Electrically tunable nonlinear optical responses at the nanoscale remain challenging to achieve because conventional nonlinear materials lack the combination of large susceptibility, nanoscale confinement, and in situ reconfigurability. Here, we report electric-field-induced second harmonic (EFISH) generation from a nanoscale tunnel junction defined by conductive atomic force microscope lithography at the LaAlO3/SrTiO3 interface. A conducting channel written at the interface is interrupted by a nanoscale insulating gap, across which applied DC bias produces local electric fields exceeding 107 V/m. The SHG signal is spatially localized at the junction, exhibits a quadratic bias dependence described by I(2ω)∝|χ0(2)+χ(3)EDC|2 with no hysteresis, a modulation depth exceeding 380% at |VDC| = 1 V, and shows a two-lobed input-polarization pattern aligned with the junction axis, consistent with EFISH from a centrosymmetric host. Calibration against a BBO reference crystal gives |χ(3)| ≈ 1 × 10−19 m2/V2 at 6 K. These results establish cAFM-written oxide nanojunctions as a reconfigurable platform for nanoscale nonlinear optics in which the junction geometry sets the symmetry of the response and the large field-induced χ(2) of SrTiO3 provides the optical nonlinearity. Because the nonlinearity is both generated and read out within the same nanoscale gap, the junction operates simultaneously as a subwavelength source and a near-field detector of optical nonlinearity.
High-speed, high-precision non-contact distance measurement is essential for advanced manufacturing, precision engineering, micro- and nanoscale device characterization, and ultrafast dynamic metrology. Although optical frequency combs enable high-precision absolute distance measurement, extending such capability to gigahertz-frequency dynamics remains challenging. Here, we propose an electro-optic–microwave distance meter to address this challenge. An RF source referenced to a rubidium frequency standard simultaneously drives an electro-optic frequency comb to generate an optical pulse train and provides the microwave phase reference for distance measurement. The target distance is then obtained from optical–microwave phase detection. This architecture allows non-ambiguity range and phase-to-distance conversion sensitivity to be tuned by the RF frequency. In the experiment, a 10 GHz RF source defines an intrinsic optical sampling rate of 10 GSa/s and corresponds to an RF phase distance period of ∼1.5 cm. The raw single-sample standard deviation is 1.5 μm, and the Allan deviation reaches a minimum of 6 nm at an averaging time of 6.55 μs. A dynamic optical-path variation at 1 GHz, generated by electro-optic phase modulation, is further traced in real time.
Controllable infrared (IR) radiation modulation is essential for adaptive camouflage, radiative cooling, and optical information encryption. Multispectral information encryption requires the integration of spectrally distinct information channels while maintaining effective control over their optical responses. Here, we present a spectrally decoupled phase-change emitter based on Ge2Sb2Te5 (GST) that enables dual-channel visible–infrared (VIS–IR) information encryption. The emitter employs two types of IR-encoding pixels that maintain uniform VIS reflectance while allowing independent control over IR emissivity. By engineering the spatial distribution of pixel units, complex patterns and codes can be encoded in the IR emission without altering the VIS appearance. Before GST crystallization, the emissivity contrast between the two pixel types is insufficient for reliable IR image reconstruction. Thermal treatment induces the nonvolatile amorphous-to-crystalline phase transition of GST, changing its optical constants and consequently establishing a pronounced IR emissivity contrast between the different GST-thickness pixels. The encoded IR information can therefore be resolved after GST crystallization and remains readable after removal of the thermal stimulus, while remaining concealed in the VIS band. This phase-state-programmed dual-channel encoding strategy provides a pathway toward multispectral information security, high-end anti-counterfeiting, optical storage, and information authentication.
Spatially structured light fields governed by global topological invariants provide a robust platform for free-space optical communication. While conventional scalar Möbius topologies are vulnerable to phase and amplitude distortions, vectorial Möbius fields constructed via spatially resolved Stokes parameters overcome this limitation. Because atmospheric turbulence distorts spatial phases but leaves local polarization states largely unperturbed, encoding topological invariants into Stokes fields effectively cancels common-mode phase fluctuations. Advancing beyond conventional two-dimensional vectorial modes, mapping information onto the three-dimensional generalized Möbius topology of these macroscopic fields provides intrinsic structural protection against severe turbulence-induced intensity fragmentation. A free-space optical communication scheme utilizing the global topological twist number as an information carrier, therefore, circumvents the need for active wavefront compensation. By employing a polarimetric detection scheme based on filtered normalized Stokes parameters combined with digital matched filtering, this approach guarantees accurate target field identification across diverse propagation scenarios, including near-field, far-field, single-screen, and multiple-screen turbulence configurations. Evaluations demonstrate that even in strong turbulence regimes with a Rytov variance of up to 3.0, these topological signatures are robustly preserved despite severe intensity fragmentation. Consequently, this scheme achieves negligible error rates for an alphabet of four topological configurations and a transmission capacity approaching 2.8 bits per optical field utilizing a basis of ten orthogonal fields, establishing a robust physical layer framework for high-dimensional optical networks.
A plaintext-adaptive sliding-window (PASW) physical-layer encryption scheme is proposed for secure orthogonal frequency-division multiplexing (OFDM) transmission over a 60 km hollow-core fiber link. The proposed PASW mechanism enables plaintext-adaptive symbol-level permutation by dynamically varying window partitioning and permutation mapping according to symbol features. Within each window, a dual-layer permutation is performed through key-driven index mapping and quotient polynomial ring automorphism, with keys generated by a cascaded two-dimensional Hénon and iterative chaotic mapping with infinite collapses. Theoretical analysis further characterizes the influence of key PASW parameters on computational complexity, symbol diffusion capability, and window adaptation capability, providing guidance for practical parameter selection. Experimental verification is conducted in a 16-quadrature amplitude modulation (QAM) OFDM system over a 60 km hollow-core fiber link. The results show that the proposed scheme does not significantly affect the legitimate receiver performance, while the bit error rate at the unauthorized receiver remains close to 0.5 over the entire received optical power range. Security analyses show that the proposed key generation scheme exhibits high sensitivity and a large key space, while statistical evaluations verify the high randomness of the generated permutation sequences. Moreover, under 16-QAM modulation, the PASW mechanism achieves a 93.75% constellation-domain symbol change rate and a 47.33% Euclidean-distance-based unified average changing intensity, indicating effective plaintext diffusion and resistance to differential attacks.
