589 research outputs found
Investigating ultrasoundâlight interaction in scattering media
Significance: Ultrasound-assisted optical imaging techniques, such as ultrasound-modulated optical tomography, allow for imaging deep inside scattering media. In these modalities, a fraction of the photons passing through the ultrasound beam is modulated. The efficiency by which the photons are converted is typically referred to as the ultrasound modulationâs âtagging efficiency.â Interestingly, this efficiency has been defined in varied and discrepant fashion throughout the scientific literature.
Aim: The aim of this study is the ultrasound tagging efficiency in a manner consistent with its definition and experimentally verify the contributive (or noncontributive) relationship between the mechanisms involved in the ultrasound optical modulation process.
Approach: We adopt a general description of the tagging efficiency as the fraction of photons traversing an ultrasound beam that is frequency shifted (inclusion of all frequency-shifted components). We then systematically studied the impact of ultrasound pressure and frequency on the tagging efficiency through a balanced detection measurement system that measured the power of each order of the ultrasound tagged light, as well as the power of the unmodulated light component.
Results: Through our experiments, we showed that the tagging efficiency can reach 70% in a scattering phantom with a scattering anisotropy of 0.9 and a scattering coefficient of 4ââmmâ»Âč for a 1-MHz ultrasound with a relatively low (and biomedically acceptable) peak pressure of 0.47 MPa. Furthermore, we experimentally confirmed that the two ultrasound-induced light modulation mechanisms, particle displacement and refractive index change, act in opposition to each other.
Conclusion: Tagging efficiency was quantified via simulation and experiments. These findings reveal avenues of investigation that may help improve ultrasound-assisted optical imaging techniques
Value Premium In The Chinese Stock MarketïŒFree Lunch Or Paid Lunch?
In this paper we examine the time-series predictability of the book-to-market (B/M) ratio for annual and monthly portfolio returns in the Chinese stock market. We find that value premiums exist throughout our sample period of 1998 to 2008. However, the predictability of B/M appears to be unrelated with financial distress risk. In fact, value stocks are less risky than growth stocks in terms of return volatility and estimated financial distress risk. Further, our results suggest that the factor VMG, which is directly related to value premium, is not a pervasive risk measure compared to market factor and SMB. While the size effect seems to be closely related to distress risk, both size and B/M factors do not appear to be driven by financial distress risk
Sub-Optimal Moving Horizon Estimation in Feedback Control of Linear Constrained Systems
Moving horizon estimation (MHE) offers benefits relative to other estimation
approaches by its ability to explicitly handle constraints, but suffers
increased computation cost. To help enable MHE on platforms with limited
computation power, we propose to solve the optimization problem underlying MHE
sub-optimally for a fixed number of optimization iterations per time step. The
stability of the closed-loop system is analyzed using the small-gain theorem by
considering the closed-loop controlled system, the optimization algorithm
dynamics, and the estimation error dynamics as three interconnected subsystems.
By assuming incremental input/output-to-state stability ({\delta}- IOSS) of the
system and imposing standard ISS conditions on the controller, we derive
conditions on the iteration number such that the interconnected system is
input-to-state stable (ISS) w.r.t. the external disturbances. A simulation
using an MHE- MPC estimator-controller pair is used to validate the results.Comment: 6 page journal paper with 2 figure
Smith-Purcell Radiation from Low-Energy Electrons
Recent advances in the fabrication of nanostructures and nanoscale features
in metasurfaces offer a new prospect for generating visible, light emission
from low energy electrons. In this paper, we present the experimental
observation of visible light emission from low-energy free electrons
interacting with nanoscale periodic surfaces through the Smith-Purcell (SP)
effect. SP radiation is emitted when electrons pass in close proximity over a
periodic structure, inducing collective charge motion or dipole excitations
near the surface, thereby giving rise to electromagnetic radiation. We
demonstrate a controlled emission of SP light from nanoscale gold gratings with
periodicity as small as 50 nm, enabling the observation of visible SP radiation
by low energy electrons (1.5 to 6 keV), an order of magnitude lower than
previously reported. We study the emission wavelength and intensity dependence
on the grating pitch and electron energy, showing agreement between experiment
and theory. Further reduction of structure periodicity should enable the
production of SP-based devices that operate with even slower electrons that
allow an even smaller footprint and facilitate the investigation of quantum
effects for light generation in nanoscale devices. A tunable light source
integrated in an electron microscope would enable the development of novel
electron-optical correlated spectroscopic techniques, with additional
applications ranging from biological imaging to solid-state lighting.Comment: 16 pages, 4 figure
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