Μm Regime Research Articles

JWST/NIRCam Detection of the Fomalhaut C Debris Disk in Scattered Light

Observations of debris disks offer important insights into the formation and evolution of planetary systems. Though M dwarfs make up approximately 80% of nearby stars, very few M dwarf debris disks have been studied in detail—making it unclear how or if the information gleaned from studying debris disks around more massive stars extends to the more abundant M dwarf systems. We report the first scattered-light detection of the debris disk around the M4 star Fomalhaut C using JWST's Near Infrared Camera (NIRCam; 3.6 and 4.4 μm). This result adds to the prior sample of only four M dwarf debris disks with detections in scattered light and marks the latest spectral type and oldest star among them. The size and orientation of the disk in these data are generally consistent with the prior Atacama Large Millimeter/submillimeter Array submillimeter detection. Though no companions are identified, these data provide strong constraints on their presence—with sensitivity sufficient to recover sub-Saturn mass objects in the vicinity of the disk. This result illustrates the unique capability of JWST to uncover elusive M dwarf debris disks in scattered light and lays the groundwork for deeper studies of such objects in the 2–5 μm regime.

Relationship of Thermostability and Binding Affinity in Metal-binding WW-Domain Minireceptors.

The design of metallo-miniproteins advances our understanding of the structural and functional roles of metals in proteins. We recently designed a metal-binding WW domain, WW-CA-Nle, which displays three histidine residues on its surface for coordination of divalent metals Ni(II), Zn(II) and Cu(II). However, WW-CA-Nle is a molten globule in the apo state and thus showed only moderate binding affinities with Kd values in the μM regime. In this report, we hypothesize that improved thermal stability of the apo state of the metal binding WW-domain scaffold should lead to improved preorganization of the metal-binding site and consequently to higher metal-binding affinities. By redesigning WW-CA-Nle, we obtained WW-CA variants, WW-CA-min and WW-CA-ANG, which were fully folded in the apo states and displayed moderate to excellent thermostabilities in the apo and holo states. We were able to show that the improved thermal stabilities led to improved metal binding, which was reflected in Kd values that were at least one order of magnitude lower compared to WW-CA-Nle. EPR spectroscopy and ITC measurements revealed a better defined and predisposed metal binding site in WW-CA-ANG.

High-responsivity on-chip waveguide coupled germanium photodetector for 2 μm waveband

Recently, the emerging 2 μm waveband has gained increasing interest due to its great potential for a wide scope of applications. Compared with the existing optical communication windows at shorter wavelengths, it also offers distinct advantages of lower nonlinear absorption, better fabrication tolerance, and larger free carrier plasma effects for silicon photonics, which has been a proven device technology. While much progress has been witnessed for silicon photonics at the 2 μm waveband, the primary challenge still exists for on-chip detectors. Despite the maturity and compatibility of the waveguide coupled photodetectors made of germanium, the 2 μm regime is far beyond its cutoff wavelength. In this work, we demonstrate an efficient and high-speed on-chip waveguide-coupled germanium photodetector operating at the 2 μm waveband. The weak sub-bandgap absorption of epitaxial germanium is greatly enhanced by a lateral separation absorption charge multiplication structure. The detector is fabricated by the standard process offered by a commercial foundry. The device has a benchmark performance with responsivity of 1.05 A/W and 3 dB bandwidth of 7.12 GHz, which is able to receive high-speed signals with up to 20 Gbit/s data rate. The availability of such an efficient and fast on-chip detector circumvents the barriers between silicon photonic integrated circuits and the potential applications at the 2 μm waveband.

A switchable multi-wavelength mode-locked fiber laser based on a multi-mode interference device

A switchable multi-wavelength, passively mode-locked fiber laser in the 1 μm regime is demonstrated in this paper. The key principles of working include the nonlinear polarization rotation (NPR) mechanism for the mode-locked pulse generation and a single mode-multimode-single mode (SMS) structure as the spectral filter. The SMS structure is coiled on the paddles of a polarization controller (PC). Stable mode-locking operation is achieved at the fundamental repetition rate of 7.97 MHz. The pulse duration is 429 ps at the pump level of 216 mW, with the dual-wavelength laser emitting at 1037.86 nm and 1045.72 nm. By properly tuning the paddles of PCs, multi-wavelength mode-locked laser spectra can be observed switching from triple- to sextuple-wavelength, with the emission bandwidth of 27.6 nm from 1035.6 nm to 1063.2 nm.

Modelling the sub-100 fs Dy3+: Fluoride fiber laser beyond 3 μm

Generating ultrafast pulses in the mid-infrared (mid-IR) region around 3 μm is a scientific and technological frontier at present. In this work, we have investigated the sub-100 fs Dy3+: fluoride fiber laser generation beyond 3 μm regime via the hybrid mode-locking mechanism based on the semiconductor saturable absorber mirror (SESAM) and the nonlinear polarization rotation (NPR) method numerically. Compared with the single SESAM- and NPR-based mode-locking, the hybrid mode-locked Dy3+: fluoride fiber laser delivers much shorter pulses via the fast and slow mode-locking from the NPR and SESAM, respectively. When dispersion compensation is taken into the hybrid mode-locking, the ultrafast pulse of 94 fs with a corresponding time-bandwidth product (TBP) of 0.333 is achieved. In addition, laser performance optimizations are investigated. By optimizing the parameters of the gain fiber and SESAM, the shortest pulse duration of 85 fs can be obtained. The results represent a feasible solution and design guidelines for ultrafast mid-infrared fiber laser generation beyond 3 μm.

Generation of soliton operating at 1.5 µm regime using Indium Selenide based saturable absorber

We successfully accomplished a soliton pulse in an Erbium-doped fiber laser (EDFL) cavity by integrating an Indium Selenide (In2Se3)-based passive saturable absorber (SA). In2Se3 film was prepared by using a common dry cast method and owns saturable intensity, modulation depth, and non-saturable loss of 1066.57 MW/cm2, 1.1 %, and 72.2 %, respectively. By implementing In2Se3 film into the EDF laser cavity, a generation of soliton with 1.880 MHz, at the threshold pump power of 83.63 mW was achieved. A narrow pulse width of 5.51 ps and remarkable temporal stability, as demonstrated by a signal noise ratio (SNR) of 78.52 dB and peak power of 1.00 kW obtained using In2Se3 as SA operating in a 1.5-μm regime proven its potential in photonic applications.

(Invited) 3D Printed Carbon Microlattices As Electrodes in Electrochemical Cells

With unprecedented freedom in materials design, additive manufacturing (AM) technologies provide us with the capabilities to leverage from geometric complexity. If properly exploited, this allows us to design performance instead of geometry, where the latter is not user defined but a result of a computer optimization. Entirely new mathematically defined, dimensionally stable carbon materials are presented that shrink isotopically under a pyrolysis process generating complex CAD defined 3D shapes with theoretically two to three orders of magnitude increases in permeability and ohmic conductance over conventional Gas Diffusion Layers. The shown lattices can be structured with resolution in the 10 µm regime having total dimensions over several cm. Initial performance tests in full cell configurations inside a PEMFC are presented. The area of mathematically defined, ordered materials with predictable and optimized performances might shape an entirely research field relevant for low cost and high-performance energy converters. Figure 1

The role of shadowed droplets in condensation heat transfer

Dropwise condensation (DWC) is a phenomenon of common occurrence and significant utility in nature and technology. In energy applications, sustenance of DWC and avoidance of transition to film formation is directly related to efficient heat removal, ensuring high performance of related devices and processes. The efficiency of heat transfer in DWC depends on the heat transfer rates of individual droplets and the droplet size distribution. While the former can be summarily captured in engineering analysis through thermal resistance modeling, the theoretical analysis of the droplet size distribution involves assumptions often oversimplifying the complexity of droplet interactions, especially in the important sub–10 μm regime. Here, a modeling framework based on the thermal resistance approach is coupled with a dynamic model of droplet interactions: Droplet growth, coalescence, jumping, and removal of condensate due to gravity are all present during the unfolding of the phenomenon to accurately predict the droplet size distribution. The motivation is condensation experiments on superhydrophobic surfaces, namely rough aluminum substrates with a hydrophobic coating. Through the “interaction” of computations with measurements, the experimental findings are explained and critical parameters for condensation heat transfer on superhydrophobic surfaces are illuminated. Although larger droplets can be easily observed in experiments, it is shown that large droplets do not significantly affect heat transfer after reaching such state of growth. Instead, it is the small (with radius < 10 μm) and what we term “shadowed” droplets, i.e., the droplets grown in the shadow of the vertical projection area of the larger droplets, that play an important role in the heat transfer process. Due to the growth of these droplets and their concomitant shadowed coalescence and ensuing re-nucleation, the shift in the droplet size distribution towards smaller sizes is significant-enough to render the heat transfer rate rather insensitive to the presence of large droplets. In this regime, the effect of contact angle hysteresis on heat transfer is not crucial. Through the comparison with measurements, the dominant role of the density of nucleation sites on heat transfer is revealed and an estimation of the density of sites (∼105 mm−2) as a function of subcooling is extracted. Finally, the distribution of sites is found critical for heat transfer; an ordered distribution of sites outperforms random and clustered distributions.

Rare-earth Yttrium oxide as Q-switcher in fiber laser system

Yttrium Oxide (Y2O3), a rare-earth material is used as saturable absorber (SA) to induce Q-switching operation in 1.55 µm regime. The Y2O3 was embedded into polyvinyl alcohol (PVA) to form a thin film so that it can be easily integrated into an Erbium-doped fiber laser (EDFL) cavity for modulating the cavity loss. A robust and stable Q-switched pulse centered at 1560.53 nm operating wavelength has been obtained with the integration of Y2O3-PVA SA in the EDFL cavity. The repetition rate of the Q-switched pulse can be varied from 88.03 kHz to 99.22 kHz with the change of pump power from 128.92 mW to 175.87 mW while the corresponding pulse duration reduces from 3.30 μs to 2.97 μs. Also, the analyzed peak power of the pulse increased from 40.92 mW to 52.67 mW within the pump power region.

Theoretical Model of a Plasmonically Enhanced Tunable Spectrally Selective Infrared Photodetector Based on Intercalation-Doped Nanopatterned Multilayer Graphene.

We showed in past work that nanopatterned monolayer graphene (NPG) can be used for realizing an ultrafast (∼100 ns) and spectrally selective mid-infrared (mid-IR) photodetector based on the photothermoelectric effect and working in the 8-12 μm regime. In later work, we showed that the absorption wavelength of NPG can be extended to the 3-8 μm regime. Further extension to shorter wavelengths would require a smaller nanohole size that is not attainable with current technology. Here, we show by means of a theoretical model that nanopatterned multilayer graphene intercalated with FeCl3 (NPMLG-FeCl3) overcomes this problem by substantially extending the detection wavelength into the range from λ = 1.3 to 3 μm. We present a proof of concept for a spectrally selective infrared (IR) photodetector based on NPMLG-FeCl3 that can operate from λ = 1.3 to 12 μm and beyond. The localized surface plasmons (LSPs) on the graphene sheets in NPMLG-FeCl3 allow for electrostatic tuning of the photodetection wavelength. Most importantly, the LSPs along with an optical cavity increase the absorbance from about N × 2.6% for N-layer graphene-FeCl3 (without patterning) to nearly 100% for NPMLG-FeCl3, where the strong absorbance occurs locally inside the graphene sheets only. Our IR detection scheme relies on the photothermoelectric effect induced by asymmetric patterning of the multilayer graphene (MLG) sheets. The LSPs on the nanopatterned side create hot carriers that give rise to the Seebeck effect at room temperature, achieving a responsivity of V/W, a detectivity of D* = 2.3 × 109 Jones, and an ultrafast response time of the order of 100 ns. Our theoretical results can be used to develop graphene-based photodetection, optical IR communication, IR color displays, and IR spectroscopy over a wide IR range.

High-Performance, Polarization-Sensitive, Long-Wave Infrared Photodetection via Photothermoelectric Effect with Asymmetric van der Waals Contacts.

Long-wavelength infrared (LWIR) photodetection is important for heat-seeking technologies, such as thermal imaging, all-weather surveillance, and missile guidance. Among various detection techniques, photothermoelectric (PTE) detectors are promising in that they can realize ultra-broadband photodetection at room temperature without an external power supply. However, their performance in terms of speed, responsivity, and noise level in the LWIR regime still needs further improvement. Here, we demonstrated a high-performance PTE photodetector based on low-symmetry palladium selenide (PdSe2) with asymmetric van der Waals contacts. The temperature gradient induced by asymmetric van der Waals contacts even under global illumination drives carrier diffusion to produce a photovoltage via the PTE effect. A responsivity of over 13 V/W, a response time of ∼50 μs, and a noise equivalent power of less than 7 nW/Hz1/2 are obtained in the 4.6-10.5 μm regime at room temperature. Furthermore, due to the anisotropic absorption of PdSe2, the detector exhibits a linear polarization angle sensitive response with an anisotropy ratio of 2.06 at 4.6 μm and 1.21 at 10.5 μm, respectively. Our proposed device architecture provides an alternative strategy to design high-performance photodetectors in the LWIR regime by utilizing van der Waals layered materials.

Ultra-broadband 3 dB power splitter from 1.55 to 2 µm wave band.

Extending the optical communication wavelengths to 2 µm can significantly increase data capacity. Silicon photonics, which is a proven device integration technology, has made rapid progress at 2 µm recently. As a fundamental functional element in the photonic design kit, the 3 dB power splitter has been extensively studied in both the 1.55 µm and 2 µm regime. While the device is highly desirable to operate over both wave bands, the large waveguide dispersion in silicon makes it challenging. In this work, we demonstrate an ultra-broadband power splitter on silicon, which has a 0.2 dB bandwidth exceeding 520 nm from 1500 to 2020 nm according to simulations. The beam splitter is realized by a triple tapered Y-junction, and its operational bandwidth is greatly increased by subwavelength grating structure. The device has an ultra-compact footprint of only 3µm×2µm. Due to the limitations on the setup and coupling technique, we measure the device bandwidth in 1.55 µm and 2 µm wave bands. The device insertion loss is measured to be below 0.4 dB from 1500 to 1620 nm and from 1960 to 2020 nm, respectively. According to these results, the proposed device is believed to be capable of operating over a broadband from 1.55 µm and 2 µm wavelengths.

Highly efficient generation of coherent light at 2128 nm via degenerate optical-parametric oscillation.

Cryogenic operation, in conjunction with new test-mass materials, promises to reduce the sensitivity limitations from thermal noise in gravitational-wave detectors. Currently, the most advanced materials under discussion are crystalline silicon as a substrate with amorphous silicon-based coatings. However, they require operational wavelengths around 2 µm to avoid laser absorption. Here we present a light source at 2128 nm based on a degenerate optical parametric oscillator to convert light from a 1064 nm nonplanar ring-oscillator. We achieve an external conversion efficiency of (87.1±0.4)% at a pump power of 52 mW in periodically poled potassium titanyl phosphate (internal efficiency was 93%). With our approach, light from the established and existing laser sources can be efficiently converted to the 2 µm regime while retaining the excellent stability properties.

2.8 µm passively Q-switched Er:ZBLAN fiber laser with an Sb saturable absorber mirror

A Q-switched Er:ZBLAN fiber laser operating at 2.8 µm was realized by employing Sb as the saturable material. The Sb material was deposited on a gold mirror by the magnetron-sputtering deposition method to develop a saturable absorber mirror (SAM). By employing the Sb-SAM in an Er:ZBLAN fiber laser, stable Q-switching operation was achieved at central wavelength of 2799.7 nm with the repetition rates ranging from 33.3 to 58.8 kHz and the pulse duration ranging from 5.7 to 1.7 µs. The Sb-SAM still works stably under the maximum pump power of 5.6 W, with an output power of 59 mW corresponding to the pulse energy of 1.03 µJ. To our knowledge, this was the first demonstration of Sb-based saturable material in Er:ZBLAN fiber laser for mid-infrared Q-switched pulse generation operating in the 2.8 µm regime, indicating its potential applications in the mid-infrared waveband.

Efficient all-solid-state passively Q-switched SWIR Tm:YAP/KGW Raman laser

We present an all-passive efficient KGW Raman laser with an external-cavity configuration in the 2 µm spectral regime. The Raman laser was pumped by a passively Q-switched Tm:YAP laser emitting at 1935 nm. Due to the bi-axial properties of the KGW crystal, the laser exhibits stimulated Raman emission at two separate spectral lines: 2272 nm and 2343 nm. The output energies achieved at these two lines are 340 µJ/pulse and 450 µJ/pulse, accordingly. The seed to Raman laser conversion efficiencies achieved of 19.2% and 23.5%, respectively, are comparable to actively Q-switched laser arrangements. To the best of our knowledge, this is the first time an efficient Raman laser in the 2 µm regime is demonstrated in a completely passive configuration.

Lubricating condition evaluation of the rotary compressor under high ambient temperature

Abstract Propane has been suggested as one of the alternative working fluids for room air conditioners in hot climate areas because of its excellent environmental properties and lower discharge temperature though there still is concern about its flammability. The lubricating condition of rotor bearings system should also be emphasized because a heavier load and lower viscosity in the oil sump could impair the reliability of rotary compressor under high ambient temperature. In this study, in order to obtain the input of simulation, the dynamic pressures and viscosity of a R290 air conditioner are experimentally measured under several conditions. On these bases, lubricating characteristics are numerically simulated by coupling the dynamics equation of crank shaft and the modified Reynolds equation of oil film using finite element method. The results show that the load on crank under 50 °C is twice as large as that of 30 °C while the viscosity of bottom oil sump decreases from 1.89 to 1.59 mPa⋅s. Simulation results imply that the minimum oil film thickness decreased to 0.44 µm under 50 °C, in which asperity contact force peaks at 3 N when arithmetic mean roughness is 0.2 µm. Asperity contact force would significantly increase to 174 N if surface roughness is 0.6 µm and boundary lubrication regime is reached. Strategies to increase oil sump viscosity is recommended to avoid adhesive wear in this case.

Designing silicon-core fiber tapers for efficient supercontinuum generation in the greenhouse gas absorption region

We propose a tapered silicon-core optical fiber design for extending the long-wavelength edge of supercontinuum generation to obtain a high spectral density source across the 3–4.5 µm regime. The taper works by generating sufficient spectral broadening of the driving laser pulse to produce a series of pumps for nondegenerate four-wave mixing, and then opening up new phase-matching conditions to transfer the power from these pumps to a target region of mid-infrared wavelengths. We show, by simulation, that this taper design works effectively when pumped with a conventional 2.1 µm femtosecond fiber laser, significantly improving the spectral coverage obtained with a fixed-diameter fiber. Thus, these tapered silicon-core fibers offer a potential platform for an efficient all-fiber spectroscopy solution to measure greenhouse gases.

Validation of secondary fluorescence excitation in quantitative X-ray fluorescence analysis of thin alloy films

Reference-free X-ray fluorescence analysis of multilayered, alloyed thin films in the μm regime with significant secondary fluorescence contributions.

Generation of ultra-fast pulse based on bismuth saturable absorber

We demonstrate a bismuth (Bi) saturable absorber (SA) for generating ultrafast pulse. The Bi SA is fabricated by the Bi film deposited on the surface of microfibers through using magnetron sputtering. Its nonlinear optical properties are investigated. The as-prepared Bi SA has outstanding nonlinear absorption property demonstrated by the open aperture (OA) &lt;i&gt;Z&lt;/i&gt;-scan system at 1500 nm and balanced twin-detector method at 1560 nm. The nonlinear optical property of Bi SA shows that the modulation depth, the nonsaturable losses, and the saturable intensity at 1.5 μm are 14% and 79%, and 0.9 MW/cm&lt;sup&gt;2&lt;/sup&gt;, respectively. Besides, the closed aperture (CA) &lt;i&gt;Z&lt;/i&gt;-scan measurement is also implemented to estimate the nonlinear refractive index of Bi film. The Bi film shows that the typical CA/OA curve possesses the feature of peak-valley profile, meaning that the sample with a negative nonlinear refractive index is self-defocusing. In our experiments, the parameters of the nonlinear absorption coefficient &lt;i&gt;β&lt;/i&gt; and the nonlinear refractive index &lt;i&gt;n&lt;/i&gt;&lt;sub&gt;2&lt;/sub&gt; are estimated at about 2.38 × 10&lt;sup&gt;–4&lt;/sup&gt; cm/W and –1.47 × 10&lt;sup&gt;–9&lt;/sup&gt; cm&lt;sup&gt;2&lt;/sup&gt;/W according to the actual experimental data points, respectively. To further investigate its nonlinear optical property, the microfiber-based Bi SA is embedded into an erbium-doped fiber laser with a typical ring cavity structure. Based on the Bi SA device, the stable ultrafast pulses are generated at 1.5 μm with the pulse width of 357 fs, the output power of 45.4 mW, corresponding to the pulse energy of 2.39 nJ, and the signal-to-noise ratio is 84 dB. The stable soliton pulses emitting at 1563 nm are obtained with a 3-dB and 6-nm spectral bandwidth. The experimental results suggest that the microfiber-based Bi SA prepared by magnetron sputtering deposition (MSD) technique can be used as an excellent photonic device for ultrafast pulse generation in the 1.5 μm regime, and the MSD technique opens a promising way to produce high-performance SA with a large modulation depth, low saturable intensity, and high power tolerance, which are conducible to the generation of high power and ultrafast pulse with high stability.

Dual broadband infrared absorptance enhanced by magnetic polaritons using graphene-covered compound metal gratings.

A dual broadband perfect absorber based on a graphene-covered compound silver (Ag) grating structure working in the infrared (0.8-2.1 µm) regime is proposed and investigated numerically. Two distinct absorption peaks approximately 1.0 are achieved by the excitation of magnetic polaritons over a large range of incident angles from 0 to 70 degrees. The physics underlying the structure is also explained by computing interactions of electromagnetic fields with the graphene and the Ag grating. In addition, it has shown that the absorption peaks can be tuned by changing geometric parameters of the structure; however, their spectral shape and absorption remain unchanged. Furthermore, the proposed compound grating with a graphene overlay provides potential applications for infrared absorbing devices.