Imad Agha

Low control-power wavelength conversion on a silicon chip

We demonstrate controlled wavelength conversion on a silicon chip based on four-wave mixing Bragg scattering (FWM-BS). A total conversion efficiency of 5% is achieved with strongly unbalanced pumps and a controlling peak power of 55 mW, while the efficiency is over 15% when using less asymmetric pumps. The numerical simulation agrees with the experimental results. Both time domain and spectral domain noise measurements show as low as 2 dB signal-to-noise ratio (SNR) penalty because of the strong pump noise, two-photon absorption, and free-carrier absorption in silicon. We discuss how the scheme can be used to implement an all-optically controlled high-speed switch.

SU-8 nanoimprint fabrication of wire-grid polarizers using deep-UV interference lithography

We describe a new fabrication method for making wire-grid polarizers for the visible and near-IR based on deep-UV interference lithography, nanoimprint, and glancing angle deposition. We fabricated aluminum wire grids with periods ranging from 375 to 230 nm with heights from 145 to 110 nm, respectively. The measured extinction ratio was as high as 220:1 at 1064 nm. The performance of the polarizer is limited by the roughness and porosity of the Al film and the underlying SU-8 structure. This method allows patterning of wire grids on any substrate material, which makes this an attractive method for fabricating wire-grid micropolarizers in a timely and cost-effective manner.

Unraveling delocalized electrons in metal induced gap states from second harmonics

Second harmonic generation from Au-Al2O3 interfaces is analyzed to estimate the density of delocalized electrons occupying metal induced gap states (MIGS). Laser light of wavelength 810 nm is incident on an Au substrate and the second harmonic at 405 nm is monitored, where the area fraction of Al2O3 coverage on Au is precisely controlled via atomic layer deposition—from no coverage to full coverage. Extensive electromagnetic simulations are performed using a phenomenological model containing a dimensionless MIGS factor “α,” to represent the strength of the delocalized electrons in MIGS in attenuating the second harmonic signal. By fitting the model to experimental data, an α = 0.13 is obtained leading to a room temperature, areal density of delocalized electrons of (3.53 ± 0.4) × 1014 cm−2 for the Au-Al2O3 interface and representing a 44% occupancy of MIGS.

Theoertical investigation of quantum waveform shaping for single photon emitters.

We investigate a new technique for quantum-compatible waveform shaping that extends the time lens method, and relies only on phase operations. Under realistic experimental conditions, we show that it is possible to both temporally compress and shape optical waveforms in the nanosecond to tens of picoseconds range, which is generally difficult to achieve using standard dispersive pulse-shaping techniques.

Mode hybridization in lattice induced transparency for polarization-insensitive THz metasurfaces

We demonstrate multiple lattice-induced transparencies by decoupling the first-order lattice mode achieving a measured Q factor of 40 and support measurements with numerical calculations revealing bright dipole resonances.

Improving the performance of Ge2Sb2Te5 materials via nickel doping: Towards RF-compatible phase-change devices

High-speed electrical switching of Ge2Sb2Te5 (GST) remains a challenging task due to the large impedance mismatch between the low-conductivity amorphous state and the high-conductivity crystalline state. In this letter, we demonstrate an effective doping scheme using nickel to reduce the resistivity contrast between the amorphous and crystalline states by nearly three orders of magnitude. Most importantly, our results show that doping produces the desired electrical performance without adversely affecting the films optical properties. The nickel doping level is approximately 2% and the lattice structure remains nearly unchanged when compared with undoped-GST. The refractive indices in amorphous and crystalline states were obtained using ellipsometry which echoes the results of X-ray diffraction. The materials thermal transport properties are measured using time-domain thermoreflectance, showing no change upon doping. The advantages of this doping system will open up opportunities for designing electrically reconfigurable high speed optical elements in the near-infrared spectrum.High-speed electrical switching of Ge2Sb2Te5 (GST) remains a challenging task due to the large impedance mismatch between the low-conductivity amorphous state and the high-conductivity crystalline state. In this letter, we demonstrate an effective doping scheme using nickel to reduce the resistivity contrast between the amorphous and crystalline states by nearly three orders of magnitude. Most importantly, our results show that doping produces the desired electrical performance without adversely affecting the films optical properties. The nickel doping level is approximately 2% and the lattice structure remains nearly unchanged when compared with undoped-GST. The refractive indices in amorphous and crystalline states were obtained using ellipsometry which echoes the results of X-ray diffraction. The materials thermal transport properties are measured using time-domain thermoreflectance, showing no change upon doping. The advantages of this doping system will open up opportunities for designing electrica...

All-optical switch based on 4-wave mixing in Si waveguides

Four-wave mixing has been proposed as means for low-noise, low-power all-optical control of light on chip. In our recent work, we have experimentally demonstrated a transistor-like all-optical logic gate in a silicon waveguide. This gate is optimal when operated in the SWIR and mid-IR regimes.

Low switching power low-noise wavelength conversion in silicon-on-insulator waveguides

We demonstrate low-noise low-power four-wave mixing Bragg scattering in a silicon-on-insulator platform as a possible controlled all-optical switch. Efficiency and quantum noise measurements are performed to verify the viability of the device.

Single-photon-compatible spectral broadening and shaping via nonlinear mixing and phase modulation

We experimentally demonstrate spectral broadening and shaping of weak mono-exponentially decaying pulses via nonlinear mixing and phase modulation. This method is compatible with single photons wavepackets generated by quantum emitters.