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Accepted papers to appear in an upcoming issue

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Direct bandgap photoluminescence from n-type indirect GaInP alloys

Cong Wang, Bing Wang, Riko I. Made, Soon Fatt Yoon, and Jurgen Michel

Doc ID: 285889 Received 30 Jan 2017; Accepted 27 Mar 2017; Posted 29 Mar 2017  View: PDF

Abstract: This work studies tellurium (Te) doping effects on the direct bandgap photoluminescence (PL) of indirect GaxIn1-xP alloys (0.72 ≤ x ≤ 0.74). The temperature-dependent PL shows that the energy difference between direct Γ valley and indirect X valleys is reduced due to the bandgap narrowing (BGN) effect, and the direct band transition gradually dominates the PL spectra as temperature increases. Carrier thermalization has been observed for Te-doped GaxIn1-xP samples as integrated PL intensity increases with increasing temperature from 175 K to 300 K. The activation energy for carrier thermalization is reduced as doping concentration increases. Both BGN effect and carrier thermalization contribute to the carrier injection into the Γ valley. As a result, the direct band transition is enhanced in the Te-doped indirect GaxIn1-xP alloys. Therefore, the photoluminescence intensity of the Ga0.74In0.26P sample with active doping concentration of 9×1017 cm-3 is increased by five times compared to a nominally undoped sample. It is also found that the photoluminescence intensity is degraded significantly when the doping concentration is increased to 5×1018 cm-3. From cross-section transmission electron microscopy, no large dopant clusters or other extended defects were found contributing to this degradation.

Non-probe compensation of optical vortices carrying orbital angular momentum

Shiyao Fu, Tonglu Wang, Shikun ZHANG, ZheYuan Zhang, Wang Zhai, and Chunqing Gao

Doc ID: 287132 Received 21 Feb 2017; Accepted 27 Mar 2017; Posted 29 Mar 2017  View: PDF

Abstract: In this paper, we demonstrate a scheme of compensating distorted optical vortex beams carrying orbital angular momentum. By inputting the intensity profile into the Gerchberg-Saxton (GS) algorithm [1], the pre-compensation phase mask can be acquired. No additional probe beams are introduced, and all the computing is aiming at the transmitted vortex beams. The distorted vortex beams before and after pre-compensation are investigated experimentally, showing favorable compensation performance. This work will find applications in the domains as rotation detection, optical communications and so on.

Numerical simulation and temporalcharacterization of dual-pumped microringresonator-based optical frequency combs

Xiaohong Hu, Weiqiang Wang, Leiran Wang, Wenfu Zhang, Yishan Wang, and Wei Zhao

Doc ID: 286540 Received 10 Feb 2017; Accepted 22 Mar 2017; Posted 22 Mar 2017  View: PDF

Abstract: Optical frequency combs (OFCs) generation from a dual-pumped microring resonator (MRR) are numerically investigated and experimentally verified. The calculation results obtained by solving the driven and damped nonlinear Schrödinger equation indicate that an ultralow coupled pump power is required to excite the primary comb modes through non-degenerate four-wave mixing (FWM) process and when the pump power is boosted, both the comb modes intensities and spectral bandwidth increase. At low pump powers, the field intensity profile exhibits a cosine variation manner with the frequency equal to the separation of the two pumps while a roll Turing pattern is formed resulted from the increased comb modes intensities and spectral bandwidth at high pump powers. Meanwhile, we found that the power difference between the two pump fields can be transferred to the newly generated comb modes locating on both sides of the pump modes through cascaded FWM process. Experimentally, the dual-pumped OFCs were realized by coupling two self-oscillated pump fields into the MRR. The numerically calculated comb spectrum is verified by generating an OFC with a 2.0-THz mode spacing and an over 160-nm bandwidth. In addition, the forming of a roll Turing pattern at high pump powers is inferred from the measured autocorrelation trace of a 10-FSR OFC. The experimental observations accord well with the numerical predications. The proposed dual-pumped OFCs could find potential applications in a wide range of fields due to their large and tunable mode spacing, robustness and flexibility, such as arbitrary optical waveform generation, high capacity optical communication and signal processing systems.

Ultra-compact on-chip slot Bragg grating structure for small electric field detection

Wentao Qiu, Huihui Lu, Fadi BAIDA, and Maria-Pilar Bernal

Doc ID: 282945 Received 09 Jan 2017; Accepted 21 Mar 2017; Posted 22 Mar 2017  View: PDF

Abstract: In this paper, we present an ultra compact one dimensional photonic crystal (PhC) Bragg grating design on thin film lithium niobate (TFLN) slot waveguide (SWG) via 2D and 3D-FDTD simulations. 2D-FDTD simulationsare employed to tune the photonic band gap (PBG) size, PBG center, cavity resonance wavelength and the whole size of PhC etc. 3D-FDTD simulations are carried out to model the real structure by varying different geometrical parameters such as SWG height and PhC size. A moderate resonance quality factor Q of about 300 is achieved with a PhC size of only 0.5 μm ×0.7 μm ×6 μm. The proposed slot Bragg grating structure is then exploited as an electric field (E-field) sensor. The sensitivity is analyzed by 3D-FDTD simulations with a minimum detectable E-field as small as mV/m. The compact size of the proposed slot Bragg grating structure may have applications in on-chip E-field sensing and optical filtering etc.

A wavelength-swept fiber laser based on bidirectional used linear chirped fiber Bragg grating

Lin Wang, Minggui Wan, Zhenkun Shen, Xudong Wang, Yuan Cao, Xinhuan Feng, and Bai-Ou Guan

Doc ID: 286167 Received 06 Feb 2017; Accepted 15 Mar 2017; Posted 24 Mar 2017  View: PDF

Abstract: A wavelength-swept fiber laser is proposed and successfully demonstrated based on a bidirectional used linear chirped fiber Bragg grating (LC-FBG). The wavelength swept operation principle is based on intra-cavity pulse stretching and compression. The LC-FBG can introduce equivalent positive and negative dispersion simultaneously, which enables a perfect dispersion matching to obtain wide bandwidth mode-locking. Experimental results demonstrate a wavelength swept fiber laser that exhibits a sweep rate of about 5.4 MHz over a 2.1 nm range at a center wavelength of 1550 nm. It has the advantages of simple configuration and perfect dispersion matching in the laser cavity.

Spatial evolution of the near-field distribution on planar gold nanoparticles with the excitation wavelength across dipole and quadrupole modes

Quan Sun, Jinghuan Yang, Han Yu, Kosei Ueno, Hiroaki Misawa, and Qihuang Gong

Doc ID: 285502 Received 25 Jan 2017; Accepted 13 Mar 2017; Posted 15 Mar 2017  View: PDF

Abstract: We investigate the superposition properties of the dipole and quadrupole plasmon modes in the near field both experimentally by using photoemission electron microscopy (PEEM) and theoretically. In particular, the asymmetric near-field distributions on gold (Au) nanodisks and nanoblocks under oblique incidence with different polarizations are investigated in detail. The results of PEEM measurements show that the evolutions of the asymmetric near-field distributions are different between the excitation with s-polarized and p-polarized light. The experimental results can be reproduced very well by numerical simulations and can be interpreted as the superposition of the dipole and quadrupole modes with the help of analytic calculations. Moreover, we hypothesize that the electrons collected by PEEM are mainly from the plane located at the interface between the gold particles and the substrate in the PEEM experiments.

Light-driven crystallization of polystyrene micro-spheres

JING LIU and Zhiyuan Li

Doc ID: 285234 Received 20 Jan 2017; Accepted 13 Mar 2017; Posted 15 Mar 2017  View: PDF

Abstract: Investigating the dynamic crystallization processes of colloidal photonic crystals are potentially invaluable for solving a number of existing and emerging technical problems about controlled fabrication of crystals, such as size normalization, stability improvement, and acceleration of synthesis. In this paper, we report systematic high-resolution optical observation of the spontaneous crystallization of monodisperse polystyrene (PS) micro-spheres in aqueous solution into close-packed arrays in a static line optical tweezers. The experiments demonstrate that the crystal structure is mainly affected by the minimum potential energy of the system, however, the crystallization dynamics could be affected by various mechanical, physical, and geometric factors. The complicated dynamic transformation process from one-dimensional (1D) crystallization to 2D crystallization and the creation and annihilation of dislocations and defects via crystal relaxation are clearly illustrated. Two major crystal growth modes as the epitaxy growth pattern and the inserted growth pattern have been identified to play a key role in shaping the dynamics of both the 1D and 2D crystallization process. These observations offer invaluable insights for in-depth research about colloidal crystal crystallization.

Effects of the slot width and angular position on the mode splitting in slotted optical resonator

Lingling Dai, Yiheng Yin, Yanhui Hu, Biyao Yang, and Ming Ding

Doc ID: 285239 Received 19 Jan 2017; Accepted 11 Mar 2017; Posted 15 Mar 2017  View: PDF

Abstract: A novel slotted optical whispering gallery modes (WGMs) resonator, which significantly enhances the light-matter interaction and provides a promising approach for increasing the sensitivity of sensors, is theoretically and numerically investigated. In this slotted resonator, the mode splitting is generated due to the Rayleigh scattering of the slot. Remarkably, effects of the slot width and angular position on the mode splitting are mainly studied. The results exhibit that the mode splitting is a second function of the slot width and the maximum mode splitting induced by the slot deformation is achieved with . Therefore, the slotted resonator is an excellent candidate for pressure and force sensing. Besides, the influence of the slot angular position on the mode splitting is a cosine curve with the highest sensitivity of , which the optical characteristic demonstrates the slotted resonator can be used for inertial measurements.

Sensing of microparticles based on a broadband ultrasmall microcavity in a freely suspended microfiber

Yang Yu, Tinghui Xiao, Honglian Guo, and Zhiyuan Li

Doc ID: 282062 Received 05 Dec 2016; Accepted 08 Mar 2017; Posted 10 Mar 2017  View: PDF

Abstract: We theoretically design and experimentally realize a broadband ultrasmall microcavity for sensing of different number of microparticles in a freely suspended microfiber. The performance of the microcavity is predicted by the theory of one-dimensional photonic crystal and verified by the numerical simulation of finite difference time domain (FDTD) and the experimental characterization of reflection and transmission spectra. The penetrating length through the reflectors as small as about four periods is demonstrated in the numerical simulation, giving rise to the ultrasmall effective mode volume that can increase the sensitivity and spatial resolution of sensing. Moreover, the reflection band as large as 150 nm of the reflectors of the microcavity has been realized in silica optical microfiber in experiment, which highly expands the wavelength range of sensing. Our proposed microcavity integrated in a freely suspended optical fiber offers a convenient and stable method for long-distance sensing microparticles without the need of complicated coupling system and free from the influence of substrates.

Complete crossing of Fano resonances in an optical microcavity via nonlinear tuning

Martino Bernard, Fernando Ramiro-Manzano, Lorenzo Pavesi, Georg Pucker, Iacopo Carusotto, and Mher Ghulinyan

Doc ID: 285174 Received 23 Jan 2017; Accepted 08 Mar 2017; Posted 10 Mar 2017  View: PDF

Abstract: We report on the modeling, simulation and experimental demonstration of complete mode crossings of Fano resonances within chip-integrated microresonators. The continuous reshaping of resonant lineshapes is achieved via nonlinear thermo-optical tuning when the cavity-coupled optical pump is partially absorbed by the material. The locally generated heat then produces a thermal field, which influences the spatially overlapping optical modes, allowing thus to alter the relative spectral separation of resonances. Furthermore, we exploit such tunability to probe continuously the interaction between different families of quasi-degenerate modes that exhibit asymmetric Fano-interactions. As a particular case, we demonstrate for the first time a complete disappearance of one of the modal features in the transmission spectrum as predicted by U. Fano [Phys. Rev. 124, 1866 (1961)]. The phenomenon is modeled as a third order non-linearity with a spatial distribution that depends on the stored optical field and the thermal diffusion within the resonator. The performed non-linear numerical simulations are in excellent agreement with the experimental results, which confirm the validity of the developed theory.

Measurement of the electromagnetic degree of temporal coherence of unpolarized light beams

Lasse-Petteri Leppänen, Kimmo Saastamoinen, Ari Tapio Friberg, and Tero Setala

Doc ID: 285606 Received 24 Jan 2017; Accepted 07 Mar 2017; Posted 07 Mar 2017  View: PDF

Abstract: We measure the electromagnetic degree of temporal coherence and the associated coherence time for quasimonochromatic unpolarized light beams emitted by an LED, a filtered halogen lamp, and a multimode HeNe laser. The method is based on observing at the output of a Michelson interferometer the visibilities (contrasts) of the intensity and polarization-state modulations expressed in terms of the Stokes parameters. The results are in good agreement with those deduced directly from the source spectra. The measurements are repeated after passing the beams through a linear polarizer so as to elucidate the role of polarization in electromagnetic coherence. While the polarizer varies the equal-time degree of coherence consistently with the theoretical predictions and alters the inner structure of the coherence matrix, the coherence time remains almost unchanged when the light varies from unpolarized to polarized. The results are important in the areas of applications dealing with physical optics and electromagnetic interference.

Coherent optical adaptive technique improves the spatial resolution of STED microscopy in thick samples

wei yan, Yan long Yang, yu tan, Xun Chen, yang li, Junle Qu, and Tong Ye

Doc ID: 284251 Received 16 Jan 2017; Accepted 03 Mar 2017; Posted 07 Mar 2017  View: PDF

Abstract: Stimulated emission depletion microscopy (STED) is one of far-field optical microscopy techniques that can provide sub-diffraction spatial resolution. The spatial resolution of the STED microscopy is determined by the specially engineered beam profile of the depletion beam and its power. However, the beam profile of the depletion beam may be distorted due to aberrations of optical systems and inhomogeneity of specimens’ optical properties, resulting in a compromised spatial resolution. The situation gets deteriorated when thick samples are imaged. In the worst case, the sever distortion of the depletion beam profile may cause complete loss of the superresolution effect no matter how much depletion power is applied to specimens. Previously several adaptive optics approaches have been explored to compensate aberrations of systems and specimens. However, it is hard to correct the high-order optical aberrations of specimens. In this report, we demonstrate that the complicated distorted wavefront from a thick phantom sample can be measured by using the coherent optical adaptive technique (COAT). The full correction can effectively maintain and improve the spatial resolution in imaging thick samples.

Generation of sub-100 fs pulses tunable from 1700 to 2100 nm from a compact frequency-shifted Er-fiber laser

Grzegorz Sobon, Tadeusz Martynkien, Karol Tarnowski, Pawel Mergo, and Jaroslaw Sotor

Doc ID: 285468 Received 23 Jan 2017; Accepted 02 Mar 2017; Posted 07 Mar 2017  View: PDF

Abstract: We report generation of sub-100 fs pulses tunable from 1700 to 2100 nm via Raman soliton self-frequency shift. The nonlinear shift occurs in a highly nonlinear fiber, which is pumped by an Er-doped fiber oscillator. The whole system is fully fiberized, without the use of any free-space optics. Thanks to its exceptional simplicity, the setup can be considered as an alternative to mode-locked Tm- and Ho-doped fiber lasers.

Graphene-supported manipulation of surface plasmon polaritons in metallic nanowaveguides

Hua Lu, Xuetao Gan, Dong Mao, and Jianlin Zhao

Doc ID: 283635 Received 26 Dec 2016; Accepted 02 Mar 2017; Posted 07 Mar 2017  View: PDF

Abstract: We investigate the electrically controlled light propagation in the metal-dielectric-metal plasmonic waveguide with a sandwiched graphene monolayer. The theoretical and simulation results show that the propagation loss exhibits an obvious peak when the permittivity of graphene approaches epsilon-near-zero point with adjusting the gate voltage on graphene. The analog of electromagnetically induced transparency (EIT) can be generated by introducing side-coupled stubs into the waveguide. Based on the EIT-like effect, the hybrid plasmonic waveguide with a length of only 1.5 μm can work as a modulator with an extinction ratio of ~15.8 dB, which is 2.3 times larger than the case without the stubs. The active modulation of surface plasmon polariton propagation can be further improved by tuning the carrier mobility of graphene. The graphene-supported plasmonic waveguide system could find applications for the nanoscale manipulation of light and chip-integrated modulation.

A broadband on-chip integrator based on silicon photonic phase-shifted Bragg grating

Xu Wang, Feng Zhou, Siqi Yan, Yuan Yu, Jianji Dong, and Xinliang Zhang

Doc ID: 284110 Received 05 Jan 2017; Accepted 01 Mar 2017; Posted 01 Mar 2017  View: PDF

Abstract: An all-optical integrator is the key device of passive all-optical network, which has been applied for photonic bit counting, optical memory units and analogue computing, and to broaden the bandwidth of the optical device for high speed optical signal processing is still a challenge. In this work, an all-optical integrator based on silicon photonic phase-shifted Bragg grating is proposed and experimentally demonstrated, which shows a wide operation bandwidth of 750 GHz and integration time window of 9 ps. The integral operation for single pulse, in-phase pulses and π-shifted pulses with different delay have been successfully achieved.

Recombination mechanisms and thermal droop in AlGaN-based UV-B LEDs

Carlo De Santi, Matteo Meneghini, Desiree Monti, Johannes Glaab, Martin Guttmann, Jens Rass, Sven Einfeldt, Frank Mehnke, Johannes Enslin, Tim Wernicke, M Kneissl, Gaudenzio Meneghesso, and Enrico Zanoni

Doc ID: 282251 Received 07 Dec 2016; Accepted 28 Feb 2017; Posted 28 Feb 2017  View: PDF

Abstract: This paper reports a comprehensive analysis of the origin of the electroluminescence (EL) peaks and of the thermal droop in UV-B AlGaN-based LEDs. By carrying out spectral measurements at several temperatures and currents, (i) we extract information on the physical origin of the various spectral bands, and (ii) we develop a novel closed-form model based on the Shockley-Read-Hall (SRH) theory and on the ABC rate equation which is able to reproduce the experimental data on thermal droop caused by non-radiative recombination through deep levels. In the samples under test, the three electroluminescence bands are ascribed to the following processes: band-to-band recombination in the quantum wells (main EL peak), a parasitic intra-bandgap radiative transition in the quantum well barriers and a second defect-related radiative process in the p-AlGaN superlattice.

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