz Polarization Modulator Based on III-V Semiconductor Nanowires.

Progress in the terahertz (THz) region of the electromagnetic spectrum is undergoing major advances, with advanced THz sources and detectors being developed at a rapid pace. Yet, ultrafast THz communication is still to be realized, owing to the lack of practical and effective THz modulators. Here, we present a novel ultrafast active THz polarization modulator based on GaAs semiconductor nanowires arranged in a wire-grid configuration. We utilize an optical pump-terahertz probe spectroscopy system and vary the polarization of the optical pump beam to demonstrate ultrafast THz modulation with a switching time of less than 5 ps and a modulation depth of -8 dB. We achieve an extinction of over 13% and a dynamic range of -9 dB, comparable to microsecond-switchable graphene- and metamaterial-based THz modulators, and surpassing the performance of optically switchable carbon nanotube THz polarizers. We show a broad bandwidth for THz modulation between 0.1 and 4 THz. Thus, this work presents the first THz modulator which combines not only a large modulation depth but also a broad bandwidth and picosecond time resolution for THz intensity and phase modulation, making it an ideal candidate for ultrafast THz communication.

Two-Dimensional CH3NH3PbI3 Perovskite Nanosheets for Ultrafast Pulsed Fiber Lasers.

Even though the nonlinear optical effects of solution processed organic-inorganic perovskite films have been studied, the nonlinear optical properties in two-dimensional (2D) perovskites, especially their applications for ultrafast photonics, are largely unexplored. In comparison to bulk perovskite films, 2D perovskite nanosheets with small thicknesses of a few unit cells are more suitable for investigating the intrinsic nonlinear optical properties because bulk recombination of photocarriers and the nonlinear scattering are relatively small. In this research, we systematically investigated the nonlinear optical properties of 2D perovskite nanosheets derived from a combined solution process and vapor phase conversion method. It was found that 2D perovskite nanosheets have stronger saturable absorption properties with large modulation depth and very low saturation intensity compared with those of bulk perovskite films. Using an all dry transfer method, we constructed a new type of saturable absorber device based on single piece 2D perovskite nanosheet. Stable soliton state mode-locking was achieved, and ultrafast picosecond pulses were generated at 1064 nm. This work is likely to pave the way for ultrafast photonic and optoelectronic applications based on 2D perovskites.

Investigation of laser speckle noise suppression by using polymer-stabilized liquid crystals within twisted nematic cell

We propose a robust method to suppress laser speckle using a polymer-stabilized liquid crystal (PSLC) device with high initial transmittance. With applied voltage, a large modulation depth has been produced through light scattering because of the refractive index mismatch between the rotated nematic liquid crystals and the polymer networks. By using PSLCs with 5 wt % monomer, a speckle noise reduction rate of approximately 54.7% can be achieved with an applied voltage of 6 V. The lowest speckle contrast of approximately 0.025 with relatively high discrimination from the projected AF image has also been demonstrated through a wedge PSLC cell.

High performance of a passively Q-switched mid-infrared laser with Bi2Te3/graphene composite SA.

We report passively Q-switched ∼2 and ∼3 μm mid-infrared (MIR) solid-state lasers with a self-assembly solvothermal-synthesized Bi2Te3/graphene heterostructure saturable absorber (SA) for the first time. Based on the oxidation resistance and high thermal conductivity of graphene, and large modulation depth of Bi2Te3 nanosheets, two high-performance Q-switching lasers were realized. One is a Tm:YAP laser with a maximum average output power of 2.34 W and a pulse width of 238 ns at ∼2 μm. The corresponding maximum pulse peak power was 91 W, which was much improved in comparison with the pure graphene-based Tm laser. The other one is an Er:YSGG laser producing a pulse width of 243 ns, which is the shortest among the 2D SAs-based ∼3 μm solid-state lasers, as far as we know. Our results indicate that such a composite Bi2Te3/graphene material is a promising SA for generating high-performance mid-infrared pulse lasers.

Tungsten disulfide saturable absorbers for 67 fs mode-locked erbium-doped fiber lasers.

In this paper, we demonstrate 67 fs pulse emitting with tungsten disulfide (WS2) in mode-locked erbium-doped fiber (EDF) lasers. Using the pulsed laser deposition method, WS2 is deposited on the surface of the tapered fiber to form the evanescent field. The fiber-taper WS2 saturable absorber (SA) with the large modulation depth is fabricated to support the ultrashort pulse generation. The influences of the WS2 SA are analyzed through contrastive experiments on fiber lasers with or without the WS2 SA. The pulse duration is measured to be 67 fs, which is the shortest pulse duration obtained in the mode-locked fiber lasers with two dimensional (2D) material SAs. Compared to graphene, topological insulator, and other transition metal dichalcogenides (TMDs) SAs, results in this paper indicate that the fiber-taper WS2 SA with large modulation depth is a more promising photonic device in mode-locked fiber lasers with the wide spectrum and ultrashort pulse duration.

Graphene-Based Waveguide-Integrated Terahertz Modulator

One of the major difficulties in the development of optoelectronic THz modulators is finding an active material that allows for large modulation depth. Graphene is a promising candidate because in the terahertz regime it behaves as a Drude metal with conductivity that can be electrostatically tuned through the application of a gate voltage. However, the maximum absorption incurred when a terahertz signal passes through a monolayer of graphene is still only on the order of 10–20%, even for the highest practically achievable carrier concentrations. We demonstrate here a THz modulator that overcomes this fundamental limitation by incorporating a graphene sheet on the surface of a passive silicon dielectric waveguide, in which the evanescent field penetrates the graphene sheet. By applying a gate voltage to the graphene sheet, a modulation depth of up to 50% was achieved. The performance of the modulator is confirmed through electromagnetic simulations, which give further insights into the spatial structure o...

Tunable and Ultra-Small Graphene Integrated Silicon Racetrack Micro Resonator

We have demonstrated ultra-small and tunable graphene integrated racetrack silicon photonic resonators by harnessing the unique optoelectronic properties of graphene layer and the racetrack silicon resonators. Previous studies have been focused on the straight waveguide interacting with a graphene layer. To further explore how to effectively increase the interaction length between graphene layer and silicon photonic devices, this paper employs integrated racetrack silicon resonator interacting with graphene layer within the coupling region . These new configurations will reduce the footprints of the graphene integrated silicon photonics to micron size , instead of hundreds of microns or even millimeter size. We have demonstrated relatively large voltage tuning results and large modulation depth when putting the graphene layer within the coupling region of the racetrack microresonator close to the critical coupling condition. Our work offers flexibility and tunability for hybrid graphene silicon photonic systems, as well as future functional hybrid photonic devices and systems. Potential applications include emerging areas such as active silicon photonic devices for high-performance data processing, tele communications, integrated biophotonics, and low-power energy saving optoelectronic devices.

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.

Plasmonic terahertz modulator based on a grating-coupled two-dimensional electron system

Electrically driven broadband modulator with large modulation depth and high speed is in high demand to meet the technical advancing and applications in terahertz fields recently. So far, the single-particle non-resonant absorption mechanism described by the Drude conductivity has been utilized in most of the related researches but is still not efficient enough. Here we proposed and demonstrated a terahertz modulator based on the collective electron plasma excitations (plasmons) in a grating-coupled two-dimensional electron gas in GaN/AlGaN heterostructure. By switching between the resonant and non-resonant conditions of the 2D plasmon excitation enabled by applying proper gate biases, the transmission of terahertz electromagnetic waves can be efficiently manipulated. Taking advantage of its resonant characteristic combined with the strong electric field enhancement in the active region, we experimentally achieved a maximum intensity modulation depth of 93%, a 3 dB operation bandwidth of ∼400 kHz, and a small required driving voltage amplitude of 2 V at a cryogenic temperature of 8.7 K. Owing to its excellent performances, this active plasmon-based terahertz modulator may offer some promising solutions in several fields of terahertz technology in the future.

A High-Performance Nb Nano-Superconducting Quantum Interference Device with a Three-Dimensional Structure

A superconducting quantum interference device (SQUID) miniaturized into the nanoscale is promising in the inductive detection of a single electron spin. A nano-SQUID with a strong spin coupling coefficient, a low flux noise, and a wide working magnetic field range is highly desired in a single spin resonance measurement. Nano-SQUIDs with Dayem bridge junctions excel in a high working field range and in the direct coupling from spins to the bridge. However, the common planar structure of nano-SQUIDs is known for problems such as a shallow flux modulation depth and a troublesome hysteresis in current-voltage curves. Here, we developed a fabrication process for creating three-dimensional (3-D) niobium (Nb) nano-SQUIDs with nanobridge junctions that can be tuned independently. Characterization of the device shows up to 45.9% modulation depth with a reversible current-voltage curve. Owning to the large modulation depth, the measured flux noise is as low as 0.34 μΦ0/Hz1/2. The working field range of the SQUID is greater than 0.5 T parallel to the SQUID plane. We believe that 3-D Nb nano-SQUIDs provide a promising step toward effective single-spin inductive detection.

Universal Near-Infrared and Mid-Infrared Optical Modulation for Ultrafast Pulse Generation Enabled by Colloidal Plasmonic Semiconductor Nanocrystals.

Field effect relies on the nonlinear current-voltage relation in semiconductors; analogously, materials that respond nonlinearly to an optical field can be utilized for optical modulation. For instance, nonlinear optical (NLO) materials bearing a saturable absorption (SA) feature an on-off switching behavior at the critical pumping power, thus enabling ultrafast laser pulse generation with high peak power. SA has been observed in diverse materials preferably in its nanoscale form, including both gaped semiconductor nanostructures and gapless materials like graphene; while the presence of optical bandgap and small carrier density have limited the active spectral range and intensity. We show here that solution-processed plasmonic semiconductor nanocrystals exhibit superbroadband (over 400 THz) SA, meanwhile with large modulation depth (∼7 dB) and ultrafast recovery (∼315 fs). Optical modulators fabricated using these plasmonic nanocrystals enable mode-locking and Q-switching operation across the near-infrared and mid-infrared spectral region, as exemplified here by the pulsed lasers realized at 1.0, 1.5, and 2.8 μm bands with minimal pulse duration down to a few hundreds of femtoseconds. The facile accessibility and superbroadband optical nonlinearity offered by these nonconventional plasmonic nanocrystals may stimulate a growing interest in the exploiting of relevant NLO and photonic applications.

Polarization controllable multispectral symmetry-breaking absorberin mid-infrared

The versatility of mid-infrared metamaterial absorbers along with the ease of fabrication has been widely used in thermal imaging, molecule sensing, and many other applications. Controllable multispectral absorption is highly required for small footprint, multi-purpose, and real-time sensing applications. In this paper, we present the polarization control of interchangeable multispectral absorption based on the dual-band metamaterial absorber in split mode. Large modulation depth of absorption is obtained during multi-band transition through polarization control. We perform theoretical and numerical analysis to explain the results by formulating an equivalent circuit for the asymmetric cross resonator. Thermal controllability is also demonstrated to show the reversible and repeatable manipulation of absorption intensity at a given wavelength. Moreover, we characterized the limitation of this device under extreme high temperature. This work offers a design methodology for interchangeable multispectral metamaterial absorber from a new perspective by adopting polarization of incident light as a control mechanism, and this will open up possibilities for many valuable applications in the future.

Active terahertz metamaterials based on liquid-crystal induced transparency and absorption

An active terahertz (THz) liquid crystal (LC) metamaterial has been experimentally investigated for THz wave modulation. Some interesting phenomena of resonance shifting, tunable electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) have been observed in the same device structure under different DC bias directions and different incident wave polarization directions by the THz time domain spectroscopy. Further theoretical studies indicate that these effects originate from interference and coupling between bright and dark mode components of elliptically polarized modes in the LC metamaterial, which are induced by the optical activity of LC alignment controllable by the electric field as well as the changes of LC refractive index. The LC layer is indeed a phase retarder and polarization converter that is controlled by the DC bias. The THz modulation depth of the analogs of EIT and EIA effects are 18.3dB and 10.5dB in their frequency band, respectively. Electrical control, large modulation depth and feasible integration of this LC device make it an ideal candidate for THz tunable filter, intensity modulator and spatial light modulator.

Excitation of ultrasharp trapped-mode resonances in mirror-symmetric metamaterials

We experimentally demonstrate a metamaterial structure composed of two mirror-symmetric joint split ring resonators (JSRRs) that support extremely sharp trapped-mode resonance with a large modulation depth in the terahertz region. Contrary to the regular mirror-arranged SRR arrays in which both the subradiant inductive-capacitive (LC) resonance and quadrupole-mode resonance can be excited, our designed structure features a metallic microstrip bridging the adjacent SRRs, which leads to the emergence of an otherwise inaccessible ultrahigh-quality-factor resonance. The ultrasharp resonance occurs near the Wood-Rayleigh anomaly frequency, and the underlying mechanism can be attributed to the strong coupling between the in-plane propagating collective lattice surface mode originating from the array periodicity and localized surface plasmon resonance in mirror-symmetric coupled JSRRs, which dramatically reduces radiative damping. The ultrasharp resonance shows great potential for multifunctional applications such as plasmonic switching, low-power nonlinear processing, and chemical and biological sensing.

Broadband linearization in photonic time-stretch analog-to-digital converters employing an asymmetrical dual-parallel Mach-Zehnder modulator and a balanced detector.

A broadband linearization scheme for time-stretch analog-to-digital converters (TS-ADCs) is proposed based on an asymmetrical dual-parallel Mach-Zehnder modulator and a balanced detector. The theoretical and simulation results indicate that, compared with the differential and arcsine operation method generally employed in TS-ADCs, the proposed scheme has a superior performance on enhancing the spur-free dynamic range and suppressing the even-order distortions and the third-order spurs even under a large modulation depth. Additionally, the proposed scheme realizes online linearization. Therefore, it has the potential to enhance the dynamic range of a broadband TS-ADC in real time.

Enhancement of ammonia gas detection by integrating wavelength-modulated spectra across the line 992.69 cm−1

A rapid tunable diode laser (TDL) absorption sensor was developed for real-time measurements of ammonia concentration by using wavelength modulation spectroscopy (WMS) at 992.698cm−1 of the ν2 vibrational band. This line has patterns free from interference with other species in the atmosphere, and can be used for open-path detection. The 1f signal was used to normalize the 2f signal thereby eliminating the need for calibration and explaining the laser transmission variations. Using WMS with a large modulation depth and integrating the absolute value of the resulting spectra increased the limit of detection (LOD) of the sensor by a factor of seven, compared with the LOD achieved by using the maximum value of the WMS 2f signal. Furthermore, an increase by a factor of 25 compared with the direct absorption spectroscopy was achieved, which allowed obtaining LOD∼1ppb with a resolution time of <2s for the detection of NH3 in the atmosphere using a short-path cell (a 60-cm absorption cell with four passes).

&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$Q$ &lt;/tex-math&gt;&lt;/inline-formula&gt;-Switched Erbium-Doped Fiber Laser Based on Silver Nanoparticles as a Saturable Absorber

We demonstrated saturable absorbers (SAs) based on silver nanoparticles (SNPs) by depositing them on the end facet of the fiber. The SNPs-based SAs showed large modulation depths and high maximum transmittances. With one of the SNPs-based SA, we built a passively $Q$ -switched erbium-doped fiber laser (EDFL). Self-started $Q$ -switching at 1564.5 nm in the EDFL was achieved for a threshold pump power of 19.9 mW; 2.4- $\mu \text{s}$ pulses with a repetition rate of 58.5 kHz and maximum single pulse energy of 132 nJ were achieved at a pump power of 139 mW. The obtained results showed the possibility of SNPs serving as a high performance SA for a $Q$ -switched fiber laser.

Ultra-high modulation depth exceeding 2,400% in optically controlled topological surface plasmons.

Modulating light via coherent charge oscillations in solids is the subject of intense research topics in opto-plasmonics. Although a variety of methods are proposed to increase such modulation efficiency, one central challenge is to achieve a high modulation depth (defined by a ratio of extinction with/without light) under small photon-flux injection, which becomes a fundamental trade-off issue both in metals and semiconductors. Here, by fabricating simple micro-ribbon arrays of topological insulator Bi2Se3, we report an unprecedentedly large modulation depth of 2,400% at 1.5 THz with very low optical fluence of 45 μJ cm−2. This was possible, first because the extinction spectrum is nearly zero due to the Fano-like plasmon–phonon-destructive interference, thereby contributing an extremely small denominator to the extinction ratio. Second, the numerator of the extinction ratio is markedly increased due to the photoinduced formation of massive two-dimensional electron gas below the topological surface states, which is another contributor to the ultra-high modulation depth.

Numerical analysis of fast saturable absorber mode-locked Yb(3+) lasers under large modulation depth.

Numerical analysis of fast saturable absorber mode-locked Yb(3+)-doped solid state lasers is reported. The analysis includes a special case in which the spectral bandwidth of the short pulse is larger than the fluorescence bandwidth of the gain material. The relationship between the available shortest pulse duration and modulation depth for a standard bulk and thin disk laser geometries with several gain materials are shown. The characteristic phenomena observed in our previous Kerr-lens mode-locked laser experiments were reproduced in the simulation.