Μm Wavelength Research Articles

Circular lattice benzene-core PCFs with flat near-zero dispersion for low-power broad-spectrum supercontinuum generation

A circular photonic crystal fiber infiltrated with benzene with different air-hole diameters is proposed as a new supercontinuum light source. Optical properties related to dispersion, effective mode area, nonlinear coefficient, and attenuation of the fundamental mode are investigated numerically. Two optimized structures are selected and verified against supercontinuum generation (SCG) in detail. The first structure (#F1) possesses all-normal dispersion, while the second (#F2) has a zero-dispersion wavelength. The possibility of coherent, octave-spanning SCG is proved by a 40 fs pulse, 1.064 μm wavelength, and 0.45 kW of power in-coupled into the core of #F1. Otherwise, injecting a 90 fs duration, 1.5 μm wavelength, and 0.555 kW peak power pump pulse into #F2 generates a broad SC spanning 0.76–4.23 μm. With the advantages of flat near-zero dispersion, high nonlinearity, low attenuation, and low input power used for SCG, the proposed fibers may lead to new low-cost all-fiber optical systems.

Power-modulated integrated path differential absorption lidar for probing benzene concentration.

Aimed at the regional open-path detection of benzene (C 6 H 6) in the atmosphere, a power-modulated integrated path differential absorption (PM-IPDA) lidar is introduced and demonstrated. Two tunable interband cascade lasers (ICLs) with about 3.2µm wavelength are utilized to generate the required PM optical signal. These two operation central wavelengths (CWs) of the PM-IPDA lidar are, respectively, 3236.6 and 3187.1nm, which can mitigate the influence of significant gases such as H 2 O, C H 4, and HCl on the detection performance. In this work, the fast Fourier transform algorithm is used to retrieve the measured values with the time resolution of 0.1s corresponding to 104 sampling bins at the sampling rate of 100kSps/s. The modulated frequency of the PM-IPDA lidar is selected as 10kHz by laboratory experiments. The slow fluctuation characteristic of the benzene absorption spectrum within the vicinity region of 3.2µm reduces the impact of small wavelength fluctuations on the performance of PM-IPDA lidar, although a scheme modulated only the driving current causes wavelength fluctuations of ∼±0.2n m. These laboratory experiments also indicate the PM-IPDA lidar can reduce the error resulting from 1/f noise. Open-path observation experiments show that the detection limit is about 0.60m g⋅m -3 and that the PM-IPDA lidar can be used for the regional open-path real-time detection of benzene.

1.3 μm InAs/GaAs quantum-dot lasers grown on planar on-axis Si (001) substrates with high slope-efficiency and low differential resistance

We report electrically pumped continuous-wave (CW) InAs/GaAs quantum dot lasers monolithically grown on planar on-axis Si (001) substrates. Combining an asymmetric waveguide epitaxy structure with aluminium-free upper cladding layers and a symmetrical cathode chip structure, 1.3 μm band lasers with low differential resistance and high slope-efficiency have been achieved. Moreover, the optimized symmetrical cathode structure of the laser chips is used to improve the slope-efficiency by reducing the differential resistance and waste heat. The Fabry–Perot broad-stripe edge-emitting lasers with 2000 μm cavity length and 15 μm stripe width achieve a single-facet output power of 73 mW, a single-facet slope efficiency of 0.165 W A−1, and a differential resistance of 1.31 Ω at ∼1.31 μm wavelength under CW conditions at room temperature (25 °C). Importantly, these results provide an effective strategy to achieve 1.3 μm wavelength band single-mode distributed feedback lasers directly on planar on-axis Si (001) substrates with high efficiency.

Simulation and design of dual-wavelength all-optical semiconductor optical amplifier with solution-processed quantum dots

All-optical semiconductor optical amplifiers (SOAs) present a cost-effective and highly efficient solution for long-distance WDM networks. In this study, the investigation of a dual-wavelength all-optical SOA incorporating two distinct sizes of solution-processed InAs/AlAs core/shell quantum dots (QDs), operating at 1.31 and 1.55 μm wavelengths, both simultaneously and individually, has been proposed. To comprehensively evaluate the optical performance of our proposed dual-wavelength all-optical SOA, the modified coupled rate equations governing carriers dynamics and the optical photon propagation equations have been solved numerically while taking into account both homogeneous and inhomogeneous broadening, as well as fluorescence or förster resonance energy transfer (FRET) effects. This computational approach provides valuable insights into the optical behavior of dual-wavelength all-optical SOA. In our proposed structure, cost-effective solution-based methods for implementing QDs of varying sizes offer flexibility to adjust QD size across a broad range, from as small as 0.1 nm to meet specific requirements. This adjustment is made with meticulous consideration of appropriate synthesis techniques.

Concave Version Printing of High-Performance Polymer THz Filters.

The development of high Q and tunable narrowband filters that can efficiently manipulate THz beams is critical for various THz applications, such as imaging and sensing. However, for filters made of metals and dielectrics, issues such as high losses, limited tunability, and lengthy process flows exist. Here, a scalable concave version reprinting technique to mass produce high-performance microstructured polymer filters is presented. The technique is extremely simple, eliminating the demand for the use of any large equipment including injection molding and thermal press printing machines, and is reliable; in the reprinted structures, there are no defects including gaps and air bubbles. The produced narrowband filters exhibit a high Q factor of 57 with wide tunability over the THz band from ∼80 to 160 μm in wavelength. The presented technique can be adopted to realize other devices as well using polymer materials with simplicity and high precision.

Optimization of dispersions in benzene-core square photonic crystal fibers for mid-infrared supercontinuum generation with very low peak power

An ultra-broadband, very low peak power supercontinuum source is proposed based on a novel benzene-filled structured photonic crystal fiber. The modification of the structural parameters has been highly effective in improving dispersion properties, an ultra-flat all-normal dispersion is achieved with fluctuations of ±1.174 ps/(nm.km) in the 0.458 µm wavelength region. Three fibers with suitable optical properties best meet the requirements of supercontinuum generations with low peak power based on numerical simulation results. With anomalous dispersion regime, the first fiber offers the supercontinuum spectra in the mid-infrared region spanning from 0.784 µm to 3.973 µm (2.539 µm bandwidth at 30 dB) using laser pulses excited at 1.55 µm has a peak power of 150 W. A pulse with a peak power of 280 W propagates in a 5 cm length of the second fiber results in a supercontinuum spectrum covering from 0.744 µm to 1.859 (bandwidth 1.063 µm at 30 dB) at a pump wavelength of 1.3 µm with all-normal dispersion regime. A smooth, highly coherent supercontinuum spectrum spanning from 0.843 µm to 2.517 µm (bandwidth at 30 dB of 1.608 µm) is achieved when the third fiber has all-normal ultra-flat dispersion profile pumped at 1.75 µm wavelength with laser pulse peak power of 350 W. With very low peak power, these proposed three-fiber based broadband supercontinuum sources could be a low-cost alternative to traditional glass-core fiber, which has great potential for mid-infrared spectroscopy.

Comprehensive optical and thermal investigation of optimal near-infrared absorption enhancement of nano-patterned aluminum

Compelling electromagnetic (EM) properties, such as tight EM-confinement and fabrication-compatible architectures, propel the importance of metal-based nanophotonics in small-footprint devices, such as highly efficient optical absorbers. Here, we focus on boosting the absorption of optically thick bulk Aluminum (Al), the most abundant metal, solely by corrugating its surface periodically. Application-oriented, we utilize the simplest one-dimensional (1D) rectangular-grating pattern that can be remotely engraved on a large scale, e.g., using femtosecond pulsed lasers. We devise four designs for various utilizations optimized for perfect absorption around the application-abundant 1.06μm wavelength under transverse electric (TE), transverse magnetic (TM), and unpolarized (denoted TEM) irradiations. Lacking attractive plasmonic confinement properties, the s-polarization is considered electromagnetically “degenerate” to the p-polarization case, making it rarely explored in the literature of 1D-grating architectures. Nevertheless, we reveal that perfect s-polarized absorption is also attainable in such designs without adding extra layers. The designs’ performance, properties, limitations, and the propelling electromagnetic physics are theoretically and numerically discussed in-depth. A large-area sample of the TEM-optimized design was fabricated, facilitating uniquely extensive optical and radiative heating experimental investigation with realistic near-to-normal Gaussian-beam incidence. The absorption enhancement derived from wideband and single-frequency optical and radiative heating measurements agree well, also with the numerical simulations. Despite fabrication-derived thickness deviation, the design experimentally showed a 9.6-(TE), 6-(TM), and 7.8-(TEM) fold absorption enhancement over bulk Al. Considering the one-dimensional-patterning simplicity and its large-area fabrication compatibility, it is noteworthy and pioneering, particularly for the challenging s-polarization case. We also study a polarization-sensitive diffused reflection scattering pattern, evidently postulated to result from resonated surface roughness, highlighting the importance of radiative heating measurements. The elaborated original findings and methods are highly applicable alongside the prevalent material and spectrum.

Post-process annealing of MgAl2O4 spinel optics for high energy laser windows

Magnesium aluminate (spinel) has been investigated as a low-absorption material for applications such as high-strength windows, armor, and high energy laser (HEL) apertures. To develop spinel for HEL applications, it is critical to lower the absorption of the lasing medium at the operating wavelength to reduce optical path distortion. In this work, post-processing of spinel via annealing is investigated to lower the absorption at 1.064 μm. Specifically, spinel optics are annealed in ambient air, oxygen, and nitrogen at temperatures of 1000–1400 °C and their transmission, absorption and photoluminescence properties are measured. This annealing process reduces the absorption by 73% from 463 ppm/cm to 126 ppm/cm and results in optics that have transmission within 1% of the Fresnel limit over the 1.0–2.0 μm wavelength range. The resulting optical characterizations imply a direct oxidation of Fe2+ to Fe3+ and the formation of VO1+ at distorted octahedral sites to mediate this reaction that results in lower optical absorption in the near-IR (NIR). Further, performance of equibiaxial flexure testing and corresponding Weibull analysis indicates that spinel manufactured using this method can outperform sapphire windows from a design strength perspective.

Passively Q-Switched 1.5- μm Erbium-Doped All-Fibre Laser with Silver Saturable Absorber

This research examines the operation of a passively Q-switched erbium-doped all-fibre laser (EDFL) using a silver (Ag) based saturable absorber (SA) by comparing the output characteristics of the Q-switched laser with those of continuous wave all-fibre lasers. A study was done on how the laser's Q-factor is modified and affects the rate at which pulses are repeated, as well as their output power, width, and energy. The results showed that high-energy Q-switched pulses were successfully generated at the essential 1.5 µm wavelength within the pump power range of 83.6 mW to 171.7 mW, with repetition rates ranging from 33.46 kHz to 77.96 kHz. The EDFL generated a pulse with a width of 4.033 µs and an energy of 2.82 nJ at a pump power of 171.7 mW. The results indicate that Ag may potentially be a good SA for pulse production in the 1.5 µm wavelength area as well as for use in a wide range of applications.

Highly efficient and widely tunable Si3N4 waveguide-based optical parametric oscillator.

We demonstrate an efficient and widely tunable synchronously pumped optical parametric oscillator (OPO) exploiting four-wave mixing (FWM) in a silicon nitride (Si3N4) waveguide with inverted tapers. At a pump pulse duration of 2 ps, the waveguide-based OPO (WOPO) exhibited a high external pump-to-idler conversion efficiency of up to -7.64 dB at 74% pump depletion and a generation of up to 387 pJ output idler pulse energy around 1.13 μm wavelength. Additionally, the parametric oscillation resulted in a 64 dB amplification of idler power spectral density in comparison to spontaneous FWM, allowing for a wide idler wavelength tunability of 191 nm around 1.15 μm. Our WOPO represents a significant improvement of conversion efficiency as well as output energy among χ3 WOPOs, rendering an important step towards a highly efficient and widely tunable chip-based light source for, e.g., coherent anti-Stokes Raman scattering.

Non-volatile double-tunable vortex metalens design based on Sb2S3 using deep neural network and particle swarm optimization algorithm

A dual-tunable vortex metalens is proposed that leverages deep neural network (DNN) and particle swarm optimization algorithm (PSO), incorporating the use of the phase-change material Sb2S3, which enables simultaneous tunable of both the focal length of the focused vortex beam and the topological charge. The control of traditional vortex metalens relies on changing the polarisation state of the incident light; however, its control range is limited. We employed neural network algorithms to learn the complex mapping relationship between the unit structure and the phase, achieving a fast and accurate prediction of the vortex metalens's phase. Compared with traditional simulation methods, the application of this neural network significantly reduces the time consumption of the design process and avoids the need for re-simulation. Furthermore, our design transcends limitations associated with specific wavelengths and enables the rapid design of tunable metalens across the 1–1.5 μm wavelength range, offering flexibility for the development of dual-tunable metalens. Based on this scheme, we simulated vortex metalens with tunable focal length and topological charge, and the results showed that the simulation results were consistent with the theoretical predictions.

Ultra-broadband and ultra-high conversion efficiency difference frequency generation in a dual-layer thin-film LiNbO3 waveguide with reversed ferroelectric domain orientations

Although there is a great breakthrough in second harmonic generation based on the birefringence and quasi phase matching (QPM) concepts, the development of an efficient difference frequency generation (DFG) based on modal phase matching (MPM) is still in its infancy at present. In this paper, we propose an ultra-broadband and ultra-high conversion efficiency mid-infrared DFG on the basis of MPM in a dual-layer waveguide consisting of two reversed poled LiNbO3 thin-films on sapphire. The ultra-high conversion efficiency is ascribed to significantly enhanced nonlinear interaction realized by making use of spatial profile of optical nonlinearity of LN crystal produced by two reversed ferroelectric domain orientations in the dual-layer waveguide. The proposed DFG device has been modeled on the basis of a 0.98 μm pump wave and a 1.34–1.44 μm signal wave, the frequency difference of which generates a mid-infrared idler wave centered at 3.32 μm wavelength. The results show that the device exhibits excellent DFG performance, including an ultra-high nonlinear conversion efficiency 434.6 % W−1 cm−2 and an ultra-broad tuning band 340 nm meanwhile, which far exceed the performance of conventional MPM devices, even the QPM ones.

Low-loss chalcogenide photonic devices with a secondary coating method.

In the traditional dry etching process for photonic device fabrication, the etching effect is influenced in many ways, usually resulting in relatively large sidewall roughness and high transmission loss. In this study, an effective method, namely the secondary coating method, is proposed to reduce the transmission loss of a Ge-Sb-Se chalcogenide waveguide and increase the quality factor (Q-factor) of a Ge-Sb-Se chalcogenide micro-ring resonator. The Ge-Sb-Se waveguide and micro-ring resonator are fabricated by ultraviolet exposure/electron beam lithography and inductively coupled plasma etching technology. Afterward, a 10 nm-thick Ge-Sb-Se thin film is deposited by thermal evaporation. The measurements show that after secondary coating, the sidewall roughness of the waveguide is reduced from 11.96 nm to 6.52 nm, with the transmission loss reduced from 2.63± 0.19 dB/cm to 1.86± 0.11 dB/cm at 1.55 µm wavelength. Keeping an equal coupling condition with equal radius and coupling distance, the Q-factor of the micro-ring resonator is improved by 47.5% after secondary coating. All results indicate that the secondary coating method is a feasible way to generate low-loss and high Q-factor integrated chalcogenide photonic devices.

1.55 µm wavelength band photonic crystal surface emitting laser with n-side photonic crystal and operation at up to 85 °C.

We describe the structure, fabrication, and measured performance of a 1543 nm wavelength photonic crystal surface emitting laser. An asymmetric double lattice design was used to achieve single mode lasing with side mode suppression ratios >40 dB. The photonic crystal was formed using encapsulated air holes in an n-doped InGaAsP layer with an InGaAlAs active layer then grown above it. In this way a laser with a low series resistance of 0.32 Ω capable of pulsed output powers of 171 mW at 25 °C and 40 mW at 85 °C was demonstrated.

High-performance multi-junction cascade 1.3 µm quantum dot vertical cavity surface-emitting laser.

A high-performance 5-junction cascade quantum dot (QD) vertical cavity surface-emitting laser (VCSEL) with 1.3 µm wavelength was designed. The characteristics of the QD as active regions and tunnel junctions are combined to effectively increase output power. The photoelectric characteristics of single-junction, 3-junction cascade, and 5-junction cascade QD VCSELs are compared at continuous-wave conditions. Results indicate that the threshold current gradually decreases, and the output power and slope efficiency exponential increase with the increase of the number of active regions. The peak power conversion efficiency of 58.4% is achieved for the 5-junction cascade individual QD VCSEL emitter with 10 µm oxide aperture. The maximum slope efficiency of the device is 6.27 W/A, which is approximately six times than that of the single-junction QD VCSEL. The output power of the 5-junction cascade QD VCSEL reaches 188.13 mW at injection current 30 mA. High-performance multi-junction cascade 1.3-µm QD VCSEL provides data and theoretical support for the preparation of epitaxial materials.

Evidence for ionization damage in mid-wave infrared nBn detectors

The effects of 63 MeV proton and 60Co gamma irradiation on the operation of 4.3 μm cutoff nBn photodetectors are demonstrated separately, and both are shown to yield a total ionizing dose (TID) effect. The effect is shown here in an InAsSbBi nBn detector and has been observed in other bulk alloy nBn detectors, and is unusual as it is notably absent in superlattice nBn detectors. The non-antireflection coated detectors exhibit a pre-radiation quantum efficiency of 17% at 3.3 μm wavelength and a dark current density of 50 μA/cm2, or roughly 300× the Rule 07 expectation, at their ideal operating voltage of −0.4 V bias at 150 K. Step-wise proton irradiation and in situ measurement indicate that the dark current increases to about 400× Rule 07 at the highest proton dose level of 150 krad(Si) (9.10 × 1011 p+/cm2), while the quantum efficiency is degraded at a relatively faster rate than the majority of analogous detectors characterized by our lab. Both the photocurrent and dark current are also shown to exhibit a turn-on voltage magnitude reduction of 100 mV following either gamma or proton irradiation, a trend which is attributable to negative trapped charge at the barrier interface (TID effect). This theory is further supported by an observed capacitance density magnitude reduction with dose and affirmed with Silvaco TCAD simulations. Following both proton exposure and subsequent anneal and gamma exposure and subsequent anneal; dark current, photocurrent, and CV all approach their pre-radiation baseline values.

Sb2S3 as a low-loss phase-change material for mid-IR photonics

We study the mid-IR optical properties of Sb2S3 – a phase-change material (PCM) with high potential for mid-IR integrated photonics. We show that Sb2S3 exhibits a high refractive index contrast Δn ≈ 1 between its two phases and an extinction coefficient k below 10−2 at mid-IR wavelengths (up to ∼10 µm). We study the impact of 100 nm Sb2S3 cladding on SiGe-on-Si waveguides and experimentally show that it brings additional propagation loss below 1 dB/cm in the 3.3–3.9 µm wavelength range in both phases. These findings pave the way for new uses of Sb2S3 in mid-IR integrated photonics.

Research on aerosol particles formed by laser interaction with various metals

Laser technologies are widely used in modern industry for cutting, welding, surface cleaning, drilling, and more, including in the nuclear industry for processing radioactively contaminated structures. These processes often lead to the formation of aerosol particles, including toxic nanoparticles, primarily oxides, which can penetrate cellular membranes and accumulate in the body. Understanding these particle formation processes is crucial for improving gas purification systems and personal protective equipment. This study utilized a ytterbium fiber pulsed-periodic laser (1.064 µm wavelength, 20 W power, 40 kHz pulse repetition rate, 200 ns pulse duration) to process various metals: aluminum (Al), bismuth (Bi), E110 alloy (Zr-based), carbon steel (St3), and stainless steel (08X18H10T). Aerosols were analyzed using a silicon monocrystalline substrate placed near the laser impact site, cleaned thoroughly to remove any contaminants. A 3D-printed model held the substrate at a 15-degree angle in the laser processing zone. Experiments were conducted in a closed box with local exhaust, positioning the substrate to capture the finest aerosol particles. Samples were analyzed using scanning electron microscopy (SEM) on a JEOL JSM-6480LV microscope. Results showed that aerosols formed consist of submicron agglomerates of particles smaller than 100 nm, with uneven deposition indicating air-borne agglomeration. Repeated scanning formed branched structures of interconnected nanoparticle agglomerates. In conclusion, laser processing with a 1.064 µm ytterbium fiber laser produces submicron aerosols of nanoparticles, presumably oxides. The deposition pattern suggests air-borne agglomeration, independent of the target material. These findings underscore the need for enhanced air purification and protective equipment to mitigate nanoparticle accumulation risks in respiratory organs and on skin during laser equipment operation.

Laser-driven quasi-static B-fields for magnetized high-energy-density experiments

We present measurements of magnetic fields generated in laser-driven coil targets irradiated by laser pulses of nanosecond duration, 1.053 μm wavelength, 500 J energy, and ∼1015 W/cm2 intensity, at the LULI2000 facility. Using two perpendicular probing axes, proton deflectometry is used to characterize the coil current and static charge at different times. Results reveal various deflection features that can be unambiguously linked to a looping quasi-steady current of well-understood polarity or to a static charging of the coil surface. Measured currents are broadly consistent with predictions from a laser-driven diode-current source and lumped circuit model, supporting the quasi-steady assessment of the discharges. Peak magnetic fields of ∼50 T at the center of 500-μm-diameter coils, obtained at the moderate laser intensity, open up the use of such laser-driven coil targets at facilities worldwide to study numerous phenomena in magnetized high-energy-density plasmas, and its potential applications.

Color measurements of the polarized light scattered by the dust in protoplanetary disks

Context. Many reflected light images of protoplanetary disks have been obtained with ground-based high-contrast instruments. Quantitative measurements of the reflected radiation provide strong constraints on the scattering dust which can clarify the dust particle evolution in these disks and the composition of the forming planets. Aims. We derived the wavelength dependence of the polarized reflectivity (Qˆφ/I*)λ or color for a sample of disks to contrain the dust based on these data. Further we searched for systematic differences in the dust properties between subgroups of disks. Methods. We selected from the ESO archive polarized imaging for 11 protoplanetary disks obtained at visible and near-infrared wavelengths with the SPHERE/ZIMPOL and SPHERE/IRDIS high contrast instruments. All disks have bright and well-resolved structures, such as rings or spirals, which allow accurate determinations of the intrinsic polarized reflectivity Qˆφ/I* at multiple wavelengths. For this, we corrected the observations for the smearing effects caused by the point spread functions (PSFs) of the individual dataset with a novel correction procedure applicable to inclined disks. For the 11 disks, we derived a total of 31 Qˆφ/I* values for wavelengths from 0.62 μm (R band) to 2.2 μm (Ks band) and compared our results, if possible, with previous determinations. For each disk, we derived a polarized reflectivity color ηV/IR between a visible band λ < 1 μm and a near-IR band λ > 1 μm and other wavelength combinations. We also consider model calculations for the polarized reflectivity colors η for protoplanetary disks to constrain the scattering properties of the dust. Results. The polarized reflectivities for the investigated disks structures are at a typical level between Qφ/I★ ~ 0.1% to 1.0%. These values depend strongly on the observing conditions and a correction for the PSF smearing effects is essential to obtain the intrinsic values Qˆφ/I*. Corrected values Qˆφ/I* are on average about a factor of 1.6 higher than the observed values. We checked the accuracy of the PSF calibrations procedure with simulations and literature data, and show that the large systematic errors in the observational values are reduced to a relative level ΔQˆφ/Qˆφ ≈ 10% or even less. The high accuracy yields wavelength gradients for the polarized reflectivity (Qˆφ/I*)φ or colors η which are significantly different between different objects. We find in our sample for all disks around Herbig stars (HD 169142, HD 135344B, HD 100453, MWC 758, and HD 142527) a red color ηv/IR > 0.5, while four out of six disks around T Tauri stars (PDS 70, TW Hya, RX J1615, and PDS 66) are gray −0.5 < ηV/IR < 0.5. The red colors support the presence of rather compact dust grains, while the absence of blue colors (except for the near-infrared color of PDS 66) is not compatible with very porous aggregates composed of small monomers. We suspect, that the very red colors ηV/IR ≈ 2 obtained for LkCa 15 and MWC 758 could be the result of an "extra" reddening of the radiation illuminating the disk caused by absorbing hot dust near the star. We discuss the prospects of further improvements for the derivation of dust properties in these disks if the fractional polarization 〈pψ〉 or other parameters of the reflected light are also taken into account in future studies.