In this paper the asymptotic limits of negative group delay (NGD) phenomena in multi-stage RLC resonator-based circuits are discussed. A NGD-bandwidth-product limit is derived as a function of the number of stages and the out-of-band gain, which is independent of the circuit topology and can include active gain compensation. The limit is verified experimentally at microwave frequencies using a gain-compensated NGD circuit employing a parallel RLC resonator in the feedback path of a high-frequency op-amp. It is shown that, in the asymptotic limit, the NGD-bandwidth-product is proportional to the square root of the number of stages, and also to the square root of the logarithm of the out-ofband gain. The relation between the time-domain transient amplitude and the out-of-band gain is analyzed for finite-duration modulated signals, indicating an exponential increase in transient amplitudes with the square of NGD. Analysis shows that any attempt to increase the NGD of a finite-duration modulating waveform, by cascading more stages, is thwarted by the transients.

Nanostructures are becoming increasingly important for technology and basic science. 1 Fabrication techniques currently employed for material deposition include low-pressure chemical vapor deposition (LPCVD), 2 laser-assisted chemical vapor deposition (LCVD), 3 plasma-enhanced chemical vapor deposition (PECVD), 4,5 ultraviolet stereo lithography, 6 spinning, 7 dipping, 8 spraying, 9 and electrodeposition. [10][11][12][13][14][15] Electrodeposition methods have many advantages over the other stated techniques and are attractive, as they are simple, inexpensive, reproducible, and damage-free. In addition, many materials can be deposited using electrodeposition, including metals, metal alloys, conducting polymers, and semiconductors with essentially no limitations on the size of the substrate or on the thickness of the deposited material. 16 Scanning probe microscopy (SPM) such as scanning tunneling microscopy (STM), 17 atomic force microscopy (AFM), 18 and scanning electrochemical microscopy (SECM) 19 has been widely used as a tool for surface imaging with atomic resolution. Furthermore, creation of structures using SPM has lately attracted considerable attention. 20-23 Using SPM for surface modification has advantages in that the modification process can be followed in real time and submicrometer resolution can be achieved. 24-25 SPM-based nanofabrication has potential uses in applications such as high-density information storage, high-resolution lithography, and production of nanoscale integrated chemical systems and electronic devices.Several groups have employed SPM to deposit metal and polymer microstructures. [28][29][30][31] Bard et al. 32 used the SECM to deposit different metals (e.g., Cu, Ag, Au, Pd) on polymer-coated substrates, whereas Shahat and Mandler et al. used the same technique to deposit Ni(OH) 2 structures 33 from aqueous solutions by changing the pH locally on the substrate and gold patterns by the controlled dissolution of a gold ultramicroelectrode (UME) tip. 34 Wipf and Zhou 35 used the "microreagent" SECM mode to deposit conducting polyaniline patterns on different substrates. Lagraff and Gewirth 36 employed the tip of an AFM to direct the growth of nanoscopic copper protrusions, whereas Madden and Hunter used a tip-directed scheme to deposit several micrometer-scale nickel structures. 25 In tip-directed localized deposition, 32 a faradaic current flows through the solution between a UME tip and a metal substrate electrode all immersed in an ionically conducting electrolyte when a bias voltage is applied between these two electrodes. If reducible metal ions are present in the electrolyte (e.g., Cu 2ϩ ions) and the substrate electrode potential is negative with respect to the tip electrode, then the passage of the faradaic current results in the deposition of metal on the substrate and an oxidation process at the tip. The magnitude of the faradaic current is kept constant by means of a conventional feedback control that monitors the current and adjusts the interelectrode spacing according...

We present details of an apparatus for capacitive detection of biomaterials in microfluidic channels operating at microwave frequencies where dielectric effects due to interfacial polarization are minimal. A circuit model is presented, which can be used to adapt this detection system for use in other microfluidic applications and to identify ones where it would not be suitable. The detection system is based on a microwave coupled transmission line resonator integrated into an interferometer. At 1.5 GHz the system is capable of detecting changes in capacitance of 650 zF with a 50 Hz bandwidth. This system is well suited to the detection of biomaterials in a variety of suspending fluids, including phosphate-buffered saline. Applications involving both model particles ͑polystyrene microspheres͒ and living cells-baker's yeast ͑Saccharomyces cerevisiae͒ and Chinese hamster ovary cells-are presented.

In biomedical applications ranging from the study of pathogen invasion to drug efficacy assays, there is a growing need to develop minimally invasive techniques for single-cell analysis. This has inspired researchers to develop optical, electrical, microelectromechanical and microfluidic devices for exploring phenomena at the single-cell level. In this work, we demonstrate an electrical approach for single-cell analysis wherein a 1.6 GHz microwave interferometer detects the capacitance changes (DeltaC) produced by single cells flowing past a coplanar interdigitated electrode pair. The experimental and simulated capacitance changes generated by yeast cells are in close agreement. By using the capacitance changes of uniform polystyrene spheres (diameter = 5.7 microm) for calibration purposes, we demonstrate a 0.65 aF sensitivity in a 10 ms response time. Using an RC circuit, a low frequency sinusoidal potential is simultaneously superimposed on the electrode pair to generate a dielectrophoretic force that translates cells. Specifically, when yeast cells suspended in a solution of 90 ppm NaCl in deionized water are exposed to 10 kHz and 3 MHz potentials (ranging from 1-3 V(pp)), they experience negative and positive dielectrophoresis, respectively. The corresponding changes in cell elevation above the interdigitated electrodes are detected using the asymmetry of the capacitance signature produced by the cell. Cell elevation changes can be detected in less than 80 ms. The minimum detectable change in elevation is estimated to be 0.22 microm. This approach will have applications in rapid single-cell dielectrophoretic analysis, and may also prove useful in conjunction with impedance spectroscopy.

A spintronic approach is introduced to transform classic Michelson interferometry that probes the electromagnetic phase only. This method utilizes a nonlinear four-wave coherent mixing effect. A previously unknown striking relation between spin dynamics and the relative phase of electromagnetic waves is revealed. Spintronic Michelson interferometry allows direct probing of both the spin-resonance phase and the relative phase of electromagnetic waves via microspintronics. Thereby, it breaks new ground for cross-disciplinary applications with unprecedented capabilities, which we demonstrate via a powerful phase-resolved spin-resonance spectroscopy on magnetic materials and an on-chip technique for phase-resolved near-field microwave imaging.