arXiv subjects

Axel Scherer

Publications and source records attributed to Axel Scherer.

11 recordsLinked to original sources

arXiv2012

Higher-order defect mode laser in an optically thick photonic crystal slab

The use of an optically thick slab may provide versatile solutions for the realization of a current injection type laser using photonic crystals. Here, we show that a transversely higher-order defect mode can be designed to be confined by a photonic band gap in such a thick slab. Using simulations, we show that a high-Q of 10^5 is possible from a finely tuned second-order hexapole mode. Experimentally, we achieve optically pumped pulsed lasing at 1347 nm from the second-order hexapole mode with a peak threshold pump power of 88 uW.

physics.optics↗

arXiv2012

High-Q impurity photon states bounded by a photonic-band-pseudogap in an optically-thick photonic-crystal slab

We show that, taking a two-dimensional photonic-crystal slab system as an example, surprisingly high quality factors (Q) over 10^5 are achievable, even in the absence of a rigorous photonic-band-gap. We find that the density of in-plane Bloch modes can be controlled by creating additional photon feedback from a finite-size photonic-crystal boundary that serves as a low-Q resonator. This mechanism enables significant reduction in the coupling strength between the bound state and the extended Bloch modes by more than a factor of 40.

physics.optics↗

arXiv2011

A photonic crystal nanocavity laser in an optically very thick slab

A photonic crystal (PhC) nanocavity formed in an optically very thick slab can support reasonably high-Q modes for lasing. Experimentally, we demonstrate room-temperature pulsed lasing operation from the PhC dipole mode emitting at 1324 nm, which is fabricated in an InGaAsP slab with thickness (T) of 606 nm. Numerical simulation reveals that, when T > 800 nm, over 90% of the laser output power couples to the PhC slab modes, suggesting a new route towards an efficient in-plane laser for photonic integrated circuits.

physics.optics↗

arXiv2011

From vertical-cavities to hybrid metal/photonic-crystal nanocavities: Towards high-efficiency nanolasers

We provide a numerical study showing that a bottom reflector is indispensable to achieve unidirectional emission from a photonic-crystal (PhC) nanolaser. First, we study a PhC slab nanocavity suspended over a flat mirror formed by a dielectric or metal substrate. We find that the laser's vertical emission can be enhanced by more than a factor of six compared with the device in the absence of the mirror. Then, we study the situation where the PhC nanocavity is in contact with a flat metal surface. The underlying metal substrate may serve as both an electrical current pathway and a heat sink, which would help achieve continuous-wave lasing operation at room-temperature. The design of the laser emitting at 1.3 um reveals that relatively high cavity Q of over 1,000 is achievable assuming room-temperature gold as a substrate. Furthermore, linearly-polarized unidirectional vertical emission with the radiation efficiency over 50 % can be achieved. Finally, we discuss how this hybrid design relates to various plasmonic cavities and propose a useful quantitative measure of the degree of the `plasmonic' character in a general metallic nanocavity.

physics.optics↗

arXiv2006

Terahertz All-Optical Modulation in a Silicon-Polymer Hybrid System

Although Gigahertz-scale free-carrier modulators have been previously demonstrated in silicon, intensity modulators operating at Terahertz speeds have not been reported because of silicon's weak ultrafast optical nonlinearity. We have demonstrated intensity modulation of light with light in a silicon-polymer integrated waveguide device, based on the all-optical Kerr effect - the same ultrafast effect used in four-wave mixing. Direct measurements of time-domain intensity modulation are made at speeds of 10 GHz. We showed experimentally that the ultrafast mechanism of this modulation functions at the optical frequency through spectral measurements, and that intensity modulation at frequencies in excess of 1 THz can be obtained in this device. By integrating optical polymers through evanescent coupling to high-mode-confinement silicon waveguides, we greatly increase the effective nonlinearity of the waveguide for cross-phase modulation. The combination of high mode confinement, multiple integrated optical components, and high nonlinearities produces all-optical ultrafast devices operating at continuous-wave power levels compatible with telecommunication systems. Although far from commercial radio frequency optical modulator standards in terms of extinction, these devices are a first step in development of large-scale integrated ultrafast optical logic in silicon, and are two orders of magnitude faster than previously reported silicon devices.

physics.optics↗

arXiv2006

Stray Field Magnetic Resonance Tomography using Ferromagnetic Spheres

The methodology for obtaining two- and three-dimensional magnetic resonance images by using azimuthally symmetric dipolar magnetic fields from ferromagnetic spheres is described. We utilize the symmetric property of a geometric sphere in the presence of a large externally applied magnetic field to demonstrate that a complete two- or three-dimensional structured rendering of a sample can be obtained without the motion of the sample relative to the sphere. Sequential positioning of the integrated sample-sphere system in an external magnetic field at various angular orientations provides all the required imaging slices for successful computerized tomographic image reconstruction. The elimination of the requirement to scan the sample relative to the ferromagnetic tip in this imaging protocol is a potentially valuable simplification compared to previous scanning probe magnetic resonance imaging proposals.

physics.ins-det↗

arXiv2004

Nanomagnetic Planar Magnetic Resonance Microscopy Lens

The achievement of three-dimensional atomic resolution magnetic resonance microscopy remains one of the main challenges in the visualization of biological molecules. The prospects for single spin microscopy have come tantalizingly close due to the recent developments in sensitive instrumentation. Despite the single spin detection capability in systems of spatially well-isolated spins, the challenge that remains is the creation of conditions in space where only a single spin is resonant and detected in the presence of other spins in its natural dense spin environment. We present a nanomagnetic planar design where a localized Angstrom-scale point in three-dimensional space is created above the nanostructure with a non-zero minimum of the magnetic field magnitude. The design thereby represents a magnetic resonance microscopy lens where potentially only a single spin located in the focus spot of the structure is resonant. Despite the presence of other spins in the Angstrom-scale vicinity of the resonant spin, the high gradient magnetic field of the lens renders those spins inactive in the detection process.

quant-ph↗

arXiv2003

Two-Dimensional Magnetic Resonance Tomographic Microscopy using Ferromagnetic Probes

We introduce the concept of computerized tomographic microscopy in magnetic resonance imaging using the magnetic fields and field gradients from a ferromagnetic probe. We investigate a configuration where a two-dimensional sample is under the influence of a large static polarizing field, a small perpendicular radio-frequency field, and a magnetic field from a ferromagnetic sphere. We demonstrate that, despite the non-uniform and non-linear nature of the fields from a microscopic magnetic sphere, the concepts of computerized tomography can be applied to obtain proper image reconstruction from the original spectral data by sequentially varying the relative sample-sphere angular orientation. The analysis shows that the recent proposal for atomic resolution magnetic resonance imaging of discrete periodic crystal lattice planes using ferromagnetic probes can also be extended to two-dimensional imaging of non-crystalline samples with resolution ranging from micrometer to Angstrom scales.

physics.ins-det↗

arXiv2003

An atom mirror etched from a hard drive

We describe the fabrication of an atom mirror by etching of a common hard drive, and we report the observation of specular retroreflection of 11 uk cesium atoms using this mirror. The atoms were trapped and cooled above the hard drive using the mirror magneto-optical trap technique, and upon release, two full bounces were detected. The hard drive atom mirror will be a useful tool for both atom optics and quantum computation.

quant-ph↗

arXiv2002

Optimization of three-dimensional micropost microcavities for cavity quantum electrodynamics

This article presents a detailed analysis, based on the first-principles finite-difference time-domain method, of the resonant frequency, quality factor (Q), mode volume (V), and radiation pattern of the fundamental (HE11) mode in a three-dimensional distributed-Bragg-reflector (DBR) micropost microcavity. By treating this structure as a one-dimensional cylindrical photonic crystal containing a single defect, we are able to push the limits of Q/V beyond those achievable by standard micropost designs, based on the simple rules established for planar DBR microcavities. We show that some of the rules that work well for designing large-diameter microposts (e.g., high-refractive index contrast) fail to provide high-quality cavities with small diameters. By tuning the thicknesses of mirror layers and the spacer, the number of mirror pairs, the refractive indices of high and low refractive index regions, and the cavity diameter, we are able to achieve Q as high as 10^4, together with a mode volume of 1.6 cubic wavelengths of light in the high-refractive-index material. The combination of high Q and small V makes these structures promising candidates for the observation of such cavity quantum electrodynamics phenomena as strong coupling between a quantum dot and the cavity field, and single-quantum-dot lasing.

quant-ph↗

arXiv2002

Design of photonic crystal microcavities for cavity QED

We discuss the optimization of optical microcavity designs based on 2D photonic crystals for the purpose of strong coupling between the cavity field and a single neutral atom trapped within a hole. We present numerical predictions for the quality factors and mode volumes of localized defect modes as a function of geometric parameters, and discuss some experimental challenges related to the coupling of a defect cavity to gas-phase atoms.

quant-ph↗