Coin Paradox Spin-Orbit Interaction Enhances Magneto-Optical Effect and Its Application in On-Chip Integrated Optical Isolator.

We designed a simple on-chip integrated optical isolator made up of a metal-insulator-metal waveguide and a disc cavity filled with magneto-optical material to enhance the transverse magneto-optical effect through the coin paradox spin-orbit interaction (SOI). The simulation results of the non-reciprocal transmission properties of this optical structure show that a high-performance on-chip integrated optical isolator is obtained. The maximum isolation ratio is greater than 60 dB with a corresponding insertion loss of about 2 dB. The great performance of the optical isolator is attributed to the strong transverse magneto-optical effect, which is enhanced by the coin paradox SOI. Moreover, the enhancement of the transverse magneto-optical effect through the coin paradox SOI is more substantial for smaller azimuthal mode number n. Benefiting from this, the transverse magneto-optical effect remains strong in a wide wavelength range. Additionally, a smaller cavity has a stronger transverse magneto-optical effect in the same wavelength range. Our research provides a new perspective for creating highly integrated magneto-optical devices.

Highly responsive tellurium-hyperdoped black silicon photodiode with single-crystalline and uniform surface microstructure.

Femtosecond laser hyperdoped silicon, also known as the black silicon (BS), has a large number of defects and damages, which results in unstable and undesirable optical and electronic properties in photonics platform and optoelectronic integrated circuits (OEICs). We propose a novel method that elevates the substrate temperature during the femtosecond laser irradiation and fabricates tellurium (Te) hyperdoped BS photodiodes with high responsivity and low dark current. At 700 K, uniform microstructures with single crystalline were formed in the hyperdoped layer. The velocity of cooling and resolidification is considered as an important role in the formation of a high-quality crystal after irradiation by the femtosecond laser. Because of the high crystallinity and the Te hyperdoping, a photodiode made from BS processed at 700 K has a maximum responsivity of 120.6 A/W at 1120 nm, which is far beyond the previously reported Te-doped silicon photodetectors. In particular, the responsivity of the BS photodiode at 1300 nm and 1550 nm is 43.9 mA/W and 56.8 mA/W with low noise, respectively, which is valuable for optical communication and interconnection. Our result proves that hyperdoping at a high substrate temperature has great potential for femtosecond-laser-induced semiconductor modification, especially for the fabrication of photodetectors in the silicon-based photonic integration circuits.

Giant Tunable Circular Dichroism of Large-Area Extrinsic Chiral Metal Nanocrescent Arrays.

Circular dichroism (CD) is an interesting phenomenon originating from the interaction of light with chiral molecules or other nanostructures lacking mirror symmetries in three-dimensional (3D) or two-dimensional (2D) space. While the observable effects of optical chirality are very weak in most of the natural materials, they can be designed and significantly enhanced in synthetic chiral structures, where the spatial symmetry of their component are broken on a nanoscale. Therefore, fabrication of composites capable of cheap, time-saving, and giant CD is desirable for the advanced optical technologies. Here, the giant CD of large-area metal nanocrescent array structures was investigated theoretically and experimentally. The largest value of the CD spectrum measured was larger than 0.5, and the CD spectrum was tuned effectively and extensively while maintaining a large peak intensity, which can be attributed to the selective excitation of the lattice surface modes (LSMs) by circularly polarized light. The analysis of the extrinsic chiral structure shows potential applications in chiral molecule sensing and polarizing imaging.

High-Performance Free-Standing Flexible Photodetectors Based on Sulfur-Hyperdoped Ultrathin Silicon.

Flexible photodetectors (PDs) prepared with silicon-based materials have received considerable attention for their use in a wide range of portable and wearable applications. In this study, we present the first free-standing flexible PD based on sulfur-hyperdoped ultrathin silicon, which was fabricated using a femtosecond laser in a SF6 atmosphere. It is found that the fabricated device exhibits excellent performance of broadband photoresponse from 400 to 1200 nm, with a peak responsivity of 63.79 A/W @ 870 nm at a low bias voltage of -2 V, corresponding to an external quantum efficiency reaching 9092%, which surpasses most values reported for silicon-based flexible PDs. In addition, the device shows a fast response speed (rise time τr = 68 µs) and stable detection performance with good mechanical flexibility. The high-performance PD described here suggests a promising way in flexible applications for sensors, imaging systems, and optical communication systems.

Polarization-resolved edge states in terahertz topological photonic crystal.

The discovery of topological photonic states has revolutionized our understanding of electromagnetic propagation and scattering. With the introduction of topology, some attractive properties such as unidirectional propagation and robustness against defects and impurities will be endowed to photonic edge modes. In this study, two-dimensionally confined topological edge states were achieved at terahertz (THz) frequency based on an all-dielectric photonic crystal structure. Trivial and nontrivial bandgaps of two deforming honeycomb lattices as well as unidirectional topological edge states were observed. Because the topological edge states with opposite helicity propagated in opposite directions at the interface, a polarization-resolved characteristic was demonstrated here, and thus a continuously tunable power splitter was achieved. This study provides some insights for further THz topological studies and possibilities for THz integrated platforms.

Giant circular dichroism of large-area extrinsic chiral metal nanocrecents.

In this work, we demonstrate the strong extrinsic chirality of the larger-area metal nanocrescents by experiments and simulations. Our results show that the metal nanocrescent exhibits giant and tunable circular dichroism (CD) effect, which is intensively dependent on the incident angle of light. We attribute the giant extrinsic chirality of the metal nanocrescent to the excitation efficiencies difference of localized surface plasmon resonance (LSPR) modes for two kinds of circularly polarized light at a non-zero incident angle. In experiment, the largest CD of 0.37 is obtained at the wavelength of 826 nm with the incident angle of 60°. Furthermore, the CD spectra can be tuned flexibly by changing the metal nanocrescent diameter. Benefitting from the simple, low-cost and mature fabrication process, the proposed large-area metal nanocrescents are propitious to application.

Conversion from terahertz-guided waves to surface waves with metasurface.

Surface waves (SWs) have attracted a widespread attention due to the characteristic of subwavelength confinement and convenient manipulation in photonic integrated circuits. Though metasurface provides a powerful tool in realizing the conversion between freely propagating waves and surface modes in recent years, a gulf between guided waves (GWs) and SWs in terahertz (THz) range still exists as a bottleneck for on-chip photonic integrated devices. Here, we implemented the conversion from THz GWs to SWs through the coupling of a lithium niobate (LN) subwavelength waveguide and metasurface antennas on an all-feature on-chip THz integrated platform. The conversion process and transmission mode of the THz waves were directly visualized via a time-resolved imaging system. Based on the dynamic process, the formation of SWs could be clarified through analyzing the dispersion relation of propagating modes, which is in good agreement with numerical models. In further, relying on the numerical simulation, SWs were induced from the collective oscillations of the metasurface antenna array and the maximum coupling efficiency was around 62.6 percent. Our work provides an efficient approach to control of GWs, and promotes the practicability of THz surface integrated devices, including THz surface spectroscopy sensing.

Efficient generation and frequency modulation of quasi-monochromatic terahertz wave in Lithium Niobate subwavelength waveguide.

A kind of lateral excitation (LE) configuration is proposed for quasi-monochromatic terahertz generation via impulsive stimulated Raman scattering in a LiNbO3 (LN) slab waveguide by numerical simulation. In an individual waveguide, maximum efficiency frequency-selective excitation is achieved with linewidth narrower than 38 GHz when phase matching is fulfilled between the pump laser and the generated terahertz (THz) waves. As a result, the frequency and linewidth of narrowband THz waves can be tuned through changing the dispersion of THz waves, which is implemented by adjusting the thickness of host LN slab. Furthermore, Au-Air-LN-Air-Au multilayer LE structure is developed to realize a dramatic change of the dispersion to obtain quasi-monochromatic THz waves, of which the linewidth is achieved as narrow as 10 GHz. In addition, the frequency and linewidth of quasi-monochromatic THz waves are modulated dynamically by varying the distance between LN slab and Au mirrors flexibly. Consequently, the optimized LE structure is expected to boost the development of high-precision and real-time inspection and sensing.

Direct visualization of light confinement and standing wave in THz Fabry-Perot resonator with Bragg mirrors.

We report for the first time the ability to perform time resolved imaging of terahertz (THz) waves propagating within a Fabry-Perot resonator on a LiNbO3 slab. Electro-optic effect is used to record the full spatiotemporal evolution of THz fields inside the resonator. In addition to revealing the real-space behavior, the data further demonstrate the confinement and the standing wave modes of THz in the cavity in frequency domain. The experimental results are in good agreement with numerical simulations. Using the coherent imaging technique to gain real-time information about a resonator system provides a unique path to study the physics of optical cavity.

Morphological evolution of self-deposition Bi2Se3 nanosheets by oxygen plasma treatment.

Bi2Se3 nanosheets were successfully synthesized by a microwave-assisted approach in the presence of polyvinylpyrroli done at a temperature of 180 °C for 2 h. The thin film was prepared on a silicon wafer via a self-deposition process in a Bi2Se3 nanosheet ink solution using the evaporation-induced self-assembly method. The structure and morphology of the obtained products were characterized by X-ray diffraction, scanning electron microscopy (SEM), x-ray photoelectron spectroscopy, and Raman spectroscopy. The highly uniform Bi2Se3 particles could be formed by controlling the oxygen plasma treatment time. After the plasma pretreatment from 10 to 20 s, the surface of Bi2Se3 film evolved from the worm-like structure to particles. The highly uniform thin film was formed on further increasing the plasma treatment time, which is consistent with the observed SEM results. Several important processes can result in the morphological evolution of Bi2Se3 nanosheets: (1) formation of Bi2Se3 oxide layer; (2) self-assembly of oxide nanoparticles under the action of high-energy oxygen plasma; and (3) electrostatic interaction and etching mechanism.

Threshold Dependence of Deep- and Near-subwavelength Ripples Formation on Natural MoS2 Induced by Femtosecond Laser.

Deep sub-wavelength ripples (DSRs) and near sub-wavelength ripples (NSRs) with uniform periods of ~160 nm and ~660 nm generated at the MoS2-vacuum interface is reported for the first time by the processing of femtosecond laser (800 nm, 120 fs, 1 kHz) in this paper. The DSRs and NSRs formation fluence thresholds are experimentally determined as 160 mJ/cm(2) and 192 mJ/cm(2), respectively. In addition, the ripple period is insensitive to the pulse number. Moreover, Raman analyses show that the MoS2 lattice in the irradiated area does not exhibit oxidation at room environment and the crystalline representation is well preserved in NSRs region. We attribute our result to the joint interactions of the spallation and sublimation of layered MoS2 together with the laser induced surface plasmon polaritons and propose an explanation to the threshold dependence of the ripple period. Our study provides some insights for ultrafast laser-matter interactions and indicates a simple effective method for future nano-fabrication of MoS2.

Novel high-efficiency crystalline-silicon-based compound heterojunction solar cells: HCT (heterojunction with compound thin-layer).

With an amorphous silicon (a-Si:H)/crystalline silicon (c-Si) heterojunction structure, the heterojunction with intrinsic thin-layer (HIT) solar cell has become one of the most promising technologies for c-Si based solar cells. By replacing a-Si:H thin films with appropriate compound semiconductors, we propose novel heterojunction structures which allow c-Si heterojunction solar cells to possess higher power conversion efficiencies than HIT solar cells. Several promising heterojunction candidates and hetero-structures have been proposed in this work, and this kind of novel c-Si compound heterojunction solar cell is denominated HCT (heterojunction with a compound thin-layer). The feasibilities of these novel HCT structures are further investigated by theoretical approaches, and the modeling results demonstrate the device performance improvement. Finally, this paper proclaims the compound selection standards and essentials of achieving high-efficiency HCT solar cells, which are guidelines for the real device implementation.

Structure of nitrogen and zirconium co-doped titania with enhanced visible-light photocatalytic activity.

Nitrogen and zirconium co-doped TiO2 (TiO2-N-x%Zr) photocatalysts were synthesized via a sol-gel method. The existing states of the dopants (N and Zr) and their corresponding band structures were investigated via XRD, Raman, BET, XPS, TEM, FT-IR, UV-vis DRS, and PL techniques. It was found that N existed only as a surface species (NOx) and Zr(4+) was doped in a substitutional mode; the doping of Zr(4+) ions and modification of N extended the absorption into the visible region and inhibited the recombination of electrons and holes. Moreover, the excess Zr(4+) ions existed as the ZrTiO4 phase when the content of Zr was sufficiently high, which could also contribute to the separation of the charge carriers. Therefore, the TiO2-N-x%Zr samples show enhanced visible-light photocatalytic activity compared with single-doped TiO2. These results offer a paradigm for the design and fabrication of optoelectronic functional materials such as solar cells and photocatalysts.

Generation and erasure of femtosecond laser-induced periodic surface structures on nanoparticle-covered silicon by a single laser pulse.

We experimentally show that the generation and erasure of femtosecond laser-induced periodic surface structures on nanoparticle-covered silicon inducted by irradiation with a single laser pulse (800 nm, 120 fs, linear polarization) depend on the pulse fluence. We propose that this is due to competition between periodic surface structuring originating from the interference of incident light with surface plasmon polaritons and surface smoothing associated with surface melting. Experimental results are supported by theoretical analysis of transient surface modifications based on combining the two-temperature model and the Drude model.

Time-resolved photoluminescence of silicon microstructures fabricated by femtosecond laser in air.

Green photoluminescence (PL) from silicon microstructures fabricated by femtosecond laser in air was studied at different temperature by time-resolved spectroscopy. The PL decay profiles are well fitted by a stretched exponential function: I(t)=I(0)*exp[-(t/τ)ß]. The dependence of the decay time constant τ and of the stretching index ß on PL photon energy and on the temperature is investigated. A model of transport and recombination of the carriers is introduced as a possible explanation of the stretched exponential decay. The nonradiative recombination rate of the localized carriers, which is dependent on the carrier density and influenced by the trapping site density and the temperature, is deduced to be responsible for this kind of decay.

Synthesis of indium borate and its application in photodegradation of 4-chlorophenol.

Indium borate has been prepared by a sol-gel method. The structure, morphology, and photophysics of the resultant photocatalysts have been studied via the techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), and diffuse reflectance UV-visible light spectroscopy. These photocatalysts have been used to photodegrade 4-chlorophenol. The photocatalytic activity depends on the annealing temperature during preparation. It is found that borates can exhibit a high photodegradation activity under UV-light irradiation, for which the efficiency can be higher than that of as-prepared TiO(2). This is explained according to the results of fluorescence spectra and valence band X-ray photoelectron spectroscopy (XPS). It is confirmed by the results of time-resolved photoluminescence decay spectra; i.e., the lifetime of electrons and holes involved in the radiative process can be longer for the borates than that for TiO(2). This implies that indium borate can be a promising photocatalyst for future applications in treatment of environment contaminants.

Second harmonic generation of periodically poled potassium titanyl phosphate waveguide using femtosecond laser pulses.

The authors have presented in this paper the fabrication and characterization of double line written type waveguides in c-cut periodically poled potassium titanyl phosphate crystals. The waveguides were fabricated by using a femtosecond laser, and were utilized for second harmonic generation at 1064 nm. Our experiments have shown that single mode propagation was observed at optimal waveguide width of 14.5 microm. And a conversion efficiency of 39.6% can be achieved.

[Photoluminescence investigation of InAs bimodal self-assembled quantum dots state filling].

Self-assembled InAs quantum dots were prepared on GaAs(100) substrate in a solid source molecular beam epitaxy system. The distribution and topographic images of uncapped dots were studied by atomic force microscope. The statistical result shows that the quantum dots are bimodal distribution. The photoluminescence spectrum results shows that the intensity of small size quantum dots dominated, which may be due to: (1) the state density of large quantum dots lower than that of small quantum dots; (2) the carriers capture rate of large size quantum dots is small relative to that of small ones; (3) there is a large strain barrier between large quantum dots and capping layer, and the large strain is likely to produce the defect and dislocation, resulting in a probability of carriers transferring from large quantum dots to small dots that is very small with temperature increasing.