December 1999

Confocal Microscopy of Human Tissues in Vivo

Conventional histopathology for screening or diagnosing diseases and disorders is an invasive method that involves excising a small piece of tissue (biopsy), fixing, cutting into thin slices (sectioning), and staining with dyes.

Improved Responsivity of Non-Steady-State Photoinduced Electromotive Force Sensors Using Asymmetric Interdigitated Contacts for Laser-Based Ultrasound Detection

Laser-based ultrasound (LBU), enables one to "see" into opaque materials with light, using lasers to generate and sense ultrasound, without contacting a sample. LBU is a nondestructive remote diagnostic, with application to industrial inspection and process control, and has niche opportunities where conventional ultrasound, using contact transducers, cannot be used. Examples include in-situ monitoring of the wall-thickness of red-hot pipes moving 3 meters/second from a furnace, evaluation of bonds in microelectronics components during assembly, inspection of delamination in composites of complex shape, and quality control of paper products moving at 20 meters/second in a mill.

~20x Enhancement in Silicon-on-Insulator Photodetector Near ? = 800 nm

The addition of a monolayer of nanoparticles can modify significantly the optical properties of the structure that supports it. We recently offered a practical example of that assertion by using a layer of silver (Ag) nanoparticles to increase by nearly 20 times the optical absorption near wavelength λ = 800 nm of a thin silicon-on-insulator (SOI) photodetector.

High-Speed Resonant Cavity Enhanced Photodiodes

An important figure of merit for photodetectors used in optical communications is the bandwidth-efficiency (BWE) product. The quantum efficiency of conventional detector structures is governed by the absorption coefficient of the semiconductor material, meaning that thick active regions are required for high quantum efficiencies.

All-Fiber Fourier-Transform Spectrometer

Optical fiber phase gratings fabricated in the core of a fiber by ultraviolet irradiation are critical components for optical communications and sensor applications. Most applications utilize fiber gratings as narrow-band reflection filters.

Internal Characterization of Fiber Bragg Gratings

A fiber grating (FG) is a periodic perturbation of the refractive index along the fiber length which is formed by exposure of the core to an intense optical pattern. Fiber gratings work as narrow-band pass optical filters, which reflect specific wavelengths and pass the rest, providing the advantages of high selectivity, low insertion loss and no polarization sensitivity.

Interferometer Phase Distributions from a Single Image Using Wavelets

An interferometer measures the optical path difference between a reference flat and a test surface, producing a pattern of fringes whose spatial frequency is proportional to the local gradient of the test surface. To obtain the two-dimensional phase distribution (and hence the surface shape), it is necessary to resolve the sign ambiguity of the local slope. In phase-stepping interferometry, this is done by acquiring three or more accurately phase-stepped images by moving either the test surface or the reference flat with piezo actuators and grabbing images with a CCD camera. Inaccuracies in the phase-stepping, however, generate errors in the extracted phase distribution.

Two-Dimensional Corrugated Waveguides

One-dimensional corrugated waveguide structures exhibit many interesting optical properties and can be used for optical coupling, optical filtering, as well as in the distributed feedback and distributed Bragg reflector lasers. We have experimentally demonstrated two-dimensional (2D) corrugated waveguides at optical wavelengths by using 2D colloidal crystal.

High-Temperature Diode and Optically-Pumped Mid-IR Lasers with Type-II Quantum Wells

While semiconductor diodes currently dominate the world market for near-IR lasers, longer-wavelength devices emitting beyond 3 μm have yet to achieve adequate performance levels. Practical mid-IR systems will ultimately require high-power non-cryogenic sources that operate either cw or quasi-cw with a high duty cycle. Key applications include chemical and biological detection, laser spectroscopy, laser surgery, and infrared countermeasures against heat-seeking missiles.

High Power Single Mode Laser Operation With Intracavity Phase Elements

In a laser operating with many transverse modes, the output power is relatively high but the emerging output beam quality is inherently poor. Generally, improvement of the beam quality is achieved by inserting an aperture inside the resonator to reduce the number of modes.

Excitation Localization Principle For Whispering-Gallery Mode Microcavities

There is interest in utilizing whispering gallery mode (WGM) microcavities for photonic applications1 (e.g., signal processing and environmental sensing) as well as in fundamental physics studies (e.g., cavity QED). These symmetric structures (e.g., spheres and disks) are relatively easy to fabricate yet display cavity Qs far surpassing those of similar sized Fabry-Perot resonators. The exceptional Qs result from near total internal reflection of circulating waves and are limited only by minimal photon tunneling through the curved interface.

Photonic Crystal Nanocavity Lasers

By creating two-dimensional photonic crystals, which are microfabricated into InGaAsP slabs, researchers from Caltech and USC have recently defined the smallest lasers to date. When combined with high index contrast slabs in which light can be efficiently guided, microfabricated two-dimensional photonic bandgap mirrors provide the geometries needed to confine light into extremely small volumes with high Q.

Waveguide Microcavities with Photonic Crystal Mirrors

The fundamental properties of waveguide-based photonic crystals have been studied for some time and great progress in their understanding has been made.1 Reflectivities in the stop-band of photonic crystal mirrors in excess of 90%, have been observed as well as substantial pass-band transmission. The next stage is to use this knowledge for the design of active and passive devices. The study of cavities is a useful step in this direction, because cavities provide functionality in the form of filters and laser resonators as well as being versatile building blocks for miniaturized photonic integrated circuits.

High Frequency Probing of Nanometric Resolution Using Near-Field Optical Heterodyne Technology

We have developed a novel optical technique that allows for injection of high frequency millimeter wave signals up to 100 GHz to a specific local area of modern ultrahigh speed semiconductor devices and integrated circuits. This high frequency probing of nanometric resolution equips one with the capability to pinpoint origin of dynamic processes in devices, their spatial extent and their relation to interface and surface parameters, structural inhomogeneities and imperfections. The technique thus enables the experimentalists to look ever more closely into the details of ultrafast phenomena occurring in today's ultrasmall devices and circuits.

Transient Coupling of Electromagnetic Radiation To Surface Plasmons in Solid-State Structures With Time-Varying Plasma Density

Surface electromagnetic waves propagate along the interface between two media and exponentially decay with distance from the interface. The simplest interface that can guide surface waves is the boundary of a plasma or plasma-like material such as metal or a semiconductor. In this case, the surface waves are called surface plasmon polaritons or, simply, surface plasmons.

Normal Mode Coupling in Optical Lattices of Excitons In Periodic Quantum Well Structures

Aperiodic arrangement of planes of identical two-level atoms in a one-dimensional (1D) optical lattice with a period close to an integer value of half the resonance wavelength represents a fundamental model system. An example of actual physical systems under investigation are cold atoms arranged in periodic light shift potentials. The interaction of such an optical lattice with resonant light drastically changes the absorption and emission properties compared to an isolated resonance due to multiple reflections and interference.

Propagating Spatial Optical Solitons In Semiconductor Lasers

The charge carrier plasma within a broad-area semiconductor laser amplifier is an active light-emitting and highly nonlinear media in which the dynamic light-matter interaction occurs on timescales ranging from femtosecond to the nanosecond regime. Due to the spatial transport of carriers and interband polarization as well as the counter-propagation of light, the internal processes linked to amplification, absorption and relaxation occur on both spatial and temporal scales.

Harmonic Generation in Soliton-Induced Waveguides

Waveguides induced by photorefractive solitons offer several interesting applications.

Enhanced Electro-Optic Response Of Layered Composite Materials

Recent research has stressed the importance of the fabrication of composite structures as a means of obtaining materials with desirable optical properties. One such approach is to fabricate nano-composite materials. These materials are inhomogeneous on a distance scale much smaller than an optical wavelength. For this reason the optical properties of such materials can be described by "effective medium theories" in which "effective" optical constants (such as refractive indices and nonlinear susceptibilities) are obtained by performing suitable volume averages of the local optical constants. Nonetheless, nano-composite materials can possess remarkable optical properties which can be quite different from those of their constituents.

Small Form Factor Optical Fiber Connectors: Performance Comparisons

Conventional duplex fiber optic connectors, such as the SC Duplex, achieve the required alignment tolerances by threading each optical fiber through a precision ceramic ferrule. The ferrules have an outer diameter of 2.5 mm, and the resulting fiber-to-fiber spacing (or pitch) of a duplex connector is approximately 12.5 mm. Since the outer diameter of an optical fiber is only 125 μm, it should be possible to design a significantly smaller optical connector. Smaller connectors with fewer precision parts could dramatically reduce manufacturing costs and have the potential to open up new applications such as fiber to the desktop.

Linear and Circular Polarization Filters Using Sculptured Thin Films

Sculptured thin films (STFs) are a new class of nanoengineered materials whose columnar morphology is locally curved to elicit desired optical responses upon excitation. Comprising multimolecular clusters of ~3-5 nm diameter, STFs are essentially unidirectionally nonhomogeneous, locally anisotropic continuums in the visible and the infrared regimes. Periodically nonhomogeneous STFs exhibit the Bragg phenomenon, which makes them very suitable as optical filters.

Fault-Tolerant High Speed Variable Fiber-Optic Attenuator Using Micromirrors

Variable fiber-optic attenuators are the basic building blocks for several key optical systems. Presently, these attenuators are required as equalizers in wavelength division multiplexed (WDM) optical communication systems using non-uniform gain optical amplifiers. Hence, a variable fiber-optic attenuator with fast several microsecond duration speeds with high attenuation dynamic range control is a current challenge to the optical community. For centuries, an excellent choice for light control is via the use of mirrors. Mirrors provide high reflectivity over broad bandwidths, as desired in WDM systems.

Photonic Crystal Temperature Sensor

By creating two-dimensional photonic crystals, which are microfabricated into InGaAsP slabs, researchers from Caltech and USC have recently defined the smallest lasers to date. When combined with high index contrast slabs in which light can be efficiently guided, microfabricated two-dimensional photonic bandgap mirrors provide the geometries needed to confine light into extremely small volumes with high Q.

The Omniguide: An All-Dielectric Hollow Waveguide

The recent emergence of a dielectric omnidirectional multilayer structure1-3 opens new opportunities for low loss broadband guiding of light in air. Light guided in a hollow waveguide lined with an omnidirectional reflecting film will propagate primarily through air and will therefore have substantially lower absorption losses. In addition, the confinement mechanism does not have angular dependence which allows the light to be guided around sharp bends with little or no leakage.

Two-Color Nonlinear Localized Modes in Photonic Crystals

For more than a decade physicists have been working towards photonic crystals (or photonic band gap materials)—low-loss periodic dielectric structures that produce many of the same phenomena for photons as the crystalline atomic potential does for electrons. Three-dimensional photonic crystals for visible light have been successfully fabricated only within the past year or two, but it is time to think about the next step—creating tunable band-gap switches and transistors operating entirely with light

Noiseless Optical Amplification of Images

Success of precision measurements often depends on the use of amplifiers. The sensitivity of these measurements is, therefore, limited by the noise that the amplifier adds to the signal. For electronic signals, the noise floor is set by thermal fluctuations. At optical frequencies, however, the thermal noise gets negligibly small. In this case, the noise floor of a phase-insensitive amplifier (PIA)—a linear amplifier whose gain does not depend on the signal phase—is determined by a fundamental quantum limit, which arises ultimately from zero-point field fluctuation.

Single-Beam Dark Optical Traps for Cold Atoms

Cold atoms confined in far off-resonant optical dipole traps are useful for precision spectroscopy and for the study of quantum collective effects. To reduce interaction of the atoms with the trapping light, traps in which repulsive light forces confine atoms mostly in the dark were developed. These dark optical traps enable long coherence times of the trapped atoms combined with tight confinement, and therefore high atomic densities. However, most schemes for dark optical traps require a combination of several laser beams, and this experimental complexity limits their wide use. Hence, single-beam dark traps are of great interest.

Observation of Efficient Phase Conjugation Owing to Electromagnetically Induced Transparency in Solids

Since the first observation of electromagnetically induced transparency (EIT) in solids, there have been many studies of its advantage for potential applications. Among them, enhancement of four-wave mixing generation is very useful for the implementation of potential EIT applications such as dynamic optical memories for THz fiber optic backbone networks and laser target tracking using turbulence aberration correction.

What Does a Bose-Einstein Condensate Look Like?

In the early '90s, before Bose-Einstein condensation was realized in atomic gases, there were lively debates about what a condensate would look like. Some researchers thought it would absorb all the light and would therefore be "pitch black," some predicted it would be "transparent" (due to superradiant line-broadening), others predicted that it would reflect the light (due to polaritons) and be "shiny" like a mirror.

Excitonic Rabi Oscillations in Semiconductors

Coherent nonlinear optical effects are usually associated with atomic or molecular systems. This is especially true for optical Rabi oscillations, one of the most fundamental nonlinear phenomena in atomic two-level systems. The most obvious reason coherent effects are less easily observable in solid state systems, such as semiconductors, is the short decoherence times, typically of the order of picoseconds.

Coherent Control of the Polarization of an Optical Field

Recent work has shown that it is possible to use one laser beam to coherently manipulate a material's absorption1 or index of refraction associated with a second laser beam. We have demonstrated that similar techniques can be used to control the tensor properties of an initially isotropic system in such a way that one field can completely and efficiently control the polarization state of another field. Our experiments show that by varying the state of polarization of the control field, it is possible to induce linear or circular birefringence in an atomic gas, resulting in the transformation of the polarization state of a probe beam from linear to any desired polarization state with a total energy throughput greater than 94%.

Instantaneous Processing of Ultrafast Waveforms by Wave Mixing Spectrally Decomposed Waves

Ultrafast phenomena in the natural sciences can be excited, manipulated and observed with tailored ultrashort optical pulses. These ultrafast waveforms are synthesized and processed in the temporal frequency domain by spatially dispersing the frequency components in a spectral processing device (SPD) and performing operations on the spectrally decomposed wave (SDW). Waveform synthesis by SDW filtering has been demonstrated with prefabricated masks, spatial light modulators and holograms. These filters are limited in their adaptability rate—a new filter can be implemented only as fast as the modulator response time or recording time of a new hologram—typically well over a microsecond.

Polarization-locked Temporal Vector Solitons in an Optical Fiber

Temporal vector solitons have components along both birefringent axes. Despite different phase velocities due to linear birefringence, the relative phase of the components can be locked at π/2. These fragile polarization-locked vector solitons (PLVS) have been the subject of much theoretical conjecture, but have previously eluded experimental observation.

Soliton Effects in an AlGaAs Bragg Grating

Experimental and theoretical research exploring the nonlinear properties of periodic structures has intensified over the past decade within a diverse range of materials utilizing both second and third order effects. Recently we studied the nonlinear transmission characteristics of high intensity pulses propagating through 4-10 mm long gratings etched into ridge waveguides formed in AlGaAs.

Extremely-Tunable Terahertz Emission: Coherent Population Flopping in a DC-Biased Quantum Well

Terahertz (THz) light-emitters are a welcome addition to the laser engineer's palette since they offer a marriage of optics and high-speed electronics. In the field of semiconductor nanostructures, the exploration of THz generation schemes and THz-induced carrier dynamics provides a useful means to study charge-carrier transport.

Real-Time Optical Pulse Characterization Using SPIDER

The recent advances in ultrafast lasers have mandated a change in the techniques used to measure them. It is no longer sufficient to measure merely the spectrum or the auto-correlation of an ultrafast laser pulse; a more complete picture is required, and this involves determining the phase of the time-varying optical field. The ability to rapidly change the shape of the pulse is a key technology for a variety of new applications, from quantum control to nonlinear optics. Complementary to this is the ability to rapidly characterize the field so that it is possible to understand the physical effects of different pulse shapes. Recently, we have reported a powerful new tool for doing just that. The technique is called Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER) and the new implementation provides feedback in real time for either manual or computer-based control of the pulse shape.

A Femtosecond Ti:Sapphire Laser with GHz Repetition Rate

The invention of solid-state femtosecond lasers, the Ti:sapphire laser based on Kerr-lens modelocking,1 had probably the deepest impact on the development of ultra-fast spectroscopy and nonlinear optics within the last decade. Since the era of ultrafast physics based on femtosecond dye lasers in the 1980s, the solid-state systems have lead to an enormous expansion of the whole field.

Ultrafast Soliton Dynamics

Optical solitons have attracted widespread interest given their potential for telecommunication applications.1 The demand for increased bandwidth is relentless and shortening of the pulses would permit further gain

Photon Statistics in Adaptive Optics

Random aberrations on the pupil of the telescope due to atmospheric turbulence determine the angular resolution of ground-based telescopes. The compensation of the wave-front degradation before detection (adaptive optics) and the extraction of diffraction-limited information from the image series (speckle interferometry) are the two techniques able to overcome this limitation. Although adaptive optics systems with a large number of subapertures in the wave-front sensor and a large number of actuators in the deformable mirror provide the best results, they are complicated and expensive. In contrast, the use of simpler adaptive optics systems has great potential application. This is the reason why we will deal with partially compensated adaptive optics systems (fewer than one actuator per atmospheric coherence diameter) in this paper.


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