8,946 research outputs found
THz generation using a reflective stair-step echelon
We present a novel method for THz generation in lithium niobate using a
reflective stair-step echelon structure. The echelon produces a discretely
tilted pulse front with less angular dispersion compared to a high
groove-density grating. The THz output was characterized using both a 1-lens
and 3-lens imaging system to set the tilt angle at room and cryogenic
temperatures. Using broadband 800 nm pulses with a pulse energy of 0.95 mJ and
a pulse duration of 70 fs (24 nm FWHM bandwidth, 39 fs transform limited
width), we produced THz pulses with field strengths as high as 500 kV/cm and
pulse energies as high as 3.1 J. The highest conversion efficiency we
obtained was 0.33%. In addition, we find that the echelon is easily implemented
into an experimental setup for quick alignment and optimization.Comment: 19 pages, 4 figure
Laser-induced currents along molecular wire junctions
The treatment of the previous paper is extended to molecular wires.
Specifically, the effect of electron-vibrational interactions on the electronic
transport induced by femtosecond laser fields along unbiased
molecular nanojunctions is investigated. For this, the photoinduced vibronic
dynamics of trans-polyacetylene oligomers coupled to macroscopic metallic leads
is followed in a mean-field mixed quantum-classical approximation. A reduced
description of the dynamics is obtained by introducing projective lead-molecule
couplings and deriving an effective Schr\"odinger equation satisfied by the
orbitals in the molecular region. Two possible rectification mechanisms are
identified and investigated. The first one relies on near-resonance
photon-absorption and is shown to be fragile to the ultrafast electronic
decoherence processes introduced by the wire's vibrations. The second one
employs the dynamic Stark effect and is demonstrated to be highly efficient and
robust to electron-vibrational interactions.Comment: 14 pages, 10 figures. Accepted in J. Chem. Phy
Wideband THz time domain spectroscopy based on optical rectification and electro-optic sampling
We present an analytical model describing the full electromagnetic propagation in a THz time-domain spectroscopy (THz-TDS) system, from the THz pulses via Optical Rectification to the detection via Electro Optic-Sampling. While several investigations deal singularly with the many elements that constitute a THz-TDS, in our work we pay particular attention to the modelling of the time-frequency behaviour of all the stages which compose the experimental set-up. Therefore, our model considers the following main aspects: (i) pump beam focusing into the generation crystal; (ii) phase-matching inside both the generation and detection crystals; (iii) chromatic dispersion and absorption inside the crystals; (iv) Fabry-Perot effect; (v) diffraction outside, i.e. along the propagation, (vi) focalization and overlapping between THz and probe beams, (vii) electro-optic sampling. In order to validate our model, we report on the comparison between the simulations and the experimental data obtained from the same set-up, showing their good agreement
On-demand delivery of single DNA molecules using nanopipettes
Understanding the behavioral properties of single molecules or larger scale populations interacting with single molecules is currently a hotly pursued topic in nanotechnology. This arises from the potential such techniques have in relation to applications such as targeted drug delivery, early stage detection of disease, and drug screening. Although label and label-free single molecule detection strategies have existed for a number of years, currently lacking are efficient methods for the controllable delivery of single molecules in aqueous environments. In this article we show both experimentally and from simulations that nanopipets in conjunction with asymmetric voltage pulses can be used for label-free detection and delivery of single molecules through the tip of a nanopipet with “on-demand” timing resolution. This was demonstrated by controllable delivery of 5 kbp and 10 kbp DNA molecules from solutions with concentrations as low as 3 pM
Generation of microwave radiation by nonlinear interaction of a high-power, high-repetition rate, 1064-nm laser in KTP crystals
We report measurements of microwave (RF) generation in the centimeter band
accomplished by irradiating a nonlinear KTiOPO (KTP) crystal with a
home-made, infrared laser at nm as a result of optical rectification
(OR). The laser delivers pulse trains of duration up to s. Each train
consists of several high-intensity pulses at an adjustable repetition rate of
approximately GHz. The duration of the generated RF pulses is
determined by that of the pulse trains. We have investigated both microwave-
and second harmonic (SHG) generation as a function of the laser intensity and
of the orientation of the laser polarization with respect to the
crystallographic axes of KTP.Comment: 5 pages, 5 figures, to appear in Optics Letters, vol. 38 (2013
Sub-wavelength terahertz beam profiling of a THz source via an all-optical knife-edge technique
Terahertz technologies recently emerged as outstanding candidates for a variety of applications in such sectors as security, biomedical, pharmaceutical, aero spatial, etc. Imaging the terahertz field, however, still remains a challenge, particularly when sub-wavelength resolutions are involved. Here we demonstrate an all-optical technique for the terahertz near-field imaging directly at the source plane. A thin layer (<100 nm-thickness) of photo carriers is induced on the surface of the terahertz generation crystal, which acts as an all-optical, virtual blade for terahertz near-field imaging via a knife-edge technique. Remarkably, and in spite of the fact that the proposed approach does not require any mechanical probe, such as tips or apertures, we are able to demonstrate the imaging of a terahertz source with deeply sub-wavelength features (<30 μm) directly in its emission plane
Fast temporal adaptation of on-off units in the first optic chiasm of the blowfly
We recorded from spiking units in the first optic chiasm between lamina and medulla in the brain of the blowfly (Calliphora vicina). Both previously characterized neuron types, on-off units and sustaining units, were encountered. On-off units had a temporal frequency response with a lower cut-off frequency than blowfly photoreceptors. This low cut-off frequency is related to a fast temporal adaptation of the on-off units to trains of short light pulses. Temporal adaptation occurred independently for short on- and off-pulses.
On-off units only responded to stimuli of relatively large contrast. Contrasts of less than 10% gave little or no response.
Non-Reciprocal Geometric Wave Diode by Engineering Asymmetric Shapes of Nonlinear Materials
Unidirectional nonreciprocal transport is at the heart of many fundamental
problems and applications in both science and technology. Here we study the
novel design of wave diode devices by engineering asymmetric shapes of
nonlinear materials to realize the function of non-reciprocal wave
propagations. We first show analytical results revealing that both nonlinearity
and asymmetry are necessary to induce such non-reciprocal (asymmetric) wave
propagations. Detailed numerical simulations are further performed for a more
realistic geometric wave diode model with typical asymmetric shape, where good
non-reciprocal wave diode effect is demonstrated. Finally, we discuss the
scalability of geometric wave diodes. The results open a flexible way for
designing wave diodes efficiently simply through shape engineering of nonlinear
materials, which may find broad implications in controlling energy, mass and
information transports.Comment: 4 figure
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