This study explores how bending the waveguides in integrated quantum photonic technologies affects the entangled photon pair generation rate through spontaneous four-wave mixing. Photon pair generation has traditionally been enhanced by modifying the waveguide's cross section or length. However, this work uncovers a new aspect: the bending radius of the waveguide significantly influences the photon pair generation rate. By increasing the curvature of the waveguide, the photons are more tightly confined, boosting the nonlinear interaction without the expected offset from increased radiation losses. The findings reveal an optimal curvature that maximizes photon pair generation, offering a novel approach to designing compact and efficient waveguide spirals. The research considers common nonlinear materials like silicon nitride, silicon-on-insulator, and photoresists for 3D waveguides and focuses on pump wavelengths near the telecommunication range.
We show that using the electric field as a quantization variable in nonlinear optics leads to incorrect expressions for the squeezing parameters in SPDC, the conversion rates in frequency conversion, and the wrong behavior of cross- and self-phase modulation. This observation is related to the fact that if the electric field is written as a linear combination of boson creation and annihilation operators one cannot satisfy Maxwell’s equations in a nonlinear dielectric.
Recent progress in the development of direct band gap GeSn is exploited to investigate the optical injection and coherent control of spin currents in this group IV semiconductor. The analysis of these properties could provide essential information for future innovative optical photon-to-spin conversion interfaces, long-sought after for entanglement distribution. A 30-band k•p model is used to evaluate the electronic properties in the material for a relatively wide range of energies, and a linear tetrahedron method is employed for the Brillouin zone integrations. Carrier, spin, current, and spin current injection rates are calculated for a bichromatic field of frequencies ω and 2ω.
We experimentally demonstrate the feasibility of the use of integrated linearly uncoupled resonators, which are coupled solely through the nonlinear interaction, to selectively enhance or suppress nonlinear processes. This is exploited to selectively enhance dual-pump spontaneous four-wave mixing while suppressing the parasitic noise associated with single pump spontaneous four-wave mixing processes. A signal-to-noise ratio characterizing the generation of identical photon pairs of more than four orders of magnitude is reported, opening the way to a new class of integrated devices exploiting the unique properties of identical photon pairs in the same optical mode.
Conventional solid-density laser-plasma targets quickly ionize to make a plasma mirror, which largely reflects ultra-intense laser pulses. This Fresnel reflection at the plane boundary largely wastes our e
orts at ultra-intense laser/solid interaction, and limits target heating to nonlinear generation of high-energy electrons which penetrate inward. One way around this dual problem is to create a material with an anisotropic dielectric function, for instance by nanostructuring a material in such a way that it cannot support the material responses which generate a specularly reflected beam. We present linear theory for metallic and plasma nanowires, particle-incell simulations of the interaction of ultra-intense femtosecond pulses with nickel nanowires, showing penetration of laser light far deeper than a nickel skin-depth, helping to uniformly heat near-solid material to conditions of high energy-densities, and XFEL experiments giving insight into their ionization and excitation.
We perform a multidimensional characterization of a polarization-entangled photon-pair source using stimulated emission tomography (SET). We measure the frequency-resolved polarization density matrix, which is composed of thousands of individual polarization density matrices, each corresponding to a different frequency pair. The measurement exhibits detailed information about correlations that would be difficult to observe using traditional quantum state tomography. This demonstration exhibits the power of SET to characterize a source of quantum states with multi-dimensional correlations and hyper-entanglement. The SET technique can be applied to a variety of photon-pair-based sources for the optimization and engineering of quantum states.
The tremendous progress in the fabrication of highly confining silicon-on-insulator (SOI) waveguides has been very beneficial for four-wave-mixing (FWM)-based wavelength conversion applications. Nevertheless, to establish power-efficient and wideband FWM wavelength conversion, one typically requires long (cm-scale) SOI waveguides with dispersion-engineered cross-sections that do not comply with the fabrication constraints of multiproject- wafer-oriented silicon photonics foundries. In this paper, we numerically examine the opportunities for wideband wavelength conversion through FWM in a foundry-compatible SOI waveguide covered with the highly nonlinear two-dimensional material of graphene. When combining subwatt level pump powers with a short waveguide length of only a few hundreds of microns, perfectly phase-matched conversion with significant efficiencies close to 20 dB can be obtained over a more than 40 THz-wide signal band adjacent to the pump frequency. Because of the tunability of the graphene properties, it is also possible to obtain quasi-phase matched FWM conversion through a periodic sign reversal of the graphene third-order nonlinearity along the waveguide. Conversion efficiencies exceeding 30 dB can be achieved over a 3.4 THz-wide signal band that is situated as much as 58 THz away from the pump frequency. Finally, the graphene tunability also allows for switching between the perfectly phase-matched and quasi-phase-matched operation modes.
We investigate the effect of phenomenological relaxation parameters on the third order optical nonlinearity of doped graphene by perturbatively solving the semiconductor Bloch equation. We focus on the contributions of optical transitions around the Dirac points, where the widely used linear dispersion relation is a good approximation. An analytic expression for the nonlinear conductivity at zero temperature can be obtained even if relaxation is included. With this analytic formula as a starting point, we construct the conductivity at finite temperature; and we illustrate the dependence of several nonlinear optical effects, such as third harmonic generation, Kerr effects and two photon absorption, and parametric frequency conversion.
We present the design of a photonic crystal-based multilayer structure that allows to experimentally demonstrate, using attenuated total reflectance experiments, the existence of the predicted transverse electric (TE) polarized excitation in graphene. We show that this mode can be excited in a single layer of graphene, even at room temperature. Furthermore, we prove that the observed mode in the reflection spectra corresponds to the TE- polarized graphene excitation and not the Bloch Surface Wave of the photonic crystal experiencing graphene- induced loss. Finally, we point out that adding an extra layer of dielectric material on top of the structure would ensure the unambiguous identification of the TE graphene mode even in the presence of fabrication errors.
We theoretically investigate general existence conditions for broadband bulk large-wavevector (high-k) propagating waves (such as volume plasmon polaritons in hyperbolic metamaterials) in arbitrary subwavelength periodic multilayers structures. Treating the elementary excitation in the unit cell of the structure as a generalized resonance pole of reflection coefficient and using Bloch’s theorem, we derive analytical expressions for the band of large-wavevector propagating solutions. We apply our formalism to determine the high-k band existence in two important cases: the well-known metaldielectric and recently introduced graphene-dielectric stacks. We confirm that short-range surface plasmons in thin metal layers can give rise to hyperbolic metamaterial properties and demonstrate that long-range surface plasmons cannot. We also show that graphene-dielectricmultilayers tend to support high-k waves and explore the range of parameters, where this is possible, confirming the prospects of using graphene for materials with hyperbolic dispersion. The suggested formalism is applicable to a large variety of structures, such as continuous or structured microwave, terahertz (THz) and optical metamaterials, optical waveguide arrays, 2D plasmonic and acoustic metamaterials.
The emergence of synthetic diamond has enabled photonics researchers to start exploiting the unique optical properties of diamond for various applications. In this paper we numerically predict the performance of
diamond ring waveguide structures for nonlinear wavelength conversion. After examining to what extent both
dispersion-engineered phase-matching and “automatic” quasi-phase-matching can be established in diamond ring
converters, we show that these phase matching approaches can yield high conversion efficiencies for a wide range
of wavelengths in the near-infrared/mid-infrared domain, as well as in the ultraviolet/visible domain.
Propagation of large-wavevector bulk plasmonic waves in multilayer hyperbolic metamaterials (HMMs) with two levels of structuring is theoretically studied. It is shown that when the parameters of a subwavelength metal-dielectric multilayer (“substructure”) are modulated (“superstructured”) on a larger, wavelength scale, the propagation of bulk plasmon polaritons in the resulting multiscale HMM is subject to photonic band gap phenomena. A great degree of control over such plasmons can be exerted by varying the superstructure geometry. As an example, Bragg reflection and Fabry-Perot resonances are demonstrated in multiscale HMMs with periodic superstructures. More complicated, aperiodically ordered superstructures are also considered, with fractal Cantor-like multiscale HMMs exhibiting characteristic self-similar spectral signatures in the high-k band. The multiscale HMM concept is shown to be a promising platform for using high-k bulk plasmonic waves as a new kind of information carriers, which can be used in far-field subwavelength imaging and plasmonic communication.
We propose an efficient four-wave-mixing-based wavelength conversion scheme in a silicon nanowire ring whereby no
dispersion engineering of the nanowire is required. Instead, we rely on the spatial variation of the Kerr susceptibility
around the ring to quasi-phase-match the wavelength conversion process for TE polarized fields. We show through
numerical modeling that in the absence of dispersion engineering this quasi-phase-matched wavelength conversion
approach can outperform 'conventional' wavelength conversion by as much as 10 dB.
Using an empirical pseudopoential description of electron states and an adiabatic bond charge model for phonon
states, a full band structure calculation is performed for the one- and two-photon indirect optical injection of
carriers and spins in bulk silicon. The calculated one- and two-photon absorption coefficients are in agreement
with experiments. For σ- light propagation direction along 001, the carrier and spin injection rates and
the degree of spin polarization show strong valley anisotropy. The carrier injection rates in the Z valleys are
larger than that in the X valley. Furthermore, the two photon indirect carrier injection shows anisotropy and
linear-circular dichroism with respect to the light propagation direction.
A theoretical model is proposed for describing the effective nonlinearity of surface plasmons supported by a
single interface between a metal and a linear dielectric (or a vacuum). The response to the polarization fields
driving the nonlinearity is modeled using a Green function formalism for surface optics. The theory is employed
to investigate the self-phase modulation of the single-interface surface plasmon. We discuss the possibility of
using surface plasmon self-phase modulation to estimate the value of the third-order nonlinear susceptibility of
metals through experimentation.
We propose a silicon ring Raman converter in which the spatial variation of the Raman gain along the ring for TE
polarization is used to quasi-phase-match the CARS process. If in addition the pump, Stokes, and anti-Stokes waves
involved in the CARS interaction are resonantly enhanced by the ring structure, the Stokes-to-anti-Stokes conversion
efficiency can be increased by at least four orders of magnitude over that of one-dimensional perfectly phase-matched
silicon Raman converters, and can reach values larger than unity with relatively low input pump intensities. These
improvements in conversion performance could substantially expand the practical applicability of the CARS process for
optical wavelength conversion.
Diffraction-based biosensors are often based on the adsorption of a target material on a grating made of thin layers,
where the adsorption is detected by a modification of the diffracted signal. In this communication we discuss two
strategies for enhancing this detection process. The first is based on the use of grating structures made of porous
elements, where sensing is based on target molecules penetrating into the elements and modifying their effective index of
refraction. The second is a resonant process where the effectiveness of the grating is enhanced by the coupling to surface
electromagnetic states, in particular Bloch surface waves that exist at the interface between a homogeneous medium and
a photonic crystal.
We present a direct-to-device method for stamping porous silicon to produce optical microstructures. The stamping
technique utilizes a reusable silicon stamp fabricated by standard lithographic methods. Large area (9mm2) stamps are
applied to single layer thin films of porous silicon with a force on the order of 1kN. The process affords precise control
over both lateral and vertical dimensions of patterning while maintaining large area uniformity. We demonstrate tunable
imprint depths in the 10nm-120nm range as well as lateral feature sizes down to 0.25μm. Imprinted structures are
characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical diffraction
experiments. By utilizing reusable stamps and a straightforward technique, the overall process can be performed at low-cost
and high throughput. This enables a wide variety of optical microstructures to be readily fabricated. As an
example, we present a porous diffraction grating and demonstrate proof-of-concept sensing capabilities, for exposure to
water vapor as well as small molecules (3-aminopropyltriethoxysilane). Additional device structures enabled by this
fabrication process are also discussed. The stamping process is expected to be applicable to other porous materials such
as porous titania, porous alumina, and porous silica.
Bloch Surface Waves (BSWs) are propagation modes that exist at the interface between a homogeneous medium and a
photonic crystal (PhC). The confinement at the interface of the media relies on total internal reflection in the
homogeneous medium and on the photonic band gap in the PhC. The dispersion relation of BSWs can be easily tailored
through the design of the PhC. This makes BSWs extremely flexible and suitable for applications in the field of optical
sensors, light emitters, and photovoltaic devices, where the capability to confine and amplify the electromagnetic field in
micro- and nano-structures allows for the enhancement of the light-matter interaction. In particular, we present two
different configurations for the detection of Bloch surface waves in silicon nitride multilayers: attenuated total
reflectance and photoluminescence measurements. In the first, we measured a 50-fold enhancement of the diffraction
signal by a protein grating printed on the multilayer when the incident light beam is coupled to the surface waves. In the
second, we observe a significant modification of the spontaneous emission by a monolayer of rhodamine molecules
bonded to the photonic crystal surface. These results may found application in the field of optical sensors, particularly
for biosensing.
We describe the activities of the Institute for Optical Sciences (IOS) at the University of Toronto towards the establishment of a Master’s Program in Optics. The IOS was formed as a collaboration between faculty members interested in optics from the four departments of Physics, Chemistry, Electrical and Computer Engineering and Materials Science and Engineering. One of its goals is to serve as unifying entity for graduate and undergraduate programs in optical sciences. The details of the proposed graduate program will be discussed. It will be set up in the form of a collaborative university program, where students must satisfy the requirements of one of the four home departments, as well as a set of IOS-specific requirements of the program. IOS-specific activities include attending the Distinguished Visiting Scientist Series, participation in a best-research-practice mini-course, where essential research skills are discussed, as well as participation in an annual internal conference. The benefits of this interdisciplinary program, for students, faculty and relevant industries are discussed. The students will benefit from a wider exposure and a more coherent curriculum. The IOS will also serve as local community within the campus to which students could belong and network. Faculty, on the other hand, will benefit from a reduced teaching load, as redundancies among the departments will be removed.
A controllable delivery of spins in nanodevices is required for applications in spintronics technologies. A pure spin current, in which oppositely oriented spins move in opposite directions, is a phenomenon that could be used for this purpose. Various optical techniques can efficiently excite such spin currents in bulk semiconductors and nanostructures. We here propose and analyze two new optical infrared-light techniques for the injection of a pure spin current in nanostructures. The techniques are based on the intersubband light absorption (one-photon process) and stimulated Raman scattering (two-photon process). The infrared light absorption deposits approximately 100 meV for each absorption event associated
with current injection. In the spin-flip Raman process which is possible due to spin-orbit (SO) coupling, the corresponding energy transfer to the system, is on the order of 1 meV. The stimulated Raman process depends on the electron momentum, and therefore, electrons with different spins can be launched in different directions. The infrared-injected pure spin currents can be engineered by changing the Rashba spin-orbit coupling using an external bias across the quantum well. The injected spin current should be detectable by pump-probe optical spectroscopy, and thus points the way toward the design of full-optical write-and-read spintronics devices.
Optical scattering loss in sub-micron scale patterned waveguides is one of the most important physical mechanisms dictating the limitations and applications of optical devices containing such structures. Despite this,there has been little theoretical work describing the extrinsic scattering losses in photonic crystal waveguides due to random fabrication variations such as disorder and surface roughness. While much work has been devoted to the characterization of ideal, lossless photonic crystal devices, the role of extrinsic optical scattering loss has not yet been suitably addressed. We present explicit formulas that describe extrinsic optical scattering loss for arbitrary sub-micron patterned waveguides occuring due fabricated imperfections such as disorder and surface roughness. Using a real-space Green function formalism, we derive original expressions for the backscattered loss and the total transmission loss, including out-of-plane contributions. Numerical calculations for planar photonic crystal waveguides yield extrinsic loss values in overall agreement with experimental measurements reported in the literature. Additionally, our formulas offer physical insight, including scaling rules that indicate how waveguide losses may be reduced by improved design. In particular, we highlight that loss becomes unavoidably large for operating frequencies approaching the photonic bandedge.
Quantum interference of single and two photon absorption pathways connecting valence and conduction band states in a semiconductor allow one to generate spin currents with or without charge currents. The underlying principles for these generation processes are outlines. We offer experimental demonstration of pure spin currents in GaAs using two color beams configured collinearly to produce spatially homogeneous currents, or non-collinearly to produce spin current gratings.
Nonlinear propagation of optical pulses in fiber Bragg gratings is studied experimentally and with numerical simulations of the coupled mode equations. After a review of enhanced nonlinear interactions for pulse wavelengths near the short wavelength edge of the photonic bandgap associated with the grating, this study explores polarization evolution during nonlinear pulse propagation. Initial results for polarization instabilities and expectations for vector Bragg solitons are described.
Laser light has been used as a probe of atoms, molecules, and solids since the invention of the laser. The use of laser light in a more active role, to modify and process surfaces, and initiate chemical reactions, followed shortly thereafter. But usually it is the intensity and the directionality of the laser light that is employed, not necessarily its coherence, and not particularly the fact that it has a well-defined phase. 'Coherence control' can be broadly understood as the set of processes whereby light modifies matter in a way that is critically dependent on the incident light beams possessing well-defined phases. While in a laser matter is manipulated to produce light of the desired properties, in coherent control light is manipulated -- in particular, its phase and intensity is adjusted -- to produce a material response of the desired type. Of the various coherent control processes that are currently being investigated, some involve a transition in the material medium from an initial state to a final state by two or more possible processes. With each of these is associated a quantum mechanical amplitude, and hence the probability for the transition can show interference effects between the two amplitudes, just as in the familiar two-slit interference experiment the probability for the electron to be observed at a given position involves a probability that is the square of the sum of two amplitudes. In quantum interference control (QUIC), the relative phase of the two amplitudes is adjusted by adjusting the relative phase of two polarizations of a single beam, or the relative phase of two beams at different frequencies. It is this particular type of coherent control that is of interest in this communication.
We study the dynamics of phase grating formation with visible light in an optical fiber. Adopting a simple two-photon local bleaching model, we show the grating approaches an ideal grating, where the writing frequency is always in the center of the local band gap, as it evolves. The evolution at each point in the fiber can be described in terms of a universal parameter.
A self-consistent model of second harmonic generation in fibers is based upon multiphoton ionization interference effects. An atom subject to a pair of harmonically related fields can exhibit a preferred direction of photoelectron emission that is dependent upon the relative phase between the two fields. Our model focuses on defects sites that require four fundamental frequency photons for ionization. In this case, phase-matched interference effects arise between four and three photon ionization and three and two photon ionization. We also qualitatively compare the four photon case with the two photon case in which there is only one interference term. The defects are modeled using a one-dimensional picture, with plane waves for ionized electron states. Both the four and two photon cases show exponential small signal spatial gain in steady state. The saturation characteristics can be different in the two cases, however. Results for the four photon model are in reasonable agreement with experimental observations. We discuss some of the limitations of our current model and possible future enhancements.
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