SciCombinator

Discover the most talked about and latest scientific content & concepts.

Journal: Optics letters

27

We report on stacked high-contrast grating reflectors with virtually angular independent reflectance for transverse-magnetic polarized light. The investigated structure consists of two-layer pairs of amorphous silicon and silicondioxide that are designed for a wavelengths of 1550 nm. The large angular tolerance results from coupling of the two involved silicon gratings and is achieved if the modal fields in the reflectors are matched. With this approach, a reflectance of more than 96% in the entire angular spectrum is feasible. Experimentally we demonstrate a reflectance of more than 98% for incidence angles up to 60° and more than 90% up to 80°.

Concepts: Optics, Fundamental physics concepts, Light, Electromagnetic radiation, Wavelength, Polarization, Amorphous silicon, Diffraction grating

27

In this Letter, multifocus optical-resolution photoacoustic microscopy is demonstrated using wavelength tuning and chromatic aberration for depth scanning. Discrete focal zones at several depth locations were created by refocusing light from a polarization-maintaining single-mode fiber pumped by a nanosecond fiber laser. The fiber and laser parameters were chosen to take advantage of stimulated Raman scattering (SRS) in the fiber to create a multiwavelength output that could then be bandpass filtered. The collimator lens and objective lens are chosen to take advantage of chromatic aberration in which each generated SRS wavelength peak focuses at a slightly different depth. The maximum amplitude of photoacoustic signals is mapped to form C-scan images. Additionally, all wavelength peaks fired simultaneously offers improved depth-of-field structural imaging at the cost of slight degradation of mainlobe-to-sidelobe ratios. Wavelength-tuned depth scanning over more than 440 μm is demonstrated, significantly greater than the ∼100  μm depth of field predicted from our focused Gaussian beams. The improved depth of focus could be valuable for structural imaging of microvascular morphology without the need for mechanical scanning in the depth direction.

Concepts: Optics, Laser, Lens, Raman scattering, Geometrical optics, Rayleigh scattering, Depth of field, Fiber laser

26

We report on a noncontact photoacoustic imaging system utilizing an all-fiber-optic heterodyne interferometer as an acoustic wave detector. The acoustic wave generated by a short laser pulse via the photoacoustic effect and arriving at the sample surface could be detected with the fiber-optic heterodyne interferometer without physical contact or using an impedance matching medium. A phantom experiment was conducted to evaluate the proposed system, and the initial acoustic pressure distribution was calculated using a Fourier-based reconstruction algorithm. It is expected that the all-fiber-optic configuration of the proposed system can be applied as a minimally invasive diagnostic tool.

Concepts: Laser, Medical imaging, Minimally invasive, Acoustics, Pressure, Standing wave, Transmission line, Optical imaging

26

We fabricated and measured a compact 3 dB hybrid plasmonic directional coupler for silicon photonics integrated circuits with a length of 21.2 μm. The coupler has a 50∶50 coupling ratio over a spectral bandwidth of more than 100 nm around a wavelength of 1.55 μm and has an insertion loss of less than 1 dB.

Concepts: Integrated circuit, Silicon, Units of measurement, Silicon photonics

26

In this work, single-nanowire solar cells (SCs) in lying configuration with a tunable rear metallic film are presented. Compared to the standalone silicon nanowire (SiNW), a significant/broadband enhancement in the overall optical absorption of the SiNW with rear metallic nanoconfiguration was observed. The optimized fraction, corresponding to the maximal ultimate photocurrent, of the SiNW surrounded by metallic film was achieved by properly engineering the SiNW radius and metallic film thickness. The considered configuration can be effectively extended to the alternative configuration with a SiNW partially embedded in a metallic substrate, where the ultimate photocurrent density of the 100 nm radius SiNW can be improved by 75.50% relative to that of the freestanding SiNW. Our simulations verify that it is a promising route for developing highly efficient single nanowire SCs.

Concepts: Photon, Photoelectric effect, Density, Solar cell, Absorption, Photovoltaics, Germanium, The Alternative

26

We developed a novel universal eigenvalue analysis for 2D arbitrary nanostructures comprising dispersive and lossy materials. The complex dispersion relation (or complex Bloch band structure) of a metallic grating is rigorously calculated by the proposed algorithm with the finite-difference implementation. The abnormally large group velocity is observed at a plasmonic band edge with a large attenuation constant. Interestingly, we found the abnormal group velocity is caused by the leaky (radiation) loss, not by metallic absorption (ohmic) loss. The periodically modulated surface of the grating significantly modifies the original dispersion relation of the semi-infinite dielectric-metal structure and induces the extraordinarily large group velocity, which is different from the near-zero group velocity at photonic band edge. The work is fundamentally important to the design of plasmonic nanostructures.

Concepts: Fundamental physics concepts, Group velocity, Wave propagation

26

We report on an 800 nm center-wavelength metal/multilayer-dielectric grating (MMDG) with broadband, high diffraction efficiency. The trapezoidal grating ridge consists of an HfO2 layer sandwiched between two SiO2 films. Combining the advantages of SiO2 and HfO2, the grating ridge reduces the difficulties of grating ridge attainment. For such a configuration, high-performance MMDG can be successfully fabricated using the existing technology. Experimentally we demonstrated a 163 nm bandwidth MMDG with -1st-order diffraction efficiency greater than 90%. The fabricated MMDG achieved high performance as the design with large fabrication tolerances.

Concepts: Grating, Diffraction grating, High-performance computing

25

A 10  m/25  Gbps light-based WiFi (LiFi) transmission system based on a two-stage injection-locked 680 nm vertical-cavity surface-emitting laser (VCSEL) transmitter is proposed. A LiFi transmission system with a data rate of 25 Gbps is experimentally demonstrated over a 10 m free-space link. To the best of our knowledge, it is the first time a two-stage injection-locked 680 nm VCSEL transmitter in a 10  m/25  Gbps LiFi transmission system has been employed. Impressive bit error rate performance and a clear eye diagram are achieved in the proposed systems. Such a 10  m/25  Gbps LiFi transmission system provides the advantage of a communication link for higher data rates that could accelerate the deployment of visible laser light communication.

Concepts: Light, Laser, Laser diode, Data transmission, Radio, Lasers and aviation safety, Bit rate, Kilobit

25

We consider the Rabi oscillation of an atom ensemble of Gaussian spatial distribution interacting with ultrafast laser pulses. Based on an analytical model calculation, we show that its dephasing dynamics is solely governed by the size ratio between the atom ensemble and the laser beam, and that every oscillation peak of the inhomogeneously broadened Rabi flopping falls on the homogeneous Rabi oscillation curve. The results are verified experimentally with a cold rubidium vapor in a magneto-optical trap. As a robust means to achieve higher-fidelity population inversion of the atom ensemble, we demonstrate a spin-echo type Rx(π/2)Ry(π)Rx(π/2) composite interaction as well.

Concepts: Laser, Stimulated emission, Oscillation, Feedback, Quantum optics, Optical pumping, Rabi cycle, Vacuum Rabi oscillation

25

Here we report 10 laser emission lines in the attractive deep blue to cyan spectral region from an intracavity frequency doubled Raman laser. The fundamental laser field that drives the Raman laser is based on the three-level transition of Nd:YLF. A maximum extracted quasi-continuous wave (qcw) output power of 0.94 W is achieved in the deep blue to cyan spectral regime.

Concepts: Photon, Optics, Light, Laser, Raman scattering, Nonlinear optics, Lasers, Raman laser