Alan D. Bristow

Two-photon absorption and Kerr coefficients of silicon for 850–2200nm

The degenerate two-photon absorption coefficient β and Kerr nonlinearity n2 are measured for bulk Si at 300K using 200fs pulses with carrier wavelength of 850<λ<2200nm for which indirect gap transitions occur. With a broad peak near the indirect gap and maximum value of 2±0.5cm∕GW, the dispersion of β compares favorably with theoretical calculations of Garcia and Kalyanaraman [J. Phys. B 39, 2737 (2006)]. Within our wavelength range, n2 varies by a factor of 4 with a peak value of 1.2×10−13cm2∕W at λ=1800nm.

Enhanced second-harmonic generation in AlGaAs microring resonators

Highly efficient second-harmonic generation can be achieved by harnessing resonance effects in microring resonator structures. We propose an angular quasi-phase-matching scheme based on the position dependence of polarization inside the ring resonator.

Ultrafast carrier dynamics in thin-films of the topological insulator Bi2Se3

Transient reflectivity measurements of thin films, ranging from 6 to 40u2009nm in thickness, of the topological insulator Bi2Se3 reveal a strong dependence of the carrier relaxation time on the film thickness. For thicker films, the relaxation dynamics are similar to those of bulk Bi2Se3, where the contribution of the bulk insulating phase dominates over that of the surface metallic phase. The carrier relaxation time shortens with decreasing film thickness, reaching values comparable to those of noble metals. This effect may result from the hybridization of Dirac cone states at the opposite surfaces for the thinnest films.

Preparation, characterization, and electrical properties of epitaxial NbO2 thin film lateral devices

Epitaxial NbO2 (110) films, 20 nm thick, were grown by pulsed laser deposition on Al2O3 (0001) substrates. The Ar/O2 total pressure during growth was varied to demonstrate the gradual transformation between NbO2 and Nb2O5 phases, which was verified using x-ray diffraction, x-ray photoelectron spectroscopy, and optical absorption measurements. Electric resistance threshold switching characteristics were studied in a lateral geometry using interdigitated Pt top electrodes in order to preserve the epitaxial crystalline quality of the films. Volatile and reversible transitions between high and low resistance states were observed in epitaxial NbO2 films, while irreversible transitions were found in case of Nb2O5 phase. Electric field pulsed current measurements confirmed thermally-induced threshold switching.

Linear and nonlinear optical properties of Au/-polymer metallodielectric Bragg stacks

We demonstrate the fabrication of one-dimensional metallodielectric Bragg stacks (MDBSs) from 500nm spin-coated poly(methyl methacrylate) and 20nm evaporated Au layers; one-, two-, and three-bilayer structures are achieved with good homogeneity ( 90%. Nonlinear absorption was measured by a z-scan technique and is observed to be enhanced at the Bragg resonances. The ∼600nm peak of the three-bilayer MDBS is enhanced by approximately seven times compared to a single Au film. The wavelength dependence of the nonlinear enhancement can be correlated with the attenuation parameter. The experimental results are in good agreement with numerical simulations based on a transfer-matrix method employing the known physical and optical parameters. The MDBSs show strong potential as versatile and inexpensive components for optical devices.

Sum and difference frequency generation as diagnostics for leaky eigenmodes in two-dimensional photonic crystal waveguides

We experimentally demonstrate how sum frequency generation (SFG) and difference frequency generation (DFG) of 150fs pulses can be enhanced and serve as diagnostics for leaky eigenmodes in a two-dimensional GaAs photonic crystal waveguide. SFG at 795nm is obtained in reflection from s-polarized 1900nm and p-polarized 1360nm pulses, with both input beams coupled to leaky eigenmodes; the SFG is enhanced by >350× compared to that from an untextured GaAs surface. We are able to detect Drude induced subnanometer blueshifts of the SFG, corresponding to refractive index changes of ⩽10−3. DFG of 1360nm light obtained in reflection from s-polarized 1900nm and p-polarized 793nm pulses displays an enhancement of >500× via three different leaky eigenmodes. As the 793nm beam polarization is varied from p polarized to right and left circularly polarized, the DFG remains essentially linearly polarized but with a reduced, albeit different, intensity for right and left circularly polarized 795nm pulses. Futhermore, the pla...

Switchable AlxGa1−xAs all-optical delay line at 1.55μm

The authors demonstrate an on-chip optical delay in AlxGa1−xAs that operates at room temperature and 1.55μm. A nonlinear directional coupler for optical switching and a 188ps long racetrack structure provide bit delays of >100 for picosecond pulses. In the linear regime the transmission ratio of the slow channel is 74%, which reduces to 40% with increasing peak intensity. (Meanwhile, the fast channel output increases from 26% to 60%.) Switching occurs due to a nonlinear detuning of the directional coupler, limited by three-photon absorption and time-averaging effects. This proof-of-principle device can operate at up to 5GHz and is promising for optical buffering applications.

Switchable All-Optical 188-ps Delay Line in AlGaAs

An integrated AlGaAs all-optical 188-ps delay line is characterized over the C- and L- bands. Self-switching by input intensity is achieved using a nonlinear directional coupler and a racetrack delay line.

Coherently controlled ballistic charge currents in unbiased bulk silicon and single-walled carbon nanotubes

Phase-related fundamental and second harmonic femtosecond pulses induce directional charge motion in group IV materials at 300 K. THz emission reveals peak current densities of 0.5 kA/cm2 in silicon and currents of 1 nA per carbon nanotube.

THz Emission from transient electrical currents injected into semiconductors via optical quantum interference

Quantum interference between single and two photon inter-band absorption processes can lead to injection of electrical currents in direct and indirect band gap semiconductors. When the optical excitation occurs through 100 fs optical pulses the transient electrical currents emit THz radiation that can be used as a probe of the ultrafast current dynamics.