E. R. Thoen

Photonic-bandgap microcavities in optical waveguides

Confinement of light to small volumes has important implications for optical emission properties: it changes the probability of spontaneous emission from atoms, allowing both enhancement and inhibition. In photonic-bandgap (PBG) materials (also known as photonic crystals), light can be confined within a volume of the order of (λ/2n)3, where λ is the emission wavelength and n the refractive index of the material, by scattering from a periodic array of scattering centres. Until recently, the properties of two- and three-dimensional PBG structures have been measured only at microwave frequencies. Because the optical bandgap scales with the period of the scattering centres, feature sizes of around 100 nm are needed for manipulation of light at the infrared wavelength (1.54 µm) used for optical communications. Fabricating features this small requires the use of electron-beam or X-ray lithography. Here we report measurements of microcavity resonances in PBG structures integrated directly into a sub-micrometre-scale silicon waveguide. The microcavity has a resonance at a wavelength of 1.56 µm, a quality factor of 265 and a modal volume of 0.055 µm3. This level of integration might lead to new photonic chip architectures and devices, such as zero-threshold microlasers, filters and signal routers.

Ultra-compact Si-SiO 2 microring resonator optical channel dropping filters

Compact optical channel dropping filters incorporating side-coupled ring resonators as small as 3 /spl mu/m in radius are realized in silicon technology. Quality factors up to 250, and a free-spectral range (FSR) as large as 24 nm are measured. Such structures can be used as fundamental building blocks in more sophisticated optical signal processing devices.

Two-photon absorption in semiconductor saturable absorber mirrors

The nonlinear reflectivity of semiconductor saturable absorber mirrors has been investigated as a function of incident energy fluence. The presence of two-photon absorption in commonly used structures was confirmed via time-resolved differential reflectivity measurements. Theoretical calculations predict that two-photon absorption will expand the continuous-wave mode-locking stability regime against Q-switched mode-locking, yet may simultaneously induce multiple pulses in a laser cavity.

One-dimensional photonic bandgap microcavities for strong optical confinement in GaAs and GaAs/Al/sub x/O/sub y/ semiconductor waveguides

Photonic bandgap (PBG) waveguide microcavities with tightly confined resonant optical modes have been designed, fabricated using high-dielectric-contrast GaAs/Al/sub x/O/sub y/ III-V compound semiconductor structures, and characterized optically. The photonic crystal lattices are defined by one-dimensional (1-D) arrays of holes in waveguides, and a controlled defect in the spacing between two holes of an array defines a microcavity. Waveguide microcavity resonances have been studied in both monorail and suspended air-bridge geometries. Resonance states with cavity Qs as high as 360 were measured at wavelengths near 1.55 /spl mu/m, with modal volumes as small as 0.026 /spl mu/m, which corresponds to only two times (/spl lambda//2n)/sup 3/.

Erbium-ytterbium waveguide laser mode-locked with a semiconductor saturable absorber mirror

Picosecond pulses are produced using a semiconductor saturable absorber mirror in a laser based on an Er-Yb codoped planar waveguide amplifier. Continuous-wave mode-locking (CWML) with 9.8-ps pulses is obtained at repetition rates up to 100 MHz. With intracavity spectral filtering, saturable pulsewidths of 1 ps are achieved, and tunable picosecond pulses are obtained from 1534 to 1553 nm. Absorber characterization suggests that two-photon absorption within the saturable absorber mirror influences the CWML stability.

Stabilization of an active harmonically mode-locked fiber laser using two-photon absorption

Two-photon absorption provided by a semiconductor mirror structure is shown to reduce amplitude fluctuations significantly in a harmonically mode-locked fiber ring laser. Pulse dropouts are eliminated in a laser that produces picosecond pulses at a repetition rate of 2 GHz.

Coherent acoustic phonons in PbTe quantum dots

Femtosecond pump-probe studies at 1.5 μm of PbTe quantum dots in a glass matrix have revealed oscillations due to a spheroidal acoustic mode. The wavelength dependence of the observed frequencies indicates size selectivity. The frequency and damping of the mode as a function of wavelength agree with the predictions of an acoustic continuum model that suggests that the dominant damping mechanism is radiative loss from the dots to the surrounding glass.

High fluence ultrafast dynamics of semiconductor saturable absorber mirrors

The ultrafast nonlinear dynamics of InGaAs/InP semiconductor saturable absorber mirrors are investigated using reflective pump–probe measurements. At high fluence, ultrafast induced absorption begins to dominate over absorption bleaching. Above the InGaAs quantum well band gap, the differential reflectivity shows a ∼1 ps transient due to nonequilibrium carrier dynamics. Below band gap, the signal is dominated by a strong two-photon absorption component followed by induced absorption that decays with a time constant of ∼5 ps; these components are attributed to nonlinear absorption and subsequent carrier diffusion in the InP layer.

Recovery dynamics in proton-bombarded semiconductor saturable absorber mirrors

Reduction of device response time, resulting from the proton bombardment of InGaAs/InP-based semiconductor saturable absorbers, was studied experimentally using an ultrafast degenerate, cross- polarized pump-probe technique. Proton bombardment is shown to reduce device response times to ∼1 ps at low optical excitation densities. Under high excitation, the device dynamics are dominated by induced absorption. The extended recovery of highly excited carriers appears to be less sensitive to defects created by bombardment. Mode locking was demonstrated with the proton-bombarded samples in an erbium-doped fiber laser.

Ultrafast hot-carrier dynamics in semiconductor saturable absorber mirrors

Femtosecond pump-probe experiments have been used to study the ultrafast nonlinear dynamics of InGaAs/InP semiconductor saturable absorber mirrors. The relative contributions of absorption bleaching and induced absorption are investigated by varying the excitation fluence over more than four orders of magnitude, well beyond complete absorption saturation. Enhanced free carrier absorption due to highly excited carriers with an extended relaxation time of 2.8 ps dominates the differential reflectivity at ultrahigh fluences and has been studied via a two-color pump-probe measurement.