Nonlinear Optics: feature issue introduction.

This joint issue of Optics Express and Optical Materials Express showcases 29 articles that report the latest advancements in nonlinear optics. These articles include contributions from authors who participated in the Optica Nonlinear Optics Topical Meeting, which took place in Honolulu, Hawaii, from July 10th to July 14th, 2023. The conference was organized by Optica (formerly known as OSA). As an introduction, the editors provide a summary of these articles, which cover a broad range of topics in nonlinear optics, spanning from fundamental nonlinear optical concepts to novel nonlinear effects, and from innovative nonlinear materials to topics such as ultrafast optics, machine learning empowered nonlinear optics, and unconventional applications. This diverse array of contributions reflects the dynamic and interdisciplinary nature of contemporary research in the field of nonlinear optics while showcasing some of the most recent developments.

Design of integrated silicon waveguides for Raman-enhanced four-wave mixing in the telecom band.

Silicon planar waveguides are designed to maximize the wavelength conversion efficiency via the use of Raman-enhanced four-wave mixing in the telecom band. By investigating the dispersion properties of various rib waveguide structures, the optimum etch depth and width are selected to obtain efficient phase-matching for a continuous-wave pump at 1545 nm. The design benefits from good fabrication tolerance in the structural parameters, which are well within the precision of standard lithography and etching processes. Using the optimized waveguides, simulations show that it is possible to reach conversion efficiencies as high as ∼45 dB for waveguide lengths as short as 4.6 cm, with a pump power of only 130 mW. This enhancement in the conversion efficiency is about 50 dB higher than conventional values for FWM in integrated silicon photonic systems, highlighting the benefits of exploiting the coupling between the two nonlinear processes.

Raman amplification at 2.2 µm in silicon core fibers with prospects for extended mid-infrared source generation.

Raman scattering provides a convenient mechanism to generate or amplify light at wavelengths where gain is not otherwise available. When combined with recent advancements in high-power fiber lasers that operate at wavelengths ~2 µm, great opportunities exist for Raman systems that extend operation further into the mid-infrared regime for applications such as gas sensing, spectroscopy, and biomedical analyses. Here, a thulium-doped fiber laser is used to demonstrate Raman emission and amplification from a highly nonlinear silicon core fiber (SCF) platform at wavelengths beyond 2 µm. The SCF has been tapered to obtain a micrometer-sized core diameter (~1.6 µm) over a length of 6 cm, with losses as low as 0.2 dB cm-1. A maximum on-off peak gain of 30.4 dB was obtained using 10 W of peak pump power at 1.99 µm, with simulations indicating that the gain could be increased to up to ~50 dB by extending the SCF length. Simulations also show that by exploiting the large Raman gain and extended mid-infrared transparency of the SCF, cascaded Raman processes could yield tunable systems with practical output powers across the 2-5 µm range.

Design of the elusive proteinaceous oxygen donor copper site suggests a promising future for copper for MRI contrast agents.

We report the preparation and spectroscopic characterization of a highly elusive copper site bound exclusively to oxygen donor atoms within a protein scaffold. Despite copper generally being considered unsuitable for use in MRI contrast agents, which in the clinic are largely Gd(III) based, the designed copper coiled coil displays relaxivity values equal to, or superior than, those of the Gd(III) analog at clinical field strengths. The creation of this new-to-biology proteinaceous CuOx-binding site demonstrates the power of the de novo peptide design approach to access chemistry for abiological applications, such as for the development of MRI contrast agents.

Low-temperature polycrystalline silicon waveguides for low loss transmission in the near-to-mid-infrared region.

Low-temperature deposited polycrystalline silicon waveguides are emerging as a flexible platform that allows for dense optoelectronic integration. Here, the optical transmission properties of poly-silicon waveguides have been characterized from the near-to-mid-infrared wavelength regime, extending the optical transmission well beyond previous reports in the telecom band. The poly-Si waveguides with a dimension of 3 µm × âˆ¼0.6 µm have been produced from pre-patterned amorphous silicon waveguides that are post-processed through laser melting, reflowing, and crystallization using a highly localized laser induced heat treatment at a wavelength of 532 nm. Low optical transmission losses (<3 dB cm-1) have been observed at 1.55 µm as well as across the wavelength range of 2-2.25 µm, aided by the relatively large waveguide heights that are enabled by the deposition process. The results demonstrate the suitability of low-temperature poly-silicon waveguides to find wide ranging applications within integrated mid-infrared systems.

Correction: An in vitro method for inducing titan cells reveals novel features of yeast-to-titan switching in the human fungal pathogen Cryptococcus gattii.

[This corrects the article DOI: 10.1371/journal.ppat.1010321.].

An in vitro method for inducing titan cells reveals novel features of yeast-to-titan switching in the human fungal pathogen Cryptococcus gattii.

Cryptococcosis is a potentially lethal fungal infection of humans caused by organisms within the Cryptococcus neoformans/gattii species complex. Whilst C. neoformans is a relatively common pathogen of immunocompromised individuals, C. gattii is capable of acting as a primary pathogen of immunocompetent individuals. Within the host, both species undergo morphogenesis to form titan cells: exceptionally large cells that are critical for disease establishment. To date, the induction, defining attributes, and underlying mechanism of titanisation have been mainly characterized in C. neoformans. Here, we report the serendipitous discovery of a simple and robust protocol for in vitro induction of titan cells in C. gattii. Using this in vitro approach, we reveal a remarkably high capacity for titanisation within C. gattii, especially in strains associated with the Pacific Northwest Outbreak, and characterise strain-specific differences within the clade. In particular, this approach demonstrates for the first time that cell size changes, DNA amplification, and budding are not always synchronous during titanisation. Interestingly, however, exhibition of these cell cycle phenotypes was correlated with genes associated with cell cycle progression including CDC11, CLN1, BUB2, and MCM6. Finally, our findings reveal exogenous p-Aminobenzoic acid to be a key inducer of titanisation in this organism. Consequently, this approach offers significant opportunities for future exploration of the underlying mechanism of titanisation in this genus.

All-fibre heterogeneously-integrated frequency comb generation using silicon core fibre.

Originally developed for metrology, optical frequency combs are becoming increasingly pervasive in a wider range of research topics including optical communications, spectroscopy, and radio or microwave signal processing. However, application demands in these fields can be more challenging as they require compact sources with a high tolerance to temperature variations that are capable of delivering flat comb spectra, high power per tone, narrow linewidth and high optical signal-to-noise ratio. This work reports the generation of a flat, high power frequency comb in the telecom band using a 17 mm fully-integrated silicon core fibre as a parametric mixer. Our all-fibre, cavity-free source combines the material benefits of planar waveguide structures with the advantageous properties of fibre platforms to achieve a 30 nm bandwidth comb source containing 143 tones with <3 kHz linewidth, 12 dB flatness, and >30 dB OSNR over the entire spectral region.

Raman enhanced four-wave mixing in silicon core fibers.

A strong Raman enhancement to the four-wave mixing (FWM) conversion efficiency is obtained in a silicon core fiber (SCF) when pumped with a continuous-wave (CW) source in the telecom band. By tapering the SCFs to alter the core diameter and length, the role of phase-matching on the conversion enhancement is investigated, with a maximum Raman enhancement of ∼15 dB obtained for an SCF with a zero dispersion wavelength close to the pump. Simulations show that by optimizing the tapered waist diameter to overlap the FWM phase-matching with the peak Raman gain, it is possible to obtain large Raman enhanced FWM conversion efficiencies of up to ∼2 dB using modest CW pump powers over wavelengths covering the extended telecom bands.

Ge Ion Implanted Photonic Devices and Annealing for Emerging Applications.

Germanium (Ge) ion implantation into silicon waveguides will induce lattice defects in the silicon, which can eventually change the crystal silicon into amorphous silicon and increase the refractive index from 3.48 to 3.96. A subsequent annealing process, either by using an external laser or integrated thermal heaters can partially or completely remove those lattice defects and gradually change the amorphous silicon back into the crystalline form and, therefore, reduce the material's refractive index. Utilising this change in optical properties, we successfully demonstrated various erasable photonic devices. Those devices can be used to implement a flexible and commercially viable wafer-scale testing method for a silicon photonics fabrication line, which is a key technology to reduce the cost and increase the yield in production. In addition, Ge ion implantation and annealing are also demonstrated to enable post-fabrication trimming of ring resonators and Mach-Zehnder interferometers and to implement nonvolatile programmable photonic circuits.

Location-Dependent Lanthanide Selectivity Engineered into Structurally Characterized Designed Coiled Coils.

Herein we report unprecedented location-dependent, size-selective binding to designed lanthanide (Ln3+ ) sites within miniature protein coiled coil scaffolds. Not only do these engineered sites display unusual Ln3+ selectivity for moderately large Ln3+ ions (Nd to Tb), for the first time we demonstrate that selectivity can be location-dependent and can be programmed into the sequence. A 1 nm linear translation of the binding site towards the N-terminus can convert a selective site into a highly promiscuous one. An X-ray crystal structure, the first of a lanthanide binding site within a coiled coil to be reported, coupled with CD studies, reveal the existence of an optimal radius that likely stems from the structural constraints of the coiled coil scaffold. To the best of our knowledge this is the first report of location-dependent metal selectivity within a coiled coil scaffold, as well as the first report of location-dependent Ln3+ selectivity within a protein.

De novo designed coiled coils as scaffolds for lanthanides, including novel imaging agents with a twist.

For much of their history, lanthanides were thought to be biologically inert. However, the last decade has seen the discovery and development of the field of native lanthanide biochemistry. Lanthanides exhibit a variety of interesting photophysical properties from which many useful applications derive. The development of effective functional lanthanide complexes requires control of their coordination sphere; something proteins manage very effectively through their 3D metal-binding sites. α-Helical coiled coil peptides are miniature scaffolds which can be designed de novo and can retain the favourable properties of larger proteins within a much simplified system. Metal binding sites, including those which bind lanthanides can be engineered into the coiled coil sequence. This review will highlight the opportunities presented by the use of coiled coil peptides as scaffolds for lanthanide binding and the potential to control the coordination environment by simple modifications to peptide sequence. Designed lanthanide coiled coils offer opportunities to gain greater insight into native lanthanide biochemistry as well as to develop new functional complexes, including imaging agents.

Stability of Cell-Penetrating Peptide anti-VEGF Formulations for the Treatment of Age-Related Macular Degeneration.

AIM: The development of a polyarginine cell-penetrating peptide (CPP) could enable the treatment of age-related macular degeneration, with drugs like bevacizumab, to be administered using eye drops instead of intravitreal injections. Topical formulations have a vast potential impact on healthcare by increasing patient compliance while reducing the financial burden. However, as the ocular preparations may contain several doses, it is essential to understand the stability of the bevacizumab+CPP conjugate produced. MATERIALS AND METHODS: In this work, we examine the stability of a bevacizumab solution with and without cell-penetrating peptide using dynamic light scattering and circular dichroism to assess the physical stability. We use HPLC to assess the chemical stability and ELISA to assess its biological activity. We also examine the potential of the CPP to be used as an antimicrobial agent in place of preservatives in the eye drop. RESULTS: The structural stability of bevacizumab with and without the CPP was found not to be affected by temperature: samples stored at either 20°C or 4°C were identical in behavior. However, physical instability was observed after five weeks, leading to aggregation and precipitation. Further investigation revealed that the addition of the polypeptide led to increased aggregation, as revealed through dynamic light scattering and concentration analysis of the peptide through HPLC. Complexing the bevacizumab with CPP had no effect on biological stability or degradation. CONCLUSIONS: Our findings suggest that the shelf life of CPP+bevacizumab complexes is at least 38 days from its initial formulation. Currently, the mechanism for aggregation is not fully understood but does not appear to occur through chemical degradation.

Nonlinear properties of laser-processed polycrystalline silicon waveguides for integrated photonics.

We report nonlinear optical characterization of cm-long polycrystalline silicon (poly-Si) waveguides at telecom wavelengths. Laser post-processing of lithographically-patterned amorphous silicon deposited on silica-on-silicon substrates provides low-loss poly-Si waveguides with surface-tension-shaped boundaries. Achieving optical losses as low as 4 dB cm-1 enabled us to demonstrate effects of self-phase modulation (SPM) and two-photon absorption (TPA). Analysis of the spectral broadening and nonlinear losses with numerical modeling reveals the best fit values of the Kerr coefficient n2=4.5×10-18 m W-1 and TPA coefficient ßTPA=9.0×10-12 m2 W-1, which are within the range reported for crystalline silicon. On-chip low-loss poly-Si paves the way for flexible integration of nonlinear components in multi-layered photonic systems.

Silicon erasable waveguides and directional couplers by germanium ion implantation for configurable photonic circuits.

A novel technique for realization of configurable/one-time programmable (OTP) silicon photonic circuits is presented. Once the proposed photonic circuit is programmed, its signal routing is retained without the need for additional power consumption. This technology can potentially enable a multi-purpose design of photonic chips for a range of different applications and performance requirements, as it can be programmed for each specific application after chip fabrication. Therefore, the production costs per chip can be reduced because of the increase in production volume, and rapid prototyping of new photonic circuits is enabled. Essential building blocks for the configurable circuits in the form of erasable directional couplers (DCs) were designed and fabricated, using ion implanted waveguides. We demonstrate permanent switching of optical signals between the drop port and through the port of the DCs using a localized post-fabrication laser annealing process. Proof-of-principle demonstrators in the form of generic 1×4 and 2×2 programmable switching circuits were fabricated and subsequently programmed.

Tuning coordination chemistry through the second sphere in designed metallocoiled coils.

The metal hydration state within a designed coiled coil can be progressively tuned across the full integer range (3 → 0 aqua ligands), by careful choice of a second sphere terminal residue, including the lesser used Trp. Potential implications include a four-fold change in MRI relaxivity when applied to lanthanide coiled coils.

Laser-Driven Phase Segregation and Tailoring of Compositionally Graded Microstructures in Si-Ge Nanoscale Thin Films.

The ability to manipulate the composition of semiconductor alloys on demand and at nanometer-scale resolutions is a powerful tool that could be exploited to tune key properties such as the electronic band gap, mobility, and refractive index. However, existing methods to modify the composition involve altering the stoichiometry by temporal or spatial modulation of the process parameters during material growth, limiting the scalability and flexibility for device fabrication. Here, we report a laser processing method for localized tailoring of the composition in amorphous silicon-germanium (a-SiGe) nanoscale thin films on silicon substrates, postdeposition, by controlling phase segregation through the scan speed of the laser-induced molten zone. Laser-driven phase segregation at speeds adjustable from 0.1 to 100 mm s-1 allows access to previously unexplored solidification dynamics. The steady-state spatial distribution of the alloy constituents can be tuned directly by setting the laser scan speed constant to achieve indefinitely long Si1-xGex microstructures, exhibiting the full range of compositions (0 < x < 1). To illustrate the potential, we demonstrate a photodetection application by exploiting the laser-written polycrystalline SiGe microstripes, showing tunability of the optical absorption edge over a wavelength range of 200 nm. Our method can be applied to pseudobinary alloys of ternary semiconductors, metals, ceramics, and organic crystals, which have phase diagrams similar to those of SiGe alloys. This study opens a route for direct laser writing of novel devices made of alloy microstructures with tunable composition profiles, including graded-index waveguides and metasurfaces, multispectral photodetectors, full-spectrum solar cells, and lateral heterostructures.