Laser Absorption Research Articles

Simulation of the temperature distribution of kidney stones induced by thulium fiber laser and Ho: YAG laser lithotripsy.

Simulation studies on temperature distribution in laser ablation help predict ablation rates, laser settings, and thermal damage. Despite the limited number of reported numerical studies on the temperature distribution of kidney fluid, there is no simulation study for kidney stone temperature distribution. We employ a numerical approach to study the kidney stone temperature distribution and predict ablation rates, which is an important parameter for clinical lithotripsy. The study looked at how the thulium fiber laser and the Ho:YAG laser differ in terms of temperature profile and ablation depth of kidney stones like calcium oxide monohydrate. The ablation depth increased from 152.7µm to 489.7µm when the TFL laser (operated at 10Hz repetition rate and 1ms pulse width) fluence increased from 764J/cm2 to 1146J/cm2. Correspondingly, the depth increased from 21µm to 68µm for the Ho: YAG laser operated at 3Hz and 0.22ms pulse width. We attribute this to an increase in temperature with laser energy. We further investigated the effect of pulse width on ablation depth by considering three different TFL pulse widths: 0.5ms, 0.75ms, and 1ms. There was a decrease in ablation depths from 402.5µm to 242.6µm when the pulse width increased from 0.5ms to 1ms. Because of lower water absorption coefficients, the Ho:YAG laser (70mJ/10Hz) produced a smaller ablation depth and temperature profile than the thulium fiber laser (70mJ/10Hz). Experimental results from the literature validated the simulation. We found that the Ho:YAG laser worked better for ablation when it was set to 0.2J/100Hz for the Ho:YAG laser and 0.4J/50Hz for the TFL laser, which were clinical laser settings that we found in the literature. This indicates that, in addition to laser absorption by water, the laser parameters also significantly influence temperature distribution and ablation.

Nonlinear LiNbO3 topological Tamm interface state photonic crystal with static visible and intelligent laser protection

The rapid advancement of high-energy laser and LiDAR technologies imposes heightened demands on intelligent laser protection windows. Traditional laser protection devices often face difficulty in simultaneously achieving high absorption of low-energy laser and high reflection of high-energy laser. Herein, we investigate the efficacy of one-dimensional nonlinear LiNbO3 topological Tamm interface state photonic crystal for static visible and intelligent laser protection applications. The photonic crystal's intelligent laser protection property arises from the amplification of nonlinear effects facilitated by the intense electric field localization surrounding the LiNbO3 interface. At 1064 nm laser energy levels below 18.29 mJ/cm2, the protection window exhibits up to 87.03% absorption. When the laser energy increased to 83.84 mJ/cm2, the reflectance reaches 88.07% attributed to the unsatisfied amplitude matching condition caused by the LiNbO3 nonlinear effect, thereby achieving successful intelligent laser protection. Furthermore, the fabricated sample demonstrates a maximum transmittance of 47.51% in the visible wavelength range. This research provides novel insights into the development of multifunctional advanced optical elements.

Novel spectroscopy method to reveal optimal culture conditions in Escherichia coli fermenter.

Fermentation engineering is critical for mass-producing chemicals, food additives, and medicines, where optimal culture conditions maximize microbial growth and metabolite production. Although monitoring bacterial growth during fermentation is critical, there is a lack of a non-invasive and sensitive method to directly monitor the bacterial metabolism. In this paper, a novel optical monitoring method is proposed based on tunable diode laser absorption spectroscopy. First, the detecting system consisting of a laser, detection, a homemade board, and an incubator is established and verified to be able to monitor the metabolite production of CO2 in Escherichia coli through a 25-h detection period. Second, the quantitative growth rate analysis method is specified by calculating the threshold time (TT) intervals between consecutive dilution gradients, and the threshold with the least sum of residuals is chosen as the optimal threshold. Finally, alongside varied pH and temperature settings in a simulated fermenter, we elucidated the influence of these factors on E. coli metabolism curves and calculated the growth rates via TT, identifying 38°C as the optimal temperature and 7.0 as the optimal pH. This study presents a novel approach to reveal optimal culture conditions during fermentation holding promises for online real-time monitoring in the future.

Flight testing of laser-optical techniques for future optical air data sensors

AbstractOptical air data sensors, which can serve as alternatives or supplements to traditional air data systems, have been a subject of ongoing research. These remote sensing optical sensors offer the advantage of measuring primary air data parameters in undisturbed airflow. As active sensors, they can self-diagnose, associating each measurement with an uncertainty and accounting for signal loss due to factors such as icing or light blocking, thus avoiding silent false measurements. In this paper, we discuss a recent flight test campaign of optical measurement techniques for potential development into air data sensors for future aircraft. We used laser Doppler anemometry, which relies on aerosol scattering of laser light, to observe the Doppler shift of scattered light from multiple directions. This enables the reconstruction of the full wind vector and the retrieval of true air speed, angle of attack, and angle of sideslip. A second instrument used tunable diode laser absorption spectroscopy, which measures an individual transition of molecular oxygen in the A band, in order to extract the static pressure from the observed collisional broadening. The techniques were implemented as airborne research instruments on the DLR’s Dassault Falcon 20 aircraft and tested in two campaigns in 2022 under different atmospheric conditions and dynamic maneuvers. We present results from these campaigns, focusing on general sensor performance, measurement rates, accuracy, and experimental challenges that need to be addressed for future applications.

Nanosecond resolved vibrational kinetics of CO2 in CO2/N2 mixtures measured in a pulsed discharge jet

Abstract This work studies the vibrational kinetics of CO2/N2 mixtures in a nanosecond pulsed near atmospheric pressure plasma jet. Quantum-cascade laser absorption spectroscopy is used to measure the populations of rovibrationally excited states of CO2 without any assumption on a particular vibrational distribution function and with a variable time resolution of as high as 8 ns. These studies are complemented by measurements using electric field induced second harmonic generation and electrical measurements to characterize the discharge. The nanosecond pulsing, combined with fast diagnostics, allows for a temporal separation of processes on different timescales, namely, electron impact excitation (e-V) during the discharge and vibrational-vibrational (V-V) and vibrational-translational (V-T) transfer in the afterglow. During the discharge, the Fermi resonant 10 0 01 state is excited at a higher rate than the energetically close 02 2 01 state of the bending mode, leading to different effective vibrational temperatures. In the afterglow, the two vibrational temperatures converge to a combined vibrational temperature T 12 within 200 ns . This equilibration process is observed here for the first time. In the afterglow, the populations of all vibrational states increase for several 10 μ s . This can be attributed to the V-V transfer from N2 to the asymmetric stretch mode of CO2 and from the latter to the other vibrational modes of CO2. An increase of the N2 admixture, which acts as an energy reservoir for the CO2, enhances the increase of the vibrational populations of CO2 in the afterglow and slows down the decay of the asymmetric mode in the late afterglow through continuous replenishment via V-V transfer. The combined data of all diagnostics allow for a detailed comparison with kinetic models.

Diode Laser Absorption Spectroscopy and DSMC Calculations for the Determination of Species-Specific Diffusion Coefficients of a CO2-N2O Gas Mixture in the Transition Gas Regime

Multicomponent gas mixture diffusion in a microscale confined flow in the transition gas regime at Knudsen numbers (Kn) above 0.1 has potential engineering applications in gas-phase microfluidics. Although the calculation of the diffusion coefficient accounts for the influence of the concentration of other species in a multicomponent gas mixture, the higher rate of gas-wall collision at 0.1 < Kn ≤ 10 introduces additional complications not predicted by conventional calculation methods. Thus, simultaneous measurement of diffusion coefficients for multiple gas species ensures accurate estimation of the diffusion coefficient of a particular species that includes the effect of interactions with other species and wall surface conditions in a multicomponent gas mixture at Kn > 0.1. However, most experimental methods for measuring the diffusion coefficient are not species-specific and therefore cannot directly differentiate between the species diffusing in a gas mixture. Thus, this paper demonstrates a new experiment methodology consisting of a two-bulb diffusion configuration accompanied by a tunable diode laser absorption spectroscopy detection technique for species-specific, in-situ, simultaneous measurement of the effective diffusion coefficient for a CO2-N2O gas mixture in the transition gas regime. The experimental results are compared against direct simulation Monte Carlo calculations and the Bosanquet approximation showing a deviation that has not been reported in the literature before.

Bulk-structured VTe2 as a novel low-cost saturable absorber for pulsed fiber lasers

In this work, we first investigate bulk-structured vanadium telluride (VTe2) saturable absorption properties and its application in the Q-Switched pulse generation. Bulk-structured VTe2 flakes have been obtained by the liquid phase exfoliation method. The prepared VTe2-SA has good saturable absorption characteristics whose saturable intensity is 0.24 MW/cm2, and the modulation depth is 2.73 % at 1.5 μm region. Furthermore, a Q-switched pulsed fiber laser based on VTe2-SA is constructed. The repetition rate of the Q-switched pulses can be increased from 16.3 kHz to 28.2 kHz, and the corresponding pulse duration decreases from 6.8 to 3.5 μs. The maximum Q-switched single pulse energy is 91.5 nJ. This work suggests that bulk-structured VTe2 can serve as a novel low-cost SA material and open up a new platform for expanding the applications of VTe2 for ultrafast photonics.

Development and demonstration of a two-color nitric oxide vibrational temperature diagnostic using spectrally-resolved ultraviolet laser absorption

Development of a new ultraviolet (UV) laser absorption diagnostic has enabled the probing of nitric oxide (NO) in the second excited vibrational state (v” = 2) for inferences of quantum-state-specific number density and vibrational temperature time-histories. Spectroscopic modeling informed the selection of the new 246.3222 nm wavelength, as this wavelength exhibits high sensitivity for thermometry in the 2000 – 8000 K temperature range. This 246.3222 nm absorption feature consists of contributions from the R12(24.5), R11(15.5), Q22(24.5), and Q21(15.5) transitions all originating in the v” = 2 state. Absorption cross-sections at this selected wavelength were measured in reflected shock experiments for sweeps of both wavelength and temperature. The wavelength sweep investigated cross-sections over a 246.3202 – 246.3246 nm range at 4590 K, and the temperature sweep measured cross-sections over a 2500 – 7500 K range at the peak of the absorption feature (246.3222 nm). Cross-section results agree with the Stanford NO gamma-band model to within ±5%, confirming the use of the model for subsequent thermometry studies. Thermometry was demonstrated in reflected shock experiments probing the vibrational relaxation and chemical reactions in 2% NO diluted in either argon (Ar) or nitrogen (N2). These experiments leverage previous UV laser absorption diagnostics that probe NO in the ground vibrational state (v” = 0) using the R11(26.5), R12(34.5), Q21(26.5) , and Q22(34.5) transitions near 224.8155 nm and the Q11(12.5) , R12(19.5) , P21(12.5) , and Q22(19.5) transitions near 226.1026 nm, which were studied in Ref. [1]. The combination of the new diagnostic wavelength with previously validated diagnostics yields low-uncertainty vibrational temperature time-histories that are in excellent agreement with previously inferred vibrational relaxation time results from Refs. [2] and [3]. Future work will apply this two-color nitric oxide vibrational temperature diagnostic to probe the vibrational temperature of NO formed in high-temperature, shock-heated air at conditions relevant to hypersonic and reentry vehicles.

Microstructures and mechanical properties of pure copper manufactured by high-strength laser powder bed fusion

Pure copper is widely used in motor windings, heat exchangers and aerospace engines because of its high electrical and thermal conductivity. High-strength laser powder bed fusion (HS-LPBF) not only allows rapid production of components with complex geometry and high spatial resolution, but also offers various advantages such as small focal spot diameter, fine powder, and small layer thickness, providing advantages for forming complex structural parts in fields such as engines and heat exchangers. A single factor single layer experiment was performed by varying the hatch spacing (H), and the range of hatch spacing was determined according to the overlap rate. The degree of influence of process parameters on the relative density of pure copper specimens was analyzed using an orthogonal experiment, and a comparative study of the phase composition, microstructure and mechanical properties of pure copper specimens was carried out by varying the laser power. The characteristics of pure copper formed by HS-LPBF were analyzed. In addition, the effect of heat treatment on the microstructure and mechanical properties of pure copper specimens was investigated, and the fracture morphology of the specimens was observed comparatively. The results show that the HS-LPBF technique can effectively increase the energy density and improve the specific surface area of the powder and the laser absorptivity due to its small focal spot diameter, fine powder and layer thickness, thus reducing the minimum energy required to melt pure copper powder. The optimum process parameters were obtained by orthogonal experiment with a relative density of 98.1 % of the specimen. The highest hardness, ultimate tensile strength and elongation were obtained at a laser power of 260 W with 84 HV, 320 MPa and 17.8 %, respectively. This ultimate tensile strength is 18 % higher than the highest ultimate tensile strength that has been reported so far. In addition, the average grain size of the optimal specimens was 3.6 µm. Mechanical properties such as hardness, tensile strength and elongation of pure copper parts can be significantly improved by precisely controlling the process parameters, in particular laser power and hatch spacing.

Differential absorption laser spectroscopy at 8 kHz using pre-compensated current modulation

Differential absorption laser spectroscopy at 8 kHz using pre-compensated current modulation

Measurements and kinetic modeling of O2 vibrational Kknetics in O2-Ar mixtures partially dissociated by a Ns pulse discharge

Abstract Vibrational kinetics of O2 is studied during the O atom recombination in an O2-Ar mixture, partially dissociated by a burst of ns discharge pulses in a heated plasma flow reactor. The time-resolved temperature in the discharge afterglow is determined by Rayleigh scattering. Time-resolved O atom number density is measured by ps Two-Photon absorption Laser Induced Fluorescence (TALIF), calibrated in xenon. Time-resolved vibrational level populations of molecular oxygen, O2(v=8-20), are measured by ps Laser Induced Fluorescence (LIF), with the absolute calibration by NO LIF. Time-resolved ozone number density is monitored by broadband UV absorption. The results are compared with the predictions of a state-specific kinetic model. The experimental data indicate a rapid initial decay of O2(v) populations generated by electron impact in the discharge, due to the vibration-translation (V-T) relaxation by O atoms. This is followed by a slower population reduction, on the time scale much longer compared to that for V-T relaxation or vibration-vibration (V-V) exchange. Both O atoms and the O2(v) populations decay on the same time scale, indicating that chemical reactions initiated by the O atom recombination result in the generation of vibrationally excited O2 molecules. These trends are reproduced by the kinetic model, which shows that the reaction of O atoms with ozone is the dominant pathway of O2(v) generation at the present conditions. The predicted relative O2(v) populations are close to the experimental results, but absolute number densities differ from the experimental data. This is likely due to uncertainties in the absolute calibration of LIF measurements and in the spectroscopic model used in the data reduction. The present work demonstrates the capability for the absolute, time-resolved measurements of vibrationally excited O2 in recombining gas flows, to quantify the energy partition in the recombination reactions.

Broad Instantaneous Bandwidth Microwave Spectrum Analyzer with a Microfabricated Atomic Vapor Cell

We report on broad instantaneous bandwidth microwave spectrum analysis with hot Rb87 atoms in a microfabricated vapor cell in a large magnetic field gradient. The sensor is a MEMS atomic vapor cell filled with isotopically pure Rb87 and N2 buffer gas to localize the motion of the atoms. The microwave signals of interest are coupled through a coplanar waveguide to the cell, inducing spin-flip transitions between optically pumped ground states of the atoms. A static magnetic field with large gradient maps the frequency spectrum of the input microwave signals to a position-dependent spin-flip pattern on absorption images of the cell recorded with a laser beam onto a camera. In our proof-of-principle experiment, we demonstrate a microwave spectrum analyzer that has ≈1 GHz instantaneous bandwidth centered around 13 GHz, 3 MHz frequency resolution, 2 kHz refresh rate, and a −23 dBm single-tone microwave power detection limit in 1 s measurement time. A theoretical model is constructed to simulate the image signals by considering the processes of optical pumping, microwave interaction, diffusion of Rb87 atoms, and laser absorption. We expect to reach more than 25 GHz instantaneous bandwidth in an optimized setup, limited by the applied magnetic field gradient. Our demonstration offers a practical alternative to conventional microwave spectrum analyzers based on electronic heterodyne detection. Published by the American Physical Society 2024

Tunable diode laser absorption spectroscopy monitoring of the water vapor condensation process in the high-speed flow

Abstract In this work, non-invasive high-precision quantitative measurements of the water vapor condensation process have been carried out using tunable diode laser absorption spectroscopy(TDLAS) at the sub-millisecond level. The high-speed condensation of water vapor is achieved by a self-designed rarefaction wave reflection system. Combined with different sizes of experimental cavities, the condensation process is realised at various time scales of sub-milliseconds (0.2-0.66 ms). The processes of temperature and water vapor content during the high-speed condensation are measured using the water vapor absorption spectra near 7168.437 cm-1 and 7185.597 cm^-1. The experimental results show that the hypervelocity expansion flow field generated by the experimental system demonstrates good uniformity and reaches a cooling rate of 10^5 K/s, which has the same order as that of the supersonic nozzle. The condensation process is similar on different sub-millisecond timescales and the normalised temperature change curves are approximately the same. Moreover, the higher the water vapor content, the shorter the condensation time.

Quantum Coherence Control at Temperatures up to 1400 K.

Coherent quantum control at high temperatures is important for expanding the quantum world and is useful for applying quantum technologies to realistic environments. Quantum control of spins in diamond has been demonstrated near 1000 K, with the spins polarized and read out at room temperature and controlled at elevated temperatures by rapid heating and cooling. Further increase of the working temperature is challenging due to fast spin relaxation in comparison with the heating and cooling rates. Here we significantly improve the heating and cooling rates by using reduced graphene oxide as the laser absorber and heat drain and hence realize coherent quantum operation at up to 1400 K, which is higher than the Curie temperatures of all known materials. This work facilitates the use of diamond sensors to study a wide range of magnetic effects in the high-temperature regime, such as thermoremanent magnetism and magnetic shape memory effects.

Interband cascade laser absorption sensor for sensitive measurement of hydrogen chloride in smoke-laden gases using wavelength modulation spectroscopy

A tunable interband cascade laser sensor, based on wavelength modulation absorption spectroscopy near 3.73 µm, was developed to measure hydrogen chloride gas concentration in smoke-laden environments associated with the overhaul stages of firefighting. Wavelength selection near 2678cm−1 targets the P(0,9) transition within the fundamental vibrational band of HCl, chosen for its absorption strength and isolation from CO2, H2O, and CH4, as well as proximity to absorption features of other toxicant gases of interest in firefighting applications. Both scanned-wavelength direct absorption with a Voigt lineshape-fitting routine and a wavelength modulation spectroscopy absorption method are employed to recover species concentration. The laser sensor is paired with a compact commercial off-the-shelf 1 m multipass optical gas cell modified to use polished Alloy 20 steel mirrors for increased corrosion resistance against humid and acidic gases, and it is tested by sampling effluent gases from pyrolyzing and burning solid samples of polyvinyl chloride under a radiant heating apparatus in a laboratory fume hood. The wavelength modulation spectroscopy method is demonstrated to enable measurement at the near-ppm-level within a compact form-factor and to provide insights into the thermochemical pyrolysis processes that lead to the formation of hydrogen chloride when polyvinyl chloride is exposed to radiant heating.

Comparison of a Quantum Cascade Laser and an Interband Cascade Laser for the Detection of Stable Carbon Dioxide Isotopes Using Tunable Laser Absorption Spectroscopy.

Quantum cascade lasers (QCLs) and interband cascade lasers (ICLs) are widely used as light sources in tunable laser absorption spectroscopy because they emit in the mid-infrared region where many strong and characteristic absorption bands are present. In this paper, we compare the performance of these lasers emitting at about 2310.1 cm-1 to determine an optimal light source for detecting isotopic ratios of carbon dioxide (CO2). Our results show that the QCL has a higher relative intensity noise of up to 15 dBc/Hz compared to the ICL over the entire measured frequency range. In addition, it has a higher frequency fluctuation. However, the maximum tuning range of the QCL is up to 5.2 cm-1 compared to up to 3.8 cm-1 for the ICL. Both lasers lose more than half of their tuning range when the tuning rate is increased to 10 kHz. When measuring the isotope ratio of CO2, an uncertainty in the value of ‰ was achieved with the ICL and of ‰ with the QCL, both at an integration time of 0.2 s. In summary, the QCL is more appropriate for applications that require a larger spectral tuning range, such as the measurement of a complex gas mixture, while the ICL has an excellent signal-to-noise ratio and is therefore better suited for applications that require higher precision.

Efficient Thermo-Optic Switching and Modulating in Nonlocal Metasurfaces.

High-Q optical resonances in nonlocal metasurfaces, benefiting from significantly enhanced light/matter interactions, feature strong responses even under a weak external stimulus. In this work, we leverage the high-Q resonances of quasi-guided modes (QGMs) supported by a photonic crystal slab (PCS) structure to achieve efficient optical switching/modulation. The QGMs with an experimentally measured Q-factor of ∼2200 are realized by shifting every second column of air holes in a rectangular lattice within a silicon slab. At a weak illumination intensity of less than 4.0 W/cm2 from a 532 nm continuous-wave pump laser, the QGM resonance around 1550 nm experiences a pronounced spectral shift, with modulation depth exceeding 55%. This is attributed to the thermo-optic response caused by photothermal heating of the metasurface triggered by the absorption of the pump laser in silicon, which is further verified by the electrical heating approach. Our reported results showcase a simple yet effective way of tailoring light propagation in nonlocal metasurfaces.

Tunable diode laser absorption spectroscopy for gas detection with a negative curvature anti-resonant hollow-core fiber

In this study, an all-fiber gas sensing technology was proposed, which is based on tunable diode laser absorption spectroscopy in the near infrared. A negative curvature anti-resonant hollow-core fiber was used as the gas chamber and optical channel, and an opto-gas coupling miniature tee was used in the configuration. Down to ppb (parts-per-billion) level noise-equivalent concentration was achieved with a fast-response capability of less than 6 s. The results demonstrated strong long-term stability, with a relative standard deviation of approximately 2.1% over a 12-h period. This approach demonstrates a simple, robust, fast response and compact sensor configuration that contributes to better management of greenhouse gas emissions and environmental pollution.

Simultaneous measurement of carbon monoxide and carbon dioxide for combustion diagnosis using 2 μm laser absorption spectroscopy

AbstractCarbon monoxide (CO) and carbon dioxide (CO2) are products of incomplete combustion and complete combustion, respectively. Time‐resolved knowledge of CO and CO2 exhaust concentrations will aid in improving combustion strategies, control, and efficiency. To achieve online high‐precision simultaneous measurement of CO and CO2, a tunable diode laser absorption spectroscopy‐based sensor is designed and constructed. Two diode lasers with central wavelengths of 2.3 and 2.0 μm are multiplexed in time‐division to cover the absorption spectral lines of CO at 4285.00 cm−1 and CO2 at 4989.96 cm−1, respectively. Wavelength modulation spectroscopy with second harmonic detection normalized by the first harmonic (WMS‐2f/1f) is employed. A software lock‐in amplifier and inversion algorithm are implemented for data processing and concentration acquisition. The entire measurement system is based on the idea of a virtual instrument and is carried out using a high‐speed data acquisition card and a computer. Experimental verification demonstrates that the measurement accuracies of CO and CO2 were 2.47% and 2.56%, respectively, with a time resolution of 20 ms. Dynamic measurement experiments with an alcohol blast burner validate the feasibility of measuring CO and CO2 in actual combustion environments. The developed sensor demonstrates the potential for real‐time and in situ carbon emission measurement for combustion diagnosis.

Ti3C2Tx/CuO heterojunction for ultrafast photonics

Nanomaterials with promising optical, mechanical and electrical properties have garnered significant interest in photonics and electronics. However, the integration of nanomaterials with diverse characteristics for potential ultrafast photonics applications has emerged as a focal point. In this study, two-dimensional MXene (Ti3C2Tx) and CuO nanoparticles were synthesized to create heterostructure materials. The surface morphology, chemical composition and nonlinear absorption properties of the heterostructure materials were investigated. First-principle-based theoretical calculations were performed to explore the electronic and optical properties of the Ti3C2Tx/CuO heterojunction, offering insights into its essential properties and supporting the potential optoelectronic applications. Importantly, the Ti3C2Tx/CuO heterojunction effectively functioned as saturable absorbers in ultrafast lasers. Incorporating the Ti3C2Tx/CuO-based saturable absorber into a net-anomalous dispersion fiber cavity generated stable conventional-soliton pulses with duration of 495 fs. Additionally, adjusting cavity dispersion to net-normal allowed the Ti3C2Tx/CuO-based saturable absorber to generate dissipative soliton with a pulse width of 22 ps. The performance of Ti3C2Tx/CuO-based fiber lasers demonstrates enhancements over previous works. This study confirms that the Ti3C2Tx/CuO heterojunction is a promising nonlinear optical material for ultrafast applications and advanced MXene-based photonic devices.