Birefringent Dispersion Optimization to Achieve Superior Nonlinear Optical Phase Matching in Deeper Solar-Blind UV Band from KH2PO4 to BeH3PO5

Nonlinear-optical (NLO) crystals require birefringent phase matching (BPM), particularly in the solar-blind ultraviolet (UV) (200-280 nm) and deep-UV (100-200 nm) regions. Achieving BPM requires optimization of optical dispersion along with having large birefringence. This requirement is especially critical for structures with low optical anisotropy, including classical phosphate UV-NLO crystals like KH2PO4 (KDP). However, there is a scarcity of in-depth theoretical analysis and general design strategies based on structural chemistry to optimize dispersion. This study presents findings from a simplified dielectric model that uncover two vital factors to micro-optimize transparent optical dispersion: effective mass (m*) of excited states and effective number (N*) of photo-responsive states. Smoothing of dispersion occurs as m* increases and N* decreases. First-principles analysis of deep-UV KBe2BO3F2-family structures is used to confirm the conciseness and validity of the model. It further proposes substituting K+ with Be2+ to decrease N* and increase m* while enlarging bandgap. This will lead to improved dispersion and an overall enhancement of KDP's BPM capability. The existing BeH3PO5 (BDP) is predicted to improve the shortest BPM wavelength for second-harmonic generation, from 251 nm in KDP to 201 nm in BDP. BDP's extension into the broader UV solar-blind waveband fully supports the proposed optimization strategy.

Optical clocks at sea

Deployed optical clocks will improve positioning for navigational autonomy1, provide remote time standards for geophysical monitoring2 and distributed coherent sensing3, allow time synchronization of remote quantum networks4,5 and provide operational redundancy for national time standards. Although laboratory optical clocks now reach fractional inaccuracies below 10-18 (refs. 6,7), transportable versions of these high-performing clocks8,9 have limited utility because of their size, environmental sensitivity and cost10. Here we report the development of optical clocks with the requisite combination of size, performance and environmental insensitivity for operation on mobile platforms. The 35 l clock combines a molecular iodine spectrometer, fibre frequency comb and control electronics. Three of these clocks operated continuously aboard a naval ship in the Pacific Ocean for 20 days while accruing timing errors below 300 ps per day. The clocks have comparable performance to active hydrogen masers in one-tenth the volume. Operating high-performance clocks at sea has been historically challenging and continues to be critical for navigation. This demonstration marks a significant technological advancement that heralds the arrival of future optical timekeeping networks.

Electronic and Nonlinear Optical Properties of B(III)-Submonoazaporphyrin-π-Diimide Compounds: A Density Functional Theory Study

Three hypothetical complexes were designed using diimides (PMDI, NTCDI, and PTCDI) as the acceptor unit and B(III)-submonoazaporphyrin (1) as the donor unit. These complexes have smaller HOMO-LUMO energy gaps (3.39-3.96 eV) than pristine 1 (6.61 eV). Further, the energy gap can be tuned by changing the number of benzene rings of these diimides. Remarkably, these proposed complexes possess considerable first hyperpolarizabilities (β0) (4865-6921 a.u.), and the regularity of the β0 values remained the same in the gas phase and toluene solvent conditions. There is an inverse relationship between the energy gap and the polarizability/first hyperpolarizability. In addition, absorption spectra, frontier molecular orbitals, and hole electron distributions were obtained using time-dependent density functional theory calculations to emphasize the relationship between structure and properties. Ultraviolet-Visible absorption spectra reveals that all complexes show satisfying IR working regions. Further analysis of the first hyperpolarizability density reveals the nature of the excellent NLO properties of the studied systems. This study can provide valuable insights for the development of potential high-performance NLO molecules.

Light-wave-controlled Haldane model in monolayer hexagonal boron nitride

In recent years, the stacking and twisting of atom-thin structures with matching crystal symmetry has provided a unique way to create new superlattice structures in which new properties emerge1,2. In parallel, control over the temporal characteristics of strong light fields has allowed researchers to manipulate coherent electron transport in such atom-thin structures on sublaser-cycle timescales3,4. Here we demonstrate a tailored light-wave-driven analogue to twisted layer stacking. Tailoring the spatial symmetry of the light waveform to that of the lattice of a hexagonal boron nitride monolayer and then twisting this waveform result in optical control of time-reversal symmetry breaking5 and the realization of the topological Haldane model6 in a laser-dressed two-dimensional insulating crystal. Further, the parameters of the effective Haldane-type Hamiltonian can be controlled by rotating the light waveform, thus enabling ultrafast switching between band structure configurations and allowing unprecedented control over the magnitude, location and curvature of the bandgap. This results in an asymmetric population between complementary quantum valleys that leads to a measurable valley Hall current7, which can be detected by optical harmonic polarimetry. The universality and robustness of our scheme paves the way to valley-selective bandgap engineering on the fly and unlocks the possibility of creating few-femtosecond switches with quantum degrees of freedom.

Fluid fibers in true 3D ferroelectric liquids

We demonstrate an exceptional ability of a high-polarization 3D ferroelectric liquid to form freely suspended fluid fibers at room temperature. Unlike fluid threads in modulated smectics and columnar phases, where translational order is a prerequisite for forming liquid fibers, recently discovered ferroelectric nematic forms fibers with solely orientational molecular order. Additional stabilization mechanisms based on the polar nature of the mesophase are required for this. We propose a model for such a mechanism and show that these fibers demonstrate an exceptional nonlinear optical response and exhibit electric field-driven instabilities.

Large nonlinear optical magnetoelectric response in a noncentrosymmetric magnetic Weyl semimetal

Weyl semimetals resulting from either inversion (P) or time-reversal (T) symmetry breaking have been revealed to show the record-breaking large optical response due to intense Berry curvature of Weyl-node pairs. Different classes of Weyl semimetals with both P and T symmetry breaking potentially exhibit optical magnetoelectric (ME) responses, which are essentially distinct from the previously observed optical responses in conventional Weyl semimetals, leading to the versatile functions such as directional dependence for light propagation and gyrotropic effects. However, such optical ME phenomena of (semi)metallic systems have remained elusive so far. Here, we show the large nonlinear optical ME response in noncentrosymmetric magnetic Weyl semimetal PrAlGe, in which the polar structural asymmetry and ferromagnetic ordering break P and T symmetry. We observe the giant second harmonic generation (SHG) arising from the P symmetry breaking in the paramagnetic phase, being comparable to the largest SHG response reported in Weyl semimetal TaAs. In the ferromagnetically ordered phase, it is found that interference between this nonmagnetic SHG and the magnetically induced SHG emerging due to both P and T symmetry breaking results in the magnetic field switching of SHG intensity. Furthermore, such an interference effect critically depends on the light-propagating direction. The corresponding magnetically induced nonlinear susceptibility is significantly larger than the prototypical ME material, manifesting the existence of the strong nonlinear dynamical ME coupling. The present findings establish the unique optical functionality of P- and T-symmetry broken ME topological semimetals.

Integrated Anionic Zirconium-Organic Cage and Cationic Boron-Imidazolate Cage for Synergetic Optical Limiting

Making oppositely charged metal-organic cages (MOCs) into a tightly ordered structure may bring interesting functions. Herein, we report a novel structure composed of anionic (Zr4 L6 )8- (L=embonate) tetrahedral cages and in situ-formed cationic [Zn4 (Bim)4 ]4+ (Bim=[BH(im)3 ]- ; im=imidazole) cubic cages. Chiral transfer is observed from enantiopure (Zr4 L6 )8- cage to enantiopure [Zn4 (Bim)4 ]4+ cage. A pair of enantiomers (PTC-373(Δ) and PTC-373(Λ)) are formed. PTC-373 exhibits high chemical and thermal stabilities, affording an interesting single-crystal-to-single-crystal transformation. More importantly, the combination of ionic pair cages significantly enhances its third-order nonlinear optical property, and its thin-film exhibits an excellent optical limiting effect.

Enhanced Green Light Emission from a Silicon-Based Metal-Encapsulated Nanoplasmonic Waveguide

Integrated silicon plasmonic circuitry is becoming integral for communications and data processing. One key challenge in implementing such optical networks is the realization of optical sources on silicon platforms, due to silicon's indirect bandgap. Here, we present a silicon-based metal-encapsulated nanoplasmonic waveguide geometry that can mitigate this issue and efficiently generate light via third-harmonic generation (THG). Our waveguides are ideal for such applications, having strong power confinement and field enhancement, and an effective use of the nonlinear core area. This unique device was fabricated, and experimental results show efficient THG conversion efficiencies of η = 4.9 × 10-4, within a core footprint of only 0.24 μm2. Notably, this is the highest absolute silicon-based THG conversion efficiency presented to date. Furthermore, the nonlinear emission is not constrained by phase matching. These waveguides are envisioned to have crucial applications in signal generation within integrated nanoplasmonic circuits.

From instable directional switching to controlled unidirectional operation in a nonlinear fiber ring laser

We investigate a new phenomenon, where a reciprocal fiber ring laser switches from bidirectional to unidirectional operation above a certain pump power threshold. Significant simplifications regarding earlier experiments are presented, which for the first time allow the identification of individual nonlinear effects. We highlight the unique role of stimulated Raman scattering in triggering unidirectional operation, and that additional conditions apply. The threshold is reduced from 30 to 3.8 W, which eases potential applications.

Voltage-controlled two-dimensional Fresnel diffraction pattern in quantum dot molecules

This study explores the influence of inter-dot tunneling effects within a quantum dot molecule on the Fresnel diffraction phenomenon. Our findings indicate that the Fresnel diffraction of the output probe Gaussian field can be manipulated by adjusting the inter-dot tunneling parameter's strength and the characteristics of the coupling field. The inter-dot tunneling effect establishes a closed-loop system, setting conditions for the interference of the applied fields. We specifically examine a Laguerre-Gaussian (LG) coupling field, investigating how its properties-such as strength, value, and sign of the orbital angular momentum (OAM)-impact the Fresnel diffraction of the output probe field. Increasing the inter-dot tunneling parameter and the coupling LG field's strength allows for control over the spatial distribution of the Fresnel diffraction pattern. Notably, the inter-dot tunneling parameter can disturb the symmetry of the diffraction patterns. Additionally, considering a negative OAM for the coupling LG field transforms the diffraction pattern into its inverse shape. This suggests that, in the presence of the inter-dot tunneling effect, the Fresnel diffraction pattern is contingent on the direction of rotation of the helical phase front of the coupling LG field. Our results offer insights into quantum control of Fresnel diffraction patterns and the identification of OAM in LG beams, presenting potential applications in quantum technologies.

Organic Polar Crystals, Second Harmonic Generation, and Piezoelectric Effects from Heteroadamantanes in the Space Group R3m

Polar crystalline materials, a subset of the non-centrosymmetric materials, are highly sought after. Their symmetry properties make them pyroelectric and also piezoelectric and capable of second-harmonic generation (SHG). For SHG and piezoelectric applications, metal oxides are commonly used. The advantages of oxides are durability and hardness - downsides are the need for high-temperature synthesis/processing and often the need to include toxic metals. Organic polar crystals, on the other hand, can avoid toxic metals and can be amenable to solution-state processing. While the vast majority of polar organic molecules crystallize in non-polar space groups, we found that both 7-chloro-1,3,5-triazaadamantane, for short Cl-TAA, and also the related Br-TAA (but not I-TAA) form polar crystals in the space group R3m, easily obtained from dichloromethane solution. Measurements confirm piezoelectric and SHG properties for Cl-TAA and Br-TAA. When the two species are crystallized together, solid solutions form, suggesting that properties of future materials can be tuned continuously.

Tracking electron motion within and outside of Floquet bands from attosecond pulse trains in time-resolved ARPES

Floquet engineering has recently emerged as a technique for controlling material properties with light. Floquet phases can be probed with time- and angle-resolved photoelectron spectroscopy (Tr-ARPES), providing direct access to the laser-dressed electronic bands. Applications of Tr-ARPES to date focused on observing the Floquet-Bloch bands themselves, and their build-up and dephasing on sub-laser-cycle timescales. However, momentum and energy resolved sub-laser-cycle dynamics between Floquet bands have not been analyzed. Given that Floquet theory strictly applies in time-periodic conditions, the notion of resolving sub-laser-cycle dynamics between Floquet states seems contradictory-it requires probe pulse durations below a laser cycle that inherently cannot discern the time-periodic nature of the light-matter system. Here we propose to employ attosecond pulse train probes with the same temporal periodicity as the Floquet-dressing pump pulse, allowing both attosecond sub-laser-cycle resolution and a proper projection of Tr-ARPES spectra on the Floquet-Bloch bands. We formulate and employ this approach inab-initiocalculations in light-driven graphene. Our calculations predict significant sub-laser-cycle dynamics occurring within the Floquet phase with the majority of electrons moving within and in-between Floquet bands, and a small portion residing and moving outside of them in what we denote as 'non-Floquet' bands. We establish that non-Floquet bands arise from the pump laser envelope that induces non-adiabatic electronic excitations during the pulse turn-on and turn-off. By performing calculations in systems with poly-chromatic pumps we also show that Floquet states are not formed on a sub-laser-cycle level. This work indicates that the Floquet-Bloch states are generally not a complete basis set for sub-laser-cycle dynamics in steady-state phases of matter.

Wandering principal optical axes in van der Waals triclinic materials

Nature is abundant in material platforms with anisotropic permittivities arising from symmetry reduction that feature a variety of extraordinary optical effects. Principal optical axes are essential characteristics for these effects that define light-matter interaction. Their orientation - an orthogonal Cartesian basis that diagonalizes the permittivity tensor, is often assumed stationary. Here, we show that the low-symmetry triclinic crystalline structure of van der Waals rhenium disulfide and rhenium diselenide is characterized by wandering principal optical axes in the space-wavelength domain with above π/2 degree of rotation for in-plane components. In turn, this leads to wavelength-switchable propagation directions of their waveguide modes. The physical origin of wandering principal optical axes is explained using a multi-exciton phenomenological model and ab initio calculations. We envision that the wandering principal optical axes of the investigated low-symmetry triclinic van der Waals crystals offer a platform for unexplored anisotropic phenomena and nanophotonic applications.

Spectral-temporal-spatial customization via modulating multimodal nonlinear pulse propagation

Multimode fibers (MMFs) are gaining renewed interest for nonlinear effects due to their high-dimensional spatiotemporal nonlinear dynamics and scalability for high power. High-brightness MMF sources with effective control of the nonlinear processes would offer possibilities in many areas from high-power fiber lasers, to bioimaging and chemical sensing, and to intriguing physics phenomena. Here we present a simple yet effective way of controlling nonlinear effects at high peak power levels. This is achieved by leveraging not only the spatial but also the temporal degrees of freedom during multimodal nonlinear pulse propagation in step-index MMFs, using a programmable fiber shaper that introduces time-dependent disorders. We achieve high tunability in MMF output fields, resulting in a broadband high-peak-power source. Its potential as a nonlinear imaging source is further demonstrated through widely tunable two-photon and three-photon microscopy. These demonstrations provide possibilities for technology advances in nonlinear optics, bioimaging, spectroscopy, optical computing, and material processing.

Scalable parallel ultrafast optical random bit generation based on a single chaotic microcomb

Random bit generators are critical for information security, cryptography, stochastic modeling, and simulations. Speed and scalability are key challenges faced by current physical random bit generation. Herein, we propose a massively parallel scheme for ultrafast random bit generation towards rates of order 100 terabit per second based on a single micro-ring resonator. A modulation-instability-driven chaotic comb in a micro-ring resonator enables the simultaneous generation of hundreds of independent and unbiased random bit streams. A proof-of-concept experiment demonstrates that using our method, random bit streams beyond 2 terabit per second can be successfully generated with only 7 comb lines. This bit rate can be easily enhanced by further increasing the number of comb lines used. Our approach provides a chip-scale solution to random bit generation for secure communication and high-performance computation, and offers superhigh speed and large scalability.

Light People: Prof. Eric Mazur speaks about ultrafast optics and education

EDITORIAL

Prof. Eric Mazur is a great influencer over and beyond the optics community. As a physicist, he is a pioneer of ultrafast optics and was one of the inventors of colliding-pulse mode-locked laser. As an educator, he not only gave talks to thousands, but also revolutionized teaching with his globally renowned methodology "Peer Instruction". As a leader and entrepreneur, he co-founded several companies and was President of Optica (formerly the Optical Society) and currently is the Chair of the Optica Foundation. Here, Light: Science & Applications talked with Prof. Eric Mazur about his opinions on research, education and industry. The full interview video can be found in the Supplementary File.

High harmonic generation in graphene quantum dots

We study theoretically the generation of high harmonics in disk graphene quantum dots placed in linearly polarized short pulse. The quantum dots (QD) are described within an effective model of the Dirac type and the length gauge was used to describe the interaction of quantum dots with an optical pulse. The generated radiation spectra of graphene quantum dots can be controlled by varying the quantum dot size, i.e. its radius. With increasing the quantum dot radius, the intensities of low harmonics mainly decrease, while the cutoff frequency increases. The sensitivity of the cutoff frequency to the QD size increases with the intensity of the pulse.

Third Harmonic Generation in Thin NbOI2 and TaOI2

The niobium oxide dihalides have recently been identified as a new class of van der Waals materials exhibiting exceptionally large second-order nonlinear optical responses and robust in-plane ferroelectricity. In contrast to second-order nonlinear processes, third-order optical nonlinearities can arise irrespective of whether a crystal lattice is centrosymmetric. Here, we report third harmonic generation (THG) in two-dimensional (2D) transition metal oxide iodides, namely NbOI2 and TaOI2. We observe a comparable THG intensity from both materials. By benchmarking against THG from monolayer WS2, we deduce that the third-order susceptibility is approximately on the same order. THG resonances are revealed at different excitation wavelengths, likely due to enhancement by excitonic states and band edge resonances. The THG intensity increases for material thicknesses up to 30 nm, owing to weak interlayer coupling. After this threshold, it shows saturation or a decrease, due to optical interference effects. Our results establish niobium and tantalum oxide iodides as promising 2D materials for third-order nonlinear optics, with intrinsic in-plane ferroelectricity and thickness-tunable nonlinear efficiency.

Third Harmonic Generation Enhancement and Wavefront Control Using a Local High-Q Metasurface

High quality factor optical nanostructures provide a great opportunity to enhance nonlinear optical processes such as third harmonic generation. However, the field enhancement in these high quality factor structures is typically accompanied by optical mode nonlocality. As a result, the enhancement of nonlinear processes comes at the cost of their local control as needed for nonlinear wavefront shaping, imaging, and holography. Here we show simultaneous strong enhancement and spatial control over third harmonic generation with a local high-Q metasurface relying on higher-order Mie resonant modes. Our results demonstrate third harmonic generation at an efficiency of up to 3.25 × 10-5, high quality wavefront shaping as illustrated by a third harmonic metalens, and a flatband, angle independent, third harmonic response up to ±11° incident angle. The demonstrated high level of local control and efficient frequency conversion offer promising prospects for realizing novel nonlinear optical devices.

RSPSSL: A novel high-fidelity Raman spectral preprocessing scheme to enhance biomedical applications and chemical resolution visualization

Raman spectroscopy has tremendous potential for material analysis with its molecular fingerprinting capability in many branches of science and technology. It is also an emerging omics technique for metabolic profiling to shape precision medicine. However, precisely attributing vibration peaks coupled with specific environmental, instrumental, and specimen noise is problematic. Intelligent Raman spectral preprocessing to remove statistical bias noise and sample-related errors should provide a powerful tool for valuable information extraction. Here, we propose a novel Raman spectral preprocessing scheme based on self-supervised learning (RSPSSL) with high capacity and spectral fidelity. It can preprocess arbitrary Raman spectra without further training at a speed of ~1 900 spectra per second without human interference. The experimental data preprocessing trial demonstrated its excellent capacity and signal fidelity with an 88% reduction in root mean square error and a 60% reduction in infinite norm ([Formula: see text]) compared to established techniques. With this advantage, it remarkably enhanced various biomedical applications with a 400% accuracy elevation (ΔAUC) in cancer diagnosis, an average 38% (few-shot) and 242% accuracy improvement in paraquat concentration prediction, and unsealed the chemical resolution of biomedical hyperspectral images, especially in the spectral fingerprint region. It precisely preprocessed various Raman spectra from different spectroscopy devices, laboratories, and diverse applications. This scheme will enable biomedical mechanism screening with the label-free volumetric molecular imaging tool on organism and disease metabolomics profiling with a scenario of high throughput, cross-device, various analyte complexity, and diverse applications.

Versatile photonic molecule switch in multimode microresonators

Harnessing optical supermode interaction to construct artificial photonic molecules has uncovered a series of fundamental optical phenomena analogous to atomic physics. Previously, the distinct energy levels and interactions in such two-level systems were provided by coupled microresonators. The reconfigurability is limited, as they often require delicate external field stimuli or mechanically altering the geometric factors. These highly specific approaches also limit potential applications. Here, we propose a versatile on-chip photonic molecule in a multimode microring, utilizing a flexible regulation methodology to dynamically control the existence and interaction strength of spatial modes. The transition between single/multi-mode states enables the "switched-off/on" functionality of the photonic molecule, supporting wider generalized applications scenarios. In particular, "switched-on" state shows flexible and multidimensional mode splitting control in aspects of both coupling strength and phase difference, equivalent to the a.c. and d.c. Stark effect. "Switched-off" state allows for perfect low-loss single-mode transition (Qi ~ 10 million) under an ultra-compact bend size (FSR ~ 115 GHz) in a foundry-based silicon microring. It breaks the stereotyped image of the FSR-Q factor trade-off, enabling ultra-wideband and high-resolution millimeter-wave photonic operations. Our demonstration provides a flexible and portable solution for the integrated photonic molecule system, extending its research scope from fundamental physics to real-world applications such as nonlinear optical signal processing and sixth-generation wireless communication.

Nonlinear topological symmetry protection in a dissipative system

We investigate experimentally and theoretically a system ruled by an intricate interplay between topology, nonlinearity, and spontaneous symmetry breaking. The experiment is based on a two-mode coherently-driven optical resonator where photons interact through the Kerr nonlinearity. In presence of a phase defect, the modal structure acquires a synthetic Möbius topology enabling the realization of spontaneous symmetry breaking in inherently bias-free conditions without fine tuning of parameters. Rigorous statistical tests confirm the robustness of the underlying symmetry protection, which manifests itself by a periodic alternation of the modes reminiscent of period-doubling. This dynamic also confers long term stability to various localized structures including domain walls, solitons, and breathers. Our findings are supported by an effective Hamiltonian model and have relevance to other systems of interacting bosons and to the Floquet engineering of quantum matter. They could also be beneficial to the implementation of coherent Ising machines.

Intrinsic dichroism in amorphous and crystalline solids with helical light

Amorphous solids do not exhibit long-range order due to the disordered arrangement of atoms. They lack translational and rotational symmetry on a macroscopic scale and are therefore isotropic. As a result, differential absorption of polarized light, called dichroism, is not known to exist in amorphous solids. Using helical light beams that carry orbital angular momentum as a probe, we demonstrate that dichroism is intrinsic to both amorphous and crystalline solids. We show that in the nonlinear regime, helical dichroism is responsive to the short-range order and its origin is explained in terms of interband multiphoton assisted tunneling. We also demonstrate that the helical dichroism signal is sensitive to chirality and its strength can be controlled and tuned using a superposition of OAM and Gaussian beams. Our research challenges the conventional knowledge that dichroism does not exist in amorphous solids and enables to manipulate the optical properties of solids.

Giant electric field-induced second harmonic generation in polar skyrmions

Electric field-induced second harmonic generation allows electrically controlling nonlinear light-matter interactions crucial for emerging integrated photonics applications. Despite its wide presence in materials, the figures-of-merit of electric field-induced second harmonic generation are yet to be elevated to enable novel device functionalities. Here, we show that the polar skyrmions, a topological phase spontaneously formed in PbTiO3/SrTiO3 ferroelectric superlattices, exhibit a high comprehensive electric field-induced second harmonic generation performance. The second-order nonlinear susceptibility and modulation depth, measured under non-resonant 800 nm excitation, reach ~54.2 pm V-1 and ~664% V-1, respectively, and high response bandwidth (higher than 10 MHz), wide operating temperature range (up to ~400 K) and good fatigue resistance (>1010 cycles) are also demonstrated. Through combined in-situ experiments and phase-field simulations, we establish the microscopic links between the exotic polarization configuration and field-induced transition paths of the skyrmions and their electric field-induced second harmonic generation response. Our study not only presents a highly competitive thin-film material ready for constructing on-chip devices, but opens up new avenues of utilizing topological polar structures in the fields of photonics and optoelectronics.