Optimal Pulse Shape Research Articles

Simulating strong field control of axial chirality using optimal control theory

We propose a strong-field based method to control the chirality of molecules that exhibit torsion, illustrating the possibility of converting a racemate into a pure enantiomer at elevated temperatures. Optimal control theory is applied to design a laser pulse that will maximize the enantiomeric ratio achieved, considering both the case of a fixed, linear polarization and the case of a tunable polarization. Our simulations show the possibility of converting 99% and 99.5% of the population into a desired enantiomer for the fixed and tunable polarization solutions, respectively, deriving interesting insights regarding the conversion dynamics from the optimized pulse shape. Finally, we discuss several potential applications of the proposed approach, including a study of time-resolved racemization and a chiral switch.

Implementation of a truncated cusp filter for real-time digital pulse processing in nuclear spectrometry

In this work the transfer function for a truncated cusp infinite impulse response (IIR) filter has been derived. The truncated cusp filter, when given an exponential pulse in the presence of white noise, outputs a truncated cusp pulse which is optimum pulse shape under the constraint of finite pulse duration. However it features a pole outside of the unit-circle and is therefore inherently unstable. To overcome this instability a switching algorithm with two alternating poles has been applied. The filter has been simulated with MATLAB and Simulink and implemented on-board a FPGA. The pulses from a waveform generator and silicon drift detector (SDD) have been also processed by the filter and results compared with theory. We find that the simulation and measurements were in agreement and show that energy resolution due to electronic noise can be improved by up to 7.5% using a truncated cusp instead of triangular filter. An improvement of up to 2.8% in overall energy resolution was measured with a typical X-ray spectrometer based on a SDD detector with a nominal energy resolution of 150eV at energies of MnKα line.

Pulse chirp effects in ultrafast laser-induced breakdown spectroscopy

When compared to many other sensitive methods for material detection, such as inductively coupled mass spectroscopy and thermal ionization mass spectroscopy, laser-induced breakdown spectroscopy (LIBS) typically exhibits a lower signal-to-noise ratio (SNR), resulting in higher detection limits. Increasing the SNR of LIBS would improve the ability to characterize the sample composition with increased accuracy and speed and reduce the amount of material needed to perform analysis. We have been investigating the effect of simple ultrashort laser pulse shaping on the SNR of LIBS. Our goal is to control the dynamics of the ionization and recombination processes in the laser-produced plasma to favorably affect the SNR associated with the line emission from the plasma. Pulse shaping is performed using an acousto-optic programmable dispersive filter. An adaptive learning algorithm is being developed to automate the pulse shape optimization process for maximization of LIBS SNR in nuclear security-relevant material characterization scenarios. We report a 27 % increase of the SNR for non-gated LIBS measurements of uranium by utilizing simple pulse shaping limited exclusively to excess quadratic spectral phase of the laser pulse.

Effect of the laser pulse temporal shape on the hole boring efficiency

Laser hole boring is a process of ion expulsion from the path of an intense laser beam under the effect of the ponderomotive force. It is considered as a method for more efficient electron energy deposition in fast ignition of inertial fusion reactions. Analysis of the electron dynamics on the laser–plasma interface shows that the hole boring efficiency can be significantly enhanced by an appropriate temporal shaping of the laser pulse. The pulse shape optimization is carried out with a newly developed fast quasi-neutral particle-in-cell (PIC) code and verified with the full PIC simulations.

Optically engineered ultrafast pulses for controlled rotations of exciton qubits in semiconductor quantum dots

Shaped ultrafast pulses designed for controlled-rotation (C-ROT) operations on exciton qubits in semiconductor quantum dots are demonstrated using a quantum control apparatus operating at ∼1 eV. Optimum pulse shapes employing amplitude and phase shaping protocols are implemented using the output of an optical parametric oscillator and a programmable pulse shaping system, and characterized using autocorrelation and multiphoton intrapulse interference phase scan techniques. We apply our pulse characterization results and density matrix simulations to assess the fundamental limits on the fidelity of the C-ROT operation, providing a benchmark for the evaluation of sources of noise in other quantum control experiments. Our results indicate the effectiveness of pulse shaping techniques for achieving high fidelity quantum operations in quantum dots with a gate time below 1 ps.

Endurance Enhancement and High Speed Set/Reset of 50 nm Generation HfO2Based Resistive Random Access Memory Cell by Intelligent Set/Reset Pulse Shape Optimization and Verify Scheme

Endurance Enhancement and High Speed Set/Reset of 50 nm Generation HfO₂Based Resistive Random Access Memory Cell by Intelligent Set/Reset Pulse Shape Optimization and Verify Scheme

Endurance Enhancement and High Speed Set/Reset of 50 nm Generation HfO2 Based Resistive Random Access Memory Cell by Intelligent Set/Reset Pulse Shape Optimization and Verify Scheme

This paper proposes a verify-programming method for the resistive random access memory (ReRAM) cell which achieves a 50-times higher endurance and a fast set and reset compared with the conventional method. The proposed verify-programming method uses the incremental pulse width with turnback (IPWWT) for the reset and the incremental voltage with turnback (IVWT) for the set. With the combination of IPWWT reset and IVWT set, the endurance-cycle increases from 48 ×103 to 2444 ×103 cycles. Furthermore, the measured data retention-time after 20 ×103 set/reset cycles is estimated to be 10 years. Additionally, the filamentary based physical model is proposed to explain the set/reset failure mechanism with various set/reset pulse shapes. The reset pulse width and set voltage correspond to the width and length of the conductive-filament, respectively. Consequently, since the proposed IPWWT and IVWT recover set and reset failures of ReRAM cells, the endurance-cycles are improved.

Electronic coherence within the semiclassical field-induced surface hopping method: strong field quantum control in K2

We demonstrate that the semiclassical field-induced surface hopping (FISH) method (Mitrićet al., Phys. Rev. A: At., Mol., Opt. Phys., 2009, 79, 053416.) accurately describes the selective coherent control of electronic state populations. With the example of the strong field control in the potassium dimer using phase-coherent double pulse sequences, we present a detailed comparison between FISH simulations and exact quantum dynamics. We show that for short pulses the variation of the time delay between the subpulses allows for a selective population of the desired final state with high efficiency. Furthermore, also for pulses of longer time duration, when substantial nuclear motion takes place during the action of the pulse, optimized pulse shapes can be obtained which lead to selective population transfer. For both types of pulses, the FISH method almost perfectly reproduces the exact quantum mechanical electronic population dynamics, fully taking account of the electronic coherence, and describes the leading features of the nuclear dynamics accurately. Due to the significantly higher computational efficiency of FISH as a trajectory-based method compared to full quantum dynamics simulations, this offers the possibility to theoretically investigate control experiments on realistic systems including all nuclear degrees of freedom.

Control of Ionization in the Interaction of Strong Laser Fields with Dense Nanoplasmas

AbstractDifferent experimental methods to maximize the yield of highly charged ions in silver and xenon clusters interacting with intense and ultra‐short optical laser pulses are discussed. Theoretically, the interaction of strong laser fields with clusters is investigated within the nanoplasma model. The time evolution of the laser intensity has been parametrized. The free optimization of the parameters with a genetic algorithm is an effective but expensive tool to control the plasma dynamics. Comparison is given to the parametric control method in which pulse separation and relative intensity ratio of double‐pulses are varied. This method delivers in the case of silver and xenon clusters pulses quite close to the optimal pulse shape (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nano-Optical Control of Hot-Spot Field Superenhancement on a Corrugated Silver Surface

Coherent control of ultrafast nano-optical excitations of a corrugated silver surface is demonstrated by means of predetermined few-parameter scans and adaptive polarization laser pulse shaping. “Hot spots” in the multiphoton photoemission signals are enhanced and manipulated with a high contrast. Switching between separated and closely spaced hot spots is shown. The latter allows controlling the shape of hot spots and yields improved nanofocusing and “purification” of the photoemission signals. Complex pulse shapes were obtained in adaptive optimizations whose features were reproducible in repeated runs. Predetermined few-parameter control scans provide insight into the interpretation of optimal pulse shapes. The results indicate the existence of long coherence lifetimes on a corrugated silver surface. This combination of collective strong nanoplasmonic near-field enhancement and long-lived coherence may be used to achieve an even stronger field enhancement (“superenhancement”) making these hot spots ideal candidates for future nanophotonic, spectroscopic, sensor and quantum information applications. In addition the observation of such long coherence lifetimes is relevant to the understanding of surface-enhanced spectroscopies such as single-molecule Raman spectroscopy.

Evolutionary optimization of rotational population transfer

We present experimental and numerical studies on control of rotational population transfer of NO($J=1/2$) molecules to higher rotational states. We are able to transfer 57$%$ of the population to the $J=5/2$ state and 46$%$ to $J=9/2$, in good agreement with quantum mechanical simulations. The optimal pulse shapes are composed of pulse sequences with delays corresponding to the beat frequencies of states on the rotational ladder. The evolutionary algorithm is limited by experimental constraints such as volume averaging and the finite laser intensity used, the latter to circumvent ionization. Without these constraints, near-perfect control ($>$98$%$) is possible. In addition, we show that downward control, moving molecules from high to low rotational states, is also possible.

Diffusion-based physical channel identification in molecular nanonetworks

Nanonetworking is an emerging field of research at the intersection of nanotechnology and communication networks. Molecular Communication (MC) is a bio-inspired paradigm, where nanonetworks, i.e., the interconnection of nanodevices, are implemented based on the exchange of molecules. Within this paradigm, one of the most promising techniques is diffusion-based MC, which relies on free diffusion to transport the molecules from a transmitter to a receiver. In this work, we explore the main characteristics of diffusion-based MC through the use of N3Sim, a physical simulation framework for MC which allows the simulation of the physics underlying the diffusion of molecules in different scenarios. Through the results obtained with N3Sim, the Linear Time Invariant (LTI) property is proven to be a valid assumption for the normal diffusion-based MC scenario. Moreover, diffusion-based noise is observed and evaluated with reference to existing stochastic models. Furthermore, the optimal pulse shape for diffusion-based MC is found to be a narrow spike. Finally, four different pulse-based coding techniques are compared in terms of the available bandwidth, ISI and energy consumption for communication; On–Off Keying is found to be the most suitable scheme in the evaluated scenario.

Coherent control of the ultrafast dissociative ionization dynamics of bromochloroalkanes

We report on the coherent control of the ultrafast ionization and fragmentation dynamics of the bromochloroalkanes C(2)H(4)BrCl and C(3)H(6)BrCl using shaped femtosecond laser pulses. In closed-loop control experiments on bromochloropropane (C(3)H(6)BrCl) the fragment ion yields of CH(2)Cl(+), CH(2)Br(+), and C(3)H(3)(+) are optimized with respect to that of the parent cation C(3)H(6)BrCl(+). The fragment ion yields are recorded in additional experiments in order to reveal the energetics of cation fragmentation, where laser-produced plasma radiation is used as a tunable pulsed nanosecond vacuum ultraviolet radiation source along with photoionization mass spectrometry. The time structure of the optimized femtosecond laser pulses leads to a depletion of the parent ion and an enhancement of the fragment ions, where a characteristic sequence of pulses is required. Specifically, an intense pump pulse is followed by a less intense probe pulse where the delay is 0.5 ps. Similarly optimized pulse shapes are obtained from closed-loop control experiments on bromochloroethane (C(2)H(4)BrCl), where the fragment ion yield of CH(2)Br(+) is optimized with respect to that of C(2)H(4)BrCl(+) as well as the fragment ion ratios C(2)H(2)(+)/CH(2)Br(+) and C(2)H(3)(+)/C(2)H(4)Cl(+). The assignment of the underlying control mechanism is derived from one-color 804 nm pump-probe experiments, where the yields of the parent cation and several fragments show broad dynamic resonances with a maximum at Δt = 0.5 ps. The experimental findings are rationalized in terms of dynamic ionic resonances leading to an enhanced dissociation of the parent cation and some primary fragment ions.

Phase-locked ions and foil transparency in the radiation pressure acceleration regime

Abstract The transverse expansion of a thin foil accelerated in the RPDA regime can be exploited in order to increase the ion energy and the acceleration efficiency at the expense of decreasing the number of accelerated particles. In the relativistic regime, the ions become phase-locked with respect to the electromagnetic wave. The use of an optimal laser pulse shape makes it possible to keep the expanding foil opaque to the laser radiation. This provides a new approach in order to enhance the energy of laser accelerated ions significantly.

Evaluation of real-time digital pulse shapers with various HPGe and silicon radiation detectors

Abstract Real-time digital pulse shaping techniques allow synthesis of pulse shapes that have been difficult to realize using the traditional analog methods. Using real-time digital shapers, triangular/trapezoidal filters can be synthesized in real time. These filters exhibit digital control on the rise time, fall time, and flat-top of the trapezoidal shape. Thus, the trapezoidal shape can be adjusted for optimum performance at different distributions of the series and parallel noise. The trapezoidal weighting function (WF) represents the optimum time-limited pulse shape when only parallel and series noises are present in the detector system. In the presence of 1/F noise, the optimum WF changes depending on the 1/F noise contribution. In this paper, we report on the results of the evaluation of new filter types for processing signals from CANBERRA high purity germanium (HPGe) and passivated, implanted, planar silicon (PIPS) detectors. The objective of the evaluation is to determine improvements in performance over the current trapezoidal (digital) filter. The evaluation is performed using a customized CANBERRA digital signal processing unit that is fitted with new FPGA designs and any required firmware modifications to support operation of the new filters. The evaluated filters include the Cusp, one-over-F (1/F), and pseudo-Gaussian filters. The results are compared with the CANBERRA trapezoidal shaper.

Blind Separation of Two Users Based on User Delays and Optimal Pulse-Shape Design

A wireless network is considered, in which two spatially distributed users transmit narrow-band signals simultaneously over the same channel using the same power. User separation is achieved by oversampling the received signal and formulating a virtual multiple-input multiple-output (MIMO) system based on the resulting polyphase components. Because of oversampling, high correlations can occur between the columns of the virtual MIMO system matrix which can be detrimental to user separation. A novel pulse-shape waveform design is proposed that results in low correlation between the columns of the system matrix, while it exploits all available bandwidth as dictated by a spectral mask. It is also shown that the use of successive interference cancelation in combination with blind source separation further improves the separation performance.

Unlimited energy gain in the laser-driven radiation pressure dominant acceleration of ions

The energy of the ions accelerated by an intense electromagnetic wave in the radiation pressure dominated regime can be greatly enhanced by a transverse expansion of a thin target. The expansion decreases the number of accelerated ions in the irradiated region increasing the energy and the longitudinal velocity of the remaining ions. In the relativistic limit, the ions become phase locked with respect to the electromagnetic wave resulting in an unlimited ion energy gain. This effect and the use of optimal laser pulse shape provide a new approach for greatly enhancing the energy of laser accelerated ions.

Shaping and timing gradient pulses to reduce MRI acoustic noise

A method to reduce the acoustic noise generated by gradient systems in MRI has been recently proposed; such a method is based on the linear response theory. Since the physical cause of MRI acoustic noise is the time derivative of the gradient current, a common trapezoid current shape produces an acoustic gradient coil response mainly during the rising and falling edge. In the falling edge, the coil acoustic response presents a 180 degrees phase difference compared to the rising edge. Therefore, by varying the width of the trapezoid and keeping the ramps constant, it is possible to suppress one selected frequency and its higher harmonics. This value is matched to one of the prominent resonance frequencies of the gradient coil system. The idea of cancelling a single frequency is extended to a second frequency, using two successive trapezoid-shaped pulses presented at a selected interval. Overall sound pressure level reduction of 6 and 10 dB is found for the two trapezoid shapes and a single pulse shape, respectively. The acoustically optimized pulse shape proposed is additionally tested in a simulated echo planar imaging readout train, obtaining a sound pressure level reduction of 12 dB for the best case.

Self-referenced spectral interferometry

A new femtosecond pulse characterization, named self-referenced spectral interferometry, is introduced. Based on linear spectral interferometry, the reference pulse is self created from the pulse being characterized. This self reference results from pulse shaping optimization and non-linear temporal filtering.

Closed-loop control of vibrational population in CO2+

An adaptive closed-loop feedback system is used to determine the optimal pulse shapes for manipulating the branching ratio of CO2+ following ionization by an intense laser pulse. For this target, selecting between the CO2+ and C+ + O+ final states requires control of the vibrational population distribution in the transient CO2+. The ability to both suppress and enhance CO2+ relative to C+ + O+ is observed, with shaped pulses surpassing a transform-limited pulse by factors of about 10 for suppression and 2 for enhancement. When optimizing small channels, such as non-dissociative CO2+, we demonstrate that a feedback signal obtained via a pulse counting technique is more robust than the more typical current mode signal collection. Furthermore, we demonstrate how the pulse counting technique allows control of a coincidence channel, specifically C+ + O+, by using logical electronic gates. Using these coincidence signals allows more specific final states to be incorporated into closed-loop control.