Ahmed El Halawany

Coherence measurements of scattered incoherent light for lensless identification of an object’s location and size

In the absence of a lens to form an image, incoherent or partially coherent light scattering off an obstructive or reflective object forms a broad intensity distribution in the far field with only feeble spatial features. We show here that measuring the complex spatial coherence function can help in the identification of the size and location of a one-dimensional object placed in the path of a partially coherent light source. The complex coherence function is measured in the far field through wavefront sampling, which is performed via dynamically reconfigurable slits implemented on a digital micromirror device (DMD). The impact of an object - parameterized by size and location - that either intercepts or reflects incoherent light is studied. The experimental results show that measuring the spatial coherence function as a function of the separation between two slits located symmetrically around the optical axis can identify the object transverse location and angle subtended from the detection plane (the ratio of the object width to the axial distance from the detector). The measurements are in good agreement with numerical simulations of a forward model based on Fresnel propagators. The rapid refresh rate of DMDs may enable real-time operation of such a lensless coherency imaging scheme.

Enhanced UV upconversion emission using plasmonic nanocavities

Strongly enhanced upconversion emission is experimentally demonstrated from an ensemble of β-NaYF₄:Gd³⁺/Yb³⁺/Tm³⁺ @NaLuF₄ core-shell nanoparticles trapped in judiciously designed plasmonic nanocavities. Using cross-shape silver nanocavities, 170-fold enhancement is obtained at UV band around 345 nm.

Passive PT-Symmetric Metasurfaces With Directional Field Scattering Characteristics

We show that passive parity-time (PT) symmetric metasurfaces can be utilized to appropriately engineer the resulting far-field characteristics. Such PT-symmetric structures are capable of eliminating diffraction orders in specific directions, while maintaining or even enhancing the remaining orders. A systematic methodology is developed to implement this class of metasurfaces in both one-dimensional and two-dimensional geometries. In two-dimensional systems, PT symmetry can be established by employing either H-like diffractive elements or diatomic oblique Bravais lattices.

Chalcogenide fibers for improved reliability of active infrared sensing systems (Conference Presentation)

Defense sensing systems must be both productive and robust to accomplish their mission. Active infrared sensing devices consist of many components such as the active medium, mirrors, beam-splitters, modulators, gratings, detectors, etc. Each of these components is subject to damage by the laser beam itself or environmental factors. Misalignment of these components due to vibration and temperatures changes can also reduce performance. The result is a complex and expensive system subject to multiple points of degradation or complete failure. However, beam confinement or “no free-space optics” via fiber transmission and even component assembly within the fiber itself can achieve reliability and low cost for sensing systems with reduced component count and less susceptibility to misalignment. We present measurements of high-power infrared laser beam transmission in chalcogenide fibers. The fiber compositions were As39S61 for the core and As38.5S61:5 for the cladding, resulting in a numerical aperture of 0.2. A polyetherimide jacket provided structural support. Multiwatt CW transmission was demonstrated in near single-mode 12 micron core fiber. Efficient coupling of quantum cascade lasing into anti-reflection coated chalcogenide fiber was also demonstrated. Efficient beam transport without damage to the fiber required careful coupling only into core modes. Beams with M2 ≥ 1.4 and powers higher than 1 W produced damage at the fiber entrance face. This was most likely due to heating of the highly absorptive polymer jacket by power not coupled into core modes. We will discuss current power limitations of chalcogenide fiber and schemes for significantly increasing power handling capabilities.

Mid-infrared performance of single mode chalcogenide fibers

Due to the intrinsic absorption edge in silica near 2.4 μm, more exotic materials are required to transmit laser power in the IR such as fluoride or chalcogenide glasses (ChGs). In particular, ChG fibers offer broad IR transmission with low losses < 1 dB/m. Here, we report on the performance of in-house drawn multi-material chalcogenide fibers at four different infrared wavelengths: 2053 nm, 2520 nm and 4550 nm. Polymer clad ChG fibers were drawn with 12.3 μm and 25 μm core diameters. Testing at 2053 nm was accomplished using a > 15 W, CW Tm:fiber laser. Power handling up to 10.2 W with single mode beam quality has been demonstrated, limited only by the available Tm:fiber output power. Anti-reflective coatings were successfully deposited on the ChG fiber facets, allowing up to 90.6% transmission with 12.2 MW/cm2 intensity on the facet. Single mode guidance at 4550 nm was also demonstrated using a quantum cascade laser (QCL). A custom optical system was constructed to efficiently couple the 0.8 NA QCL radiation into the 0.2 NA ChG fiber, allowing for a maximum of 78% overlap between the QCL radiation and fundamental mode of the fiber. With an AR-coated, 25 μm core diameter fiber, >50 mW transmission was demonstrated with > 87% transmission. Finally, we present results on fiber coupling from a free space Cr:ZnSe resonator at 2520 nm.

Omni-resonant optical micro-cavity

Optical cavities transmit light only at discrete resonant frequencies, which are well-separated in micro-structures. Despite attempts at the construction of planar ‘white-light cavities’, the benefits accrued upon optically interacting with a cavity – such as resonant field buildup – have remained confined to narrow linewidths. Here, we demonstrate achromatic optical transmission through a planar Fabry-Pérot micro-cavity via angularly multiplexed phase-matching that exploits a bio-inspired grating configuration. By correlating each wavelength with an appropriate angle of incidence, a continuous spectrum resonates and the micro-cavity is rendered transparent. The locus of a single-order 0.7-nm-wide resonance is de-slanted in spectral-angular space to become a 60-nm-wide achromatic resonance spanning multiple cavity free-spectral-ranges. The result is an ‘omni-resonant’ planar micro-cavity in which light resonates continuously over a broad spectral span. This approach severs the link between the resonance bandwidth and the cavity-photon lifetime, thereby promising resonant enhancement of linear and nonlinear optical effects over broad bandwidths in ultrathin devices.

Electrically driven single photon source at high temperature

We present a theoretical model for an electrically driven single photon source operating at high temperatures. We show that decoherence, which is usually the main obstacle for operating single photon sources at high temperatures, ensures an efficient operation of the presented electrically driven single photon source at high temperatures. The single-photon source is driven by a single electron source attached to a heterostructure semiconductor nanoring. The electrons dynamics in the nanoring and the subsequent recombination with the hole is described by the generalized master equation with a Hamiltonian based on tight-binding model, taking into account the electron-LO phonon interaction. As a result of decoherence, an almost 100% single photon emission with a strong antibunching behavior i.e. g(2)(0) << 1 at high temperature up to 300 K is achieved.