Light emission from gold and silver thin films in a scanning tunneling microscope: role of contamination and interpretation of grain structure in photon maps

Scanning tunnelling microscope (STM) tip-induced light emission from Au and Ag has been studied. Thin film samples similar to100nm thick were prepared by thermal evaporation at 0.5nm/s onto a room-temperature glass substrate to produce grains of 20-50nm in lateral dimension at the surface. Light emission from the samples in the STM was quasi-simultaneously recorded with the topography, at 1.8V tip bias and 3-40nA current, alternating pixel by pixel at the same bias. Typically, a surface scan range of 150 nm x 150 nm was surveyed. Au, W and PtIr tips were used.

Engineering Nonclassicality in a Mechanical System through Photon Subtraction

Nonclassical states of a mechanical mode at nonzero temperature are achieved in a scheme that combines radiation-pressure coupling to a light field and photon subtraction. The scheme embodies an original and experimentally realistic way to obtain mesoscopic quantumness by putting together two mature technologies for quantum control. The protocol is quasi-insensitive to mechanical damping.

Comment on 'Implications on clinical scenario of gold nanoparticle radiosensitization in regards to photon energy, nanoparticle size, concentration and location'

A recent paper by Lechtman et al (2011 Phys. Med. Biol. 56 4631-47) presented Monte Carlo modelling of gold nanoparticle dose modification. In it, they predict that the introduction of gold nanoparticles has the strongest effect with x-rays at kilovoltage energies, and that negligible increases in dose are expected at megavoltage energies. While these results are in agreement with others in the literature (including those produced by our group), the conclusion that '(goldnanoparticle) radiosensitization using a 6 MV photon source is not clinically feasible' appears to conflict with recently published experimental studies which have shown radiosensitization using 6 MV x-ray sources with relatively low gold concentrations. The increasing disparity between theoretical predictions of dose enhancement and experimental results in the field of gold nanoparticle radiosensitization suggests that, while the ability of gold nanoparticles to modify dose within a tumour volume is well understood, the resulting radiosensitization is not simply correlated with this measure. This highlights the need to validate theoretical predictions of this kind against experimental measurements, to ensure that the scenarios and values being modelled are meaningful within a therapeutic context.

Photon Production from the Vacuum Close to the Superradiant Transition: Linking the Dynamical Casimir Effect to the Kibble-Zurek Mechanism

The dynamical Casimir effect (DCE) predicts the generation of photons from the vacuum due to the parametric amplification of the quantum fluctuations of an electromagnetic field. The verification of such an effect is still elusive in optical systems due to the very demanding requirements of its experimental implementation. We show that an ensemble of two-level atoms collectively coupled to the electromagnetic field of a cavity, driven at low frequencies and close to a quantum phase transition, stimulates the production of photons from the vacuum. This paves the way to an effective simulation of the DCE through a mechanism that has recently found experimental demonstration. The spectral properties of the emitted radiation reflect the critical nature of the system and allow us to link the detection of the DCE to the Kibble-Zurek mechanism for the production of defects when crossing a continuous phase transition.

Two-photon double ionization of He

We determine generalized cross sections for two-photon double ionization of He in the photon energy region between 40.7 and 47 eV where absorption of two photons can lead to non-sequential double ionization only. The present cross sections, obtained in R-matrix Floquet theory, agree with cross sections obtained from time-dependent calculations. By examining the ratio of two-photon double ionization to two-photon single ionization, we demonstrate that core excitation effects at an intensity of 10(13) W cm(-2) are relatively unimportant at 45 eV, but that they are significant at other photon energies.

Photon collisions with atoms and ions within an intermediate-energy R-matrix framework

Recent experimental advances in light technology necessitate the availability of sophisticated theoretical models which can incorporate an accurate treatment of double-electron continua. We describe here a new intermediate-energy R-matrix approach to photoionisation and photo-double-ionisation and illustrate its feasibilty by application to photoionisation and photo-double-ionisation of He, and photodetachment and photo-double-detachment of H^-. Results are shown to be in excellent agreement with previous theoretical and experimental studies. This work is a key step in the development of a multipurpose R-matrix code for multiple-electron ejection. © 2012 American Physical Society

IMAGING OF SURFACE-PLASMON PROPAGATION AND EDGE INTERACTION USING A PHOTON SCANNING TUNNELING MICROSCOPE

We report the direct imaging of surface plasmon propagation on thin silver films using the photon scanning tunneling microscope. It is found that the surface plasmon remains tightly confined in the original launch direction with insignificant scattering to other momentum states. A propagation length of 13.2 mum is measured at lambda = 632.8 nm. We also present images showing the interaction of a surface plasmon with the edge of the metal film supporting it. The most remarkable feature is the absence of a specularly reflected beam.

Anisotropy parameters in two-color two-photon above-threshold ionization

We employ the time-dependent R-matrix (TDRM) method to calculate anisotropy parameters for positive and negative sidebands of selected harmonics generated by two-color two-photon above-threshold ionization of argon. We consider odd harmonics of an 800-nm field ranging from the 13th to 19th harmonic, overlapped by a fundamental 800-nm IR field. The anisotropy parameters obtained using the TDRM method are compared with those obtained using a second-order perturbation theory with a model potential approach and a soft photon approximation approach. Where available, a comparison is also made to published experimental results. All three theoretical approaches provide similar values for anisotropy parameters. The TDRM approach obtains values that are closest to published experimental values. At high photon energies, the differences between each of the theoretical methods become less significant.

Time-dependent R-matrix theory applied to two-photon double ionization of He

We introduce a time-dependent R-matrix theory generalized to describe double-ionization processes. The method is used to investigate two-photon double ionization of He by intense XUV laser radiation. We combine a detailed B-spline-based wave-function description in an extended inner region with a single-electron outer region containing channels representing both single ionization and double ionization. A comparison of wave-function densities for different box sizes demonstrates that the flow between the two regions is described with excellent accuracy. The obtained two-photon double-ionization cross sections are in excellent agreement with other cross sections available. Compared to calculations fully contained within a finite inner region, the present calculations can be propagated over the time it takes the slowest electron to reach the boundary.

Resonantly Enhanced Multi-Photon Ionization Spectrum of the Neutral Green Fluorescent Protein Chromophore

The photophysics of the green fluorescent protein is governed by the electronic structure of the chromophore at the heart of its β-barrel protein structure. We present the first two-color, resonance-enhanced, multiphoton ionization spectrum of the isolated neutral chromophore in vacuo with supporting electronic structure calculations. We find the absorption maximum to be 3.65 ± 0.05 eV (340 ± 5 nm), which is blue-shifted by 0.5 eV (55 nm) from the absorption maximum of the protein in its neutral form. Our results show that interactions between the chromophore and the protein have a significant influence on the electronic structure of the neutral chromophore during photoabsorption and provide a benchmark for the rational design of novel chromophores as fluorescent markers or photomanipulators.