Shreyas B. Raghunathan

Plasmon switching: Observation of dynamic surface plasmon steering by selective mode excitation in a sub-wavelength slit

We report a plasmon steering method that enables us to dynamically control the direction of surface plasmons generated by a two-mode slit in a thin metal film. By varying the phase between different coherent beams that are incident on the slit, individual waveguide modes are excited. Different linear combinations of the two modes lead to different diffracted fields at the exit of the slit. As a result, the direction in which surface plasmons are launched can be controlled. Experiments confirm that it is possible to distribute an approximately constant surface plasmon intensity in any desired proportion over the two launching directions. We also find that the anti-symmetric mode generates surface plasmons more efficiently than the fundamental symmetric mode.

A Prototype PZT Matrix Transducer With Low-Power Integrated Receive ASIC for 3-D Transesophageal Echocardiography

This paper presents the design, fabrication, and experimental evaluation of a prototype lead zirconium titanate (PZT) matrix transducer with an integrated receive ASIC, as a proof of concept for a miniature three-dimensional (3-D) transesophageal echocardiography (TEE) probe. It consists of an array of 9 × 12 piezoelectric elements mounted on the ASIC via an integration scheme that involves direct electrical connections between a bond-pad array on the ASIC and the transducer elements. The ASIC addresses the critical challenge of reducing cable count, and includes front-end amplifiers with adjustable gains and microbeamformer circuits that locally process and combine echo signals received by the elements of each 3 × 3 subarray. Thus, an order-of-magnitude reduction in the number of receive channels is achieved. Dedicated circuit techniques are employed to meet the strict space and power constraints of TEE probes. The ASIC has been fabricated in a standard 0.18-μm CMOS process and consumes only 0.44 mW/channel. The prototype has been acoustically characterized in a water tank. The ASIC allows the array to be presteered across ±37° while achieving an overall dynamic range of 77 dB. Both the measured characteristics of the individual transducer elements and the performance of the ASIC are in good agreement with expectations, demonstrating the effectiveness of the proposed techniques.

Experimental demonstration of an intensity minimum at the focus of a laser beam created by spatial coherence: application to the optical trapping of dielectric particles.

In trying to manipulate the intensity distribution of a focused field, one typically uses amplitude or phase masks. Here we explore an approach, namely, varying the state of spatial coherence of the incident field. We experimentally demonstrate that the focusing of a Bessel-correlated beam produces an intensity minimum at the geometric focus rather than a maximum. By varying the spatial coherence width of the field, which can be achieved by merely changing the size of an iris, it is possible to change this minimum into a maximum in a continuous manner. This method can be used, for example, in novel optical trapping schemes, to selectively manipulate particles with either a low or high index of refraction.

Correlation singularities in partially coherent electromagnetic beams

We demonstrate that coherence vortices, singularities of the correlation function, generally occur in partially coherent electromagnetic beams. In successive cross sections of Gaussian Schell-model beams, their locus is found to be a closed string. These coherence singularities have implications for both interference experiments and correlation of intensity fluctuation measurements performed with such beams.

Topological reactions of correlation functions in partially coherent electromagnetic beams

It was recently shown that so-called coherence vortices, singularities of the two-point correlation function, generally occur in partially coherent electromagnetic beams. We study the three-dimensional structure of these singularities and show that in successive cross sections of a beam a rich variety of topological reactions takes place. These reactions involve, apart from vortices, the creation or annihilation of dipoles, saddles, maxima and minima of the phase of the correlation function. Since these reactions happen generically, i.e., under quite general conditions, these observations have implications for interference experiments with partially coherent, electromagnetic beams.

Plasmon Switching: Observation of Dynamic Surface Plasmon Steering by Selective Mode Excitation in a Sub-wavelength Slit

By selectively exciting the modes in a sub-wavelength slit in a gold film, an approximately constant surface plasmon intensity can be distributed in any desired proportion over two launching directions.

Design of a miniature ultrasound probe for 3D transesophageal echocardiography

The relatively large size of most of the current 3D Transesophageal Echocardiography probes (TEE) enables their usage in adults for short procedures only. In this paper, we propose a new miniature 3D TEE probe with a head volume of 1 cm3, that would be suitable for use in neonates or for prolonged transnasal use in adults. We focus on partitioned designs, in which a minority of transmit elements is directly wired out and the majority of receive elements connect to a limited number of receive cables via an ASIC that performs a nine-fold data reduction in receive mode. The designs are motivated based on the pressure and beam width of the transmitted field, and the resolution, grating lobes and side lobes in the received field.

Effective permittivity of finite inhomogeneous objects

A generalization of the S-parameter retrieval method for finite three-dimensional inhomogeneous objects under arbitrary illumination and observation conditions is presented. The effective permittivity of such objects may be rigorously defined as a solution of a nonlinear inverse scattering problem. We confirm analytically and observe numerically effects that were previously reported in the one-dimensional strongly inhomogeneous slabs: the nonuniqueness of the effective permittivity and its dependence on the illumination and observation conditions, and the geometry of the object. Moreover, we show that, although the S-parameter retrieval of the effective permittivity is scale free at the level of problem statement, the exact solution of this problem either does not exist or is not unique. Using the results from the spectral analysis we describe the set of values of the effective permittivity for which the scattering problem is ill-posed. Unfortunately, real nonpositive values, important for negative refraction and invisibility, belong to this set. We illustrate our conclusions using a numerical reduced-order inverse scattering algorithm specifically designed for the effective permittivity problem.

Low-power receive electronics for a miniature real-time 3D ultrasound probe

Transesophageal echocardiography (TEE) involves the use of an ultrasound transducer mounted at the tip of an endoscope to make ultrasonic images of the human heart from the esophagus. In conventional TEE probes, each transducer element is wired-out by a micro-coaxial cable to an external imaging system. To obtain real-time three-dimensional images, however, a two-dimensional transducer array with more than 1000 elements is required. Direct wiring of so many elements through an endoscope is not feasible, so channel reduction should be performed locally. In this paper, we present an application-specific integrated circuit (ASIC) that includes low-noise amplifiers, programmable-gain amplifiers and micro-beamformer circuits that locally process and combine the signals received by sub-groups of the transducer array. Thus, an order-of-magnitude channel reduction is achieved. The acoustic characterization of the prototype ASIC with a co-integrated transducer array will be presented.

A front-end ASIC with receive sub-array beamforming integrated with a 32 × 32 PZT matrix transducer for 3-D transesophageal echocardiography

This paper presents a power- and area-efficient front-end ASIC that is directly integrated with an array of 32 × 32 piezoelectric transducer elements to enable the next-generation miniature ultrasound probes for real-time 3-D transesophageal echocardiography. The 6.1 × 6.1 mm2 ASIC, implemented in a low-voltage 0.18 μm CMOS process, effectively reduces the number of cables required in the probes narrow shaft by means of 96 sub-array beamformers, which have a compact element-matched layout and employ mismatch-scrambling to enhance the dynamic range. The ASIC consumes less than 230 mW while receiving and its functionality has been successfully demonstrated in a 3-D imaging experiment.