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Accepted papers to appear in an upcoming issue

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Third Harmonic Generation in the Semi-Infinite Phase-Matching Limit

Ekaterina Sergan and George Gibson

Doc ID: 297316 Received 08 Jun 2017; Accepted 19 Jul 2017; Posted 19 Jul 2017  View: PDF

Abstract: We expand upon previously published (Gibson et al., Opt. Lett., Vol. 37, pg. 3279, 2012) experimental results on third harmonic generation with a focused gaussian beam in the semi-infinite limit, and compare these results with an analytical treatment of the phase-matching integral and simulations to investigate limits to the conversion efficiency. In the tight-focusing limit for a gaussian beam the phase-matching integral vanishes in the lowest order non-linear term, leading to poor conversion efficiency and mode quality. However, we utilize two techniques to break the phase-matching symmetry to dramatically improve the conversion efficiency and mode. Both methods inhibit third harmonic generation after the focus, preventing the cancellation of the harmonic radiation. We show that new limits on the conversion efficiency are set by induced non-linear phase-mismatch and ionization.

Coupling characteristics of twin-core few-mode all-solid photonic bandgap fiber and their application in mode separation

Hongye Li, Zhi Wang, Yange Liu, Hu Liang, and Simeng Han

Doc ID: 292852 Received 14 Apr 2017; Accepted 18 Jul 2017; Posted 18 Jul 2017  View: PDF

Abstract: We theoretically investigate the coupling characteristics of a few-mode twin-core all-solid photonic bandgap fiber (AS-PBGF). We show that extrema in the coupling length, reversal of the ordering of supermodes, and complete decoupling can occur in high-order modes. Using significant differences in the coupling characteristics between different modes, we can separate degenerate mode pairs such as LP11a and LP11b, and LP21a and LP21b, and also modes with the same azimuthal quantum number, e.g., LP01 and LP02. By setting a suitable photonic-crystal period, we can realize an optical fiber coupler with mode separation properties in the C-band.

Fundamental limits of the single nanoobject detection performance of optical fiber tapers

Mario Chemnitz, Matthias Zeisberger, and Markus Schmidt

Doc ID: 295059 Received 04 May 2017; Accepted 17 Jul 2017; Posted 18 Jul 2017  View: PDF

Abstract: Detecting the scattering process of individual nanoobjects using optical fibers is a promising fast, reliable and label-free detection scheme with strong potential in fields such as bioanalytics or disease diagnostics. Here we investigate the influence of single nanoobject scattering on the transmission of the fundamental mode of fiber tapers. We use an analytic equation for the key parameter related to particle induced scattering – the effective modal scattering area – and reveal optimal structural and spectral dependencies and limitations. As examples we determine the minimal detectable particle diameter of two sorts of nanoparticles, showing that silica tapers with diameter of the order of 500 nm allow straightforward detection of plasmonic nanoparticles and polymer beads with diameters of 40 nm and 150 nm, respectively.

Particle-wave dichotomy in quantum Monte Carlo: unlocking the quantum correlations

Ivan Christov

Doc ID: 294775 Received 28 Apr 2017; Accepted 17 Jul 2017; Posted 19 Jul 2017  View: PDF

Abstract: Here, a dichotomy of particles and waves is employed in a quantum Monte Carlo calculation of interacting electrons. Through the creation and propagation of concurrent stochastic ensembles of walkers in physical space and in Hilbert space one can correctly predict the ground state and the real-time evolution of a single electron interacting with larger quantum system. It is shown that such walker ensembles can be constructed straightforwardly through a stochastic sampling (windowing) applied to the mean-field approximation. Our calculations reveal that the ground state and the real-time evolution of the probability distributions and the decoherence due to the Coulomb interaction in presence of strong ultrashort laser pulse can be accounted for correctly by calculating the density matrix of the electron, without referencing to the quantum many-body state of the whole system.

Vectorial analysis of a surface plasmon mode diffracted by a thick dielectric lens

Fahimeh Armin and Mir Mirsalehi

Doc ID: 297064 Received 02 Jun 2017; Accepted 17 Jul 2017; Posted 17 Jul 2017  View: PDF

Abstract: Using the vectorial form of Huygens-Fresnel principle, we have studied the diffraction of a surface plasmon mode propagating on a metal-dielectric interface as it passes through a dielectric thick lens. This method, unlike the scalar diffraction theory, provides the interference and focusing patterns of the surface mode in the near field. Also, we have calculated the phase delay of the surface plasmon mode behind the lens without using the thin lens assumption. Based on the developed equations, we have analyzed the effects of lens thickness and aperture size on in-plane focusing of the surface plasmon.

Coupler-free surface polariton excitation and propagation with cold four level atomic medium

Rasoul Sadighi-Bonabi, saeed asgarnezhad, and Chao Hang

Doc ID: 295764 Received 11 May 2017; Accepted 16 Jul 2017; Posted 18 Jul 2017  View: PDF

Abstract: The possibility of the direct excitation of surface polaritons (SPs) by the free space laser fields at the interface of negative-index metamaterial (NIMM) layer and a bottom layer of colded double‎- ‎Lambda type atomic medium ‎is investigated‎. ‎T‎he giant field enhancement (up to ‎$‎‎|E/E_{0}|^{2}‎\approx ‎4.1‎$‎) together with suppressed Ohmic loss of the NIMM layer in a wide transparency window of a double electromagnetically induced transparency (EIT) ‎result in the SPs generation. The excitation efficiency of this SPs can be effectively enhanced ‎b‎y applying the unidirectional atomic motion, modulation of the coupling laser characteristics and using a probe laser with proper incident angle‎. ‎Based ‎on ‎the ‎special ‎modulation ‎of ‎the ‎driven ‎fields ‎parameters, ‎the ‎group ‎velocity switching ‎of ‎the ‎SPs ‎is ‎observed.‎ In the nonlinear propagation regime of the SPs, a‎ giant Kerr nonlinearity is produced by applying a negative unidirectional atomic velocity‎. The controllable optical Kerr effect in this coupler free scheme results in the lossless propagation of the nonlinear SPs. It is revealed that the EIT based system can be used for stable propagation of one-dimensional (1+1)D Bright and Dark superluminal polaritonic solitons‎. ‎It is also shown that the proposed scheme could be easily realized in an experiment and hence can be used for various ultrafast optical devices‎.

Propagating bound states in the continuum at the surface of a photonic crystal

Zhen Hu and Ya Yan Lu

Doc ID: 295827 Received 11 May 2017; Accepted 16 Jul 2017; Posted 18 Jul 2017  View: PDF

Abstract: Bound states in the continuum (BICs) are trapped or guided modes with their frequencies within the radiation continuum. On periodic structures, BICs have interesting properties and potentially important applications. It is known thatBICs can exist on the surface of a photonic crystal (PhC), and they are distinctively different from the well-known surface Bloch modes below the lightline. However, for a given structure with specific geometric and materialparameters, it is difficult to predict whether BICs exist or not. In this paper, using an efficient computational method, we calculate BICs at the surface of a two dimensional PhC consisting of dielectric rods, and determine theexistence domain in the plane of the refractive index and the radius of the surface rods. The boundary of the existence domain reveals that the BICs cease to exist when the bulk PhC can no longer confine light. In addition, the frequency and wavenumber of the BIC can approach the lightline, leading to bound states on the lightline and special highly confined surface Bloch modes below the lightline.

Dynamical tuning of terahertz meta-lens assisted by graphene

Zongduo Huang, Bin Hu, Weiguang Liu, Juan Liu, and Yongtian Wang

Doc ID: 291778 Received 31 Mar 2017; Accepted 16 Jul 2017; Posted 19 Jul 2017  View: PDF

Abstract: Metasurfaces, which are composed of arrays of scatters, can introduce abrupt change of optical properties. However, they lack flexibility to achieve active control. Here we propose a meta-lens with tunable focal length assisted by graphene at terahertz frequencies. The meta-lens consists of a monolayer graphene and a gold film etched with rectangle apertures with different lengths and rotation angles. By uniformly adjusting the gate voltage applying on the graphene layer, the chemical potential of graphene changes from 0.1eV to 0.3eV, and the focal length is changed by 1.25λ continually. This meta-lens can focus the beam to a spot size in order of one wavelength and works well even at wide angles of incidence.

Thermal tuning of spectral emission from optically trapped liquid-crystal droplet resonators

Alexandr Jonas, Zdenek Pilat, Jan Jezek, Silvie Bernatova, Tomáš Fořt, Pavel Zemanek, Mehdi Aas, and Alper Kiraz

Doc ID: 295240 Received 11 May 2017; Accepted 14 Jul 2017; Posted 17 Jul 2017  View: PDF

Abstract: Surfactant-stabilized emulsion droplets of liquid crystals (LCs) suspended in water and labeled with a fluorescent dye form active, anisotropic optofluidic microresonators hosting whispering gallery modes (WGMs) which are excited due to the contrast of refractive index between the LC droplets and the surrounding aqueous medium. In addition, owing to the refractive index contrast, such LC emulsion droplets can be stably trapped in three-dimensions using optical tweezers, enabling long-term investigation of their spectral characteristics. We explore various combinations of fluorescently dyed LC droplets and host liquid - surfactant systems and show that the WGM emission spectra of optical resonators based on optically trapped LC emulsion droplets can be largely and (almost) reversibly tuned by controlled changes of the ambient temperature. Depending on the actual range of temperature modulation and LC phase of the studied droplet, thermally-induced effects can either lead to phase transitions in the LC droplets or cause modifications of their refractive index profile without changing their LC phase. Our results indicate feasibility of this approach for creating miniature thermally-tunable sources of coherent light that can be manipulated and stabilized by optical forces.

Revisiting the boundary conditions for second-harmonic generation at metal-dielectric interfaces

Kothakapu reddy, Parry Chen, Antonio Fernández-Domínguez, and Yonatan Sivan

Doc ID: 296549 Received 23 May 2017; Accepted 13 Jul 2017; Posted 17 Jul 2017  View: PDF

Abstract: We study second-harmonic generation (SHG) arising from surface nonlinearity at a metal-dielectric interface using a spectral decomposition method. Since our method avoids the need to consider the generalized boundary condition across the metal-dielectric interface in the presence of a perpendicular surfacesource, we retrieve the known discontinuity of the tangential component of the electric field for a general geometry, based on a purely mathematical argument. Further, we reaffirm the standard convention of the implementation of this condition, namely, that the surface dipole source radiates as if placedoutside the metal surface for arbitrary geometries. We also study and explain the spectral dependence of the discontinuity of the tangential component of the electric field at second harmonic. Finally, we note that the default settings of the commercial numerical package COMSOL Multiphysics fail to accountfor the discontinuous tangential component. We provide a simple recipe that corrects the boundary condition within theseexisting settings.

AlGaAs waveguide microresonators for efficient generation of quadratic frequency combs

maria parisi, Natalia Morais, Iolanda Ricciardi, Simona Mosca, Tobias Hansson, Stefan Wabnitz, Giuseppe Leo, and Maurizio De Rosa

Doc ID: 291722 Received 20 Apr 2017; Accepted 13 Jul 2017; Posted 17 Jul 2017  View: PDF

Abstract: We propose a flexible design for directional quasi-phase matching in AlGaAs waveguide resonators, for implementing recently demonstrated optical frequency combs in cavity enhanced second-harmonic generation systems. We numerically study the onset of internally pumped optical parametric oscillations that trigger the subsequent comb formation. We also perform a coherence analysis of a particular class of numerically simulated optical frequency comb, corresponding to stable temporal cavity field patterns,revealing the high degree of coherence associated to such stable solutions, a key feature for many refined applications of optical frequency combs. Our analysis shows that efficient generation of coherent frequency combs is possible with threshold powers in the microwatt range and path lengths up to several mm, thus enabling the practical realization of new on-chip frequency comb synthesizers, fully integrable in more complex photonic circuits.

Generation of narrowband terahertz radiation by an ultrashort laser pulse in a bulk LiNbO₃ crystal

Eugene Mashkovich, Sergey Sychugin, and Michael Bakunov

Doc ID: 297256 Received 05 Jun 2017; Accepted 12 Jul 2017; Posted 12 Jul 2017  View: PDF

Abstract: We show that a femtosecond laser pulse propagating in a LiNbO₃ crystal as a superposition of ordinary and extraordinary waves can efficiently generate narrowband terahertz radiation in a quasi-phase-matched regime. The generation mechanism is similar to that in periodically poled crystals, although it occurs in a bulk crystal. In particular, a 600 fs pulse from Yb-doped laser amplifier can generate radiation at ~0.5 THz with bandwidth of 3.6 GHz and efficiency ~10¯⁵. Frequency tuning can be achieved by tilting the laser beam in the crystal. The predicted effect can be used for creating tunable spectrally bright terahertz sources.

Resonant excitation of transverse patterns in broad-area lasers by periodic temporal pump modulation

Anton Krents, Nonna Molevich, and Dmitry Anchikov

Doc ID: 296591 Received 23 May 2017; Accepted 11 Jul 2017; Posted 12 Jul 2017  View: PDF

Abstract: We demonstrate numerically that the temporal periodic modulation of the pump can excite regular transverse spatiotemporal intensity patterns in broad-area lasers. The Floquet stability analysis is used to characterize dynamics of modulated systems. Pattern formation occurs when the modulation frequency is approximately equal to the relaxation frequency or twice the value of this frequency. Instability of homogeneous oscillations leads to the formation of regular optical patterns such as stripes and hexagons. The characteristic sizes of the observed patterns are in good agreement with predictions of the Floquet analysis.

Dual-wavelength pulsed dynamics in Nd:GdVO4 laser with Cr4+:YAG saturable absorber: roles of pump rate and spot size

Kun-Guei Hong, Yu-Chia Lu, and Ming-Dar Wei

Doc ID: 296943 Received 30 May 2017; Accepted 11 Jul 2017; Posted 12 Jul 2017  View: PDF

Abstract: The dual-wavelength pulse dynamics at 1.06 μm and 1.34 μm were reported based on the mechanism of mixed spiking and passive Q-switching. When the dynamical behaviors were classified based on the 1.34-µm spiking pulse evolutions in one duration of 1.06-μm Q-switched pulse train, the multiple-pulse, leading one-on-one, multiple-duration pulse, and simultaneously one-on-one dynamics were sequently generated by increasing the pump power. When the spot size of 1.34μm emission was varied, the dynamical pulse behaviors occurred in sequence but the sizes of regions were changed. The experiments agreed with the numerical results.

Control of optical bistability in nonlinear regime of the two-sided cavity quantum electrodynamics

Liyong Wang, Mingsheng Zhan, Yifu Zhu, and Zheng Tan

Doc ID: 297190 Received 02 Jun 2017; Accepted 11 Jul 2017; Posted 12 Jul 2017  View: PDF

Abstract: We investigate the optical bistability behavior of a two-sided cavity quantum electrodynamics (CQED) system. The nonlinear input-output relation of the atom-cavity system coupled by two input light fieldsfrom two ends of the cavity can be controlled by a control light from the free space. Different from the common optical bistability phenomenon in the atom-cavity system, two separate bistability regions are observed. The bistsbility threshold and hysteresis loop can be well controlled by the control light. Due to field interference effect, the output light intensity becomes different as varying the relative phase of two incident fields. The output field intensity at the threshold of the first bistability region approaches zero with increasing the detuning or lowering the intensity of the control light, thus the perfect photon absorption condition is extended and a broadband near-perfect photon absorber is realized. Furthermore,we show an asymmetrical transmission feature due to field interference.

Weak scattering of electromagnetic random source of circular frames upon a deterministic medium

Xun Wang, Liu Zr, and Kelin Huang

Doc ID: 294989 Received 04 May 2017; Accepted 09 Jul 2017; Posted 11 Jul 2017  View: PDF

Abstract: By application of the theory of first-order Born approximation, analytical expressions of the cross spectral density matrix of a newly-proposed electromagnetic random source of circular frames scattered upon a deterministic medium in the far zone are derived. In terms of the analytical formulas obtained, the scattered spectral properties of both single circular frame and superposed circular frames are numerically investigated. Results show that the scattered spectral properties of the targeted optical source in the far zone would be significantly influenced by the effective radius σR of scattering medium, the observation azimuth φ, in addition to the parameters of the incident source, such as the index M, the polarization states, and the correlation widths. It is pointed out here that this is for the first time, to the best of our knowledge, that the influence of azimuth φ on the properties of scattered spectral has been analyzed, and some meaningful and interesting results have been obtained in this paper.

Bias-free and Antenna-Coupled CW Terahertz Array Emitter with Anomalous Schottky Barriers

mohammad javad mohammad-zamani, Morteza Ftahipour, Mohammad Neshat, fakhroddin nazari, and mahdi ghaemi

Doc ID: 292585 Received 10 Apr 2017; Accepted 08 Jul 2017; Posted 11 Jul 2017  View: PDF

Abstract: We present a new bias-free antenna-coupled CW THz photomixer with interdigitated electrodes. Each finger pair is made of metal/semiconductor/metal (MSM) electrodes with dis-similar Schottky barriers. The two dis-similar metals in each MSM have a barrier height of difference (ΔΦΒ) and a finite lateral spacing (s). In the proposed THz emitter, not only the optical absorption and hence photogeneration is enhanced by the surface resonant modes, but also the high built-in field amplifies the THz current. Furthermore, by having a large radiation resistance of the antenna integrated to array electrodes, it is possible to achieve the maximum power of 13 mW from a dipole antenna coupled to the proposed electrode array with 10×10 µm² active area and pitch of Λ=800 nm. This is more than 7×10⁴ times larger THz power than the highest THz power radiated from the array emitters of the same area that contains the bias-free antenna-less array of far-field emitters with double pitch size. Such THz sources can pave the way to various biomedical applications such as endoscopic imaging without a need for hazardous external circuitry for biasing, reducing the patient health risk.

Extremely Low Loss Porous-Core Photonic Crystal Fiber with Ultra Flat Dispersion in THz Regime

Md. Shariful Islam, Mohammad Faisal, and S. M. Razzak

Doc ID: 293013 Received 18 Apr 2017; Accepted 08 Jul 2017; Posted 11 Jul 2017  View: PDF

Abstract: An extremely low loss porous-core fiber with nearly zero dispersion flattened over a wide band of frequency in the terahertz (THz) regime is presented in this paper. A novel structure of hexagonal airholes both in the core and the cladding is introduced to provide an overwhelming reduction in bulk material absorption loss and confinement loss. Numerical analysis shows that, an effective material loss (EML) as low as 0.0206 1/cm and a very flat dispersion of ±0.16 ps/THz/cm can be obtained from the proposed fiber in the frequency range of 0.98–1.64 THz. Within the whole frequency band the fiber operates in single mode region and shows a variation in total loss of only ±0.01 1/cm.

Spectral compression of chirp-free optical pulses in fibers with exponentially increasing dispersion

Qian Li, Taiwei Zhang, and Mingfeng Li

Doc ID: 294744 Received 27 Apr 2017; Accepted 08 Jul 2017; Posted 11 Jul 2017  View: PDF

Abstract: We numerically demonstrate high degree spectral compression of chirp-free optical pulses in fibers with exponentially increasing dispersion. Both the spectral compression factor and brightness enhanced factor can exceed 10 and the suggested fiber is a 0.8 km dispersion exponentially increasing fiber. We find that the input pulses finally evolve into the self-similar soliton as they propagate in the fiber. The pulse chirp is generated although the input pulse is chirp-free, and the detailed evolution of chirp parameter is discussed. The exponentially increasing dispersion has enabled the efficient spectral compression and some key parameters such as the initial pulse width, initial dispersion value, and dispersion exponentially increasing rate also affect spectral compression process for both hyperbolic secant and Gaussian input pulses. The wavelength tunability and the effect of third-order dispersion are also discussed. We also give the comparison between the numerical results from the generalized nonlinear Schrödinger equation and pulse parameter equations.

On-chip high sensitivity temperature sensor based on the gain-loss coupled micro-resonators

Wenxiu Li, Hao Zhang, Jiaming Liu, Jian Lin, xia xue, Xiaofu Zhang, Xianfan Xu, Anping Huang, and Zhisong Xiao

Doc ID: 290866 Received 16 Mar 2017; Accepted 06 Jul 2017; Posted 11 Jul 2017  View: PDF

Abstract: Asymmetrical Fano resonance spectral transmission with large slope can be used for ultra-sensitivity temperature sensing. In this paper, a very sharp asymmetrical Fano resonance is achieved by coupled gain-loss cavities. The sensitivity for detecting temperature can be enhanced up to 10⁷/K, eight orders of magnitude compared with two indirectly coupled loss cavities, in addition to the resolution of 10¯¹³ K. Thermal sensing mechanism based on gain-loss coupled optical cavities is investigated theoretically. The integrated structure shows potential to be an ultra-sensitivity temperature micro-sensor.

Nonlinear propagation of laser filamentation in a predesigned zigzagging fashion induced by initial modulation

Yuze Hu, Nie Jinsong, and Ke SUN

Doc ID: 292982 Received 18 Apr 2017; Accepted 05 Jul 2017; Posted 05 Jul 2017  View: PDF

Abstract: We theoretically demonstrate the formation and control of novel snake filaments in air. In particular, the snake filaments can be elongated with large input beam size in the phase modulation direction, and they share almost the same-zigzagging fashion along the propagation direction. The specific fields of satellite lobes make great contributions on alternating the accelerating direction of the main lobe, namely the snake filament. As a result, the filament accelerates in a certain transverse area within two-phase boundaries that are called phase wells. Furthermore, for the spatiotemporal dynamics, a strip structure is sustained during the filamentation process of snake filaments. Results presented here are expected to have significant impacts on the wide range of fields in nonlinear optics.

Photocurrent Transients of Thin Film Solar Cells

Mehrnoush Mokhtarimehr and Svetlana Tatarkova

Doc ID: 291632 Received 29 Mar 2017; Accepted 04 Jul 2017; Posted 06 Jul 2017  View: PDF

Abstract: The absorber layers of thin film kesterite solar cells were analyzed using Intensity Modulated Photocurrent Spectroscopy (IMPS). The technique allows fast and efficient quantitative and qualitative assessment of the solar cell photo-activity and, in addition to estimating the rates of electrons/holes’ generation and recombination in the absorbing layer. The significant variations in photo performance are linked to the rate of photo-induced holes and electrons extraction/recombination but the significant role of surface optical effects can be revealed via manipulations with the phase of incident light.

Wigner function and entanglement dynamics of Two-atom two-mode nonlinear Jaynes-Cummings model

Hassan Safari, Mahnaz Ghorbani, and Mohammad Javad Faghihi

Doc ID: 290336 Received 08 Mar 2017; Accepted 15 Jun 2017; Posted 16 Jun 2017  View: PDF

Abstract: In this paper, a model in which two moving atoms interact with a two-mode field with nondegenerate two-photon transitions is studied in the presence of intensity-dependent coupling. To discuss the entanglement dynamics between subsystems as well as nonclassicality of the system, the time-dependent form of the state vector of the system is exactly obtained. The dynamics of entanglement between the atoms and the field is evaluated by the von Neumann entropy. To examine the degree of entanglement between the atoms, concurrence and negativity are obtained. Then, in order to understand the nonclassicality feature of the state vector of the system, the two-mode Wigner-Weyl quasi-probability distribution function is precisely derived. It is deduced from the numerical results that the domain and the maximum amount of the degree of entanglement and nonclassicality of the system can be appropriately tuned by suitably adopting the nonlinearity function and the field-mode structure parameters.

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