Small Linewidth Research Articles

Spaser Nanoparticles for Ultranarrow Bandwidth STED Super-Resolution Imaging.

Super-resolution microscopy, as a powerful tool of seeing abundant spatial details, typically can only distinguish a few distinct targets at a time due to the spectral crosstalk between fluorophores. Spaser (i.e., surface plasmon laser) nanoprobes, which confine lasing emission into nanoscale, offer an opportunity to eliminate such obstacle. Here, realized is narrow band stimulated emission depletion (STED) nanoscopy on spaser nanoparticles by collecting the coherent spasing signals. Demonstrated are the physics concept and feasibility of erasing spaser emission by using a depletion beam to suppress the population inversion, which lays the foundation of spaser-based STED super-resolution. Thanks to the small size (47 nm) and narrow spectral linewidth (3.8 nm) of the spaser nanoparticles, a 74 nm spatial resolution in STED imaging within an acquisition bandwidth of 10 nm is finally obtained. These spaser nanoparticles, if multiplexing with different wavelengths, in principle, allow for spectral-multiplexed imaging, sensing, cytometry, and light operation of a large number of targets all at once.

Hybrid Dielectric-Plasmonic Nanocomposite Arrays for Bulk and Local Refractive Index Sensing

Plasmonics-based biosensors are often limited by material losses in the form of joule heating while all dielectric nanoparticles systems have relatively smaller local electric field enhancements. For efficient sensing, it is desirable to have a system with high sensitivity but with minimal losses. Here, we demonstrate, using numerical simulations, the capability of a hybrid dielectric-plasmonic system for refractive index sensing applications. We show that the optical resonances of such a hybrid system have smaller linewidths and the peak wavelengths are tunable. Bulk as well as local refractive index sensing are demonstrated in this work. Owing to large sensitivities of 300 nm/RIU with a figure of merit (FOM) of 10, the hybrid photonic-plasmonic systems presented here are promising materials for future biosensing applications.

Dual-ring dual-wavelength fiber laser sensor for simultaneous measurement of refractive index and ambient temperature with improved discrimination and detection limit.

In this paper, an in-line Mach-Zehnder interferometer embedded in a fiber Bragg grating (FBG) is fabricated by a tiny offset-core splicing technique and inserted into a fiber ring laser to effectively improve the detection limit and discrimination of sensing. Through fine power adjustment, the stable operation of dual-wavelength lasing is completed in a single FBG for the first time to our knowledge, and simultaneous measurement of refractive index (RI) and ambient temperature is achieved with high discrimination due to the RI immunity of the FBG. The experimental results indicate that the sensitivities of RI and temperature are -30.57 nm/RIU and 131.1pm/°C, respectively, with very low intensity drift. Furthermore, the over 20 times improvement of the RI detection limit is obtained owing to the large extinction ratio (>30 dB) and small linewidth (≤0.13 nm).

Influence of the polarization anisotropy on the linewidth enhancement factor and reflection sensitivity of 1.55- μ m InP-based InAs quantum dash lasers

This work investigates the effect of the polarization anisotropy on the linewidth enhancement factor and the reflection sensitivity of InAs/InP quantum dash semiconductor lasers. The results show that the small linewidth enhancement factor and high stability against external optical feedback are obtained for nanostructures oriented perpendicular to the cavity axis as opposed to those oriented parallel to the cavity axis. Effective simulations on the critical feedback level of these two lasers are also in agreement with experimental results. Such anisotropy is attributed to the polarization dependence of the transition matrix element in these quantum nanostructures.

Are Narrow-line Seyfert 1 Galaxies Powered by Low-mass Black Holes?

Abstract Narrow-line Seyfert 1 galaxies (NLS1s) are believed to be powered by the accretion of matter onto low-mass black holes (BHs) in spiral host galaxies with BH masses M BH ∼ 106–108 M ⊙. However, the broadband spectral energy distribution of the γ-ray-emitting NLS1s are found to be similar to flat-spectrum radio quasars. This challenges our current notion of NLS1s having low M BH. To resolve this tension of low M BH values in NLS1s, we fitted the observed optical spectrum of a sample of radio-loud NLS1s (RL-NLS1s), radio-quiet NLS1s (RQ-NLS1s), and radio-quiet broad-line Seyfert 1 galaxies (RQ-BLS1s) of ∼500 each with the standard Shakura–Sunyaev accretion disk (AD) model. For RL-NLS1s we found a mean log( ) of 7.98 ± 0.54. For RQ-NLS1s and RQ-BLS1s we found mean log( ) of 8.00 ± 0.43 and 7.90 ± 0.57, respectively. While the derived values of RQ-BLS1s are similar to their virial masses, for NLS1s the derived values are about an order of magnitude larger than their virial estimates. Our analysis thus indicates that NLS1s have M BH similar to RQ-BLS1s and their available virial M BH values are underestimated, influenced by their observed relatively small emission line widths. Considering Eddington ratio as an estimation of the accretion rate and using , we found the mean accretion rate of our RQ-NLS1s, RL-NLS1s, and RQ-BLS1s as , and , respectively. Our results therefore suggest that NLS1s have BH masses and accretion rates that are similar to BLS1s.

Multifunctional Electronic Textiles Using Silver Nanowire Composites.

Textiles represent an appealing platform for continuous wearable applications due to the exceptional combination of compliance, water vapor permeability, and comfortableness for long-term wear. We present mechanically and electrically robust integration of nanocomposites with textiles by laser scribing and heat press lamination. The simple and scalable integration technique enables multifunctional E-textiles without compromising the stretchability, wearability, and washability of textiles. The textile-integrated patterns exhibit small line width (135 μm), low sheet resistance (0.2 Ω/sq), low Young's modulus, good washability, and good electromechanical performance up to 50% strain, which is desirable for wearable and user-friendly electronic textiles. To demonstrate the potential utility, we developed an integrated textile patch comprising four dry electrophysiological electrodes, a capacitive strain sensor, and a wireless heater for electrophysiological monitoring, motion tracking, and thermotherapy, respectively. Beyond the applications demonstrated in this paper, the materials and methods presented here pave the way for various other wearable applications in health care, activity tracking, rehabilitation, sports medicine,and human-machine interactions.

Microwave to optical photon conversion via fully concentrated rare-earth-ion crystals

Most investigations of rare earth ions in solids for quantum information have used rare earth ion doped crystals. Here we analyse the conversion of quantum information from microwave photons to optical frequencies using crystals where the rare earth ions, rather than being dopants, are part of the host crystal. The potential of large ion densities and small linewidths makes such systems very attractive in this application. We show that, as well as high efficiency, large bandwidth conversion is possible. In fact, the collective coupling between the rare earth ions and the optical and microwave cavities is large enough that the limitation on the bandwidth of the devices will instead be the spacing between magnon mode modes in the crystal.

Optimizing optically injected semiconductor lasers for periodic dynamics with reduced sensitivity to perturbations.

Optically injected semiconductor lasers generate limit-cycle dynamics of period-one and period-two oscillations at microwave frequencies. At specific operating points, the periodic oscillations can be rendered insensitive to small-signal fluctuation in the operating conditions of the master and slave lasers. Periodic oscillations with low-sensitivity to fluctuation in the slave laser bias current, injection strength, or detuning frequency can be enhanced through tailoring the intrinsic laser parameters. Here, through numerical calculations, the effects of each intrinsic parameter on the various low-sensitivity operating points are demonstrated through detailed maps as functions of the operating parameters. For enhanced low sensitivity to bias-current and injection-strength fluctuations, a laser with a small linewidth enhancement factor is favored. Conversely, a large linewidth enhancement factor enhances low sensitivity to detuning-frequency fluctuations. Lasers with a more negative value of the gain saturation factor confine the regions of low sensitivity to fluctuations in the bias current and detuning frequency to the extrema of the periodic oscillation frequency. The laser relaxation rates that enhance the periodic-oscillation dynamic regions of an optically injected semiconductor laser, in turn, increases the regions of the various types of low-sensitivity operating points.

Direct writing of Cu-based fine micropatterns using femtosecond laser pulse-induced sintering of Cu2O nanospheres

Cu-based micropatterns were directly fabricated using femtosecond laser pulse-induced sintering of Cu2O nanospheres. Cu2O nanospheres synthesized by the polyol method were spin-coated on glass substrates. By evaluating the nonlinear optical absorption of the Cu2O nanosphere films using the Z-scan method, the two-photon absorption coefficient was estimated as ∼70 cm GW−1, which is adequate in comparison with that of bulk Cu2O. Although Cu and Cu2O composite micropatterns were obtained at a high scan speed of 100–1000 μm s−1, CuO micropatterns were formed at a low scan speed of 1 μm s−1. Compared with the femtosecond laser sintering of CuO nanoparticles using linear optical absorption, the width of line patterns obtained using Cu2O nanospheres was smaller at each pulse energy. These results suggest that two-photon absorption induced a decrease of the absorbed area, providing the small line width. This direct writing technique is useful for selective fabrication of Cu-based fine micropatterns.

Improving the spectral performance of extended cavity diode lasers using angled-facet laser diode chips

We present and compare theoretical and experimental results on the electro-optical performance of extended cavity diode lasers (ECDLs) that employ two ridge waveguide designs for the single-transverse mode GaAs laser diode chip. One facet of the laser diode chips serves as a partially reflective output coupler for the laser cavity. The other facet constitutes an intra-cavity interface which introduces spurious optical feedback to the laser diode chip. The waveguide designs differ with respect to the suppression of this spurious feedback. The first design employs a straight ridge waveguide intersecting both facets at normal incidence. The intra-cavity facet is anti-reflection coated and features a residual intensity reflectivity of the order $$10^{-4}$$ . The second design employs a bent ridge waveguide intersecting the anti-reflection-coated intra-cavity facet at an appropriate angle. This provides an additional suppression of the spurious intensity reflection to a value estimated to be less than $$10^{-6}$$ . We compare the electro-optical performance of both designs theoretically and experimentally. The utilization of a bent waveguide results in an improved spectral stability and purity, specifically a higher side mode suppression and a small intrinsic spectral linewidth over the whole investigated current range, of the external cavity diode laser without sacrificing other parameters such as the output power. The external cavity diode lasers under study exhibit no degradation of the measured frequency noise power spectra and intrinsic linewidths even if there is a drop of the side mode suppression ratio provided that it is not reduced to a very small value. Thus, the usage of a more readily accessible straight waveguide chip in an ECDL could be sufficient if only a limited tuning range and a particularly compact assembly are needed. For spectroscopic applications requiring a small intrinsic spectral linewidth over a large frequency range a bent waveguide chip could be the better choice.

Small-scale physical and chemical structure of diffuse and translucent molecular clouds along the line of sight to Sgr B2

Context. The diffuse and translucent molecular clouds traced in absorption along the line of sight to strong background sources have so far been investigated mainly in the spectral domain because of limited angular resolution or small sizes of the background sources. Aims. We aim to resolve and investigate the spatial structure of molecular clouds traced by several molecules detected in absorption along the line of sight to Sgr B2(N). Methods. We have used spectral line data from the EMoCA survey performed with the Atacama Large Millimeter/submillimeter Array (ALMA), taking advantage of its high sensitivity and angular resolution. The velocity structure across the field of view is investigated by automatically fitting synthetic spectra to the detected absorption features, which allows us to decompose them into individual clouds located in the Galactic centre (GC) region and in spiral arms along the line of sight. We compute opacity maps for all detected molecules. We investigated the spatial and kinematical structure of the individual clouds with statistical methods and perform a principal component analysis to search for correlations between the detected molecules. To investigate the nature of the molecular clouds along the line of sight to Sgr B2, we also used archival Mopra data. Results. We identify, on the basis of c-C3H2, 15 main velocity components along the line of sight to Sgr B2(N) and several components associated with the envelope of Sgr B2 itself. The c-C3H2 column densities reveal two categories of clouds. Clouds in Category I (3 kpc arm, 4 kpc arm, and some GC clouds) have smaller c-C3H2 column densities, smaller linewidths, and smaller widths of their column density PDFs than clouds in Category II (Scutum arm, Sgr arm, and other GC clouds). We derive opacity maps for the following molecules: c-C3H2, H13CO+, 13CO, HNC and its isotopologue HN13C, HC15N, CS and its isotopologues C34S and 13CS, SiO, SO, and CH3OH. These maps reveal that most molecules trace relatively homogeneous structures that are more extended than the field of view defined by the background continuum emission (about 15′′, that is 0.08–0.6 pc depending on the distance). SO and SiO show more complex structures with smaller clumps of size ~5–8′′. Our analysis suggests that the driving of the turbulence is mainly solenoidal in the investigated clouds. Conclusions. On the basis of HCO+, we conclude that most line-of-sight clouds towards Sgr B2 are translucent, including all clouds where complex organic molecules were recently detected. We also conclude that CCH and CH are good probes of H2 in both diffuse and translucent clouds, while HCO+ and c-C3H2 in translucent clouds depart from the correlations with H2 found in diffuse clouds.

Flat-band spin dynamics and phonon anomalies of the saw-tooth spin-chain system Fe2O(SeO3)2

Fe$^{3+}$ $S = 5/2$ ions form saw-tooth like chains along the $a$ axis of the oxo-selenite Fe$_2$O(SeO$_3$)$_2$ and an onset of long-range magnetic order is observed for temperatures below $T_C = 105$ K. This order leads to distinct fingerprints in phonon mode linewidths and energies as resolved by Raman scattering. In addition, new excitations with small linewidths emerge below $T = 150$ K, and are assigned to two-magnon scattering processes with the participation of flat-band and high energy magnon branches. From this a set of exchange coupling constants is estimated. The specific ratio of the saw-tooth spine-spine and spine-vertex interactions may explain the instability of the dimer quantum ground state against an incommensurate 3D magnetic order.

II-VI Based Inorganic-Organic Hybrid Quantum Structures with High Degree of Structural Ordering, Long-Term Stability, and Novel Properties

Translational symmetry ensures phase coherency of a physical process among different units of a crystal, and thus produces collective quantum effects beyond the sum of the units. Any significant physical and/or chemical fluctuation, which typically exists in a semiconductor alloy or self-assembled or artificially grown nanostructure array, would hinder our ability to study and use the collective behavior relying on this coherency. Man-made structures remain as one of the central interests since semiconductor superlattices were proposed by Esaki and Tsu in 1970. Unfortunately, man-made structures with both genuine long- and short-range order are rare. Here a new family of inorganic-organic hybrid quantum structures provides us a unique opportunity to explore coherency quantum phenomena in man-made structures. These self-assembled hybrid structures are ultra-thin slabs or atomic chains of II-VI semiconductors interconnected or coordinated by small organic molecules, exhibiting very high degree of structural perfectness comparable to a typical binary semiconductor, as manifested in a very small XRD and Raman linewidth. They exhibit several extraordinary properties, for instance, exceedingly strong excitonic absorption, large exciton binding energies, and zero thermal expansion. By varying either the inorganic or organic component, one can tune the coupling between the inorganic units to achieve quasi-1-D, quasi-2-D, and 3-D super-structures with tailored material properties for different basic research interests and potential applications (e.g., room temperature exciton-polariton condensation, high efficiency UV emission and detection, p-type transparent conducting material). Furthermore, some hybrid structures, such as β-ZnTe(en)0.5, have been found to retain the same structural perfectness after being stored under ambient condition over a decade, despite others show varying degrees of structural degradation. This finding indicates that the long-term stability of the material depends on both the intrinsic effect, the strength of chemical bonding, and the extrinsic effect, such as the presence of defects in the initial structure.

Junction-free copper wires with submicron linewidth for large-area high-performance transparent electrodes

Electrospinning and wet etching facilitate large-area fabrication of junction-free copper wires with the smallest linewidth of 550 nm.

Engineering surface lattice resonance of elliptical gold nanodisk array for enhanced strain sensing.

We demonstrate an elliptical gold nanodisk array (GNA) for engineering the spectral profile of surface lattice resonance (SLR). The nanodisk's shape has a great impact on SLR. Small linewidth of 20 nm at an aspect ratio of 1.17, as well as large wavelength tuning of 64 nm within 4% strain via different orientations and polarizations, are achieved experimentally. The enhanced wavelength response of 6.93 nm per 1% strain variation for elliptical GNA is 2.4 times better than that for general circular GNA. Furthermore, the strain sensing for elliptical GNA approaches is 5.7 times greater than that for circular GNA.

Large Area Three‐Dimensional Photonic Crystal Membranes: Single‐Run Fabrication and Applications with Embedded Planar Defects

AbstractThree‐dimensional photonic crystals (3D PhCs) enable light manipulations in all three spatial dimensions, however, real world applications are still faced with challenges in fabrication. Here, a facile fabrication strategy for 3D silicon PhCs with a simple cubic (SC) lattice structure is presented, which exhibits a complete photonic bandgap at near‐infrared wavelengths of around 1100 nm. The fabrication process is composed of standard deep ultra‐violet stepper lithography, followed by a single‐run modified plasma etch process. By applying a direct dry etch release step at the end of the 3D structural etch process, the fabricated 3D PhCs can be released and transferred in the form of a membrane onto other substrates such as glass, polymers, or even substrates with engineered surface. The thickness of the demonstrated membranes is around 2 µm and the size can be up to a few millimeters. A high reflectivity is observed at the stop band frequency, and a planar defect is introduced during the etching process resulting in an optical resonance mode with a small linewidth of around 30 nm. The structure constitutes an optical bandpass filter and can be used as a sensor for organic solvents.

High-resolution SOFIA/EXES Spectroscopy of SO2 Gas in the Massive Young Stellar Object MonR2 IRS3: Implications for the Sulfur Budget.

Sulfur has been observed to be severely depleted in dense clouds leading to uncertainty in the molecules that contain it and the chemistry behind their evolution. Here, we aim to shed light on the sulfur chemistry in young stellar objects (YSOs) by using high-resolution infrared spectroscopy of absorption by the ν 3 rovibrational band of SO2 obtained with the Echelon-Cross-Echelle Spectrograph on the Stratospheric Observatory for Infrared Astronomy. Using local thermodynamic equilibrium models we derive physical parameters for the SO2 gas in the massive YSO MonR2 IRS3. This yields a SO2/H abundance lower limit of 5.6 ± 0.5 × 10-7, or >4% of the cosmic sulfur budget, and an intrinsic line width (Doppler parameter) of b < 3.20 km s-1. The small line widths and high temperature (T ex = 234 ± 15 K) locate the gas in a relatively quiescent region near the YSO, presumably in the hot core where ices have evaporated. This sublimation unlocks a volatile sulfur reservoir (e.g., sulfur allotropes as detected abundantly in comet 67P/Churyumov-Gerasimenko), which is followed by SO2 formation by warm, dense gas-phase chemistry. The narrowness of the lines makes formation of SO2 from sulfur sputtered off grains in shocks less likely toward MonR2 IRS3.

4 × 128-Gb/s PDM-DMT signal transmission over 1440-km SSMF with high phase noise tolerance.

In this paper, we propose an intensity modulation and coherent detection scheme with high phase noise tolerance for polarization division multiplexing (PDM) discrete multi-tone (DMT) signal transmission by employing Kramers-Kronig (KK) detection and digital carrier regeneration (DCR). At the transmitter side, DMT signal is modulated by a Mach-Zehnder modulator (MZM) setting bias around the null point and transmitted with the suppressed optical carrier. At the receiver side, a directly modulated lasers (DMLs) locating at the edge of DMT signal is used as the local oscillator (LO) for coherent detection. For signal recovery, KK detection is first used to reduce the signal to signal beating noise. Digital optical carrier is then regenerated by the DCR scheme and the DMT signal could be recovered by enveloping detection with the regenerated digital carrier, which can achieve high laser line-width tolerance and mitigate the residual phase noise caused by KK detection. To verify the effectiveness of the proposed scheme, we compare the KK and DCR based receiver digital signal processing (DSP) with conventional receiver DSP by using both DMLs with larger linewidth (~10 MHz) or external cavity lasers (ECLs) with smaller linewidth (less than 100 kHz) as optical carrier and LO. The results show that KK and DCR can mutually improve the system performance with ECLs as optical carrier and LO. Moreover, it is shown that the signal using DMLs cannot be recovered without the DCR method due to the high laser line-width of DML. Finally, we successfully demonstrate 4 × 128-Gb/s KK and DCR based PDM-DMT signal transmission over 1440-km SSMF by employing DMLs as optical carrier and LO.

Analysis of the optical feedback dynamics in InAs/GaAs quantum dot lasers directly grown on silicon

This work reports on a systematic investigation of the influence of optical feedback in InAs/GaAs quantum dot lasers epitaxially grown on silicon. The boundaries associated to the onset of the critical feedback level corresponding to the first Hopf bifurcation are extracted at different bias conditions with respect to the onset of the first excited state transition. Overall, results show that quantum dot lasers directly grown onto silicon are much more resistant to optical feedback than quantum well lasers, mostly resulting from a small linewidth enhancement factor of high-quality quantum dot material. However, results also unveil that the onset of the critical feedback level strongly depends on the excited-to-ground-state ratio, hence a figure of merit showing that a small ratio of the excited-to-ground-state lasing thresholds is not beneficial for maintaining a high degree of stability. This work brings further insights in the understanding of quantum dot laser physics and is useful for designing feedback resistant lasers for isolator-free transmission in metro, access, and data center optical networks, as well as for integrated photonics.

Spectral Tomography for the Line-of-sight Structures of the Taurus Molecular Cloud 1

Abstract We clarify the line-of-sight structure of the Taurus Molecular Cloud 1 (TMC-1) on the basis of the CCS (J N = 43−32) and HC3N (J = 5−4) spectral data observed at a very high velocity resolution and sensitivity of ΔV ≃ 0.0004 km s−1 (=61 Hz) and ΔT mb ≃ 40 mK. The data were obtained toward the cyanopolyyne peak with ∼30 hr integration using the Z45 receiver and the PolariS spectrometer installed in the Nobeyama 45 m telescope. Analyses of the optically thin F = 4−4 and 5−5 hyperfine lines of the HC3N emission show that the spectra consist of four distinct velocity components with a small line width (≲0.1 km s−1) at V LSR = 5.727, 5.901, 6.064, and 6.160 km s−1, which we call A, B, C, and D, respectively, in the order of increasing LSR velocities. Utilizing the velocity information of the four velocity components, we further analyzed the optically thicker CCS spectrum and the other hyperfine lines of the HC3N emission by solving the radiative transfer to investigate how the four velocity components overlap along the line of sight. Results indicate that they are located in the order of A, B, C, and D from far side to near side to the observer, indicating that TMC-1 is shrinking, moving inward as a whole.