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Accepted papers to appear in an upcoming issue

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Time-domain numerical modeling of THz receivers based on photoconductive antennas

Enrique Moreno, MARIO PANTOJA, Juan Bautista Roldan Aranda, Amelia Bretones, Salvador González García, and Zahra Hemmat

Doc ID: 237616 Received 27 Apr 2015; Accepted 26 Jul 2015; Posted 29 Jul 2015  View: PDF

Abstract: We present here a simulator that solves the main semiconductor charge and transport equations coupled to Maxwell equations to study receivers based on photoconductive antennas (R-PCAs). Making use of this tool we were able to correctly characterize the operation of these antennas. In doing so, we compared simulations with the results of the semi-empirical expression $I_{THz}(t)\propto σ(t)* E_{THz}(t)$ employed to evaluate the detected photo-current by means of the convolution between the photoconductivity in the receiver and the electric field linked to the emitter antenna. We were able to accurately reproduce experimental data with our simulation tool. These kind of tools are essential to model photoconductive antennas, a fundamental step needed for the development of terahertz time domain spectroscopy (THz-TDS) applications based on PCAs.

Temporal pulse division in hollow fiber compressors

Hermance Jacqmin, Aurelie Jullien, Brigitte Mercier, and Rodrigo Lopez-Martens

Doc ID: 243475 Received 22 Jun 2015; Accepted 25 Jul 2015; Posted 29 Jul 2015  View: PDF

Abstract: We propose a detailed analysis of a temporal multiplexing technique suited to the post-compression of energetic femtosecond laser pulses in gas-filled hollow-core fibers down to the few-cycle regime. Twofold temporal division and combination is achieved using two birefringent plates with specific crystallographic orientation. We demonstrate a simple interferometric method for measuring the relative spectral phase between two replicas, which gives a measure of the phase-mismatch in the combining plate as well that induced by eventual cross-phase modulation or ionization during propagation in the fiber. We present the experimental conditions required for producing few-cycle pulses with high fidelity. This passive combination technique will aid the energy scaling of hollow fiber compressors to the multi-milliJoule level.

Second harmonic generation of focused ultrashort x-ray pulses

Shimon Yudovich and Sharon Shwartz

Doc ID: 240512 Received 07 May 2015; Accepted 24 Jul 2015; Posted 24 Jul 2015  View: PDF

Abstract: We study the process of x-ray second harmonic generation (SHG) from focused ultrashort pulses. We numerically simulate and derive approximated analytical expressions for the efficiency and tolerances of the SHG process, considering the effects of temporal and spatial walk-off, and the coupling to the linear Bragg scattering. We show that for the recently observed x-ray SHG in diamond, the simultaneous Bragg diffraction of the generated second harmonic is negligible. However, spatiotemporal walk-off effects are crucial components regarding the wave propagation aspects of this process, due to the highly noncollinear phase-matching geometry.

Coherent Artifact Study of Two-Dimensional Spectral Shearing Interferometry

Michelle Rhodes, Madhuri Mukhopadhyay, Jonathan Birge, and Rick Trebino

Doc ID: 242984 Received 15 Jun 2015; Accepted 24 Jul 2015; Posted 24 Jul 2015  View: PDF

Abstract: We study multi-shot intensity-and-phase measurements of unstable trains of ultrashort pulses using two-dimensional spectral shearing interferometry (2DSI). We find that, like other interferometric ultrashort-laser pulse-measurement methods, it measures only the coherent artifact and so yields effectively only a lower bound to the pulse length. We also attempt to identify warning signs of pulse-shape instability in 2DSI and find that it can signal instability with reduced fringe visibility, although this effect is very small when using the small spectral shears appropriate for large temporal ranges. We conclude that 2DSI should be used with caution, and large shear measurements or alternative techniques should be used to verify the stability of the pulse train.

A measure for carrier shocking

Paul Kinsler

Doc ID: 243231 Received 16 Jun 2015; Accepted 23 Jul 2015; Posted 24 Jul 2015  View: PDF

Abstract: I propose a definition for a "shocking coefficient" S intended to make determinations of the degree of waveform shocking, and comparisons thereof, more quantitative. This means we can avoid having to make ad hoc judgements on the basis of the visual comparison of wave profiles.

Design of ultra-broadband graphene absorber using circuit theory

Amin Khavasi

Doc ID: 238348 Received 17 Apr 2015; Accepted 23 Jul 2015; Posted 24 Jul 2015  View: PDF

Abstract: I propose a novel method for designing a broadband THz absorber by using periodic arrays of graphene ribbons on a Salisbury-screen-like structure. The recently proposed analyitcal circuit model of graphene arrays is used for obtaining analytical expressions for the input admittance of the proposed device. The input admittance is then adjusted to be almost matched to free space in a wide frequency range. Conequently, it is demonstrated that bandwidth of 90% absorption can be extended up to 100% of the central frequency with only one layer of patterned graphene.

Dissipative preparation of three-atom entanglement state via quantum feedback control

Ai-Dong Zhu, Hong-Fu Wang, Shou Zhang, Wen-Mei Sun, Zhao Jin, Shi-Lei Su, and Yan Liang

Doc ID: 236682 Received 23 Mar 2015; Accepted 21 Jul 2015; Posted 22 Jul 2015  View: PDF

Abstract: Two schemes are presented for generating steady three-atom Greenberger-Horne-Zeilinger(GHZ) and W states in a strongly dissipative cavity via quantum feedback control. The quantum feedback control is only applied to a single atom based on quantum-jump detection to improve the fidelity of the target state. Thus cavity decay plays a key role in obtaining the target state. The required interaction time need not be accurately controlled.

Advanced regime of the non-collinear two-phonon acousto-optical interaction governed by elastic waves of finite amplitude and optical spectrum analysis

Adan Omar Arellanes Bernabe and Alexandre Shcherbakov

Doc ID: 237104 Received 31 Mar 2015; Accepted 21 Jul 2015; Posted 22 Jul 2015  View: PDF

Abstract: Principally new features of square-law nonlinearity peculiar to the non-collinear two-phonon acousto-optical interaction governed by elastic waves of finite amplitude in birefringent crystals are revealed and studied. An additional degree of freedom represented by the dispersive birefringence factor, which can be distinguished within this nonlinear phenomenon, is found and characterized. This physical degree of freedom gives us a one-of-a-kind opportunity to apply the two-phonon acousto-optical interaction in practice for the first time. The needed theoretical analysis is developed and proof-of-principle experiments, performed with a specially designed unique wide-aperture acousto-optical cell made of the calomel (α-Hg₂Cl₂) single crystal, are presented. The results of experiments with this acousto-optical cell confirm the elaborated theory with numerical estimations and allow the proposed application to optical spectrum analysis with the doubled resolution.

Impact of structural asymmetry on the efficiency of triple-core photonic crystal fiber for all-optical logic operation

Vasantha Jayakantha Raja R., Uthayakumar Thangaraj, Philippe Grelu, and Kuppusamy Porsezian

Doc ID: 241484 Received 21 May 2015; Accepted 20 Jul 2015; Posted 20 Jul 2015  View: PDF

Abstract: We investigate the nonlinear dynamics and steering performance of structurally asymmetric triple-core photonic crystal fiber (ATPCF) for the accomplishment of efficient all-optical logic gates. We study two kinds of ATPCF, one with planar and the other with triangular core setting. The effective mode indices are obtained through finite element method. The dynamics and steering characteristics of ATPCF are numerically explored via coupled nonlinear Schrödinger equations. The extinction ratios for the various logic gate operations are determined in presence of suitable control signal. ATPCF are found to demonstrate efficient logic gate operation, as well as they highlight the practical issue of geometrical tolerance in PCF design

Optimized ancillae generation for ultra-broadband 2D spectral-shearing interferometry

Cristian Manzoni, Rocio Borrego-Varillas, Aurelio Oriana, Federico Branchi, Sandro De Silvestri, and Giulio Cerullo

Doc ID: 241597 Received 26 May 2015; Accepted 19 Jul 2015; Posted 20 Jul 2015  View: PDF

Abstract: We introduce a new scheme which overcomes the challenges of ancillae preparation in two-dimensional spectrally-sheared interferometry. In our approach the sheared monochromatic ancillae are generated by spectral filtering in the Fourier plane of a pulse shaper. This scheme allows direct and precise measurement of the spectral shear, greatly improving the accuracy and reliability of spectral phase retrieval. By characterizing few-optical-cycle pulses spanning nearly two octaves of bandwidth, we prove the applicability of our method to (sub)-single-cycle pulses.

Broadband Terahertz Modulators using Self-gated Graphene Capacitors

Coskun Kocabas, Nurbek Kakenov, OSMAN BALCI, Hakan Altan, and Emre Ozan Polat

Doc ID: 240535 Received 08 May 2015; Accepted 19 Jul 2015; Posted 20 Jul 2015  View: PDF

Abstract: We demonstrate a terahertz intensity modulator using a graphene supercapacitor which consists of two large area graphene electrodes and electrolyte medium. The mutual electrolyte gating between the graphene electrodes provides a very efficient electrostatic doping with Fermi energies of 1 eV and charge density of 8x1013 cm-2. We show that, the graphene supercapacitor yields more than 50% modulation between 0.1 to 1.4 THz with operation voltages less than 3 V. The low insertion losses, high modulation depth over a broad spectra and the simplicity of the device structure are the key attributes of graphene supercapacitors for THz applications.

Experimentally realization of focal field engineering of the azimuthally polarized beams modulated by multi-azimuthal masks

Peng Li, Jianlin Zhao, Sheng Liu, yi zhang, and Xie Gaofeng

Doc ID: 240605 Received 12 May 2015; Accepted 16 Jul 2015; Posted 20 Jul 2015  View: PDF

Abstract: This paper experimentally investigates the controlling of focal field of azimuthally polarized (AP) Gaussian beam with multi-azimuthal masks. Based on the angular diffraction theory, an explicit model is established to describe the intensity and polarization distributions of the focal fields. It reveals that the focal fields of the modulated AP beams are closely related to the structure of the mask. By designing the mask structure parameters, we could consciously manage the intensity and polarization distributions of the focal fields of an AP Gaussian beam. Our experimental results also demonstrate the feasibility of controlling focal field characteristics by engineering mask structure, and suggest an effective approach for manipulating the focal field of vector beams.

Theoretical predictions and experimental suggestions for refraction behaviors occurring at lossy interfaces

Jiangwei Chen and Ruiqi Wang

Doc ID: 239910 Received 27 Apr 2015; Accepted 16 Jul 2015; Posted 29 Jul 2015  View: PDF

Abstract: According to the energy streamline method, refraction of energy flow of the transverse magnetic (electric) polarized electromagnetic waves occurring at the lossy/lossless interface may be either positive or negative depending on the sign of the real part of permittivity (permeability) of the lossy medium. On the other hand, based on the reflection and refraction formulas developed by us, it is demonstrated that refraction behaviors of electromagnetic wave occurring at the lossy/lossless interface relate to the difference between electric damping angle and magnetic one of the lossy medium. Furthermore, the two-dimensional transmission lines with adjustable effective permittivity and permeability are proposed to be applied to experimentally test these predictions. This work provides systematically the basic refraction properties of electromagnetic waves occurring at lossy/lossless interfaces, may be useful to deeply understand the observed refraction behaviors associated with the lossy interfaces and fully test the validity of reflection and refraction theories.

Analysis of Integrated MIM-based Plasmonic Devices Using a Transmission-Line Formulation

Mahmood Shahabadi, Zahra Serahati, and Mohsen Rajaei

Doc ID: 241428 Received 27 May 2015; Accepted 15 Jul 2015; Posted 16 Jul 2015  View: PDF

Abstract: A rigorous and time-efficient method for numerical analysis of plasmonic integrated devices has been proposed. In this technique, the structure under investigation is periodically repeated along one direction, and then is analyzed using a Transmission-Line Formulation. The obtained numerical results for various plasmonic devices are compared with those generated by an FDTD analysis. The proposed technique does not require domain discretization and absorbing layers; thus, it excells an FDTD analysis in terms of computation time and memory resources.

Dressed dynamics of two time-reversed shapes of Airy pulses in relaxing nonlinear medium

Xiaohui Shi, Xiquan Fu, Yanfeng Bai, Chao Tan, Ziyang Zhang, and rui wang

Doc ID: 241773 Received 27 May 2015; Accepted 13 Jul 2015; Posted 14 Jul 2015  View: PDF

Abstract: We investigate the dressed dynamics of two kinds of asymmetric Airy pulses with time-reversed shapes in relaxing nonlinear medium. Airy tails act as low intensity “dressing” pulses, refuel Airy main lobe and modify propagation dynamics of Airy pulses. It is demonstrated that soliton sheds from the main lobe of Airy pulses and the spectrum maximum intensity are periodically manifested due to the effect of self-phase modulation. The intrinsically asymmetric nature of Airy pulses is revealed through the nonlinear generation of Raman soliton self-frequency shift and the generation of static solitons driven by the soliton fission processes and the interactions with dressing Airy tails. The resulting Raman induced frequency shift can be controlled by using time-reversed Airy pulses and varying truncation coefficients. Numerical results show that the frequency shift can be enhanced with a larger input peak power as well.

Two plasmonic mode excitation using a double step rectangle grating

Yosuke Mishima, hideaki habara, and Kazuo Tanaka

Doc ID: 237285 Received 03 Apr 2015; Accepted 13 Jul 2015; Posted 13 Jul 2015  View: PDF

Abstract: Excitation of plasmonic modes is investigated on the grating having a double step rectangle profile. Whereas a single step grating excites one side of plasmonic mode, the double step profile can induce two-mode surface plasmon polaritons simultaneously. Mode wavelength of the double step grating can be individually controlled by selecting rule density and depths of steps. In addition, the field intensity on the double step grating has wider and stronger distributions than that appeared on the single step grating.

Modeling of supersonic diode pumped alkali lasers

Boris Barmashenko, Eyal Yacoby, Oren Sadot, Karol Waichman, and Salman Rosenwaks

Doc ID: 240712 Received 11 May 2015; Accepted 10 Jul 2015; Posted 13 Jul 2015  View: PDF

Abstract: Three-dimensional computational fluid dynamics (3D CFD) modeling of supersonic diode pumped alkali lasers (DPALs), taking into account fluid dynamics and kinetic processes in the lasing medium, is reported. For a supersonic Cs DPAL with laser section geometry and resonator parameters similar to those of the 1-kW flowing-gas subsonic Cs DPAL [A.V. Bogachev et al., Quantum Electron. 42, 95 (2012)] the maximum value of lasing power, ~ 7 kW, is 25% higher than that achievable in the subsonic case. Comparison between semi-analytical and 3D CFD models for Cs shows that the latter predicts much higher maximum achievable laser power than the former. Optimization of the laser parameters using 3D CFD modeling shows that very high power and optical-to-optical efficiency, 35 kW and 82%, respectively, can be achieved in the Cs supersonic device pumped by a collimated cylindrical (0.5 cm diameter) beam. Application of end- or transverse-pumping by collimated rectangular (large cross section ~ 2 – 4 cm2) beam makes it possible to obtain even higher output power, > 250 kW, for ~ 350 kW pumping power. The main processes limiting the power of Cs supersonic DPAL are saturation of the D2 transition and large ~ 40% losses of alkali atoms due to ionization, whereas the influence of gas heating is negligibly small. For supersonic K DPAL both gas heating and ionization effects are shown to be unimportant and the maximum achievable power, ~ 40 kW and 350 kW, for pumping by ~ 100 kW cylindrical and ~ 700 kW rectangular beam, respectively, are higher than those achievable in the Cs supersonic laser. The power achieved in the supersonic K DPAL is two times higher than for the subsonic version with the same resonator and K density at the inlet, the maximum optical-to-optical efficiency being 82%. The power of the mechanical pump required for closed-cycle operation of supersonic DPALs was estimated for constant and variable area ratio diffusers. The nucleation rate of the alkali atoms during supersonic expansion was calculated and found to be negligible.

Realizing high-quality, ultra-large momentum states and ultrafast topological transitions using semiconductor hyperbolic metamaterials

Salvatore Campione, Ting S. Willie Luk, Sheng Liu, and Michael Sinclair

Doc ID: 242382 Received 09 Jun 2015; Accepted 09 Jul 2015; Posted 14 Jul 2015  View: PDF

Abstract: We employ both the effective medium approximation (EMA) and Bloch theory to compare the dispersion properties of semiconductor hyperbolic metamaterials (SHMs) at mid-infrared frequencies and metallic hyperbolic metamaterials (MHMs) at visible frequencies. This analysis reveals the conditions under which the EMA can be safely applied for both MHMs and SHMs. We find that the combination of precise nanoscale layering and the longer infrared operating wavelengths puts the SHMs well within the effective medium limit and, in contrast to MHMs, allows the attainment of very high photon momentum states. In addition, SHMs allow for new phenomena such as ultrafast creation of the hyperbolic manifold through optical pumping. In particular, we examine the possibility of achieving ultrafast topological transitions through optical pumping which can photo-dope appropriately designed quantum wells on the femtosecond time scale.

Analysis of Q factors of structural imperfections in triangular-cross-sectional nanobeam photonic crystal cavities

Yuki Yamaguchi, Seungwoo Jeon, Bong-Shik Song, Yoshinori Tanaka, Takashi Asano, and Susumu Noda

Doc ID: 241403 Received 21 May 2015; Accepted 09 Jul 2015; Posted 09 Jul 2015  View: PDF

Abstract: We present comprehensive and quantitative analysis of the effect of structure imperfections on quality (Q) factors in triangular-cross-sectional nanobeam photonic crystal cavities. We investigated the influence of nine-type imperfect factors on Q factors for the cavities. Our analysis revealed that ideal Q factor of 2.0×10^6 decreases significantly to 4.0×10^4 due to the influence of the structural imperfections. The Q factor can be improved to over 4.5×10^5 by reducing the influence of the structural asymmetry with respect to the center of the nanobeam.

Transverse distinguishability of entangled photons with arbitrarily shaped spatial near- and far-field distributions

Robert Elsner, Dirk Puhlmann, Axel Heuer, Ralf Menzel, and Gregor Pieplow

Doc ID: 243886 Received 01 Jul 2015; Accepted 07 Jul 2015; Posted 09 Jul 2015  View: PDF

Abstract: Entangled photons generated by spontaneous parametric down conversion are ubiquitous in quantum optics. In general they exhibit a complex spatial photon count distribution. This spatial structure is responsible for seemingly surprising results concerning e.g. complementarity such as the apparent simultaneous observation of interference fringes $V$ and which-way information $D$ at a double slit as recently reported by Menzel et al. We implement a complete quantitative model of the SPDC interaction which fully incorporates the effects of crystal anisotropies, phase matching and the pump beam structure and allows for arbitrary manipulations of the SPDC light in the near- and far-fields. This enables us to establish an upper bound $D^2+V^2 \leq 1.47$ for the experimental parameters reported by Menzel et al. We report new experimental results which agree excellently with these theoretical predictions. The new model enables a detailed quantitative analysis of this surprising result and the fair sampling interpretation of biphotons passing a double slit.

Friedmann-Robertson-Walker transformational technique in paraxial wave optics

Giuseppe Della Valle, Stefano Longhi, and Davide Gatti

Doc ID: 237865 Received 14 Apr 2015; Accepted 07 Jul 2015; Posted 13 Jul 2015  View: PDF

Abstract: We introduce in optics a transformational technique inspired by the Friedmann-Robertson-Walker cosmology. We found that the extended covariant scaling transformation used in the Friedmann-Robertson-Walker theory can be applied to light-wave phenomena under scalar paraxial approximation, and an optical analogy with the early Universe is established for a parabolic gradient-index lens. Most importantly, our method provides new ways to propagate light beams as well as novel guidelines for advanced optical structure design, as discussed in some application examples. In a first example we study Talbot imaging in different configurations comprising illumination with spherical beams or with Gaussian beams and propagation in homogeneous or inhomogeneous gradient-index media. In a second example we derive an exact analytical solution for the propagation of self-accelerating parabolic beams in parabolic gradient-index media. Finally, in a third example we show how the new insightful transformational method can be exploited to design lossless non-adiabatic waveguide tapers and adapters.

Improved Combined Tangential Formulation for Electromagnetic Analysis of Penetrable Bodies

Diego Martínez Solís, Jose Taboada, Fernando Obelleiro, and Oscar Rubiños López

Doc ID: 237941 Received 10 Apr 2015; Accepted 07 Jul 2015; Posted 08 Jul 2015  View: PDF

Abstract: The surface integral equation (SIE) method is one of the most popular numerical methods for the electromagnetic analysis of any kind of homogeneous and piecewise homogeneous dielectric, metallic, and composite objects. This work is focused on the tangential formulations (usually more accurate than the normal ones) paying special attention to their capability of modeling the plasmonic behavior of metals at optical and quasi-optical frequencies, in which the localized surface plasmon resonances (LSPRs) bring out significant accuracy problems. The authors present a new tangential SIE, the improved combined tangential formulation (ICTF), in which the fast convergence behavior of the original CTF is combined with a high accuracy, due to the first kind nature of the new ICTF. Some numerical results are shown to illustrate the behavior and accuracy of the proposed formulation even when dealing with highly resonant plasmonic problems.

Laplace-Fourier analysis and instabilities of a gainy slab

Hans Olaf Hågenvik and Johannes Skaar

Doc ID: 240299 Received 05 May 2015; Accepted 07 Jul 2015; Posted 29 Jul 2015  View: PDF

Abstract: The idealization of monochromatic plane waves leads to considerable simplifications in the analysis of electromagnetic systems. However, for active systems this idealization may be dangerous due to the presence of growing waves. Here we consider a gainy slab, and use a realistic incident beam, which is both causal and has finite width. This clarifies some apparent paradoxes arising from earlier analyses of this setup. In general it turns out to be necessary to involve complex frequencies ω and/or complex transversal wavenumbers k_x, even for the case with a weakly amplifying slab which does not lase. Simultaneously real ω and k_x cannot describe amplified waves in a slab which is infinite in the transversal direction. We also show that the only possibility to have an absolute instability for a finite width beam, is if a normally incident plane wave would experience an instability.

Design of the ultra-compact slotted photonic crystal nanobeam cavity for biosensing

Tong Lin, Guangya Zhou, Fook Siong Chau, Jie Deng, and Xingwang Zhang

Doc ID: 241639 Received 26 May 2015; Accepted 04 Jul 2015; Posted 06 Jul 2015  View: PDF

Abstract: Here we elucidate a novel label-free biosensor in aqueous environments based on the slotted photonic crystal nanobeam cavity. A slot is introduced in the center of the photonic crystal nanobeam cavity which reduces the mode volume by an order of 10 without sacrificing the quality factor and sensitivity. This sensor is characteristic of a ultra-small mode volume of 0.076(λ/n)3 and shows its prominent advantage in single nanoparticle detection which the minimum detectable nanoparticle radius is around 1 nm. And its quality factor is as high as 6.08×106 while its sensitivity is around 460 nm/RIU which is promising for refractive index sensing. This sensor can also be integrated into a high density array of sensor networks due to the unparalleled fabrication advantages.

Observation of Triple-Dressing on photonic band gap of optically driven hot atoms

Zhiguo Wang, Dan Zhang, Mengqin Gao, Zakir Ullah, Yiqi Zhang, Haixia Chen, and Yanpeng Zhang

Doc ID: 235101 Received 25 Feb 2015; Accepted 01 Jul 2015; Posted 29 Jul 2015  View: PDF

Abstract: We experimentally investigate transmission of the probe field, the reflected four wave mixing photonic band gap signal and fluorescence signal in a five level atomic system. Also, we compared the single-dressing, double-dressing and triple-dressing on the three types of signals. These three types of signals can be controlled by frequency detunings, powers and relative phases of the dressing beams, and finally it is confirmed that energy of system satisfies the law of conservation. Such scheme could have potential applications in amplification processing of triode and quantum information processing.

Local circular polarizations in nanostructures induced by linear polarization via optical near-fields

Makoto Naruse, Takeharu Tani, Tetsuya Inoue, Hideki Yasuda, Hirokazu Hori, and Masayuki Naya

Doc ID: 238095 Received 14 Apr 2015; Accepted 27 Jun 2015; Posted 09 Jul 2015  View: PDF

Abstract: We previously reported [Naruse, et al. Sci. Rep. 4, 6077, 2014] that the geometrical randomness of disk-shaped silver nanoparticles, which exhibit high reflection at near-infrared wavelengths, serves as the origin of a particle-dependent localization and hierarchical distribution of optical near-fields in the vicinity of the nanostructure. In this study, we show that the induced polarizations are circular, particularly at resonant wavelengths. We formulate optical near-field processes between nanostructures, accounting for their polarizations and geometries, and attribute circular polarization to the layout-dependent phase difference between the electrical susceptibilities associated with longitudinal and transverse-electric components. This study clarifies the fundamental optical properties of random nanostructured matter and offers generic theoretical concepts for implementing nanoscale polarizations of optical near-fields.

Bessel beam CARS microscopy

Juergen Popp, Sandro Heuke, Fisseha Bekele Legesse, Denis Akimov, Uwe Hubner, Jan Dellith, and Michael Schmitt

Doc ID: 241823 Received 26 May 2015; Accepted 26 Jun 2015; Posted 07 Jul 2015  View: PDF

Abstract: We report about laser scanning CARS microscopy using a Bessel beam to enhance the lateral resolution. This study covers a numerical investigation that predicts an improvement in lateral resolution by a factor of up to 1.33 while the axial depth of view increases up to a factor of 2 for an ideal Bessel beam serving both as pump and probe. Further, we present a simple experimental implementation of a Bessel-like beam by adding only two axicons and a telescope to a conventional CARS laser scanning microscopy setup. The predicted resolution improvement is demonstrated for well-scattering, structured polymer samples.

Ultraviolet and visible upconversion luminescence in Y2O3:Er3+, Gd3+ microcrystals by 1.55-µm excitation

Yuhong Zhang, Zhenwen Dai, Zhiguo Zhang, Shuaibing Li, and feng qin

Doc ID: 238463 Received 21 Apr 2015; Accepted 19 Jun 2015; Posted 21 Jul 2015  View: PDF

Abstract: Upconversion (UC) emission spectra of Y2O3:Er3+(3 mol%), Gd3+(0, 10, 20 and 40 mol%) microcrystals were studied under 1.55 µm excitation. Ultraviolet (UV) UC from the 2L15/2, 2H9/2, 4D1/2, 4D3/2, 4D7/2, 4D5/2, 2D5/2, 2P1/2, 2P3/2, 4G11/2, 2G9/2 levels in Er3+ and from the 6D1/2, 6P7/2 levels in Gd3+ were obtained. Enhancements of UV UC emissions of Er3+ were observed in Y2O3:Er3+, Gd3+. The collaboration of energy transfer and energy back transfer between Er3+ and Gd3+ ions was thought as the possible mechanism for the enhancements. This phenomenon would provide a new idea for investigating enhancement of UC radiations in rare-earth ions doped materials. The dependences of UC emissions on pumping power and dopant concentration were also studied for understanding possible UC mechanisms.

Impact of nonlinear loss on Stimulated Brillouin Scattering

Christian Wolff, Philipp Gutsche, Michael Steel, Benjamin Eggleton, and Chris Poulton

Doc ID: 241140 Received 21 May 2015; Accepted 07 Jun 2015; Posted 29 Jul 2015  View: PDF

Abstract: We study the impact of two-photon absorption (2PA) and fifth-order nonlinear loss such as 2PA-induced free-carrier absorption in semiconductors on the performance of Stimulated Brillouin Scattering devices. We formulate the equations of motion including effective loss coefficients, whose explicit expressions are provided for numerical evaluation in any waveguide geometry. We find that 2PA results in a monotonic, algebraic relationship between amplification, waveguide length and pump power, whereas fifth-order losses lead to a non-monotonic relationship. We define a figure of merit for materials and waveguide designs in the presence of fifth-order losses. From this, we determine the optimal waveguide length for the case of 2PA alone and upper bounds for the total Stokes amplification for the case of 2PA as well as fifth-order losses. The analysis is performed analytically using a small-signal approximation and is compared to numerical solutions of the full nonlinear modal equations.

Creation of electron-positron pairs in oscillating electric fields

Q. Charles Su, Wanyang Wu, Feng He, and Rainer Grobe

Doc ID: 237683 Received 07 Apr 2015; Accepted 06 Jun 2015; Posted 29 Jul 2015  View: PDF

Abstract: By solving the time-dependent Dirac equation numerically, we calculate the rate of electron-positron pairs that are created from the vacuum by an oscillating field. The data permit us to examine the validity of approximate but analytical expressions derived from the WKB-based description. If two characteristic parameters (E/c^3 and ω/c^2) are less than 1 or the parameter γ (=ωc/Ε) is small (tunneling region) this description predicts the true rate very well, where E, ω and c are the amplitude, frequency and velocity of the external field, respectively. In the multi-photon region (where E/c^3 is small and γ is large) perturbative methods become valid. We also analyze the spatial distributions of the created particles in the three regions of interaction (γ<<1, γ>>1 and γ≈1), which also reflect the tunneling and multiphoton based mechanisms for pair creation.

Saturable and inverse saturable absorption in multi-walled-carbon-nano-tubes-doped fast Sol-gel hybrid glasses

Raphi Dror, ZeeV Burshtein, Raz Gvishi, and Mariana Pokrass

Doc ID: 232480 Received 14 Jan 2015; Accepted 10 Apr 2015; Posted 10 Apr 2015  View: PDF

Abstract: Dynamics of saturable absorption and inverse saturable absorption of Multi walled carbon nano tubes (MWCNT)-doped fast Sol-gel hybrid glasses was studied by optical transmission of 532-nm laser beam pulses at two extreme conditions: 6-ns long temporally isolated single pulses up to 0.02 J/cm2, and a pulse train at different intensities in the range 0.72-10.7 kW/cm2 (quasi-CW illumination). The temporally isolated single pulses were analysed by the slow saturable absorber limit; The quasi-CW illumination at steady state was analysed by the fast inverse saturable absorber limit. A fresh, corrected solution to the rate equation was developed for the latter. The time dependence of reaching steady state absorption in the quasi-CW case was analysed by a numerical simulation of a state model. The single-pulse case analysis yields a ground-state density N=5.1×1015cm-3, a ground-state absorption cross-section σgs=5.5×10-15cm2 , and a first excited-state absorption cross-section σes1=3.6×10-15cm2. The quasi-CW case analysis is consistent with the occurrence of the MWCNT transition upon illumination into a highly absorbing/scattering state ("plasma" state). The analysis yields a ground-state concentration estimation N=3.2×1015cm-3, and a ground-state absorption cross-section σgs=6.3×10-15cm2. The "plasma" lower-state absorption/ scattering cross-section is σes2=2.3×10-14cm2. The light-intensity dependent transition rate between the first MWCNT excited-state and its "plasma" lower state followed the relation τ-1inc[s-1]=12.25exp(4×10-7I[W/cm2]); the transition rate between the lowest "plasma" state and the MWCNT ground-state, was also light-intensity dependent, following the relation τ-1[s-1]=3.5exp(2.54×10-7I[W/cm2]).

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