Optical Propagation Direction Research Articles

Nanophotonic resistive switch based on tapered copper-silicon structure with low power and high extinction ratio

A non-volatile nanophotonic resistive switch in tapered copper-on-silicon structure is proposed with a potential of electrical writing and optical readout. Incorporating tapered hybrid plasmonic structure in the device enables variable areas of the density of conduction filaments in the direction of optical propagation causing enhanced interaction of light with the lossy metal filaments. The tapered Cu-SiO2-Si is shown to have large optical extinction ratio where high mobility of copper ions in thin SiO2 leads to the low power operation. Optical readout of the resistive switching is demonstrated in the 10 nm thin ion conducting layer (SiO2) at a 1550 nm wavelength with well-defined hysteresis curve. Switching between off and on states is attained by voltage induced annihilation/formation of the nanoscale metal filament inside a thin layer of SiO2 sandwiched between copper and silicon. An optical extinction ratio of 18 dB is reported for a 30 μm long device. In addition, the proposed nanophotonic switch exhibits an intrinsic stochastic property of set voltage reduction in each set-reset cycle. High extinction ratio, reduced set voltage, low power consumption, and non-volatility make the device excellent choice for applications in realizing ultra-compact on-chip devices for optical switching, modulation, and neuromorphic computing.

The importance of molecular axis alignment and symmetry-breaking in photoelectron elliptical dichroism.

Photoelectron angular distributions (PADs) produced from the photoionization of chiral molecules using elliptically polarized light exhibit a forward/backward asymmetry with respect to the optical propagation direction. By recording these distributions using the velocity-map imaging (VMI) technique, the resulting photoelectron elliptical dichroism (PEELD) has previously been demonstrated as a promising spectroscopic tool for studying chiral molecules in the gas phase. The use of elliptically polarized laser pulses, however, produces PADs (and consequently, PEELD distributions) that do not exhibit cylindrical symmetry about the propagation axis. This leads to significant limitations and challenges when employing conventional VMI acquisition and data processing strategies. Using novel photoelectron image analysis methods based around Hankel transform reconstruction tomography and machine learning, however, we have quantified-for the first time-significant symmetry-breaking contributions to PEELD signals that are of a comparable magnitude to the symmetric terms in the multiphoton ionization of (1R,4R)-(+)- and (1S,4S)-(-)-camphor. This contradicts any assumptions that symmetry-breaking can be ignored when reconstructing VMI data. Furthermore, these same symmetry-breaking terms are expected to appear in any experiment where circular and linear laser fields are used together. This ionization scheme is particularly relevant for investigating dynamics in chiral molecules, but it is not limited to them. Developing a full understanding of these terms and the role they play in the photoionization of chiral molecules is of clear importance if the potential of PEELD and related effects for future practical applications is to be fully realized.

ANSYS Workbench simulation of glass welding by femtosecond laser pulses

In this work, temperature field, residual stress, and strain field of two-fused-silica substrate welding by femtosecond laser pulses were simulated by ANSYS Workbench (a software of finite element analysis) using a transient thermal model together with a static structural model. A Gaussian heat source was periodically deposited at the interface of the samples and was moved at a constant velocity perpendicular to the optical propagation direction. The maximum residual stress was just 16.474 KPa at a time of 20 s through a simulation analysis. This was not sufficient to cause cracks in the fused silica. In addition, the influence of a magnetic field on the welding process was studied. The value of the residual stress and strain apparently decreased with an external magnetic field. In a real experiment, the maximum bonding strength reached 76.6 MPa under the same conditions as in simulation.

Real-time identification of the singleness of a trapped bead in optical tweezers.

Beads trapped in optical tweezers are aligned along the optical propagation direction, which makes it difficult to determine the number of beads with bright-field microscopy. This problem also dramatically influences the measurement of the optical trapping based single-molecule force spectroscopy. Here, we propose a video processing approach to count the number of trapped micro-objects in real time. The approach uses a normalized cross-correlation algorithm and image enhancement techniques to amplify a slight change of the image induced by the entry of an exotic object. As tested, this method introduces a ∼10% change per bead to the image similarity, and up to four beads, one-by-one falling into the trap, are identified. Moreover, the feasibility of the above analysis in a moving trap is investigated. A movement of the trap leads to a fluctuation of less than 2% for the similarity signal and can be ignored in most cases. The experimental results prove that image similarity measurement is a sensitive way to monitor the interruption, which is very useful, especially during experiments. In addition, the approach is easy to apply to an existing optical tweezers system.

(Invited) Heterogeneous Photonic Integration by Direct Wafer Bonding

The silicon photonics platform has emerged as a promising integration technique for use in many applications, particularly datacenter communications, which is expected to produce demand for large numbers of low cost photonic integrated circuits (PICs). Silicon-based PICs are almost universally based on silicon-on-insulator (SOI) wafers, which are enabled by wafer bonding technology. Silicon (Si) is useful as a waveguide material due to its high index contrast, low intrinsic absorption, and mature manufacturing infrastructure. However, it does not strongly interact with light in wavelength ranges of interest for data transmission. Heterogeneous integration of InP-based materials with Si using wafer bonding technology is able to overcome this limitation and has allowed the realization of a wide range of photonic devices on Si1. Beyond integration of InP with Si, wafer bonding enables complex vertical integration with of a wide variety of materials, such as silicon nitride (Si3N4) ultra-low loss waveguides, and magnetic and nonlinear materials. A crystalline Si layer transferred to an ultra-low loss Si3N4 waveguide wafer by bonding allows for integration of highly confined Si photonic waveguide with low-loss passive components in Si3N4. Any device that can be realized on an SOI wafer can be integrated with the Si3N4 waveguide. Multiple die bonding of InP chips on the Si layer can add lasers, amplifiers, modulators, and photodetectors to the circuit. Lateral tapers of the waveguides allows coupling between the layers with loss below 0.5 dB. This technology was used to combine a Si3N4 arrayed waveguide grating (AWG) wavelength multiplexer with high-speed 50 Gb/s heterogeneous Si/InP photodetectors to form a 400 Gb/s wavelength division multiplexing (WDM) receiver2. A cross section of the photodetector is shown in Figure 1. Bonding technology can allow the introduction of additional circuit functions that are unavailable using common semiconductor materials. One such device is an optical isolator, which allows light to experience loss that is dependent on the direction of optical propagation. This optical nonreciprocity can be achieved by bonding a magneto-optic material such as cerium substituted yttrium iron garnet (Ce:YIG) on Si waveguides, and applying a magnetic field through the material. Using this phenomenon, it is possible to fabricate optical isolators as well as circulators for use in optical sensors, amplifiers, and WDM systems. Our design consists of a Si microring with a bonded Ce:YIG die on top, shown schematically in Figure 2. A metallic microstrip is deposited on the backside of the bonded die following substrate removal. By applying a current through this microstrip, we generate a magnetic field, which causes a resonance split between the forward and backwards propagating light. Thus, optical isolators with up to 32 dB of isolation have been demonstrated3. Lithium niobate (LN) is a material of paramount importance for nonlinear optics, thanks to its large nonlinear coefficient and broad transparent window (0.4-5.0 µm)4. It has attracted attention for use in PICs, because the compact scale of integrated waveguides results in high photon densities that effectively enhance nonlinear interaction. Recently, the LN-on-insulator (LNOI) technology has been successfully demonstrated by ion injection and heterogeneous bonding. This allows the implementation of LN thin films with submicron thickness on various material platforms5. We present here a quasi-phase matched, ultra-low-loss S3N4-LN platform, which reduces the loss of integrated LN waveguides down to a value comparable with best results of bulk LN waveguides 6 (Figure 3). Bonding of LN to silicon is also possible, allowing integration of LN-based devices with SOI photonics. Wafer bonding technology allows the realization of complex and highly integrated photonic devices. The ability to combine multiple materials such as multiple-die bonding for integration of lasers, modulators, and detectors enables PICs with a wide variety of functions. Vertical integration through bonding of high-quality thin films, such as silicon nitride, lithium niobate, crystalline silicon, and thermal silicon dioxide can be used to construct integrated waveguides with useful properties and high performance. 1. M. J. R. Heck et al., IEEE J. Sel. Top. Quantum Electron. 19, 6100117 (2013). 2. M. Piels, J. F. Bauters, M. L. Davenport, M. J. R. Heck, and J. E. Bowers, J. Light. Technol. 32, 817–823 (2014). 3. D. Huang et al., Optical Fiber Communications (2016). 4. G. D. Boyd, R. C. Miller, K. Nassau, W. L. Bond, and A. Savage, Appl. Phys. Lett. 5, 234–236 (1964). 5. L. Chen, Q. Xu, M. G. Wood, and R. M. Reano, Optica 1, 112 (2014). 6. K. R. Parameswaran et al., Opt. Lett. 27, 179–181 (2002). Figure 1

Miniaturized bending-free solitons by restoring symmetry in periodically biased photorefractives

We consider optical propagation through a centrosymmetric photorefractive crystal with the externally applied bias voltage modulated along the optical propagation direction. We analytically prove that, if the modulation scale is smaller than the optical diffraction length, the resulting effective nonlinearity has an even parity in the transverse plane for an even-symmetric intensity profile and supports bending-free solitons down to few-micrometer beam widths. Numerical integration of the full photorefractive model for light-matter interaction allows us to confirm the feasibility of these miniaturized solitons and, for longer modulation periods, to investigate the excitation of self-trapped wiggling optical beams.

Optofluidic trapping and transport on solid core waveguides within a microfluidic device

In this work we demonstrate an integrated microfluidic/photonic architecture for performing dynamic optofluidic trapping and transport of particles in the evanescent field of solid core waveguides. Our architecture consists of SU-8 polymer waveguides combined with soft lithography defined poly(dimethylsiloxane) (PDMS) microfluidic channels. The forces exerted by the evanescent field result in both the attraction of particles to the waveguide surface and propulsion in the direction of optical propagation both perpendicular and opposite to the direction of pressure-driven flow. Velocities as high as 28 mum/s were achieved for 3 mum diameter polystyrene spheres with an estimated 53.5 mW of guided optical power at the trapping location. The particle-size dependence of the optical forces in such devices is also characterized.

Novel low‐loss wide‐angle Y‐junctions of optical waveguides with graded‐index profiles

AbstractA low‐loss wide‐angle Y‐junction with graded‐index optical waveguides which can change the optical propagation direction, is proposed. The optical power is divided equally into two branching waveguides with a wide angle. Device performance is examined numerically by the finite‐difference beam propagation method (FDBPM). The simulated results show that the output optical power can be more than 96%, even if a branching angle is as large as 53.1°. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 41: 156–159, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20078

Optical measurement of wide bandwidth ultrasound fields

A technique for measuring instantaneous three-dimensional pressure in a wideband ultrasound pulse is presented. A collimated laser pulse impinges on the ultrasound field, and the optical intensity in a diffraction plane subsequent to this scattering event is imaged onto a CCD camera and stored. Timing is controlled precisely, allowing the acousto-optic interaction to occur at the same point in the temporal evolution of the sound field each time the experiment is performed. After interaction with the ultrasound pulse, the optical phase is approximately proportional to the ray sum of instantaneous pressure in the pulse (taken in the direction of optical propagation). Optical phase immediately subsequent to passage through the ultrasound pulse is computed from recorded optical intensity via a phase retrieval algorithm. The experiment is performed at several angles as the ultrasound transducer is rotated about an axis normal to the optical axis of the imaging system. Tomographic reconstruction algorithms are applied to the retrieved optical phase data, computing pressure at all points in the ultrasound pulse. The resulting pulse may be propagated forward or backward in time under a source-free model. Reconstructions of the field from a 2.5-MHz array transducer compare well with hydrophone measurements.

Electrooptic effect calculations: simplified procedure for arbitrary cases

To aid in the design of electrooptic devices, a simplified procedure is introduced for determining the principal indices of refraction and the orientations of the index ellipsoid when an electric field is applied. The approach employs the general Jacobi method, and it is applicable to an arbitrary isotropic, uniaxial, or biaxial crystal class with an arbitrary direction of applied field. It includes a straightforward approach for labeling the new principal dielectric axes so as to produce the minimum global rotation of the ellipsoid from the zerofield principal dielectric axes. The necessary calculations can be easily implemented with a pocket calculator and are often found to be more accurate than those obtained with larger computers using standard library math packages. Furthermore, for analyzing device performance, simple analytic expressions for the orientations of the fast and slow polarization axes and the fast and slow indices of refraction are derived for a given arbitrary direction of optical propagation and any arbitrary direction of applied electric field. The calculational procedure is applicable to linear and quadratic electrooptic effects and the photoelastic effect in isotropic, uniaxial, and biaxial crystals. Illustrative examples for GaAs, LiNbO(3), and KDP are presented.

Extending the wavelength range of fundamental laser sources

The ability to extend the wavelength range of fundamental laser sources provides the opportunity to match a light source to a specific chemical or biological interaction wavelength. The most commonly used technique to alter laser wavelengths involves mixing two optical waves in a nonlinear crystal to produce the sum frequency. Energy is efficiently transferred from the two input waves to the output wave only when a proper phase-matching condition is maintained. Special cases arise when the two input waves have the same optical frequency (i.e., are components of the same wave) or have in themselves a harmonic relationship with a common fundamental source. The first instance results in an output wavelength equal to exactly one-half that of the fundamental input and is called second harmonic generation (SHG). In the second instance, combining the fundamental and second harmonic of a laser will result in third harmonic generation (THG). THG is a two-stage process, a doubling stage followed by a mixing stage. Phase matching is accomplished by utilizing the birefringence of the nonlinear crystal to overcome the refractive index dispersion from one wavelength to another. The indices are precisely tuned to the values desired for a specific set of wavelengths either by altering the optical propagation direction with respect to the crystal optic axis (angle matching) or by changing the temperature of the crystal and utilizing the subsequent index change (temperature matching). Whichever method is used, the physical properties of individual crystal species impose a range limitation on phase matching that requires that several crystals be used for broad wavelength coverage. Laser sources used in the study of fast reactions must be characterized by short pulse widths. In addition, to achieve maximum efficiency in harmonic generating and mixing applications, the laser should have a narrow line width, high peak power, and low beam divergence. Spectral brightness (W/cm2 sterad A) is a single parameter relating the three variables. For efficient harmonic generation and mixing, the laser must exhibit a high spectral brightness. Among the more common and flexible lasers that exhibit high spectral brightness are Nd:YAG, ruby, N2-pumped dye lasers and Nd:YAG-pumped dye lasers. Techniques used to generate short-pulse outputs include Q-switching and mode-locking. A wavelength region of high interest to biologists, chemists, physicists, and spectroscopists is the ultraviolet (UV), since this is a region of high interaction of light with molecules. Several techniques are employed to extend the wavelength range of tunable or fixed-frequency laser sources to the ultraviolet. SHG is the most direct approach. In those cases where the desired wavelength regions fall within the umbrella of 90° phase-matching of KDP homologues (see figure), an extremely efficient tem-