Peak Wavelength Research Articles

Passively mode-locked fiber lasers with broadband FeOOH saturable absorber

In this contribution, we synthesized iron oxide hydroxide (FeOOH) nanomaterials and successfully demonstrated the application of FeOOH-based saturable absorbers in ultrafast photonics at 1 μm and 1.5 μm. Various characterizations were conducted to analyze the morphological and structural features of FeOOH nanomaterials. Subsequently, a tapered fiber coated with FeOOH was integrated into the resonator as a saturable absorber, exhibiting the significant saturable absorption effect with modulation depths of 1.26 % at 1 μm and 2.61 % at 1.5 μm, respectively. In an Yb-doped fiber laser, stable noise-like mode-locking pulses were achieved with a pulse duration of 519 fs at the operating wavelength of 1037.2 nm with a full-width at half-maximum (FWHM) spectral bandwidth of 4.7 nm, yielding a maximum single pulse energy of 1.23 nJ with a high signal-to-noise ratio (SNR) of 47.8 dB. Meanwhile, the conventional soliton pulses were produced in the passively mode-locked Er-doped fiber laser, featuring a pulse duration of 1.02 ps, and an SNR of 42.5 dB at a peak wavelength of 1559.25 nm with a FWHM bandwidth of 2.8 nm. This work demonstrates the considerable nonlinear optical properties of FeOOH nanomaterials in the near-infrared range, proposing a promising saturable absorber for ultrafast photonics applications.

Mid-infrared imaging spectroscopic measurements of C2H4 frost simulating the outer solar system environments

In the dense and cold atmosphere of Titan, the presence of C2H4 haze has been confirmed by the observations of spacecraft. In the present study, original cryogenic experimental equipment was developed to simulate the low-temperature solid formation of C2H4 in combination with in-situ infrared spectroscopic measurements. As a result, out-of-plane bending vibration ν7 of solid-phase C2H4 located at ~ 10.5 μm was successfully detected with high sensitivity, and two-dimensional spectrographs of C2H4 at low temperatures were obtained. The obtained spectra of C2H4 can be fitted to the double Lorentzian function with various heights, central wavelengths, and full widths at half the maximum (FWHM) of the two-component Lorentzian functions. They were classified into three types using the fitting parameters. However, their spectral shapes are different from the amorphous, metastable, and crystalline forms obtained by the previous laboratory experiment in terms of the distance of two peak wavelengths and FWHM. The results may link to understanding the spectral band properties of C2H4 condensation in the haze component of Titan.Graphical

Real-Time Tracking of Carbon Dioxide Concentration Using an Optical Microsphere Resonator Sensor.

Whispering gallery mode resonator sensors are nondisruptive optical sensors that can detect and monitor perturbations in a gaseous environment. Through its resonant properties of peak wavelength, amplitude, and quality factor (Q factor), changes in concentration can be quantified within seconds and monitored over days with great stability. In addition, the small footprint, low cost, and high sensitivity are ideal properties for a disposable sensor that can be utilized in extreme environments. The large Q factor of the resonant cavity enables long interaction lengths and amplifies the effect of small changes in the background refractive index, which is detectable in picometer shifts of the resonance wavelength. However, this measurement is susceptible to changes in other environmental factors such as temperature, pressure, and humidity, which manifest on the picometer wavelength scale, reinforcing the need to decouple the variables. In this work, we compare the spectral response of different diameter resonators to carbon dioxide, nitrogen, and its mixtures, observing the spectral shifting and broadening of the cavity resonance near 1550 nm. In addition, the effect of environmental temperature on spectral shifting due to the thermo-optic effect is characterized and quantified. Lastly, the gas concentrations are changed in real time to showcase the tracking and recovery capabilities of the resonator sensor.

Ultrahigh Polarization of Emitted Light by Quenched Emissive Material

AbstractThe emission of highly polarized light by organic light‐emitting diodes (OLEDs) is crucial for various applications; however, achieving such emission requires the use of polarizers, which reduce the device efficiency and durability. In this study, ultrahigh‐polarized light with a polarization ratio (PR) of 407:1 is achieved via the rapid thermal quenching (RTQ) of OLEDs. Poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) films, molybdenum disulfide (MoS2) nanosheets, and tungsten disulfide (WS2) nanosheets are applied as hole transport layers and surface‐alignment layers through surface treatment, including rubbing and/or ion‐beam exposure to align the molecules of the emissive materials, namely, {poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT). Quenching is performed at 240 °C, at which F8BT has a nematic liquid crystal (NLC) phase with high molecular ordering. Upon quenching, the high molecular ordering of the NLC‐phased emissive layer instantaneously froze via RTQ, leading to the emission of highly polarized light. Consequently, the PEDOT:PSS‐, MoS2‐, and WS2‐based OLEDs exhibit ultrahigh PRs of 407:1, 349:1, and 328:1, respectively, for photoluminescence at a 540 nm peak wavelength. In contrast, for electroluminescence, the PEDOT:PSS‐based OLED exhibits a high PR of 395:1. This result is the best reported to date and is comparable to the PRs generated by polarizers, indicating that quenching can be utilized for the development of ultrahigh‐polarized light‐emitting devices.

Tunable Enhanced Second-Harmonic Generation in InP-InAsP Quantum Well Nanomembranes.

Second-harmonic generation (SHG) offers a convenient approach for infrared-to-visible light conversion in tunable nanoscale light sources and optical communication. Semiconductor nanostructures offer rich possibilities to tailor their nonlinear optical properties. In this study, strong second-harmonic generation in InP nanomembranes with InAsP quantum well (QW) is demonstrated. Compared with bulk InP, up to 100 times enhancement of SHG is achieved in the short-wave infrared range. This enhancement is shown to be predominantly induced by the resonance-enhanced absorption and quantum confinement of fundamental wavelengths in the InAsP QW. The thin nanomembrane structure will also provide nanocavity enhancement for second-harmonic wavelengths. The enhanced SHG peak wavelengths can also be tuned by changing the QW composition. These findings provide an effective strategy for enhancing and manipulating the second-harmonic generation in semiconductor quantum-confined nanostructures for on-chip all-optical applications.

Low-Leakage Current Core-Shell AlGaN Nanorod LED Device Operating in the Ultraviolet-B Band.

Despite the considerable potential of AlGaN-based ultraviolet-B light-emitting diodes (UV-B LEDs) in various applications such as phototherapy, UV curing, plant growth, and analytical technology, their development is still ongoing due to low luminescence efficiency. In this study, we introduced a novel epitaxial growth mechanism to effectively control the height and thickness of AlGaN multiple wells (MWs) on AlGaN nanorod structures using horizontal reactor-based metal-organic chemical vapor deposition (MOCVD). By adjusting the H2 carrier gas flow rate, we could control the growth boundary layer's thickness, successfully separating the AlGaN well and p-AlGaN layer from the substrate. Cathodoluminescence (CL) measurements confirmed the stability of the core-shell AlGaN quantum wells as a highly stable nonpolarized structure, with the wavelength peak remaining almost unchanged under various injection currents. Furthermore, transmission electron microscopy (TEM) provided clear evidence of differentiation, highlighting the distinct formation of the 275 nm AlGaN core and the 295 nm AlGaN shell structure. The developed AlGaN MW structure, characterized by these rectification features, not only demonstrated a significantly improved electroluminescence (EL) peak intensity but also exhibited a much lower leakage current compared to the conventional core-shell AlGaN structure. The newly proposed growth mechanism and advanced nonpolarized core-shell AlGaN structure are expected to serve as excellent alternatives for substantially enhancing the efficiency of the next generation of high-efficiency UV LEDs.

Development of broadband ultraviolet pulsed laser using Ce:LiCAF crystal to determine SO\(_2\) gas concentration by differential absorption spectroscopy

Earth’s atmosphere is a mixture of many gases which include pollutants such as SO2, NO2, NO, and O3 that have absorption in the ultraviolet (UV) wavelength region. Therefore, the development of broadband UV laser sources that are useful in a differential absorption spectroscopy (DOAS) system for environmental research is necessary. In this research, we present the DOAS system using the Ce:LiCAF laser for determining SO2 gas concentration in the atmosphere. The Ce:LiCAF laser has a full width at half maximum of 2 nm with a wavelength range from 286 to 291 nm and a peak wavelength of 288.5 nm. The results show that the DOAS system accurately determines the gas concentration with a measurement error of 6 %. This result can serve as the basis for developing practical DOAS systems with the ability to monitor a wide range of gasses and survey many other types of pollutants.

Red vision in animals is broadly associated with lighting environment but not types of visual task.

Red sensitivity is the exception rather than the norm in most animal groups. Among species with red sensitivity, there is substantial variation in the peak wavelength sensitivity (λmax) of the long wavelength sensitive (LWS) photoreceptor. It is unclear whether this variation can be explained by visual tuning to the light environment or to visual tasks such as signalling or foraging. Here, we examine long wavelength sensitivity across a broad range of taxa showing diversity in LWS photoreceptor λmax: insects, crustaceans, arachnids, amphibians, reptiles, fish, sharks and rays. We collated a list of 161 species with physiological evidence for a photoreceptor sensitive to red wavelengths (i.e. λmax ≥ 550 nm) and for each species documented abiotic and biotic factors that may be associated with peak sensitivity of the LWS photoreceptor. We found evidence supporting visual tuning to the light environment: terrestrial species had longer λmax than aquatic species, and of these, species from turbid shallow waters had longer λmax than those from clear or deep waters. Of the terrestrial species, diurnal species had longer λmax than nocturnal species, but we did not detect any differences across terrestrial habitats (closed, intermediate or open). We found no association with proxies for visual tasks such as having red morphological features or utilising flowers or coral reefs. These results support the emerging consensus that, in general, visual systems are broadly adapted to the lighting environment and diverse visual tasks. Links between visual systems and specific visual tasks are commonly reported, but these likely vary among species and do not lead to general patterns across species.

Behavioral responses of cave-roosting bats to artificial light of different spectra and intensities: Implications for lighting management strategy

Artificial light at night has become an emerging environmental pollutant, posing a serious threat to biodiversity. Cave-roosting animals are vulnerable to light pollution due to long-term adaptation to nocturnal niches, and the problem is especially severe in the context of cave tourism and limestone mining. Mitigating the adverse impacts of artificial light on cave-dwelling animals presents a challenge. This study aimed to assess the relative contributions of spectral parameters and light intensity to the emergence behavior of nine cave-roosting bat species: Rhinolophus macrotis, Rhinolophus pearsonii, Rhinolophus rex, Rhinolophus pusillus, Rhinolophus siamensis, Rhinolophus sinicus, Hipposideros armiger, Myotis davidii, and Miniopterus fuliginosus. We manipulated light spectra and intensities through light-emitting diode (LED) lighting and gel filters at the entrance of bat roost. We monitored nightly passes per species to quantify bat emergence under the dark control and ten lighting conditions (blue, green, yellow, red, and white light at high and low intensities) using ultrasonic recording. Our analyses showed that the number of bat passes tended to be reduced in the presence of white, green, and yellow light, independent of light intensity. In contrast, the number of bat passes showed no pronounced differences under the dark control, blue light, and red light. The number of bat passes was primarily affected by LED light's blue component, red component, peak wavelength, and half-width instead of light intensity. These results demonstrate that spectral parameters of LED light can significantly affect emergence behavior of cave-dwelling bats. Our findings highlight the importance of manipulating light colors to reduce the negative impacts of light pollution on cave-roosting bats as a function of their spectral sensitivity. We recommend the use of gel filters to manage existing artificial lighting systems at the entrance of bat-inhabited caves.

Femtosecond laser ablation (fs-LA) XPS – A novel XPS depth profiling technique for thin films, coatings and multi-layered structures

Sputter depth profiling has been employed for XPS/AESdepth profilingsincethe late 1960s. However, for many materials, ion beam induced damage distorts the chemical state information and chemical composition, limiting the value of the analysis. A novelmethodology is presented in which XPS depth profiles are generated using a 160 fs pulse length, 1030 nm peak wavelength femtosecond laser in place of the traditional ion gun. Femtosecond laser ablation (fs-LA) XPS depth profiles are compared with argon monatomic and cluster ion beam depth profilesfor different classes of materials, including ceramics, semiconductors, polymers and metals. In all cases, the XPS spectra recorded following femtosecond laser ablation are fully representative of the original chemical composition and chemical state, with no ablation induced damage. The technique is also shown to be very versatile with ablation rates of <20 nm per pulse being achieved for thin films but profiles to depths of >30 μm are also realisable in practical time scales.fs-LA XPS depth profiling promises to be a very exciting new methodology for the XPS community, complementing sputter depth profiling but avoiding the chemical damage induced by ion beams and offering valuable new depth profiling capabilities.

Size‐Dependent Characteristics of InGaN‐Based Blue and Green Micro‐Light‐Emitting Diodes

This study focuses on investigating the size‐dependent characteristics (chip diameter: 100–10 μm) of InGaN‐based blue and green micro‐light‐emitting diodes (μLEDs) with respect to their electrical and optical properties. The LED's epistructure is grown using the metal–organic chemical vapor deposition technique, preceded by simulation and optimization using SiLENSe LED simulator. The cross‐sectional transmission electron microscopy study confirms the epistructure. The Photoluminescence mapping of the epistructure reveals that the blue emission is at 461 nm (full width at half maximum [FWHM] ≈ 15.5 nm) and the green emission is at 510 nm (FWHM ≈ 17.5 nm). The study analyzes the size‐dependent external quantum efficiency (EQE), optical power, and peak wavelength shift between blue and green μLEDs. The electroluminescence (EL) study reveals that the redshift of the EL peak wavelength of μLEDs is attributed to the release of strain at reduced chip sizes. The size‐dependent EQE study shows a reduction of EQE with shrunken chip size, attributed to the increase of nonradiative Shockley–Read–Hall recombination. Finally, the study estimates the characteristics temperature of the μLEDs using temperature‐dependent EL measurement, revealing that blue μLEDs exhibit a significantly large value of characteristic temperature ≈1926 K.

The lower correlated color temperature with higher illuminance nocturnal light environment improves cognitive performance and sleep quality

Well-designed nocturnal light environment could boost performance on the cognitive tasks and promote sleep quality after light exposure. We optimized and fabricated a four-channel mixed white light with peak wavelengths of 429, 523, 591, and 621 nm. Comparing with common white light emitting diode (LED) (5798 K, 212.7 lx), the mixed white light has lower correlated color temperature (CCT) (2799 K), higher illuminance (356.2 lx), similar melanopic illuminance, and better color fidelity. We conducted experiments on 14 healthy young subjects (7 males and 7 females; age 18–25 years) to investigate the effects of nocturnal light environments on the cognitive performances and sleep quality. In consistent with the α-opic flux model, the mixed white light with higher illuminance shrinks the pupil size and has less melanopic flux, which results in more melatonin for subjects. More melatonin before sleep benefits the sleep quality by decreasing arousal times and improving sleep continuity. The higher illuminance light environment also promotes the inhibition ability and working memory. Moreover, it presents better color discrimination and less visual fatigue. Therefore, the lower CCT with higher illuminance nocturnal light environment effectively promotes both cognitive performances in the evening and the following sleep quality.

In-vitro Approaches to Investigate the Detrimental Effect of Light on Dopaminergic Neurons

Our recent study revealed that fluorescent lamp light can penetrate deep into the brain of mice and rats leading to the development of typical histological characteristics associated with Parkinson’s disease such as the loss of dopamine neurons in the substantia nigra. Monochromatic LED lights were thus used in this work to deepen our knowledge on the effects of the major wavelength peaks of fluorescent light on mouse and human dopaminergic cells. In particular, we exposed immortalized dopaminergic MN9D neuronal cells, primary cultures of mouse mesencephalic dopaminergic cells and human dopaminergic neurons differentiated from induced pluripotent stem cells (hiPSC) to different LED light wavelengths. We found that chronic exposure to LED light reduced overall undifferentiated MN9D cell number, with the most significant effects observed at wavelengths of 485 nm and 610 nm. Moreover, LED light especially at 610 nm was able to negatively impact on the survival of mouse mesencephalic dopaminergic cells and of human dopaminergic neurons derived from hiPSC. Notably, differentiated MN9D dopaminergic cells, which closely resemble mature dopamine neuronal phenotype, acutely exposed for 3 h at 610 nm, showed a clear increase in ROS production and cytotoxicity compared to controls undifferentiated MN9D cells. These increases were even more pronounced by the co-treatment with the oxidative agent H2O2. Collectively, these findings suggest that specific wavelengths, particularly those capable of penetrating deep into the brain, could potentially pose an environmental hazard in relation to Parkinson's disease.

Broadband miniaturized spectrometers with a van der Waals tunnel diode

Miniaturized spectrometers are of immense interest for various on-chip and implantable photonic and optoelectronic applications. State-of-the-art conventional spectrometer designs rely heavily on bulky dispersive components (such as gratings, photodetector arrays, and interferometric optics) to capture different input spectral components that increase their integration complexity. Here, we report a high-performance broadband spectrometer based on a simple and compact van der Waals heterostructure diode, leveraging a careful selection of active van der Waals materials- molybdenum disulfide and black phosphorus, their electrically tunable photoresponse, and advanced computational algorithms for spectral reconstruction. We achieve remarkably high peak wavelength accuracy of ~2 nanometers, and broad operation bandwidth spanning from ~500 to 1600 nanometers in a device with a ~ 30×20 μm2 footprint. This diode-based spectrometer scheme with broadband operation offers an attractive pathway for various applications, such as sensing, surveillance and spectral imaging.

In-vivo studies on PDMS-embedded fiber Bragg grating based smart laparoscopic grasper

Minimally invasive surgeries are often associated with the risk of tissue damage due to improper force feedback mechanisms to notify surgeons regarding the force thresholds. This work evaluates the grasping forces during tissue handling to offer force feedback and assist surgeons during tissue manipulation. A polydimethylsiloxane embedded fiber Bragg grating sensor was developed and integrated at the tip of the laparoscopic grasper to measure the grasping force. Simulation studies have been performed to investigate the performance of the embedded grating sensor. The sensitivity of the sensor embedded within polydimethylsiloxane was found to be around 31.4 pm N−1 which was better compared to that of a silicone embedded counterpart. In-vivo studies were performed with soft tissue phantoms prepared with silicone rubber interspersed with different concentrations of mineral oil. Experimental results reveal a decrease in the peak wavelength shift as the tissue gets softer which is consistent with the nature of grasping force for softer tissues.

Thermally Stable NIR Emission from Disordered Ba5La3MgAl3O15:Cr3+ with Peak Wavelength Beyond 900 nm

AbstractBroadband near‐infrared (NIR) emissions in the long wavelength region with simultaneous high efficiency and thermal stability are desirable for NIR phosphor‐converted light‐emitting diodes (NIR pc‐LEDs) light sources. Herein, a novel broadband NIR‐emitting phosphor is reported, Ba5La3MgAl3O15:Cr3+ (BLMAO:Cr3+), which exhibits a NIR emission band spanning 700–1300 nm with maximum peak wavelength (λmax) at 925 nm and full width at half maximum of 160 nm. The optimized phosphor achieves an internal quantum efficiency of 66%, and the integrated emission intensity at 423 K remains 61% of that at room temperature, which surpasses most Cr3+‐activated phosphors emitting in the long‐wavelength NIR region (λmax &gt; 900 nm). The NIR luminescence properties are elucidated on the basis of density functional calculations of the multiple Cr3+ local structures as generated by the Ba/La occupation disorder in the host material. The applications of the NIR phosphor in pc‐LED devices are demonstrated, showing potential in night‐vision and nondestructive examination. The results indicate that BLMAO:Cr3+ is a promising phosphor with highly efficient and thermally stable broadband NIR emission for NIR LED light sources.

Towards a Miniaturized Photoacoustic Sensor for Transcutaneous CO2 Monitoring.

A photoacoustic sensor system (PAS) intended for carbon dioxide (CO2) blood gas detection is presented. The development focuses on a photoacoustic (PA) sensor based on the so-called two-chamber principle, i.e., comprising a measuring cell and a detection chamber. The aim is the reliable continuous monitoring of transcutaneous CO2 values, which is very important, for example, in intensive care unit patient monitoring. An infrared light-emitting diode (LED) with an emission peak wavelength at 4.3 µm was used as a light source. A micro-electro-mechanical system (MEMS) microphone and the target gas CO2 are inside a hermetically sealed detection chamber for selective target gas detection. Based on conducted simulations and measurement results in a laboratory setup, a miniaturized PA CO2 sensor with an absorption path length of 2.0 mm and a diameter of 3.0 mm was developed for the investigation of cross-sensitivities, detection limit, and signal stability and was compared to a commercial infrared CO2 sensor with a similar measurement range. The achieved detection limit of the presented PA CO2 sensor during laboratory tests is 1 vol. % CO2. Compared to the commercial sensor, our PA sensor showed less influences of humidity and oxygen on the detected signal and a faster response and recovery time. Finally, the developed sensor system was fixed to the skin of a test person, and an arterialization time of 181 min could be determined.

Mid-wavelength infrared focal plane array based on type II InAs/GaSb superlattices on InP substrate

We report the growth and characterization of band gap engineered mid-wavelength infrared photodiode, pBiBn, based on type-II InAs/GaSb superlattices on InP substrate. InP substrates are attractive material because they have high transmittance in the mid-wavelength infrared region and near thermal expansion coefficient to Si read-out IC (ROIC). Misfit dislocations due to lattice mismatch between GaSb and InP were successfully suppressed by thick GaSb buffer layer and two step growth of it. The photoluminescence (PL) spectrum from the absorption layers exhibits a peak wavelength of 5.01 μm at 77 K. The distinct PL peak was observed even at 120 K, indicating the excellent crystalline quality of T2SLs grown on an InP substrate. The dark current densities of 292 × 10−6 A/cm2 at 77 K and cut off wavelength of 5.15 μm at 80 K were measured, respectively. A clear image was obtained by fabricated focal plane array of 640 × 512 pixels with 15 μm pixel pitch, and NEDT and defective pixel rate was 50 mK and less than 1 % at 100 K, respectively.

Growth, characterization, and efficient laser operation of Czochralski- and micro-pulling-down-grown Yb3+:YScO3 mixed sesquioxides

We report on the growth, spectroscopy and laser operation of Yb3+-doped mixed sesquioxide crystals. Various Yb3+-doped crystals with compositions close to (Y0.5Sc0.5)2O3 have been successfully grown by the Czochralski method and by the micro-pulling down (µ-PD) method. Our spectroscopic investigations reveal broadened stimulated emission and absorption cross-section spectra originating from the structural disorder of the mixed crystals. We find the peak wavelengths to shift by ∼1.6 nm between Y:Sc-ratios of 54:46 and 46:54 and confirm a linear relation of the peak position with the lattice constant of the host composition. In the laser experiments, we obtain highly efficient continuous-wave laser operation under pumping with an optically-pumped semiconductor laser (OPSL) at ∼975 nm, reaching slope efficiencies of up to 89% at optical-to-optical efficiencies exceeding 80% at laser wavelengths between 1037 nm and 1086 nm.

Dual ligands synergy enables thermal and moisture stability-enhanced blue quasi-2D perovskite for efficient light-emitting diodes

Quasi-2D metal halide perovskite is rising for efficient blue light-emitting diodes due to strong quantum confinement and tunable emission, while a challenge remains in its poor stability of n-phase distribution under external stresses. Herein, the stable blue quasi-2D perovskite under heat and moisture conditions is achieved by dual ligand engineering. The Lewis base moieties − SO3− from taurine (3S) bind tightly with perovskite edges, inhibiting Ostwald ripening of the perovskite phase caused by the heat, and 3, 3-diphenylpropylamine (DPPA) provides a strong hydrophobicity to the perovskite film, suppressing phase disproportionation induced by the moisture. Accordingly, the blue perovskite film, modified synergistically by the 3S and DPPA dual ligands, maintains its initial photoluminescence quantum yield (PLQY) after over 200 h with 70 °C heating, and only exhibiting a redshift in PL peak wavelength less than 10 nm. Exposing under environment condition of 20 % relative humidity for 300 h, the blue perovskite film still retains its initial PL peak wavelength with PLQY exceeding 50 % of the initial value. This achievement establishes a notable strategy for enhancing both the thermal and moisture stability of blue quasi-2D perovskite. Furthermore, the light-emitting diodes based on the perovskite films display the enhanced operating stability. This work offers an efficient strategy for stabilizing blue quasi-2D perovskite and the corresponding devices for the display application.