Tunable electro-optic Microlens array. II. Cylindrical geometry

Although the concept of an artificial compound eye has been discussed in the literature, its optical arrangement has never been widely adopted for optical design. A design is presented for a tunable gradient-index microlens array, believed to be new, induced electro-optically inside a cylindrical shell. The transparent electrodes on the both sides of the shell are positioned such that the electrodes on the opposite side compensate the phase delay from the electrodes on the front side for a normally incident plane wave, thus suppressing the intrinsic electrode diffraction for the device without applied voltage. The original technique of the electric field calculation was developed to analyze the induced refractive index inside the shell for two types of electro-optic (EO) ceramics: with linear and with quadratic EO effects. For the linear effect it was shown that for given EO coefficients, electric field strength and intrinsic refractive index, the electrode number should exceed a certain amount to make the focal distance less than the cylinder radius. The quadratic effect provides higher sensitivity to the type of the diffracted wave polarization. It was shown how the quadratic coefficient ratio R(12)/R(11) affects the focal-length difference between TE and TM light polarization.     

Tunable superconducting nanoinductors

We characterize inductors fabricated from ultra-thin, approximately 100 nm wide strips of niobium (Nb) and niobium nitride (NbN). These nanowires have a large kinetic inductance in the superconducting state. The kinetic inductance scales linearly with the nanowire length, with a typical value of 1 nH μm(-1) for NbN and 44 pH μm(-1) for Nb at a temperature of 2.5 K. We measure the temperature and current dependence of the kinetic inductance and compare our results to theoretical predictions. We also simulate the self-resonant frequencies of these nanowires in a compact meander geometry. These nanowire inductive elements have applications in a variety of microwave frequency superconducting circuits.     

Tunable microfluidic microlenses

A novel type of liquid microlens, bounded by a microfabricated, distensible membrane and activated by a microfluidic liquid-handling system, is presented. By use of an elastomer membrane fabricated by spin coating onto a dry-etched silicon substrate, the liquid-filled cavity acts as a lens whereby applied pressure changes the membrane distension and thus the focal length. Both plano-convex and plano-concave lenses, individual elements as well as arrays, were fabricated and tested. The lens surface roughness was seen to be approximately 9 nm rms, and the focal length could be tuned from 1 to 18 mm. This lens represents a robust, self-contained tunable optical structure suitable for use in, for example, a medical environment.     

Tunable aqueous virtual micropore

A charged microparticle can be trapped in an aqueous environment by forming a narrow virtual pore--a cylindrical space region in which the particle motion in the radial direction is limited by forces emerging from dynamical interactions of the particle charge and dipole moment with an external radiofrequency quadrupole electric field. If the particle satisfies the trap stability criteria, its mean motion is reduced exponentially with time due to the viscosity of the aqueous environment; thereafter the long-time motion of particle is subject only to random, Brownian fluctuations, whose magnitude, influenced by the electrophoretic and dielectrophoretic effects and added to the particle size, determines the radius of the virtual pore, which is demonstrated by comparison of computer simulations and experiment. The measured size of the virtual nanopore could be utilized to estimate the charge of a trapped micro-object.     

Tunable diode laser instruments

Tunable diode lasers operate typically between 10 – 100 K and their properties, being temperature dependant, require temperature stability within a fraction of a mK. The construction of tunable diode lasers is described together with ways of achieving the operational temperature. Laboratory and field results are discussed for a range of spectral instruments.     

Tunable photonic crystal microcavities

We present a method for tuning a photonic crystal microcavity by modulating the index of refraction of the lattice sites within and surrounding the microcavity. The index of refraction can be actively modulated after infiltrating anisotropic liquid crystals into a two-dimensional photonic crystal lattice of air cylinders in silicon. We analyze the Q-factors and resonance frequencies of a tunable photonic crystal microcavity by considering various methods of index modulation. These tunable cavities are incorporated in a channel drop filter to demonstrate their enhancement of wavelength division multiplexing photonic crystal applications.     

Tunable superhydrophobic electrospun nonwoven mat

Superhydrophobicity is often described in terms of water contact angles higher than 150° along with lower roll-off angles. Surface roughness is one of the main parameters responsible for inducing superhydrophobicity on any material. One such intriguing material is superhydrophobic electrospun nonwoven mat but the design parameters for creating surface roughness are unclear primarily due to its irregular surface. Here, this irregular nature of superhydrophobic electrospun mat is probed through a simple analytical model based on the first principles of stereology revealing the important design parameters as porosity and fibre orientation distribution. Theoretically, we have shown that simply enhancing the porosity of an electrospun mat having intrinsic hydrophilic fibres, can still demonstrate the superhydrophobic behaviour.     

Tunable Resonators for Quantum Circuits

We have designed, fabricated and measured high-Q λ/2 coplanar waveguide microwave resonators whose resonance frequency is made tunable with magnetic field by inserting a DC-SQUID array (including 1 or 7 SQUIDs) inside. Their tunability range is 30% of the zero field frequency. Their quality factor reaches up to 3×10⁴. We present a model based on thermal fluctuations that accounts for the dependence of the quality factor with magnetic field.      

A Stable Tunable Microwave Source

This paper describes a prototype model of a phase-stabilized backward wave oscillator (BWO). The BWO is locked to a harmonic of a crystal oscillator by employing an automatic phase control loop. Tuning is accomplished by pulling the crystal oscillator and switching different crystals into the oscillator circuit. Power spectrum plots are taken in open-and closed-loop conditions, and methods of analyzing these plots are described. With the proper loop bandwidth, the BWO will assume the frequency stability of the reference oscillator. The phase-locked BWO is analyzed by a conventional feedback control technique. Each element in the loop is assigned a transfer function. From this the open-loop transfer function is determined. It is then shown that the improvement in short-term frequency stability is directly proportional to the open-loop transfer function and, hence, the open-loop gain. Utilizing plots of the absolute magnitude and phase angle of the open-loop gain vs. frequency, conclusions are drawn as to the maximum amount of gain which is possible while still maintaining system stability. Possible implementation of a series equalizer is considered in conjunction with this. Comments are also made on the relationship of the closed-loop bandwidth to the system lock-in range and system response to incremental changes in frequency.     

Electronically Tunable RC Sinusoidal Oscillators

This paper presents two types of active configurations for realizing electronically tunable RC sinusoidal oscillators. The type-1 network employs two grounded scaled resistances KR1 and KR2, where K is a scaling factor. The frequency of oscillation Wo is controlled conveniently by adjusting K, since Wo appears in the form W0 = l/K/?R1C1R2C2. For realizing the scaled resistances, an active configuration is proposed, which realizes KRi = Ri/1 + f(VB), where f(VB) denotes a function of a controlling voltage VB. Thus the frequency tuning can be effected by controlling a voltage VB. The type-2 oscillator uses two periodically switched conductances. It is shown that the tuning of oscillation frequency can be done by varying the pulsewidth-to-period ratio (?/T) of the periodically switched conductances.