Maziar P. Nezhad

Electrically pumped sub-wavelength metallo-dielectric pedestal pillar lasers

Electrically driven subwavelength scale metallo-dielectric pedestal pillar lasers are designed and experimentally demonstrated. The metallo-dielectric cavity significantly enhances the quality factor (Q > 1500) of the wavelength and subwavelength scale lasers and the pedestal structure significantly reduces the threshold gain (< 400 cm(-1)) which can potentially enable laser operation at room temperature. We observed continuous wave lasing in 750 nm gain core radius laser at temperatures between 77 K and 140 K with a threshold current of 50 μA (at 77 K). We also observed lasing from a 355 nm gain core radius laser at temperatures between 77 K and 100 K.

Implementation of a graded-index medium by use of subwavelength structures with graded fill factor

We present a novel configuration for the implementation of subwavelength-based graded-index devices. The proposed concept is based on the etching of one-dimensional subwavelength gratings into a high-index slab waveguide to achieve the desired effective index distribution. A graded-index profile can be achieved by gradually modifying the duty ratio of the grating along the horizontal axis, while the beam is confined in the vertical direction by the slab waveguide. On the basis of this concept, novel graded-index lenses and waveguides are both proposed and characterized numerically by use of finite-difference time-domain and finite-element analysis. The proposed devices can be used for guiding, imaging, optical signal processing, mode matching, coupling, and other applications while offering the intrinsic advantages of on-chip integration such as miniaturization, eliminating the need to align each component separately, and compatibility with standard microfabrication techniques for manufacturability.

On the measurement of the Pockels effect in strained silicon

We measure the voltage-dependent phase shift in silicon waveguides strained by a silicon nitride layer and show that, in our measurements, the phase shift is due to free carrier accumulation inside the waveguides. Nonetheless, inverting the applied voltage also inverts the applied phase shift-an effect due to a quasi-static surface charge in the silicon nitride. Since the measured effect is on the same order as recently published second-order nonlinearities attributed to the Pockels effect, inclusion of these carrier-based effects in the analysis of experimental data is of paramount importance.

Excitation and direct imaging of surface plasmon polariton modes in a two-dimensional grating

We describe the simultaneous excitation and direct far-field imaging of the scattering from surface plasmon polariton modes in a two-dimensional metallic hole array grating. Conditions for the coupling and imaging are discussed, where the coupling is shown to be consistent with both measured and calculated dispersion relations. Excitation is accomplished at several different wavelengths (from 1.31 to 1.57μm), incidence angles, and grating periods, enabling the observation of a number of distinct modes with various in-plane wave vectors.

Wavelength selective coupler with vertical gratings on silicon chip

We propose a wavelength selective coupler using vertical grating structure on silicon chip and discuss the operation principle and the analytic design procedure. The transmission spectra expected by the procedure agree with finite difference time domain simulation results. We then fabricate the designed wavelength selective coupler and demonstrate the expected operation. The available wavelength-division-multiplexing bandwidth of this device is not limited by free spectral range as in ring resonators. We also propose some applications of the wavelength selective coupler.

Purcell effect in sub-wavelength semiconductor lasers.

We present a formal treatment of the modification of spontaneous emission rate by a cavity (Purcell effect) in sub-wavelength semiconductor lasers. To explicitly express the assumptions upon which our formalism builds, we summarize the results of non-relativistic quantum electrodynamics (QED) and the emitter-field-reservoir model in the quantum theory of damping. Within this model, the emitter-field interaction is modified to the extent that the field mode is modified by its environment. We show that the Purcell factor expressions frequently encountered in the literature are recovered only in the hypothetical condition when the gain medium is replaced by a transparent medium. Further, we argue that to accurately evaluate the Purcell effect, both the passive cavity boundary and the collective effect of all emitters must be included as part of the mode environment.

Continuous-Wave Band Translation Between the Near-Infrared and Visible Spectral Ranges

Wavelength conversion based on degenerate four-wave mixing in a photonic crystal fiber with two zero-dispersion wavelengths is investigated both theoretically and experimentally. The proposed concept of universal band translation in a single-pass traveling-wave structure offers a wavelength band rather than a single-wavelength mapping between distant spectral ranges. Near-infrared signals are modulated using both harmonic and pseudorandom bit sequences and translated to the visible optical band. Multiple-channel translation, which produces wavelength-division-multiplexed idlers in the visible band, is demonstrated for the first time. The performance of the translation process is measured both spectrally and temporally for both single- and multiple-channel signals

Etch-free low loss silicon waveguides using hydrogen silsesquioxane oxidation masks

An etch-free fabrication technique for creating low loss silicon waveguides in the silicon-on-insulator material system is proposed and demonstrated. The approach consists of local oxidation of a silicon-on-insulator chip covered with a e-beam patterned hydrogen silsesquioxane mask. A single oxidation step converts hydrogen silsesquioxane to a glass-like compound and simultaneously defines the waveguides, bypassing the need for any wet or dry etching steps. The spectral response of ring resonators fabricated using this technique was used to characterize the waveguide losses. Intrinsic Q-factors as high as 1.57 × 10(6), corresponding to a waveguide loss of 0.35 dB/cm, were measured.

Optical Bistability in a Silicon Waveguide Distributed Bragg Reflector Fabry–Pérot Resonator

We demonstrate optical bistability in a silicon waveguide Fabry-Pérot resonator formed by a pair of distributed Bragg reflectors. In the bistable regime, the output power of the resonator ceases to be uniquely determined by the input power because multiple powers within the cavity satisfy the resonance condition. Pulsating behavior is observed within the resonator output, and is attributed to noise within the experimental setup driving the resonator between the multiple allowed output powers.

Compact chip-scale filter based on curved waveguide Bragg gratings

We propose a method for miniaturization of filters based on curved waveguide Bragg gratings, so that long structures can be packed into a small area on a chip. This eliminates the stitching errors introduced in the fabrication process, which compromise the performance of long Bragg gratings. Our approach relies on cascading curved waveguide Bragg gratings with the same radius of curvature. An analytical model for the analysis of these devices was developed, and a filter based on this model was designed and fabricated in a silicon on insulator platform. The filter had a total length of 920μm, occupied an area of 190μm×114μm, and exhibited a stop band of 1.7nm at 1.55μm and an extinction ratio larger than 23dB.