Effective medium theory for Kapitza stratified media

Materials with low optical trigger threshold and large THz field modulation depth are highly desirable for THz switches and modulators. VO2, a classical strongly correlated oxide undergoing a sharp metal–insulator transition (MIT), is advantageous for its large THz field modulation depth by virtue of MIT. However, for optically triggered VO2‐based THz switches and modulators, high pump laser intensity is normally required to trigger the MIT, adding considerable costs to THz systems. Reducing optical trigger threshold of VO2 is of substantial interest in the THz science. Here, van der Waals (vdW) heteroepitaxial VO2/mica films are reported with extremely low optical trigger threshold and large THz field modulation depth. Such VO2 films can be used to fabricate low cost but high performance THz switches and modulators. The vdW‐bonded VO2 film–substrate interface allows for negligible disturbance to the VO2 lattice by the mica substrate and reduces thermal conduction to the substrate, boosting the MIT and thus, reducing the pump threshold.

A novel micromachined thermal emitter for fast transient temperature operation is presented. Compared to most commercial available thermal emitters, the one here presented is able to operate in a pulsed mode. This allows the use of lock-in techniques or pyrodetectors in the data acquisition without the use of an optical chopper for light modulation. Therefore, these types of thermal emitters are very important for small filter photometers. Several hot-plate suspension concepts were studied in order to find a design with excellent mechanical stability and high thermal decoupling. In contrary to the classical spider suspension design, a novel approach based on a non-axis-symmetric design is presented. The thermal emitters are fabricated using silicon on insulator technology and KOH-etching. The emitters are heated with Pt-meanders. For temperature determination an additional Pt-structure is deposited onto the hot-plates. The emitters are mounted in TO-5 housings using a ceramic adhesive and gold wire bonding. The used operation temperature is 750°C. In pulsed operation it’s important to have a large modulation depth in terms of thermal radiation intensity in the needed spectral range. The maximal reachable modulation depth ranges from ambient temperature to steady state temperature. A modulation frequency of 5 Hz still allows using nearly the maximum modulation depth. A parameterized finite element model was realized and adapted to the measured data. This was the basis for the numerical optimization procedure for a new improved design.

Recently, the versatility of metasurfaces for pixel-wise wave regulation prompts emerging metadevices with spatially variant controllability, especially the spatial light modulator (SLM), with reliable prospects for applications in many scenarios. Somehow, the modulation depth and operating bandwidth remain elusive technical challenges, restricting the application scenarios and performance. In this paper, we propose a metasurface-based scheme for broadband terahertz (THz) SLM with large modulation depth as well as spatially varying programmable control. The phase change chalcogenide of Ge2Sb2Te5 (GST) is integrated into meta-annulus units for electrically switchable and independent control of each pixel. For proof-of-concept experiments, a THz SLM composed of an array of 5 × 5 addressable pixels with electrically switchable transmittance is demonstrated. Finally, the feasibility of our scheme is confirmed by simulations and experiments with an average modulation depth (MD) of 87.8% in the range of 0.36 THz to 0.57 THz, potentially providing a remarkable feature for broadband applications in THz imaging, sensing and communication, etc.

In this work we investigated the possibility of generating subpicosecond pulses as a result of a modulation instability of continuous wave signals with a large modulation depth. In our case a large modulation depth of a continuous wave is achieved by using a cylindrical waveguides with a running refractive index wave (RRIW). Here, the entire cascade fiber system is a cylindrical waveguides RRIW connected in series with a section of passive fiber with anomalous dispersion. To achieve high peak power values in the generated pulse train, it is necessary to add a section of active fiber with normal dispersion to the stage. It is shown that, as a result of the regime of a induce modulation instability, pulses with a peak power that are orders of magnitude higher than the power of the incoming pump wave can be formed.

Silicon photonic modulators are an essential element in providing fast and massive connectivity to the data-centric world. Ever-increasing data usage requires them to be smaller, faster, and easier to fabricate. Graphene with exceptional properties has been emerging as a material for such next-generation silicon photonic modulators, and a variety of graphene-based photonic or plasmonic modulators have been realized and verified. However, due to weak light-graphene interaction in them, they have a modulation depth smaller than 0.16 dB/μm, which is similar to those of existing germanium-silicon electroabsorption modulators. This work reports a graphene-covered hybrid plasmonic waveguide that has truly strong light-graphene interaction. The hybrid plasmonic waveguide is realized with standard CMOS technology and efficiently coupled to a conventional Si waveguide. To prove the strong light-graphene interaction, solid-electrolyte gating is used to modulate the intensity of the waveguide although its modulation speed is slow. It is demonstrated that the waveguide has a remarkably large modulation depth of 0.276 dB/μm even though just one single-layer graphene covers the waveguide. This demonstration opens the door to the waveguide covered with a graphene-oxide-graphene capacitor, which may have a larger modulation depth and a large 3-dB bandwidth, and it is theoretically analyzed. This work may be the solid base for a graphene-based silicon photonic modulator which is theoretically expected to surpass current silicon photonic modulators.

Ultra-compact dynamically reconfigurable modulation of optical transmission has been widely studied by using subwavelength-spaced resonant metasurface structures containing reconfigurable optical materials. However, it has been difficult to achieve high transmissivity, large modulation depth, and broad bandwidth simultaneously with the conventional resonance-based metasurface schemes. Here, we propose a reconfigurable phase-transition diffractive grating, made of thick VO2 ridge waveguides, for achieving the above-mentioned three goals simultaneously in the near-infrared range. Based on the large dielectric-to-plasmonic transition characteristic of VO2 in the near-infrared range, diffraction directivity of dual-VO2 ridge waveguide is designed to be tuned by thermally driven phase transition of VO2 for transverse electrically polarized illumination. Then, the diffractive VO2 ridge waveguide grating composed of the periodically arranged dual VO2 ridge waveguides is designed with on-state efficiency around 0.3 and minimum modulation depth about 0.35 over a broad bandwidth of 550 nm (1100-1650 nm). The working principle and excellent modulation performance are thoroughly verified through numerical and experimental studies.

We demonstrate a bismuth (Bi) saturable absorber (SA) for generating ultrafast pulse. The Bi SA is fabricated by the Bi film deposited on the surface of microfibers through using magnetron sputtering. Its nonlinear optical properties are investigated. The as-prepared Bi SA has outstanding nonlinear absorption property demonstrated by the open aperture (OA) <i>Z</i>-scan system at 1500 nm and balanced twin-detector method at 1560 nm. The nonlinear optical property of Bi SA shows that the modulation depth, the nonsaturable losses, and the saturable intensity at 1.5 μm are 14% and 79%, and 0.9 MW/cm<sup>2</sup>, respectively. Besides, the closed aperture (CA) <i>Z</i>-scan measurement is also implemented to estimate the nonlinear refractive index of Bi film. The Bi film shows that the typical CA/OA curve possesses the feature of peak-valley profile, meaning that the sample with a negative nonlinear refractive index is self-defocusing. In our experiments, the parameters of the nonlinear absorption coefficient <i>β</i> and the nonlinear refractive index <i>n</i><sub>2</sub> are estimated at about 2.38 × 10<sup>–4</sup> cm/W and –1.47 × 10<sup>–9</sup> cm<sup>2</sup>/W according to the actual experimental data points, respectively. To further investigate its nonlinear optical property, the microfiber-based Bi SA is embedded into an erbium-doped fiber laser with a typical ring cavity structure. Based on the Bi SA device, the stable ultrafast pulses are generated at 1.5 μm with the pulse width of 357 fs, the output power of 45.4 mW, corresponding to the pulse energy of 2.39 nJ, and the signal-to-noise ratio is 84 dB. The stable soliton pulses emitting at 1563 nm are obtained with a 3-dB and 6-nm spectral bandwidth. The experimental results suggest that the microfiber-based Bi SA prepared by magnetron sputtering deposition (MSD) technique can be used as an excellent photonic device for ultrafast pulse generation in the 1.5 μm regime, and the MSD technique opens a promising way to produce high-performance SA with a large modulation depth, low saturable intensity, and high power tolerance, which are conducible to the generation of high power and ultrafast pulse with high stability.

In this paper, a nematic liquid crystal (NLC)-based tunable terahertz (THz) plasmonic metamaterials (MMs) with large modulation depth (MD) and low insertion loss (IL) is designed and experimentally verified at THz frequencies. The proposed structure includes two-layered MM that is immersed in LC. The metal MM is used directly as electrode. The tunable device with a 46×46 array of sub-wavelength circular air loops was fabricated on a quartz glass substrate, with 2×2 cm2 area and 220 µm thickness. The obtained results show that the amplitude MD and IL for normally incident electromagnetic (EM) waves are about 96% and 1.19 dB at 421.2 GHz, respectively, when the bias voltage applied to the NLC layer varies from 0 to 16 V. Meanwhile, the transmission peak frequency gradually decreases from 421.2 to 381.8 GHz, and the frequency tunability (FT) of the proposed structure is greater than 9.35%. This study provides a potential solution for THz modulators, filters, and switches.

Based on Kukhtarev's equations, we derive the formulae ofintensity-coupling coefficient, the phase-coupling coefficient and thegroup velocity at large modulation depth. It is theoretically shownthat the signal beam can be amplified after passing through thephotorefractive crystal and the group velocity reduced to m/s, evencm/s. Meanwhile, we also analyse the influence of the thermalexcitation rate and the large signal effects on optical amplificationand reduction of light propagation in photorefractive two-wave mixing.

In this Letter, high-quality WS2 film and MoS2 film were vertically stacked on the tip of a single-mode fiber in turns to form heterostructure (WS2-MoS2-WS2)-based saturable absorbers with all-fiber integrated features. Their nonlinear saturable absorption properties were remarkable, such as a large modulation depth (∼16.99%) and a small saturable intensity (6.23 MW·cm-2). Stable pulses at 1.55μm with duration as short as 296fs and average power as high as 25mW were obtained in an erbium-doped fiber laser system. The results demonstrate that the proposed heterostructures own remarkable nonlinear optical properties and offer a platform for adjusting nonlinear optical properties by stacking different transition-metal dichalcogenides or modifying the thickness of each layer, paving the way for engineering functional ultrafast photonics devices with desirable properties.

Criteria for optimal implementation of the volume holographic interconnections utilizing photorefractive materials are presented. To study the photorefractive recording mechanism which yields optimal implementation of the volume holographic interconnections, analytic solutions of the steady-state space-charge fields in photorefractive crystals are derived for small laser intensity at large modulation depth. Numerical solutions for the photorefractive crystal BSO are obtained without the assumption of small laser intensity, and they are compared with the analytic solutions. The data regions which can be applied to optimize the volume holographic interconnections are obtained.

Vanadium dioxide (VO2), due to its characteristic semiconductor–metal transition (SMT) at 341 K, is a promising material for optical modulators in the terahertz (THz) regime. In this study, doping Co ions into epitaxial VO2 films have been demonstrated to acquire SMT with a reduced critical temperature, a large modulation depth, and a narrow transition region in the THz regime, which would benefit the applications of VO2 film in THz switches and memory devices. Utilizing Co ions, a 77% THz modulation ratio within a 3 °C transition region appeared when the doping content was 4.0 at. %. Moreover, more doping content induced two different phases in VO2 films, identified by X-ray diffraction (XRD), which would decrease the THz modulation ratio. Theoretical analysis based on the Drude-Smith model and the Bruggeman effective medium theory predict that the phase containing more Co ions might be transparent to THz waves and void of switching ability in the temperature range from 30 °C to 80 °C. This phase separation may be responsible for the complex correlation between the Co doping concentration and the SMT properties.

Dynamics of single mode semiconductor lasers with passive dispersive reflectors

We propose a silicon Fabry–Perot planar waveguide modulator structure consisting of two Bragg reflectors to form the cavity. The Bragg reflectors are nanoscale trenches in the waveguide fabricated using electron beam lithography and reactive ion etching. Compared to conventional waveguide modulator designs, large modulation depth can be achieved with much smaller modulator length using high-finesse Fabry–Perot cavity, leading to much less loss and higher speed. This modulator design can also be utilized as an externally tunable spectral filter.

Transparent Thermally Tunable Microwave Absorber Prototype Based on Patterned VO2 Film