KEYWORDS: Atomic force microscopy, Silicon, Semiconductor lasers, Near field optics, Mode locking, Waveguides, Near field, Mirrors, Active optics, Raman spectroscopy
Optical characterization at the nanoscale currently requires instruments such as NSOM/TERS, or hybrid AFM with specialized far-field optical microscopes that are quite complicated and do not provide any time-resolved data. We have demonstrated a novel class of probes for Scanning Probe Microscopy (SPM) - an Ultrafast Pulsed Atomic Force Microscopy Optical Probe (UFP AAOP) that will enhance characterization capabilities at the nanoscale and provide an exciting opportunity for obtaining both space- and time-resolved chemical information simultaneously. In the UFP AAOP design, a two-section quantum-dot mode-locked laser is monolithically integrated with an SPM probe fabricated from GaAs, with a nanoscale opening at the apex of the tip as the output aperture. With UFP AAOP, the light is supplied through the tip; hence, there is no scattered far-field light and thus significantly reduced background. Furthermore, the difficulties associated with laser alignment onto the tip and with imaging the signal onto a detector are avoided with the UFP AAOP. The UFP AAOP provides pulses with less than 4 ps duration and spatial resolution better than 300 nm at 1240 nm wavelength. It is potentially possible to reduce the pulse width to ~ 0.3 ps and to improve lateral resolution to ~ 1 nm. These unique optical probes will perform the functions of conventional AFM probes and simultaneously provide information about chemical properties of the sample at the nanoscale together with time-resolved spectroscopy. UFP AAOP will facilitate the creation of a new microscopy/spectroscopy instrument with combined single-molecule spatial resolution and ultrafast time-resolved capability.
Greatly enhanced high-speed modulation performance has been recently predicted in numerical calculations for a novel injection-locking scheme involving a distributed Bragg reflector master laser monolithically integrated with a unidirectional whistle-geometry semiconductor micro ring laser. In this work, we confirm the unidirectionality of the whistle-geometry configuration through rigorous three-dimensional finite-difference time-domain (FDTD) simulation by showing a strong asymmetry in photon lifetimes between the two counter propagating modes. At the same time, we explain why this result does not violate the Helmholtz reciprocity principle.
Frequency chirp simulations based on rate equation analysis are performed for strongly injection-locked ring lasers for
applications in optical communication and rf photonics. Different scenarios are explored and compared, including weak
injection of a single ring laser, strong injection of a single ring laser, and strong injection of cascaded ring lasers.
Together with previously published results, these simulations confirm that the novel whistle-geometry ring laser scheme
can provide, especially when modulation signal is applied to both ring lasers in the cascaded arrangement, a greatly
improved performance in terms of modulation response and frequency chirp, and more flexibility in the design of ultrahigh
speed transmitters.
Conformal transformation and the beam propagation method are employed for the simulation of deeply-etched bent and
serpentine waveguides on an InP substrate with an active region consisting of double InGaAs quantum wells. The modal
properties are analyzed and the bending and transition losses are extracted from the propagation results. For the
serpentine waveguides, the transition losses increase significantly for waveguides with bending radii less than 12 μm, yet
careful choice of the transition point at which the curvature reverses is shown to minimize the losses induced by the
transition. Effects of etch depth are also considered and conclusion that it is necessary to etch through the active region
for deeply-etched bent waveguides is drawn.
Nonidentical multiple quantum wells (MQWs) had been widely used for broadening the emission or gain bandwidth of semiconductor optical amplifiers (SOAs). However, the carrier distribution among the MQWs is not uniform, leading to nonuniform gain contributed from different QWs. Thus using nonidentical MQWs for broadband purpose is not intuitively straightforward. Several factors need to be carefully considered. Those factors include the QW sequence, electron/hole transport time across the separate confinement hetero-structure, as well as carrier capture time. In this work, we will discuss the design of MQWs for broadband SOAs. With properly designed nonidentical MQWs, the emission bandwidth could be nearly 400 nm. Also, the tuning range of semiconductor lasers could be extended to be over 200 nm.
A broadly tunable range of 200 nm is achieved in external cavity semiconductor laser using a Fabry-Perot laser diode as an amplifier. The broadband tuning is possible due to two reasons. First, the gain bandwidth is broadened using proper design of nonidentical quantum-well structure. Second, carriers distribute over a broad bandwidth, leading to the reduction of gain over a narrow bandwidth. Thus self-oscillation of uncoated Fabry-Perot laser didoes is suppressed, but there is still gain for external-cavity configuration. A tuning range covering from 1340nm to 1540nm is then achieved.
A newly developed multi-functional microscope named Morphinscope and which possesses versatile metrology functions such as that of a confocal microscope, a photon tunneling microscope, a laser based phase-shifting interferometry microscope, and an ellipsometer is presented. This microscope can switch between its various measurement functions by simply rotating its turret, which makes it a low-cost choice for surface analysis. This instrument satisfies the design goal of providing the user with a versatile instrument that can undertake various metrology functions with only one instrument. This design point circumvents a major limitation facing today's surface analyses, i.e., once after a defect is identified, the effort in locating the same defect again with a different instrument is a difficult if not impossible task. As a phase shifting interferometer has in inherent drawback in reconstructing the phase map of a non-homogeneous surface, an ellipsometer can come to the rescue by measuring the complex index of refraction of the surface. More specifically, the Fresnel equation can be used to calculate the phase change of inhomogeneous surfaces due to reflection in order to retrieve the measured complex index of refraction. Combining this understanding, this Morphinscope has the possibility of retrieving the surface profile of a non-homogeneous media.
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