Linewidth Δλ Research Articles

Detecting single nanoparticles using fiber-tip nanophotonics

Sensing nano-objects, from nanoparticles to molecules, has become a crucial need in environmental monitoring, medical diagnostics, and drug development. Detection of single particles and molecules is highly desirable, as it provides specific information on size, dynamics, and interactions. Current nanophotonic implementations rely on complex optical readout schemes, limiting their application in the field. Here we demonstrate a nanophotonic fiber-tip sensor with a compact sensor footprint and a simple readout scheme. We leverage advanced design methods to simultaneously achieve a small mode volume V m =0.74(λ/n)3, narrow linewidth Δλ=0.4nm, and a large modulation ΔR≈20% in reflection from the fiber. This unique combination of properties opens the way to sensing weak nanoscale perturbations in the vicinity of the fiber tip. In particular, we experimentally demonstrate the real-time detection of single 50 nm nanoparticles. This opens a route towards real-time sensing of single nanoparticles, and potentially single molecules, in environmental monitoring and diagnostics.

Midwave resonant cavity infrared detectors (RCIDs) with suppressed background noise.

We report a resonant cavity infrared detector (RCID) with an InAsSb/InAs superlattice absorber with a thickness of only ≈ 100 nm, a 33-period GaAs/Al0.92Ga0.08As distributed Bragg reflector bottom mirror, and a Ge/SiO2/Ge top mirror. At a low bias voltage of 150 mV, the external quantum efficiency (EQE) reaches 58% at the resonance wavelength λres ≈ 4.6 µm, with linewidth δλ = 19-27 nm. The thermal background current for a realistic system scenario with f/4 optic that views a 300 K scene is estimated by integrating the photocurrent generated by background spanning the entire mid-IR spectral band (3-5 µm). The resulting specific detectivity is a factor of 3 lower than for a state-of-the-art broadband HgCdTe device at 300 K, where dark current dominates the noise. However, at 125 K where the suppression of background noise becomes critical, the estimated specific detectivity D* of 5.5 × 1012 cm Hz½/W is more than 3× higher. This occurs despite a non-optimal absorber cut-off that causes the EQE to decrease rapidly with decreasing temperature, e.g., to 33% at 125 K. The present RCID's advantage over the broadband device depends critically on its low EQE at non-resonance wavelengths: ≤ 1% in the range 3.9-5.5 µm. Simulations using NRL MULTIBANDS indicate that impact ionization in the bottom contact and absorber layers dominates the dark current at near ambient temperatures. We expect future design modifications to substantially enhance D* throughout the investigated temperature range of 100-300 K.

Irregular Spatial Distributions of Spectral Line Parameters in the Middle Solar Chromosphere Revealed from Analysis of Solar Flash Mg i b 2 Spectra

This paper presents the analytical results of solar Mg i b 2 flash spectra, obtained by the prototype Fiber Arrayed Solar Optic Telescope in process of the 2013 Gabon total solar eclipse. The analysis reveals irregular distributions of the spectral line parameters like ratio of line source function to continuum one β, ratio of line emissivity to continuum emissivity ζ, ratio of the continuum opacity to the line opacity r 0, line center optical depth τ 0, the line width Δλ D , and the line-of-sight velocity v los, while the approximately spherical symmetry can be found in the maps of integrated line intensity and continuum intensity. These irregular distributions originate from those of line profile features like the maximum intensity, the line width and line center wavelength. It is also found from the recovered line center optical depth τ 0 that in the middle chromosphere, the optical depth is not small due to non-ignorable absorption and the long light path along the line-of-sight. Finally, we show that the excessive broadening of spectral lines can be due to co-existence of multiple radiative sources with different line-of-sight velocities unresolved in one detector pixel.

High-Performance Plasmonic Nanolasers with a Nanotrench Defect Cavity for Sensing Applications

Recent developments in small footprint plasmonic nanolasers show promise for active optical sensing with potential applications in various fields, including real-time and label-free biochemical sensing, and gas detection. In this study, we demonstrate a novel hybrid plasmonic crystal nanolaser that features a ZnO nanowire placed on Al grating surfaces with a nanotrench defect nanocavity. The lasing action of gain-assisted defect nanocavity overcomes the ohmic loss parasitically in the plasmonic nanostructures. Therefore, the plasmonic nanolaser exhibits an extremely small mode volume, a narrow linewidth Δλ, and a high Purcell factor that can facilitate the strong interaction between light and matter. This can be used as a refractive index sensor and is highly sensitive to local changes in the refractive indices of ambient materials. By careful design, the near-ultraviolet nanolaser sensors have significant sensing performances of glucose solutions, revealing a high sensitivity of 249 nm/RIU and high resol...

Surface Emission Vertical Cavity Transistor Laser

We demonstrate the first surface emission vertical cavity transistor laser (VCTL) operation with an InGaP heterojunction bipolar transistor incorporating the InGaAs quantum-wells in the base and vertical distributed Bragg reflectors. The transistor collector I-V characteristics show gain (β = ΔI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> /ΔI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> ) compression, β decreasing from 0.52 to 0.47, due to the base recombination shifting from spontaneous to stimulated with increasing base current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> >; I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> ). The surface emission VCTL threshold current is I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> ~ 3 mA for a cavity of 9×6 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> lateral dimensions. The laser spectra at IB = 10 mA exhibit two peaks at 975.12 and 975.22 nm with linewidth Δλ ~ 0.7 Å.

Laser-induced fluorescence diagnostic for measuring magnetic fields (abstract)

A novel, spatially resolved diagnostic is being developed to measure magnetic fields associated with intense ion beam propagation through a low-pressure gas, as is envisioned for ion-driven inertial confinement fusion. The diagnostic technique uses laser-induced fluorescence spectroscopy, and can be varied to measure either small or large fields. Small fields, as expected in ballistic beam transport, produce Zeeman shifts, ΔλZ, much smaller than the transition linewidth Δλ. High sensitivity to measure these sub-Doppler shifts is achieved by a variation on the Babcock technique.Reference 1 Large fields, as expected in self-pinched beam transport, produce ΔλZ larger than Δλ. These ΔλZ will be resolved using a Fabry–Pérot interferometer, in a novel technique similar to that for small fields. Results from benchtop experiments using calibrated B fields for the small-field technique will be presented. Available results from a proof-of-principle experiment for the large-field technique will also be presented.

The results of lasing linewidth narrowing on the VEPP-3 storage ring optical klystron

Lasing in visible and ultraviolet ranges was obtained using the optical klystron installed on VEPP-3 storage ring in 1988 [I.B. Drobyazko et al., Nucl. Instr. and Meth. A282 (1989) 424] with minimum relative linewidth Δλ λ = 10 −4 . To decrease the linewidth we have performed the following experiments. The optical cavity of the VEPP-3 storage ring optical klystron has been updated to install intracavity selective elements. We used the simplest selective element — a glass plate with parallel faces as a natural interferometer. Using a 1.2 mm thick glass plate installed inside the optical cavity we have reached lasing with a very narrow linewidth. The minimum relative lasing linewidth, 2.7 × 10 −6 ( λ = 6250 A ̊ , Δλ = 0.017 A ̊ ), was 30 times narrower than the minimum one without the plate ( Δλ = 0.6 A ̊ ). The average power was the same in both cases. Experiments on lasing linewidth narrowing with the use of the plates with different thickness are under way.

Monochromatic x-ray emission of a fully ionized hydrogen plasma

Subnanosecond emission phenomena have been studied at a modified 1 kJ Mather type plasma focus device. High energy particle beams, nonthermal infrared and microwave emission as well as anisotropic soft X-rays are observed. In the plasma a hydrodynamic instability leads to a final radius of r ⋍ 50 μ m and a length of l = 500 μm through which a current of l = 200 kA is driven. Induced by the high power density, microinstabilities modulate the plasma density periodically. Due to particle-wave interaction, electrons and ions are accelerated to energies beyond 1 MeV. The electron beam is emitted in pulses with a pulse period of about 10–30 ps and a pulse width in the range of 1–3 ps. Simultaneously, anisotropic emission of soft X-rays at a wavelength of about λ = 1 nm into a cone with a half angle θ = 0.2 rad is observed. The relative line width Δλ λ = 2×10 −3 , indicates a temporal coherent emission of X-radiation by the fully ionized hydrogen plasma. Assuming an FEL-like process as generation mechanism of the soft X-rays, the periods length of the density structure and the relativistic factor γ of the electron beam are determined as λ ⋍ 17 nm and γ ⋍ 3 . The observed phenomena in the plasma can be theoretically described as a self-organization process in terms of a three-wave-coupling model of a beam-plasma system.

Superradiative rare gas halide lasers excited by electric discharge

High-power stimulated radiation was obtained in three fluorides of rare gases by exciting a high pressure mixture of He+R+SF6 (R is argon, krypton, xenon) by a pulse electric discharge (mixture pressure ranged from 1 to 10 atm). The ArF laser superradiance spectrum of about 15 A total width with the centre at λ=1932 A is given. The ArF laser power wasP≈104 W. The KrF laser generated 2 ns pulses of about 2.5 MW power at λ=2486 A with a linewidth Δλ≈10 A. This laser efficiency in relation to the stored energy of an accumulator is about 0.2%. In the XeF laserP=200 to 300 kW, τ=3 to 3.5 ns, λ=3511, 3531 A. High density of superradiance power is observed. The prospects of increase of mean power of excimer lasers are discussed.