All-optical Control Research Articles

Terahertz Metagrating Emitters with Beam Steering and Full Linear Polarization Control.

We report the realization of broadband THz plasmonic metagrating emitters for simultaneous beam steering and all-optical linear polarization control. Two types of metagratings are designed and experimentally demonstrated. First, the plasmonic meta-atoms are arranged in a metagrating with a binary phase modulation which results in the nonlinear generation of THz waves to the ±1 diffraction orders, with complete suppression of the zeroth order. Complete tunability of the diffracted THz linear polarization direction is demonstrated through simple rotation of the pump polarization. Then, the concept of lateral phase shift is introduced into the design of the metagratings using interlaced phase gradients. By controlling the spatial shift of the submetagrating, we are able to continuously control the linear polarization states of the generated THz waves. This method results in a higher nonlinear diffraction efficiency relative to binary phase modulation. These functional THz metagratings show exciting promise to meet the challenges associated with the current diverse array of applications utilizing THz technology.

All-optical control of excitons in semiconductor quantum wells

Applying the Floquet theory, we developed the method to control excitonic properties of semiconductor quantum wells (QWs) by a high-frequency electromagnetic field. It is demonstrated, particularly, that the field induces the blue shift of exciton emission from the QWs and narrows width of the corresponding spectral line. As a consequence, the field strongly modifies optical properties of the QWs and, therefore, can be used to tune characteristics of the optoelectronic devices based on them.

A Kerr polarization controller

Kerr-effect-induced changes of the polarization state of light are well known in pulsed laser systems. An example is nonlinear polarization rotation, which is critical to the operation of many types of mode-locked lasers. Here, we demonstrate that the Kerr effect in a high-finesse Fabry-Pérot resonator can be utilized to control the polarization of a continuous wave laser. It is shown that a linearly-polarized input field is converted into a left- or right-circularly-polarized field, controlled via the optical power. The observations are explained by Kerr-nonlinearity induced symmetry breaking, which splits the resonance frequencies of degenerate modes with opposite polarization handedness in an otherwise symmetric resonator. The all-optical polarization control is demonstrated at threshold powers down to 7 mW. The physical principle of such Kerr effect-based polarization controllers is generic to high-Q Kerr-nonlinear resonators and could also be implemented in photonic integrated circuits. Beyond polarization control, the spontaneous symmetry breaking of polarization states could be used for polarization filters or highly sensitive polarization sensors when operating close to the symmetry-breaking point.

Photochromic switching of narrow-band lattice resonances.

Narrow-band resonances supported by a variety of periodic metallic or dielectric nanostructures have great potential applications in light sources, optical sensors, and switches or modulators. Here we report the switching of narrow-band lattice resonances in a mirror-backed two-dimensional array of dielectric nanopillars. The nanopillar is composed of a silica core and photochromic coating. By exposure to ultraviolet light, the photochromic molecules can be turned into a state that is highly absorptive around the wavelength of the lattice resonance. Because the lattice resonance has enhanced the near-fields concentrated on the tops of dielectric nanopillars, the absorptive coating can destroy this resonance. The absorptive state of the photochromic molecules can be recovered to a transparent state by exposure to visible light. We fabricate the device and characterize the change of reflection spectra to demonstrate the reversible switching of lattice resonances by exposure to ultraviolet and visible light alternately. An all-optical control of the narrow-band photoluminescence is further demonstrated by combining a fluorescent dye with the photochromic molecules.

Terahertz-assisted even harmonics generation in silicon

SummaryWhen a biased electric/light field is applied to centrosymmetric crystals like silicon, the broken symmetry creates even-order harmonics radiation which can reveal key insights into the material. Recently, the second harmonic has been generated by THz-induced symmetry breaking, but the observation of higher-order radiation remains largely unexplored. Here, we demonstrate picosecond-level ultrafast, nondestructive symmetry manipulation of silicon crystal by using a 500 kV/cm intense terahertz (THz) electric field. The THz-induced fourth harmonic of the infrared probe is also observed and characterized for the first time. In addition, we find that the even-order harmonics show no dependence on the THz field direction thus it allows for sub-cycle symmetry manipulations. Our study paves the way toward ultrafast all-optical crystal symmetry control in the future high-speed electronics and photonics.

Pulse shaping for on-demand emission of single Raman photons from a quantum-dot biexciton

Semiconductor quantum dots embedded in optical cavities are promising on-demand sources of single photons. Here, we theoretically study single photon emission from an optically driven two-photon Raman transition between the biexciton and the ground state of a quantum dot. The advantage of this process is that it allows all-optical control of the properties of the emitted single photon with a laser pulse. However, with the presence of other decay channels and excitation-induced quantum interference, on-demand emission of the single Raman photon is generally difficult to achieve. Here we show that laser pulses with non-trivial shapes can be used to maintain excitation conditions for which with increasing pulse intensities the on-demand regime is reached. To provide a realistic picture of the achievable system performance, we include phonon-mediated processes in the theoretical caluclations. While preserving both high photon purity and indistinguishability, we find that although based on a higher-order emission process, for realistic system parameters on-demand Raman photon emission is indeed achievable with suitably tailored laser pulses.

Light modulation based on the enhanced Kerr effect in molybdenum disulfide nanostructures with curved features.

A novel type of molybdenum disulfide (MoS2) nanoparticles (NPs) was chemically synthesized, which possessed curved features with three-dimensional (3D) freedom compared with planar two-dimensional (2D) materials. Due to the introduction of curved features, the synthesized NPs exhibited a strongly enhanced nonlinear refractive index (n2 ∼ 10-5 cm-2 W-1) and third-order susceptibility (χ(3) ∼ 10-7 esu), which were experimentally verified by the spatial self-phase modulation effect in the visible wavelength range. Both the nonlinear parameters were two orders of magnitude higher than their planar MoS2 nanostructure counterparts. In addition, the relative change of the effective nonlinear refractive index Δn2/n2 was found to be distinctly dependent on the intensity of the applied electromagnetic field. Moreover, an all-optical modulation was experimentally realized based on the spatial cross-phase modulation effect. Our results demonstrate planar MoS2 materials with 3D features as potential candidates for next generation all-optical applications and open a substantial approach for the design of efficient nanomaterials with favorable optical nonlinearity.

Selective rotational control in mixtures of molecular super-rotors.

We demonstrate experimentally a method of all-optical selective rotational control in gas mixtures. Using an optical centrifuge-an intense laser pulse whose linear polarization rotates at an accelerated rate, we simultaneously excite two different molecular species to two different rotational frequencies of choice. The new level of control is achieved by shaping the centrifuge spectrum according to the rotational spectra of the centrifuged molecules. The shaped optical centrifuge releases one molecular species earlier than the other, therefore separating their target rotational frequencies and corresponding rotational states. The technique is applicable to molecules with non-overlapping rotational spectra in the frequency range of interest and will expand the utility of rotational control in the studies of the effects of molecular rotation on collisions and chemical reactions.

Single-photon nonlinearity at room temperature.

The recent progress in nanotechnology1,2 and single-molecule spectroscopy3-5 paves the way foremergent cost-effective organic quantum optical technologies with potential applications in useful devices operating at ambient conditions. We harness a π-conjugated ladder-type polymer strongly coupled to a microcavity forming hybrid light-matter states, so-called exciton-polaritons, to create exciton-polariton condensates with quantum fluid properties. Obeying Bose statistics, exciton-polaritons exhibit an extreme nonlinearity when undergoing bosonic stimulation6, which we have managed to trigger at the single-photon level, thereby providing an efficient way for all-optical ultrafast control over the macroscopic condensate wavefunction. Here, we utilize stable excitons dressed with high-energy molecular vibrations, allowing for single-photon nonlinear operation at ambient conditions. This opens new horizons for practical implementations like sub-picosecond switching, amplification and all-optical logic at the fundamental quantum limit.

All-Optical Linear-Polarization Engineering in Single and Coupled Exciton-Polariton Condensates

We demonstrate all-optical linear polarization control of exciton-polariton condensates in anisotropic elliptical optical traps. The cavity inherent TE-TM splitting lifts the ground state spin degeneracy with emerging fine structure modes polarized linear parallel and perpendicular to the trap major axis with the condensate populating the latter. Our findings show a new type of polarization control with exciting perspectives in both spinoptronics and studies on extended systems of interacting nonlinear optical elements with anisotropic coupling strength and adjustable fine structure.

All-optical graphene-on-silicon slot waveguide modulator based on graphene’s Kerr effect

All-optical graphene-based optical modulators have recently attracted much attention because of their ultrafast and broadband response characteristics (bandwidth larger than 100 GHz) in comparison with the previous graphene-based optical modulators, which are electrically tuned via the graphene Fermi level. Silicon photonics has some benefits such as low cost and high compatibility with CMOS design and manufacturing technology. On the other hand, graphene has a unique large nonlinear Kerr coefficient, which we calculate using graphene's tight-binding model based on the semiconductor Bloch equations. Its real and imaginary parts are negative at the wavelength of 1.55 µm and EF=0.1eV. To simultaneously use the benefits mentioned above, we present an all-optical, CMOS-compatible, and graphene-on-silicon slot (GOSS) waveguide extinction and phase modulator that consists of two different geometries. The first one consists of a one-stage GOSS waveguide with a single layer of graphene. To increase the light-graphene interaction and consequently enhance the modulation efficiency (ME), another stage of the GOSS waveguide is placed over the first one. This two-stage configuration is called a graphene-on-silicon double-slot (GOSDS) waveguide. The ME, insertion loss (IL), and modulation depth (MD) for a 12.5 µm GOSDS waveguide modulator with a double layer of graphene can reach 0.241 dB/µm, 1.31 dB, and 77%, respectively, at optical pump intensities about 9MWcm-2. Our design has a smaller waveguide length (17.6 times) than the previous all-optical graphene-on-silicon ribbon waveguide extinction modulator and high MD (about 2 times) in comparison with a graphene-clad microfiber all-optical extinction modulator. Compared with an all-optical Mach-Zehnder interferometer phase modulator, our design has short graphene coated waveguide length (≈0.1 times) and low local optical intensities (≈0.043 times) needed for π phase shift. This study may promote the design and realization of high-performance, wideband, compact, and all-optical control on a single chip with a reasonable contrast level.

Ultrafast Optomagnonics in Ferrimagnetic Multi-Sublattice Garnets

This review discusses the ultrafast magnetization dynamics within the gigahertz to terahertz frequency range in ferrimagnetic rare-earth iron garnets with different substitutions. In these garnets, the roles of spin–orbit and exchange interactions have been detected using femtosecond laser pulses via the inverse Faraday effect. The all-optical control of spin-wave and Kaplan–Kittel exchange resonance modes in different frequency ranges is shown. Generation and localization of the electric field distribution inside the garnet through the metal-bound surface plasmon-polariton strongly enhance the amplitude of the exchange resonance modes. The exchange resonance mode in yttrium iron garnets was observed using circularly polarized Raman spectroscopy. The results of this study may be utilized in the development of a wide class of optomagnonic devices in the gigahertz to terahertz frequency range.

Observation of Mechanical Faraday Effect in Gaseous Media.

We report the experimental observation of the rotation of the linear polarization of light propagating in a gas of fast-spinning molecules (molecular superrotors). In the observed effect, related to Fermi's prediction of "polarization drag" by a rotating medium, the vector of linear polarization tilts in the direction of molecular rotation. We use an optical centrifuge to bring the molecules in a gas sample to ultrafast unidirectional rotation and measure the polarization drag angles of the order of 10^{-4} rad (with an experimental uncertainty about 10^{-6} rad) over the propagation distance of the order of 1mm in a number of gases under ambient conditions. We demonstrate an all-optical control of the drag magnitude and direction and investigate the robustness of the mechanical Faraday effect with respect to molecular collisions.

All optical control of magnetization in quantum confined ultrathin magnetic metals

All-optical control dynamics of magnetization in sub-10 nm metallic thin films are investigated, as these films with quantum confinement undergo unique interactions with femtosecond laser pulses. Our theoretical analysis based on the free electron model shows that the density of states at Fermi level (DOSF) and electron–phonon coupling coefficients (Gep) in ultrathin metals have very high sensitivity to film thickness within a few angstroms. We show that completely different magnetization dynamics characteristics emerge if DOSF and Gep depend on thickness compared with bulk metals. Our model suggests highly efficient energy transfer from femtosecond laser photons to spin waves due to minimal energy absorption by phonons. This sensitivity to the thickness and efficient energy transfer offers an opportunity to obtain ultrafast on-chip magnetization dynamics.

Giant All-Optical Modulation of Second-Harmonic Generation Mediated by Dark Excitons

All-optical control of nonlinear photonic processes in nanomaterials is of significant interest from a fundamental viewpoint and with regard to applications ranging from ultrafast data processing to spectroscopy and quantum technology. However, these applications rely on a high degree of control over the nonlinear response, which still remains elusive. Here, we demonstrate giant and broadband all-optical ultrafast modulation of second-harmonic generation (SHG) in monolayer transition-metal dichalcogenides mediated by the modified excitonic oscillation strength produced upon optical pumping. We reveal a dominant role of dark excitons to enhance SHG by up to a factor of ∼386 at room temperature, 2 orders of magnitude larger than the current state-of-the-art all-optical modulation results. The amplitude and sign of the observed SHG modulation can be adjusted over a broad spectral range spanning a few electronvolts with ultrafast response down to the sub-picosecond scale via different carrier dynamics. Our results not only introduce an efficient method to study intriguing exciton dynamics, but also reveal a new mechanism involving dark excitons to regulate all-optical nonlinear photonics.

All-Optical Experimental Control of High-Harmonic Photon Energy

We generate high-order harmonics in gaseous medium with tunable photon energy using time domain interferometry of double pulses in a non-collinear generation geometry. The method is based on the fact that the generated harmonics inherit certain spectral properties of the driving laser. The two temporally delayed ultrashort laser pulses, identical in all parameters, are produced by a custom-made split-and-delay unit utilizing wave front splitting without a significant energy loss. The arrangement is easy to implement in any attosecond pulse generation beamline, and is suitable for the production of an extreme ultraviolet source with simply and quickly variable central photon energy, useful for a broad range of applications.

High spectro-temporal compression on a nonlinear CMOS-chip

Optical pulses are fundamentally defined by their temporal and spectral properties. The ability to control pulse properties allows practitioners to efficiently leverage them for advanced metrology, high speed optical communications and attosecond science. Here, we report 11× temporal compression of 5.8 ps pulses to 0.55 ps using a low power of 13.3 W. The result is accompanied by a significant increase in the pulse peak power by 9.4×. These results represent the strongest temporal compression demonstrated to date on a complementary metal–oxide–semiconductor (CMOS) chip. In addition, we report the first demonstration of on-chip spectral compression, 3.0× spectral compression of 480 fs pulses, importantly while preserving the pulse energy. The strong compression achieved at low powers harnesses advanced on-chip device design, and the strong nonlinear properties of backend-CMOS compatible ultra-silicon-rich nitride, which possesses absence of two-photon absorption and 500× larger nonlinear parameter than in stoichiometric silicon nitride waveguides. The demonstrated work introduces an important new paradigm for spectro-temporal compression of optical pulses toward turn-key, on-chip integrated systems for all-optical pulse control.

Synthesis and dissociation of soliton molecules in parallel optical-soliton reactors

Mode-locked lasers have been widely used to explore interactions between optical solitons, including bound-soliton states that may be regarded as “photonic molecules”. Conventional mode-locked lasers normally, however, host at most only a few solitons, which means that stochastic behaviours involving large numbers of solitons cannot easily be studied under controlled experimental conditions. Here we report the use of an optoacoustically mode-locked fibre laser to create hundreds of temporal traps or “reactors” in parallel, within each of which multiple solitons can be isolated and controlled both globally and individually using all-optical methods. We achieve on-demand synthesis and dissociation of soliton molecules within these reactors, in this way unfolding a novel panorama of diverse dynamics in which the statistics of multi-soliton interactions can be studied. The results are of crucial importance in understanding dynamical soliton interactions and may motivate potential applications for all-optical control of ultrafast light fields in optical resonators.

All-Optical Nonlinear Control of Circularly Polarized Light in Birefringent Fibers

Polarization state manipulation and tunability are two highly desirable objectives of utmost importance for the development of many applications, especially in optical fiber-based systems. In this article, we present in detail a comprehensive theory for describing and controlling the nonlinear polarization conversion of circularly polarized light in birefringent optical fibers (BOFs). We use our theory to provide a nonlinear model that allows obtaining the accurate input powers with which we can tune the most common continuous-wave polarization states at the BOF output. To analyze the polarization instability of the output optical signals, we define a parameter that gives a magnitude of the polarization instability as a function of the input power. The analytical results we provide agree accurately with the results obtained through numerical techniques. Our theory opens up new perspectives and provides considerable physical insight on the all-optical polarization control with BOFs.

All-Optical Control of a Single Resonance in a Graphene-On-Silicon Nanobeam Cavity Using Thermo-Optic Effect

We propose and demonstrate on-chip all-optical control of a single resonance in a graphene-on-silicon nanobeam cavity using thermo-optic (TO) effect. In the design, the graphene sheet covered on the nanobeam cavity is patterned in a “U” shape, which enables the device to maintain a constant quality factor and a high extinction ratio (ER) after graphene transfer. Benefiting from the ultra-small optical mode volume of the nanobeam cavity and the high thermal conductivity of graphene, efficient all-optical control of a single resonance is implemented with a tuning efficiency of 0.0346 nm/mW. All-optical switching is also demonstrated with a switching power of 47 mW at a high ER of ~16 dB. Moreover, the 10%-90% switching times are 3.24 μs and 5.52 μs for the rising edge and the falling edge, respectively. Such graphene-assisted device potentially could be a good candidate to realize efficient on-chip all-optical control of an individual resonance with a high ER.