Ani Khachatrian

Assembling programmable FRET-based photonic networks using designer DNA scaffolds

DNA demonstrates a remarkable capacity for creating designer nanostructures and devices. A growing number of these structures utilize Förster resonance energy transfer (FRET) as part of the devices functionality, readout or characterization, and, as device sophistication increases so do the concomitant FRET requirements. Here we create multi-dye FRET cascades and assess how well DNA can marshal organic dyes into nanoantennae that focus excitonic energy. We evaluate 36 increasingly complex designs including linear, bifurcated, Holliday junction, 8-arm star and dendrimers involving up to five different dyes engaging in four-consecutive FRET steps, while systematically varying fluorophore spacing by Förster distance (R0). Decreasing R0 while augmenting cross-sectional collection area with multiple donors significantly increases terminal exciton delivery efficiency within dendrimers compared with the first linear constructs. Förster modelling confirms that best results are obtained when there are multiple interacting FRET pathways rather than independent channels by which excitons travel from initial donor(s) to final acceptor.

Fabrication of terahertz metamaterials by laser printing

A laser printing technique was used to fabricate split-ring resonators (SRRs) on Si substrates for terahertz (THz) metamaterials and their resonance behavior evaluated by THz time-domain spectroscopy. The laser-printed Ag SRRs exhibited sharp edge definition and excellent thickness uniformity, which resulted in an electromagnetic response similar to that from identical Au SRR structures prepared by conventional photolithography. These results demonstrate that laser printing is a practical alternative to conventional photolithography for fabricating metamaterial structures at terahertz frequencies, since it allows their design to be easily modified and optimized.

Single-Event Effect Performance of a Commercial Embedded ReRAM

We show the single-event effect characteristics of a production-level embedded resistive memory. The resistive memory under investigation is a reduction-oxidation random access memory embedded inside a microcontroller. The memory structure consists of Ir top electrode, Ta2O5-δ/TaOx metal-oxide, and TaN bottom electrode. The radiation testing focused on the resistive memory array and peripheral circuits, while other portions of the microcontroller were shielded against the ion beam. We found that the resistive memory array is hardened against heavy ion and pulsed-laser-induced bit upsets. However, the microcontroller is susceptible to single-event functional interrupts due to single-event upsets in the resistive memory peripheral control circuits, which comprise of CMOS elements. Furthermore, the resistive memory architecture is not susceptible to functional failures during write, which is problematic for flash memories due to radiation-induced charge pump degradation.

Resonance energy transfer in DNA duplexes labeled with localized dyes.

The growing maturity of DNA-based architectures has raised considerable interest in applying them to create photoactive light harvesting and sensing devices. Toward optimizing efficiency in such structures, resonant energy transfer was systematically examined in a series of dye-labeled DNA duplexes where donor-acceptor separation was incrementally changed from 0 to 16 base pairs. Cyanine dyes were localized on the DNA using double phosphoramidite attachment chemistry. Steady state spectroscopy, single-pair fluorescence, time-resolved fluorescence, and ultrafast two-color pump-probe methods were utilized to examine the energy transfer processes. Energy transfer rates were found to be more sensitive to the distance between the Cy3 donor and Cy5 acceptor dye molecules than efficiency measurements. Picosecond energy transfer and near-unity efficiencies were observed for the closest separations. Comparison between our measurements and the predictions of Förster theory based on structural modeling of the dye-labeled DNA duplex suggest that the double phosphoramidite linkage leads to a distribution of intercalated and nonintercalated dye orientations. Deviations from the predictions of Förster theory point to a failure of the point dipole approximation for separations of less than 10 base pairs. Interactions between the dyes that alter their optical properties and violate the weak-coupling assumption of Förster theory were observed for separations of less than four base pairs, suggesting the removal of nucleobases causes DNA deformation and leads to enhanced dye-dye interaction.

Concurrent Modulation of Quantum Dot Photoluminescence Using a Combination of Charge Transfer and Förster Resonance Energy Transfer: Competitive Quenching and Multiplexed Biosensing Modality

An emerging trend with semiconductor quantum dots (QDs) is their use as scaffolds to assemble multiple energy transfer pathways. Examples to date have combined various competitive and sequential Förster resonance energy transfer (FRET) pathways between QDs and fluorescent dyes, luminescent lanthanide complexes, and bioluminescent proteins. Here, we show that the photoluminescence (PL) of QD bioconjugates can also be modulated by a combination of FRET and charge transfer (CT), and characterize the concurrent effects of these mechanistically different pathways using PL measurements at both the ensemble and the single particle level. Peptides were distally labeled with either a fluorescent dye that quenched QD PL through FRET or a ruthenium(II) phenanthroline complex that quenched QD PL through electron transfer. The labeled peptides were assembled around a central CdSe/ZnS QD at different ratios, tuning the relative rates of FRET and CT, which were competitive quenching pathways. The concurrent effects of FRET and CT were predictable from a rate analysis that was calibrated to the isolated effects of each of these pathways. Notably, the dye/QD PL intensity ratio reflected changes in the relative rate of FRET but was approximately independent of CT. In turn, the sum of the QD and dye PL intensities, when adjusted for quantum yields, reflected changes in the relative rate of CT quenching, approximately independent of FRET. The capacity for multiplexed sensing of protease activity was demonstrated using these two orthogonal detection channels. Combined CT-FRET configurations with QDs are thus promising for applications in bioanalysis, sensing, and imaging, and may prove useful in other photonic applications.

Design of Radiation-Hardened RF Low-Noise Amplifiers Using Inverse-Mode SiGe HBTs

A SiGe RF low-noise amplifier (LNA) with built-in tolerance to single-event transients is proposed. The LNA utilizes an inverse-mode SiGe HBT for the common-base transistor in a cascode core. This new cascode configuration exhibits reduced transient peaks and shorter transient durations compared to the conventional cascode one. The improved SET response was verified with through-wafer two-photon absorption pulsed-laser experiments and supported via mixed-mode TCAD simulations. In addition, analysis of the RF performance and the reliability issues associated with the inverse-mode operation further suggests this new cascode structure can be a strong contender for space-based applications. The LNA with the inverse-mode-based cascode core was fabricated in a 130 nm SiGe BiCMOS platform and has similar RF performance to the conventional schematic-based LNA, further validating the proposed approach.

Modeling and Investigations on TID-ASETs Synergistic Effect in LM124 Operational Amplifier From Three Different Manufacturers

The synergistic effect between Total Ionizing Dose (TID) and Analog Single Event Transient (ASET) in LM124 operational amplifiers (opamps) from three different manufacturers is investigated. This effect is clearly identified on only two manufacturers by three, highlighting manufacturer dependent. In fact, significant variations were observed on both the TID sensitivity and the ASET response of LM124 devices from different manufacturers. Hypotheses are made on the cause of the differences observed. A previously developed ASET simulation tool is used to model the transient response. The effects of TID on devices are taken into account in the model by injecting the variations of key electrical parameters obtained during Co60 irradiation. An excellent agreement is observed between the experimental responses and the model outputs, independently of the TID level, the bias configuration and the manufacturer of the device.

Simulation of Laser-Based Two-Photon Absorption Induced Charge Carrier Generation in Silicon

Numerical simulation software is used to calculate quantitatively the two-photon absorption-induced carrier-density distributions generated under conditions that are experimentally relevant for single-event effects studies. The results provide valuable insight into how the magnitudes and shapes of the charge carrier distributions evolve over a large range of experimental conditions and the impacts this has for different device geometries. Furthermore, values of integrated charge are determined that can be more directly correlated with experimental observables.

Single-Event Transient and Total Dose Response of Precision Voltage Reference Circuits Designed in a 90-nm SiGe BiCMOS Technology

This paper presents an investigation of the impact of single-event transients (SETs) and total ionization dose (TID) on precision voltage reference circuits designed in a fourth-generation, 90-nm SiGe BiCMOS technology. A first-order uncompensated bandgap reference (BGR) circuit is used to benchmark the SET and TID responses of these voltage reference circuits (VRCs). Based on the first-order BGR radiation response, new circuit-level radiation-hardening-by-design (RHBD) techniques are proposed. An RHBD technique using inverse-mode (IM) transistors is demonstrated in a BGR circuit. In addition, a PIN diode VRC is presented as a potential SET and TID tolerant, circuit-level RHBD alternative.

A Dosimetry Methodology for Two-Photon Absorption Induced Single-Event Effects Measurements

A pulsed-laser dosimetry approach for two-photon absorption (TPA) single-event effects (SEE) measurements is presented. Development and implementation of three online beam monitors is described. The beam monitors permit characterization of the primary laser beam parameters of interest: the pulse energy delivered to the device under test, the pulse duration, and the focused laser spot size. A direct consequence of this methodology is the ability to monitor continuously the operating point of the TPA SEE pulsed-laser beamline and to make the necessary adjustments when parameters drift, either during an experiment or between experiments.