Observation of discrete-light temporal refraction by moving potentials with broken Galilean invariance.

Refraction is a basic beam bending effect at two media's interface. While traditional studies focus on stationary boundaries, moving boundaries or potentials could enable new laws of refractions. Meanwhile, media's discretization plays a pivotal role in refraction owing to Galilean invariance breaking principle in discrete-wave mechanics, making refraction highly moving-speed dependent. Here, by harnessing a synthetic temporal lattice in a fiber-loop circuit, we observe discrete time refraction by a moving gauge-potential barrier. We unveil the selection rules for the potential moving speed, which can only take an integer v = 1 or fractional v = 1/q (odd q) value to guarantee a well-defined refraction. We observe reflectionless/reflective refractions for v = 1 and v = 1/3 speeds, transparent potentials with vanishing refraction/reflection, refraction of dynamic moving potential and refraction for relativistic Zitterbewegung effect. Our findings may feature applications in versatile time control and measurement for optical communications and signal processing.

In-line attosecond photoelectron holography for single photon ionization.

The momentum distribution of photoelectrons in H2+ molecules subjected to an attosecond pulse is theoretically investigated. To better understand the laser-molecule interaction, we develop an in-line photoelectron holography approach that is analogous to optical holography. This approach is specifically suitable for extracting the amplitude and phase of the forward-scattered electron wave packet in a dissociating molecule with atomic precision. We also extend this approach to imaging the transient scattering cross-section of a molecule dressed by a near infrared laser field. This attosecond photoelectron holography sheds light on structural microscopy of dissociating molecules with high spatial-temporal resolution.

On-Chip Photonic Localization in Aharonov-Bohm Cages Composed of Microring Lattices.

Flatband localization endowed with robustness holds great promise for disorder-immune light transport, particularly in the advancement of optical communication and signal processing. However, effectively harnessing these principles for practical applications in nanophotonic devices remains a significant challenge. Herein, we delve into the investigation of on-chip photonic localization in AB cages composed of indirectly coupled microring lattices. By strategically vertically shifting the auxiliary rings, we successfully introduce a magnetic flux of π into the microring lattice, thereby facilitating versatile control over the localization and delocalization of light. Remarkably, the compact edge modes of this structure exhibit intriguing topological properties, rendering them strongly robust against disorders, regardless of the size of the system. Our findings open up new avenues for exploring the interaction between flatbands and topological photonics on integrated platforms.

Selecting mode by the complex Berry phase in non-Hermitian waveguide lattices.

Bloch oscillations (BOs) in a parity-time (PT)-symmetric Su-Schrieffer-Heeger (SSH) waveguide array are theoretically investigated. We show that the BOs are amplified or damped even for the systems to exhibit entirely real energy bands. The amplified and damped BOs stem from the complex Berry phase and closely relate to the topological properties of the lattice. For the topological nontrivial lattice, the amplification and attenuation of BOs are much more prominent than the trivial case and the output Bloch mode can be selected. Furthermore, we propose an experimental scheme and perform a numerical simulation based on a bent waveguide array. Our work uncovers the impact of the topological properties on the dynamics of the bulk Bloch modes and unveils a horizon in the study of non-Hermitian physics. The mode selection induced by the complex Berry phase may also find application in integrated photonic devices such as the mode filter.

Proton tunneling in the dissociation of H2+ and its asymmetric isotopologues driven by circularly polarized THz laser pulses.

Light-induced deprotonation of molecules is an important process in photochemical reactions. Here, we theoretically investigate the tunneling deprotonation of H2+ and its asymmetric isotopologues driven by circularly polarized THz laser pulses. The quasi-static picture shows that the field-dressed potential barrier is significantly lowered for the deprotonation channel when the mass asymmetry of the diatomic molecule increases. Our numerical simulations demonstrate that when the mass symmetry breaks, the tunneling deprotonation is significantly enhanced and the proton tunneling becomes the dominant dissociation channel in the THz driving fields. In addition, the simulated nuclear momentum distributions show that the emission of the proton is directed by the effective vector potential for the deprotonation channel and, meanwhile, the angular distribution of the emitting proton is affected by the alignment and rotation of the molecule induced by the rotating field.

Energy-dependent photoion angular distributions in two-body Coulomb explosions of molecules.

We experimentally study two-body Coulomb explosions of CO2, O2, and CH3Cl molecules in intense femtosecond laser pulses. We observe an obvious variation in the ionic angular distribution of the fragments with respect to the kinetic energy releases (KERs). Using a classical model based on ab initio potential energy curves, we find that the dependence of the ionic angular distribution on the KER is relevant to the fact that the accurate potential energy deviates significantly from the value determined by applying the Coulomb interaction approximation at a relatively small internuclear distance of the molecule. We show that the KER-dependent ionic angular distribution provides an effective way to determine the critical internuclear distance at which the Coulomb interaction approximation holds or breaks down without relying on the knowledge of the accurate potential energy curves.

Unusual Pressure-Induced Self-Trapped Exciton to Free Exciton Transfer in Chiral 2D Lead Bromide Perovskites.

The phenomenon of pressure-induced emission alterations related to complex excitonic dynamics in 2D lead halide perovskites (LHPs) has gained considerable attention for understanding their structure-property relationship and obtaining inaccessible luminescence under ambient conditions. However, the well-known pressure-induced emissions are limited to the formation of self-trapped excitons (STEs) due to the structural distortion under compression, which goes against the advantage of the highly pure emission of LHPs. Here, the pressure-induced detrapping from STEs to free excitons (FEs) accompanied by the dramatic transition from broadband orangish emission to narrow blue emission has been achieved in chiral 2D LHPs and R- and S-[4MeOPEA]2PbBr4, (4MeOPEA = 4-methoxy-α-methylbenzylammonium). The combined experimental and calculated results reveal that the distortion level of PbBr6 octahedra of R- and S-[4MeOPEA]2PbBr4 exhibits an unusually significant reduction as the applied pressure increases, which leads to decreased electron-phonon coupling and self-trapped energy barrier and consequently enables the detrapping of STEs to FEs. This work illustrates the dramatic exciton transfer in 2D LHPs and highlights the potential for realizing highly efficient and pure light emissions by manipulating the structural distortion via strain engineering.

260 fs, 403 W coherently combined fiber laser with precise high-order dispersion management.

An ultrafast fiber laser system comprising two coherently combined amplifier channels is reported. Within this system, each channel incorporates a rod-type fiber power amplifier, with individual operations reaching approximately 233 W. The active-locking of these coherently combined channels, followed by compression using gratings, yields an output with a pulse energy of 504 µJ and an average power of 403 W. Exceptional stability is maintained, with a 0.3% root mean square (RMS) deviation and a beam quality factor M2 < 1.2. Notably, precise dispersion management of the front-end seed light effectively compensates for the accumulated high-order dispersion in subsequent amplification stages. This strategic approach results in a significant reduction in the final output pulse duration for the coherently combined laser beam, reducing it from 488 to 260 fs after the gratings compressor, while concurrently enhancing the energy of the primary peak from 65% to 92%.

Photonic skin-topological effects in microring lattices.

We investigate the non-Hermitian Hofstadter-Harper model composed of microring resonators, in which the non-Hermitian skin effect (NHSE) is particularly analyzed. The effect is achieved through the interaction between well-designed gain-loss layouts and artificial gauge fields. Remarkably, we reveal the emergence of a hybrid skin-topological effect (HSTE), where only the original topological edge modes convert to skin modes while bulk modes remain extended. By changing the distributions of gauge fields, we show the NHSE can manifest itself in bulk modes and be localized at specific edges. Using the equivalence of sites in the bulk or at boundaries to 1D SSH chains, we analyze the potential cancellation of NHSE in these configurations. Additionally, we demonstrate a new, to the best of our knowledge, type of HSTE in topological insulators which emerge at any gain-loss interfaces. The study may improve the understanding of the NHSE behavior in 2D topological systems and provide a promising avenue for tuning light propagation and localization.

High harmonic generation in solids: particle and wave perspectives.

High harmonic generation (HHG) from gas-phase atoms (or molecules) has opened up a new frontier in ultrafast optics, where attosecond time resolution and angstrom spatial resolution are accessible. The fundamental physical pictures of HHG are always explained by the laser-induced recollision of particle-like electron motion, which lay the foundation of attosecond spectroscopy. In recent years, HHG has also been observed in solids. One can expect the extension of attosecond spectroscopy to the condensed matter if a description capable of resolving the ultrafast dynamics is provided. Thus, a large number of theoretical studies have been proposed to understand the underlying physics of solid HHG. Here, we revisit the recollision picture in solid HHG and show some challenges of current particle-perspective methods, and present the recently developed wave-perspective Huygens-Fresnel picture for understanding dynamical systems within the ambit of strong-field physics.

Enhanced efficiency of launching hyperbolic phonon polaritons in stacked α-MoO3 flakes.

In this work, we reported a systemic study on the enhanced efficiency of launching hyperbolic phonon polaritons (PhPs) in stacked α-phase molybdenum trioxide (α-MoO3) flakes. By using the infrared photo-induced force microscopy (PiFM), real-space near-field images (PiFM images) of mechanically exfoliated α-MoO3 thin flakes were recorded within three different Reststrahlen bands (RBs). As referred with PiFM fringes of the single flake, PiFM fringes of the stacked α-MoO3 sample within the RB 2 and RB 3 are greatly improved with the enhancement factor (EF) up to 170%. By performing numerical simulations, it reveals that the general improvement in near-field PiFM fringes arises from the existence of a nanoscale thin dielectric spacer in the middle part between two stacked α-MoO3 flakes. The nanogap acts as a nanoresonator for prompting the near-field coupling of hyperbolic PhPs supported by each flake in the stacked sample, contributing to the increase of polaritonic fields, and verifying the experimental observations Our findings could offer fundamental physical investigations into the effective excitation of PhPs and will be helpful for developing functional nanophotonic devices and circuits.

Multidimensional synthetic frequency lattice in the dynamically modulated waveguides.

Here we propose an effective method to construct a higher-dimensional synthetic frequency lattice with an optical waveguide under dynamic modulation. By applying the traveling-wave modulation of refractive index modulation with two different frequencies that are not mutually commensurable, a two-dimensional frequency lattice could be formed. The Bloch oscillations (BOs) in the frequency lattice is demonstrated by introducing a wave vector mismatch of the modulation. We show that the BOs are reversible only as the amounts of wave vector mismatch in orthogonal directions are mutually commensurable. Finally, by employing an array of waveguides with each under traveling-wave modulation, a 3D frequency lattice is formed and its topological effect of one-way frequency conversion is revealed. The study offers a versatile platform for exploring higher-dimensional physics in concise optical systems and may find great application in optical frequency manipulations.

Photonic Möbius topological insulator from projective symmetry in multiorbital waveguides.

The gauge fields dramatically alter the algebraic structure of spatial symmetries and make them projectively represented, giving rise to novel topological phases. Here, we propose a photonic Möbius topological insulator enabled by projective translation symmetry in multiorbital waveguide arrays, where the artificial π gauge flux is aroused by the inter-orbital coupling between the first (s) and third (d) order modes. In the presence of π flux, the two translation symmetries of rectangular lattices anti-commute with each other. By tuning the spatial spacing between two waveguides to break the translation symmetry, a topological insulator is created with two Möbius twisted edge bands appearing in the bandgap and featuring 4π periodicity. Importantly, the Möbius twists are accompanied by discrete diffraction in beam propagation, which exhibit directional transport by tuning the initial phase of the beam envelope according to the eigenvalues of translation operators. This work manifests the significance of gauge fields in topology and provides an efficient approach to steering the direction of beam transmission.

Photonic Floquet Landau-Zener tunneling and temporal beam splitters.

Landau-Zener tunneling (LZT), i.e., the nonadiabatic transition under strong parameter driving in multilevel systems, is ubiquitous in physics, providing a powerful tool for coherent wave control both in quantum and classical systems. While previous works mainly focus on LZT between two energy bands in time-invariant crystals, here, we construct synthetic time-periodic temporal lattices from two coupled fiber loops and demonstrate dc- and ac-driven LZTs between periodic Floquet bands. We show that dc- and ac-driven LZTs display distinctive tunneling and interference characteristics, which can be harnessed to realize fully reconfigurable LZT beam splitter arrangements. As a potential application to signal processing, we realize a 4-bit temporal beam encoder for classical light pulses using a reconfigurable LZT beam splitter network. Our work introduces and experimentally demonstrates a new class of reconfigurable linear optics circuits harnessing Floquet LZT, which may find versatile applications in temporal beam control, signal processing, quantum simulations, and information processing.

Clean hundred-µJ-level sub-6-fs blue pulses generated with helium-assisted solid thin plates.

In this work, 85 µJ, 5.5 fs pulses spanning 350-500 nm with 96% energy concentrated on the main pulse are generated by pulse compression using a helium-assisted, two-stage solid thin plate apparatus. To the best of our knowledge, these are the highest energy sub-6 fs blue pulses obtained to date. Furthermore, during the spectral broadening process, we observe that solid thin plates are much more easily damaged by blue pulses in a vacuum than in a gas-filled environment at the same field intensity. Helium, with the highest ionization energy and extremely low material dispersion, is adopted to create a gas-filled environment. Thus, the damage to solid thin plates is eliminated, and high-energy, clean pulses can be obtained with only two commercially available chirped mirrors in a chamber. Furthermore, the excellent output power stability of 0.39% root mean square (rms) fluctuations over 1 h is maintained. We believe that few-cycle blue pulses at the hundred-µJ level can open the door to numerous new ultrafast and strong-field applications in this spectral region.

Reconfigurable refraction manipulation at synthetic temporal interfaces with scalar and vector gauge potentials.

Photonic gauge potentials, including scalar and vector ones, play fundamental roles in emulating photonic topological effects and for enabling intriguing light transport dynamics. While previous studies mainly focus on manipulating light propagation in uniformly distributed gauge potentials, here we create a series of gauge-potential interfaces with different orientations in a nonuniform discrete-time quantum walk and demonstrate various reconfigurable temporal-refraction effects. We show that for a lattice-site interface with the potential step along the lattice direction, the scalar potentials can yield total internal reflection (TIR) or Klein tunneling, while vector potentials manifest direction-invariant refractions. We also reveal the existence of penetration depth for the temporal TIR by demonstrating frustrated TIR with a double lattice-site interface structure. By contrast, for an interface emerging in the time-evolution direction, the scalar potentials have no effect on the packet propagation, while the vector potentials can enable birefringence, through which we further create a "temporal superlens" to achieve time-reversal operations. Finally, we experimentally demonstrate electric and magnetic Aharonov-Bohm effects using combined lattice-site and evolution-step interfaces of either scalar or vector potential. Our work initiates the creation of artificial heterointerfaces in synthetic time dimension by employing nonuniformly and reconfigurable distributed gauge potentials. This paradigm may find applications in optical pulse reshaping, fiber-optic communications, and quantum simulations.

Experimental observation of the spectral self-imaging effect with a four-wave mixing time lens.

Here we use a four-wave mixing time lens to demonstrate the spectral self-imaging effect for a frequency comb. The time lens is built by imposing a temporal quadratic phase modulation onto the input signal pulses, which corresponds to a frequency comb in the Fourier spectrum. The modulation is implemented by a Gaussian pump pulse propagating in an external single-mode fiber. Both the signal and pump pulses are injected into a highly nonlinear fiber and four-wave mixing Bragg scattering occurs. We observe periodic revivals of the input frequency comb as the pump pulse propagates periodic distances. The comb-spacing is squeezed at fractional ratios to its original value. Meanwhile, the central-frequency undergoes redshifts and blueshifts subject to the scattered frequencies. We also find that the envelope width of input pulses has an effect on the output spectrum width. The study may find great applications in spectral reshaping and frequency metrology used for optical communication and signal processing.

High-order dynamic localization and tunable temporal cloaking in ac-electric-field driven synthetic lattices.

Dynamic localization (DL) of photons, i.e., the light-motion cancellation effect arising from lattice's quasi-energy band collapse under a synthetic ac-electric-field, provides a powerful and alternative mechanism to Anderson localization for coherent light confinement. So far only low-order DLs, corresponding to weak ac-fields, have been demonstrated using curved-waveguide lattices where the waveguide's bending curvature plays the role of ac-field as required in original Dunlap-Kenkre model of DL. However, the inevitable bending losses pose a severe limitation for the observation of high-order DL. Here, we break the weak-field limitation by transferring lattice concepts from spatial to synthetic time dimensions using fiber-loop circuits and observe up to fifth-order DL. We find that high-order DLs possess superior localization and robustness against random noise over lower-order ones. As an exciting application, by judiciously combining low- and high-order DLs, we demonstrate a temporal cloaking scheme with flexible tunability both for cloak's window size and opening time. Our work pushes DL towards high-order regimes using synthetic-lattice schemes, which may find potential applications in robust signal transmission, protection, processing, and cloaking.

Chiral Third-Harmonic Metasurface for Multiplexed Holograms.

Chiral nonlinear metasurfaces could natively synergize nonlinear wavefront manipulation and circular dichroism, offering enhanced capacity for multifunctional and multiplexed nonlinear metasurfaces. However, it is still quite challenging to simultaneously enable strong chiral response, precise wavefront control, high nonlinear conversion efficiency, and independent functions on spins and chirality. Here, we propose and experimentally demonstrate multiplexed third-harmonic (TH) holograms with four channels based on a chiral Au-ZnO hybrid metasurface. Specifically, the left- and right-handed circularly polarized (LCP and RCP) components of the TH holographic images can be designed independently under the excitation of an LCP (or RCP) fundamental beam. In addition, the TH conversion efficiency is measured to be as large as 10-5, which is 8.6 times stronger than that of a bare ZnO film with the same thickness. Thus, our work provides a promising platform for realizing efficient and multifunctional nonlinear nanodevices.

Two-photon-pumped amplified spontaneous emission from Ruddlesden-Popper perovskite flakes.

Herein, we report the two-photon pumped amplified spontaneous emission (ASE) in the 2D RPPs flakes at room temperature. We prepared high-quality (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, 5) flakes by mechanical exfoliating from the fabricated crystals. We show that the (BA)2(MA)n-1PbnI3n+1 flakes display a tunable two-photon pumped emission from 527 nm to 680 nm, as n increases from 1 to 5. Furthermore, we demonstrated two-photon pumped ASE from the (BA)2(MA)n-1PbnI3n+1 (n = 3, 4, 5) flakes. The two-photon pumped ASE thresholds of the RPPs are lower than lots of the other semiconductor nanostructures, indicating an excellent performance of the RPPs for two-photon pumped emission. In addition, we investigated the pump-wavelength-dependent two-photon pumped ASE behaviors of the RPPs flakes, which suggest that the near-infrared laser in a wide wavelength range can be converted into visible light by the frequency upconversion process in RPPs. This work has opened new avenues for realizing nonlinearly pumped ASE based on the RPPs, which shows great potential for the applications in wavelength-tunable frequency upconversion.