Engineering topological interface states in metal-wire waveguides for broadband terahertz signal processing.

Innovative terahertz waveguides are in high demand to serve as a versatile platform for transporting and manipulating terahertz signals for the full deployment of future six-generation (6G) communication systems. Metal-wire waveguides have emerged as promising candidates, offering the crucial advantage of sustaining low-loss and low-dispersion propagation of broadband terahertz pulses. Recent advances have opened up new avenues for implementing signal-processing functionalities within metal-wire waveguides by directly engraving grooves along the wire surfaces. However, the challenge remains to design novel groove structures to unlock unprecedented signal-processing functionalities. In this study, we report a plasmonic signal processor by engineering topological interface states within a terahertz two-wire waveguide. We construct the interface by connecting two multiscale groove structures with distinct topological invariants, i.e., featuring a π-shift difference in the Zak phases. The existence of this topological interface within the waveguide is experimentally validated by investigating the transmission spectrum, revealing a prominent transmission peak in the center of the topological bandgap. Remarkably, we show that this resonance is highly robust against structural disorders, and its quality factor can be flexibly controlled. This unique feature not only facilitates essential functions such as band filtering and isolating but also promises to serve as a linear differential equation solver. Our approach paves the way for the development of new-generation all-optical analog signal processors tailored for future terahertz networks, featuring remarkable structural simplicity, ultrafast processing speeds, as well as highly reliable performance.

Coupled Electron-Nuclear Dynamics Induced and Monitored with Femtosecond Soft X-ray Pulses in the Amino Acid Glycine.

The coupling of electronic and nuclear motion in polyatomic molecules is at the heart of attochemistry. The molecular properties, transient structures, and reaction mechanism of these many-body quantum objects are defined on the level of electrons and ions by molecular wave functions and their coherent superposition, respectively. In the present contribution, we monitor nonadiabatic quantum wave packet dynamics during molecular charge motion by reconstructing both the oscillatory charge density distribution and the characteristic time-dependent nuclear configuration coordinate from time-resolved Auger electron spectroscopic data recorded in previous studies on glycine molecules [Schwickert et al. Sci. Adv. 2022, 8, eabn6848]. The electronic and nuclear motion on the femtosecond time scale was induced and probed in kinematically complete soft X-ray experiments at the FLASH free-electron laser facility. The detailed analysis of amplitude, instantaneous phase, and instantaneous frequency of the propagating many-body wave packet during its lifecycle provides unprecedented insight into dynamical processes beyond the Born-Oppenheimer approximation. We are confident that the refined experimental data evaluation helps to develop new theoretical tools to describe time-dependent molecular wave functions in complicated but ubiquitous non-Born-Oppenheimer photochemical conditions.

Femtosecond laser assisted selective etching of microchannels in lithium niobate.

Here, we report on the fabrication of cm-long microchannels in LiNbO3 by selective etching of femtosecond laser inscribed tracks using hydrofluoric acid. We achieved a 1 cm long microchannel after 300 h of etching a single track inscribed into the volume along the optical axis of LiNbO3. Furthermore, we investigated the dependence of the etching behavior on various writing parameters. Highly selective etching with a selectivity up to 104 was achieved and a functional relationship between the etched depth and time was found. Thus, our results set the first milestone for future fabrication of 3D-hollow microstructures in the volume of LiNbO3 combining its outstanding physical properties such as the strong nonlinearity as well as the acousto- and electrooptic properties with both microfluidic and photonic structures in a monolithic setup.

Charge-induced chemical dynamics in glycine probed with time-resolved Auger electron spectroscopy.

In the present contribution, we use x-rays to monitor charge-induced chemical dynamics in the photoionized amino acid glycine with femtosecond time resolution. The outgoing photoelectron leaves behind the cation in a coherent superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay. Temporal modulation of the Auger electron signal correlated with specific ions is observed, which is governed by the initial electronic coherence and subsequent vibronic coupling to nuclear degrees of freedom. In the time-resolved x-ray absorption measurement, we monitor the time-frequency spectra of the resulting many-body quantum wave packets for a period of 175 fs along different reaction coordinates. Our experiment proves that by measuring specific fragments associated with the glycine dication as a function of the pump-probe delay, one can selectively probe electronic coherences at early times associated with a few distinguishable components of the broad electronic wave packet created initially by the pump pulse in the cation. The corresponding coherent superpositions formed by subsets of electronic eigenstates and evolving along parallel dynamical pathways show different phases and time periods in the range of ( - 0.3 ± 0.1 ) π ≤ ϕ ≤ ( 0.1 ± 0.2 ) π and 18.2 - 1.4 + 1.7 ≤ T ≤ 23.9 - 1.1 + 1.2 fs. Furthermore, for long delays, the data allow us to pinpoint the driving vibrational modes of chemical dynamics mediating charge-induced bond cleavage along different reaction coordinates.

Electronic quantum coherence in glycine molecules probed with ultrashort x-ray pulses in real time.

Here, we use x-rays to create and probe quantum coherence in the photoionized amino acid glycine. The outgoing photoelectron leaves behind the cation in a coherent superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay and by photoelectron emission from sequential double photoionization. Sinusoidal temporal modulation of the detected signal at early times (0 to 25 fs) is observed in both measurements. Advanced ab initio many-electron simulations allow us to explain the first 25 fs of the detected coherent quantum evolution in terms of the electronic coherence. In the kinematically complete x-ray absorption measurement, we monitor its dynamics for a period of 175 fs and observe an evolving modulation that may implicate the coupling of electronic to vibronic coherence at longer time scales. Our experiment provides a direct support for the existence of long-lived electronic coherence in photoionized biomolecules.

Versatile metal-wire waveguides for broadband terahertz signal processing and multiplexing.

Waveguides play a pivotal role in the full deployment of terahertz communication systems. Besides signal transporting, innovative terahertz waveguides are required to provide versatile signal-processing functionalities. Despite fundamental components, such as Bragg gratings, have been recently realized, they typically rely on complex hybridization, in turn making it extremely challenging to go beyond the most elementary functions. Here, we propose a universal approach, in which multiscale-structured Bragg gratings can be directly etched on metal-wires. Such an approach, in combination with diverse waveguide designs, allows for the realization of a unique platform with remarkable structural simplicity, yet featuring unprecedented signal-processing capabilities. As an example, we introduce a four-wire waveguide geometry, amenable to support the low-loss and low-dispersion propagation of polarization-division multiplexed terahertz signals. Furthermore, by engraving on the wires judiciously designed Bragg gratings based on multiscale structures, it is possible to independently manipulate two polarization-division multiplexed terahertz signals. This platform opens up new exciting perspectives for exploiting the polarization degree of freedom and ultimately boosting the capacity and spectral efficiency of future terahertz networks.

Multiplexing temperature-compensated open-cavity Fabry-Perot sensors at a fiber tip.

We investigate multiplexing of four highly sensitive Fabry-Perot (FP) microresonators at the tip of a single-mode optical fiber for refractive index (RI) measurements with simultaneous temperature compensation. The individual sensing elements for RI or temperature consist of either open-cavity FP resonators or solid fiber core regions fabricated by diamond-blade dicing of single-mode optical fibers, respectively. The reflectivity of the open resonators is further enhanced by matched dielectric coatings. At the same time, the solid core resonators formed by the fiber pieces between the open cavities are used as thermometers. This allows immediate compensation for temperature cross-sensitivity during RI measurements. The general performance of the sensor is demonstrated by measuring the RI of sucrose solutions, where we use phase tracking of the characteristic Fourier transform components of the backreflected optical spectrum for evaluation. The temperature sensitivity is on average 20±/∘C with an accuracy of 0.01°C, fully sufficient for biomedical applications. Meanwhile, the four RI sensing (open) cavities show high sensitivity of approximately 1160 nm/RIU. Due to the compact size of the sensor, small spatial inhomogeneities of RI can be accurately detected. If the cavities are additionally filled with molecularly imprinted polymers or coated with thin functional layers, they could also be used for the detection of trace substances in biomedical laboratory-on-a-fiber applications.

Watt-level 775†nm SHG with 70% conversion efficiency and 97% pump depletion in annealed/reverse proton exchanged diced PPLN ridge waveguides.

We report on fabrication of ridge waveguides formed in congruent periodically poled lithium niobate substrates using annealed and reverse proton exchange followed by diamond blade dicing. 1 W of second-harmonic generation at 775 nm has been obtained in a single-pass in 50 mm long ridge waveguides with internal conversion efficiency of 70%. At this power level, 97% pump depletion has been reached. Although elevated temperature operation and ridge geometry help to mitigate photorefractive damage (PRD) effects, nevertheless, at even higher second harmonic outputs significant power drop with blue shift and distortion of the SHG tuning curve have been observed indicating an onset of PRD.

Fiber-optic sensor measuring spatial distributions of refractive index and temperature.

In this work, we present a fiber sensor designed to measure simultaneously spatial inhomogeneities of the refractive index and temperature in liquid media, for example, induced by biochemical reactions. The sensor's constituent elements are Fabry-Perot microresonators fabricated in standard single-mode optical fibers by diamond blade dicing. To allow simultaneous measurements of different refractive indices, the sensor comprises two open cavities approximately 2 mm apart. With a small Si inlay inserted into one of the resonators used for temperature measurements, the sensor allows for immediate compensation of crosstalk between temperature- and composition-induced fluids' refractive index changes. The measurements were evaluated by phase tracking of the characteristic Fourier transform components of the sensor's backreflected spectra. The temperature sensitivity of the Si inlay is 0.063 rad/°C (79 pm/°C), and an accuracy of 0.01°C is obtained. Meanwhile, the two refractive index sensing (open) cavities show a sensitivity of 1168 and 1153 nm/RIU for temperature-compensated measurements. Finally, the sensor performance to measure spatial distributions is demonstrated by measuring the diffusion behavior of sucrose in water, which allows precise monitoring of hydration effects and breaking of bonds at elevated temperatures.

KLu(WO4)2/SiO2 Tapered Waveguide Platform for Sensing Applications.

This paper provides a generic way to fabricate a high-index contrast tapered waveguide platform based on dielectric crystal bonded on glass for sensing applications. As a specific example, KLu(WO4)2 crystal on a glass platform is made by means of a three-technique combination. The methodology used is on-chip bonding, taper cutting with an ultra-precise dicing saw machine and inductively coupled plasma-reactive ion etching (ICP-RIE) as a post-processing step. The high quality tapered waveguides obtained show low surface roughness (25 nm at the top of the taper region), exhibiting propagation losses estimated to be about 3 dB/cm at 3.5 m wavelength. A proof-of-concept with crystal-on-glass tapered waveguides was realized and used for chemical sensing.

Efficient Nd:Ti:LiNbO3 ridge waveguide lasers emitting around 1085 nm.

In this work, we report on efficient neodymium-doped titanium in-diffused ridge waveguide lasers in x-cut congruent LiNbO3 under excitation at 814 nm. For the sample fabrication we used our novel technique of three-side evaporation and in-diffusion for Nd and Ti incorporation into pre-defined ridges. Due to improved photorefractive damage resistance by indium tin oxide (ITO) coating we achieved stable laser operation at 1084.7 nm with a maximum output power of 108 mW and a slope efficiency of 34% exceeding the best literature values for Nd:Ti:LiNbO3 ridge waveguide lasers.

Dual parameter fiber-integrated sensor for refractive index and temperature measurement based on Fabry-Perot micro-resonators.

We report on a miniature all-fiber dual parameter sensor capable of simultaneous measurement of the refractive index (RI) and temperature of fluids and gases. The high-sensitivity sensing element is comprised of two Fabry-Perot (FP) micro-resonators fabricated in a single-mode fiber and has a total length of <100 µm. The RI sensing cavity is formed by diamond blade dicing, whereas a thinner silicon inlay glued into it serves as a temperature sensor. The sensor's performance was tested on sucrose solutions over a range of temperatures. For the evaluation of the backreflected FP spectra, phase tracking of the characteristic Fourier transform components was used. Good accuracy (0.01°C) and linearity of temperature measurement with Si inlay with sensitivity 0.097 rad/°C (85.2 pm/°C) were found, whereas the open cavity allowed for reliable temperature-compensated measurements of 10-3 RI steps with 290 rad/RIU (1130 nm/RIU) sensitivity.

Coherent propulsion with negative-mass fields in a photonic lattice.

In this Letter, we demonstrate the first, to the best of our knowledge, coherent propulsion with negative-mass fields in an optical analog. We observe a self-accelerating state, driven by a nonlinear coherent interaction of its two components that are experiencing diffractions of opposite signs in a photonic lattice, which is analogous to the interaction of two objects with opposite mass signs. Surprisingly, the coherent propulsion is highly immune to the initial phase of the two components, which is in sharp contrast with the behavior encountered in traditional coherent wave interactions. Compared to its incoherent counter-part, the coherent propulsion exhibits an enhanced acceleration.

Periodically poled ridge waveguides in KTP for second harmonic generation in the UV regime.

Waveguide circuits play a key role in modern integrated optics and provide an appealing approach to scalability in quantum optics. We report on periodically poled ridge waveguides in z-cut potassium titanyl phosphate (KTiOPO4 or KTP), a material that has recently received growing interest due to its unique dispersion properties. Ridges were defined in surface-near rubidium-exchanged KTP by use of a precise diamond-blade dicing saw. We fabricated single-mode ridge waveguides at around 800 nm which exhibit widths of 1.9-3.2 µm and facilitated type-II second harmonic generation from 792 nm to 396 nm with high efficiency of 6.6 %/W·cm2. Temperature dependence of the second harmonic process was found to be 53 pm/K. The low temperature dependence and high nonlinear conversion efficiency make our waveguides ideally suited for future operations in classical nonlinear integrated optics and integrated quantum networking applications.

Fiber-integrated refractive index sensor based on a diced Fabry-Perot micro-resonator.

We report on a fiber-integrated refractive index sensor based on a Fabry-Perot micro-resonator fabricated using simple diamond blade dicing of a single-mode step-index fiber. The performance of the device has been tested for the refractive index measurements of sucrose solutions as well as in air. The device shows a sensitivity of 1160 nm/RIU (refractive index unit) at a wavelength of 1.55 µm and a temperature cross-sensitivity of less than 10-7 RIU/°C. Based on evaluation of the broadband reflection spectra, refractive index steps of 10-5 of the solutions were accurately measured. The conducted coating of the resonator sidewalls with layers of a high-index material with real-time reflection spectrum monitoring could help to significantly improve the sensor performance.

Rb/Ba side-diffused ridge waveguides in KTP.

We report on characterization of ridge waveguides fabricated in KTP (KTiOPO4) by use of diamond-blade dicing and Rb/Ba ion exchange. The waveguides were prepared in substrates that have their z-axis in the surface plane, perpendicular to the waveguide direction. This hinders the RbBa ions from diffusion into the depth, as they are only mobile along the z-axis, and improves the waveguide's resistance against elevated temperature. Attenuation coefficients of 0.3 dB/cm (0.4 dB/cm) for TM (TE) polarization were measured at 1060 nm wavelength. Internal conversion efficiency of up to 3.3%/(W cm2) was determined for type-II SHG of 1064 nm.

Attosecond interferometry with self-amplified spontaneous emission of a free-electron laser.

Light-phase-sensitive techniques, such as coherent multidimensional spectroscopy, are well-established in a broad spectral range, already spanning from radio-frequencies in nuclear magnetic resonance spectroscopy to visible and ultraviolet wavelengths in nonlinear optics with table-top lasers. In these cases, the ability to tailor the phases of electromagnetic waves with high precision is essential. Here we achieve phase control of extreme-ultraviolet pulses from a free-electron laser (FEL) on the attosecond timescale in a Michelson-type all-reflective interferometric autocorrelator. By varying the relative phase of the generated pulse replicas with sub-cycle precision we observe the field interference, that is, the light-wave oscillation with a period of 129 as. The successful transfer of a powerful optical method towards short-wavelength FEL science and technology paves the way towards utilization of advanced nonlinear methodologies even at partially coherent soft X-ray FEL sources that rely on self-amplified spontaneous emission.

Second harmonic generation of diamond-blade diced KTiOPO4 ridge waveguides.

We report on the fabrication of ridge waveguides in KTiOPO4 nonlinear optical crystals through carbon ion irradiation followed by precise diamond blade dicing. The diced side-walls have low roughness, which allows for low propagation loss of ~1dB/cm in fabricated of ridges. The waveguide property investigation has been performed at 1064 nm as well as 532 nm, showing good guidance at both TE and TM polarizations. Based on type II phase matching configuration, efficient second harmonic generation of green light at room temperature has been realized. High conversion efficiencies of ~1.12%W-1 and ~12.4% have been obtained for frequency doubling under the pump of continuous-wave (CW) and pulsed fundamental waves at 1064 nm, respectively.

Low loss ridge waveguides in lithium niobate thin films by optical grade diamond blade dicing.

We report on the fabrication and characterization of ridge waveguides in lithium niobate thin films by diamond blade dicing. The lithium niobate thin films with a thickness of 1 µm were fabricated by bonding a He-implanted lithium niobate wafer to a SiO(2)-coated lithium niobate wafer and crystal ion slicing. Propagation losses of 1.2 dB/cm for TE and 2.8 dB/cm for TM polarization were measured at 1550 nm for a 9.28 mm long and 2.1 µm wide waveguide using the Fabry-Perot method.

Quasi-phase-matched frequency conversion in ridge waveguides fabricated by ion implantation and diamond dicing of MgO:LiNbO(3) crystals.

We report on the fabrication and second harmonic generation from a periodically-poled MgO-doped lithium niobate ridge waveguide within the telecommunication L-band. The ridge waveguide is fabricated by carbon ion implantation and the following diamond blade dicing method. A normalized second harmonic conversion efficiency of 20.3%W-1cm-2 was obtained with a total insertion loss of 4.3dB at wavelength of 1612.7nm. Our analysis shows that at least ~70% of the second-order nonlinearity was preserved after the implantation and thermal annealing process.