Experimental observation of strong photon localization in disordered photonic crystal waveguides

We demonstrate experimentally that structural perturbations imposed on highly-dispersive photonic crystal-based waveguides give rise to spectral features that bear signatures of Anderson localization. Sharp resonances with the effective Qs of over 30,000 are found in scattering spectra of disordered waveguides. The resonances are observed in a ~20-nm bandwidth centered at the cutoff of slowly-guided Bloch-modes. Their origin can be explained with interference of coherently scattered electromagnetic waves which results in the formation of a narrow impurity (or localization) band populated with spectrally distinct quasistates. Standard photon localization criteria are fulfilled in the localization band.Comment: first submitted to PRL on April 20th, 2007; 16 pages, 4 figure

Synchronous imaging for rapid visualization of complex vibration profiles in electromechanical microresonators

Synchronous imaging is used in dynamic space-domain vibration profile studies of capacitively driven, thin n+ doped poly-silicon microbridges oscillating at rf frequencies. Fast and high-resolution actuation profile measurements of micromachined resonators are useful when significant device nonlinearities are present. For example, bridges under compressive stress near the critical Euler value often reveal complex dynamics stemming from a state close to the onset of buckling. This leads to enhanced sensitivity of the vibration modes to external conditions, such as pressure, temperatures, and chemical composition, the global behavior of which is conveniently evaluated using synchronous imaging combined with spectral measurements. We performed an experimental study of the effects of high drive amplitude and ambient pressure on the resonant vibration profiles in electrically-driven microbridges near critical buckling. Numerical analysis of electrostatically driven post-buckled microbridges supports the richness of complex vibration dynamics that are possible in such micro-electromechanical devices.Comment: 7 pages, 8 figure, submitted to Physical Review

Transmission properties of a single metallic slit: From the subwavelength regime to the geometrical-optics limit

In this work we explore the transmission properties of a single slit in a metallic screen. We analyze the dependence of these properties on both slit width and angle of incident radiation. We study in detail the crossover between the subwavelength regime and the geometrical-optics limit. In the subwavelength regime, resonant transmission linked to the excitation of waveguide resonances is analyzed. Linewidth of these resonances and their associated electric field intensities are controlled by just the width of the slit. More complex transmission spectra appear when the wavelength of light is comparable to the slit width. Rapid oscillations associated to the emergence of different propagating modes inside the slit are the main features appearing in this regime.Comment: Accepted for publication in Phys. Rev.

Whispering gallery microresonators for second harmonic light generation from a low number of small molecules

Unmarked sensitive detection of molecules is needed in environmental pollution monitoring, disease diagnosis, security screening systems and in many other situations in which a substance must be identified. When molecules are attached or adsorbed onto an interface, detecting their presence is possible using second harmonic light generation, because at interfaces the inversion symmetry is broken. However, such light generation usually requires either dense matter or a large number of molecules combined with high-power laser sources. Here we show that using high-Q spherical microresonators and low average power, between 50 and 100 small non-fluorescent molecules deposited on the outer surface of the microresonator can generate a detectable change in the second harmonic light. This generation requires phase matching in the whispering gallery modes, which we achieved using a new procedure to periodically pattern, with nanometric precision, a molecular surface monolayer

Lyapunov exponent of the random Schr\"{o}dinger operator with short-range correlated noise potential

We study the influence of disorder on propagation of waves in one-dimensional structures. Transmission properties of the process governed by the Schr\"{o}dinger equation with the white noise potential can be expressed through the Lyapunov exponent $\gamma$ which we determine explicitly as a function of the noise intensity \sigma and the frequency \omega. We find uniform two-parameter asymptotic expressions for $\gamma$ which allow us to evaluate $\gamma$ for different relations between \sigma and \omega. The value of the Lyapunov exponent is also obtained in the case of a short-range correlated noise, which is shown to be less than its white noise counterpart.Comment: 20 pages, 4 figure

Emergence of semi-localized Anderson modes in a disordered photonic crystal as a result of overlap probability

In this paper we study the effect of positional randomness on transmissional properties of a two dimensional photonic crystal as a function of a randomness parameter $\alpha$ ($\alpha=0$ completely ordered, $\alpha=1$ completely disordered). We use finite-difference time-domain~(FDTD) method to solve the Maxwell's equations in such a medium numerically. We consider two situations: first a 90\degr bent photonic crystal wave-guide and second a centrally pulsed photonic crystal micro-cavity. We plot various figures for each case which characterize the effect of randomness quantitatively. More specifically, in the wave-guide situation, we show that the general shape of the normalized total output energy is a Gaussian function of randomness with wavelength-dependent width. For centrally pulsed PC, the output energy curves display extremum behavior both as a function of time as well as randomness. We explain these effects in terms of two distinct but simultaneous effects which emerge with increasing randomness, namely the creation of semi-localized modes and the shrinking (and eventual destruction) of the photonic band-gaps. Semi-localized (i.e. Anderson localized) modes are seen to arise as a synchronization of internal modes within a cluster of randomly positioned dielectric nano-particles. The general trend we observe shows a sharp change of behavior in the intermediate randomness regime (i.e. $\alpha \approx 0.5$) which we attribute to a similar behavior in the underlying overlap probability of nano-particlesComment: New published version with a new title. This article is featured on the cover of the corresponding journal (Nov. issue of EJPB

Lower bound for the spatial extent of localized modes in photonic-crystal waveguides with small random imperfections

Light localization due to random imperfections in periodic media is paramount in photonics research. The group index is known to be a key parameter for localization near photonic band edges, since small group velocities reinforce light interaction with imperfections. Here, we show that the size of the smallest localized mode that is formed at the band edge of a one-dimensional periodic medium is driven instead by the effective photon mass, i.e. the flatness of the dispersion curve. Our theoretical prediction is supported by numerical simulations, which reveal that photonic-crystal waveguides can exhibit surprisingly small localized modes, much smaller than those observed in Bragg stacks thanks to their larger effective photon mass. This possibility is demonstrated experimentally with a photonic-crystal waveguide fabricated without any intentional disorder, for which near-field measurements allow us to distinctly observe a wavelength-scale localized mode despite the smallness (∼1/1000 of a wavelength) of the fabrication imperfections

The Nanolithography Toolbox

This article introduces in archival form the Nanolithography Toolbox, a platform-independent software package for scripted lithography pattern layout generation. The Center for Nanoscale Science and Technology (CNST) at the National Institute of Standards and Technology (NIST) developed the Nanolithography Toolbox to help users of the CNST NanoFab design devices with complex curves and aggressive critical dimensions. Using parameterized shapes as building blocks, the Nanolithography Toolbox allows users to rapidly design and layout nanoscale devices of arbitrary complexity through scripting and programming. The Toolbox offers many parameterized shapes, including structure libraries for micro- and nanoelectromechanical systems (MEMS and NEMS) and nanophotonic devices. Furthermore, the Toolbox allows users to precisely define the number of vertices for each shape or create vectorized shapes using Bezier curves. Parameterized control allows users to design smooth curves with complex shapes. The Toolbox is applicable to a broad range of design tasks in the fabrication of microscale and nanoscale devices

Loss engineered slow light waveguides

Slow light devices such as photonic crystal waveguides (PhCW) and coupled resonator optical waveguides (CROW) have much promise for optical signal processing applications and a number of successful demonstrations underpinning this promise have already been made. Most of these applications are limited by propagation losses, especially for higher group indices. These losses are caused by technological imperfections ("extrinsic loss") that cause scattering of light from the waveguide mode. The relationship between this loss and the group velocity is complex and until now has not been fully understood. Here, we present a comprehensive explanation of the extrinsic loss mechanisms in PhC waveguides and address some misconceptions surrounding loss and slow light that have arisen in recent years. We develop a theoretical model that accurately describes the loss spectra of PhC waveguides. One of the key insights of the model is that the entire hole contributes coherently to the scattering process, in contrast to previous models that added up the scattering from short sections incoherently. As a result, we have already realised waveguides with significantly lower losses than comparable photonic crystal waveguides as well as achieving propagation losses, in units of loss per unit time (dB/ns) that are even lower than those of state-of-the-art coupled resonator optical waveguides based on silicon photonic wires. The model will enable more advanced designs with further loss reduction within existing technological constraints. (C) 2010 Optical Society of AmericaPublisher PDFPeer reviewe