Ultralow-light-level color image reconstruction using high-efficiency plasmonic metasurface mosaic filters

As single-photon imaging becomes progressively more commonplace in sensing applications such as low-light-level imaging, three-dimensional profiling, and fluorescence imaging, there exist a number of fields where multispectral information can also be exploited, e.g., in environmental monitoring and target identification. We have fabricated a high-transmittance mosaic filter array, where each optical filter was composed of a plasmonic metasurface fabricated in a single lithographic step. This plasmonic metasurface design utilized an array of elliptical and circular nanoholes, which produced enhanced optical coupling between multiple plasmonic interactions. The resulting metasurfaces produced narrow bandpass filters for blue, green, and red light with peak transmission efficiencies of 79%, 75%, and 68%, respectively. After the three metasurface filter designs were arranged in a 64×64 format random mosaic pattern, this mosaic filter was directly integrated onto a CMOS single-photon avalanche diode detector array. Color images were then reconstructed at light levels as low as approximately 5 photons per pixel, on average, via the simultaneous acquisition of low-photon multispectral data using both three-color active laser illumination and a broadband white-light illumination source

Si-based n-type THz Quantum Cascade Emitter

Employing electronic transitions in the conduction band of semiconductor heterostructures paves a way to integrate a light source into silicon-based technology. To date all electroluminescence demonstrations of Si-based heterostructures have been p-type using hole-hole transitions. In the pathway of realizing an n-type Ge/SiGe terahertz quantum cascade laser, we present electroluminescence measurements of quantum cascade structures with top diffraction gratings. The devices for surface emission have been fabricated out of a 4-well quantum cascade laser design with 30 periods. An optical signal was observed with a maximum between 8-9 meV and full width at half maximum of roughly 4 meV

n-type Ge/SiGe multi quantum-wells for a THz quantum cascade laser

Exploiting intersubband transitions in Ge/SiGe quantum cascade devices provides a way to integrate terahertz light emitters into silicon-based technology. With the view to realizing a Ge/SiGe Quantum Cascade Laser, we present the optical and structural properties of n-type strain-symmetrized Ge/SiGe asymmetric coupled quantum wells grown on Si(001) substrates by means of ultrahigh vacuum chemical vapor deposition. We demonstrate the high material quality of strain-symmetrized structures and heterointerfaces as well as control over the inter-well coupling and electron tunneling. Motivated by the promising results obtained on ACQWs, which are the basic building block of a cascade structure, we investigate, both experimentally and theoretically, a Ge/SiGe THz QCL design, optimized through a non-equilibrium Green's function formalism

Electron-doped SiGe Quantum Well Terahertz Emitters pumped by FEL pulses

We explore saturable absorption and terahertz photoluminescence emission in a set of n-doped Ge/SiGe asymmetric coupled quantum wells, designed as three-level systems (i.e., quantum fountain emitter). We generate a non-equilibrium population by optical pumping at the 1→3 transition energy using picosecond pulses from a free-electron laser and characterize this effect by measuring absorption as a function of the pump intensity. In the emission experiment we observe weak emission peaks in the 14–25 meV range (3–6 THz) corresponding to the two intermediate intersubband transition energies. The results represent a step towards silicon-based integrated terahertz emitters

Plasmonic filters for ambient and near infrared sensing on CMOS

The light sensors market is growing, driven largely by increased use of proximity detection and ambient light sensing (ALS) in consumer electronics. There is high demand for reduced cost and physical size of light sensors, however the spectral filter technology used on complementary metal-oxide semiconductor (CMOS) chips has not advanced significantly. Plasmonic filters have been proposed as a superior alternative offering reduced cost and thickness, among other advantages. In this work plasmonic filters are investigated in the near infrared (NIR) range for proximity sensing applications, and the visible range for ALS applications using CMOS compatible materials and fabrication processes. The plasmonic filters are thin metallic films nanostructured with an array of subwavelength holes that facilitate coupling with surface plasmon polaritons (SPP) and localised surface plasmons (LSP). They exhibit extraordinary optical transmission with peak transmission wavelengths controlled by the geometry and size of the hole array. Filters were designed on glass substrate by electromagnetic simulations using a finite-difference time-domain (FDTD) method, created using micro and nano-fabrication techniques, and then measured by microspectrophotometry to evaluate their spectral response. Following characterisation, the NIR filter was fabricated directly onto a CMOS chip and the spectral response was assessed by chip measurement for a proof-of-concept demonstration of an integrated device. The NIR plasmonic filter exhibited poor suitability on CMOS due to high order plasmonic resonances in the visible range that were enhanced by Fabry-Pérot resonances supported by the CMOS stack. The most common plasmonic filter, a circular-shaped hole nanostructure, is sensitive to angle of incidence (AOI) making it unsuitable for ALS applications. Preliminary designs for plasmonic ALS filters with low sensitivity to AOI were demonstrated, by characterisation on glass, using a cross-shaped hole nanostructure. Design dimensions that produced this quality were decreased array period and decreased ratio of the cross arm-length to arm-width, due to increased separation between the SPP and LSP resonances generated by the plasmonic hole array filter

Electron State Coupling in asymmetric Ge/SiGe quantum Wells (Conference Presentation)

No abstract available