arXiv subjects

S. W. Wu

Publications and source records attributed to S. W. Wu.

3 recordsLinked to original sources

arXiv2025

Patient-Specific Modeling of Dose-Escalated Proton Beam Therapy for Locally Advanced Pancreatic Cancer

Purpose: This study explores the feasibility of dose-escalated proton beam therapy (dPBT) for Locally Advanced Pancreatic Cancer (LAPC) patients by modeling common patient scenarios using current clinically-adopted practices. Methods: Five patient datasets were used as simulation phantoms, each with six tumour sizes, to systematically simulate treatment scenarios typical in LAPC patients. Using the Raystation treatment planning system, robustly-optimised dPBT and stereotactic ablative radiotherapy (SABR) treatment plans were created with a 5 mm margin allowing for intra- and inter-fraction anatomical changes. following clinically-adopted protocols. Safe dose-escalation feasibility is assessed with dose metrics, tumour control (TCP) and normal tissue complication probabilities (NTCP) for average and worst-case intra-fraction motion scenarios. Significance testing was performed using a paired student's t-test. Results: Dose-escalation feasibility is largely dependent on tumour size and proximity to critical structures. Minimal therapeutic benefit was observed for patients with greater than 4.5 cm tumours, however for tumours less than or equal to 4.5 cm dPBT TCPs of 45-90% compared to SABR TCPs of 10-40% (p<0.05). The worst-case scenario dPBT TCP was comparable to SABR. Hypofractioned dPBT further improved this result to greater than 90% (p<0.05) for tumours less than or equal to 4.5 cm. Conclusion: Safe dPBT is feasible for patients with targets up to the median size and see a significant therapeutic benefit compared to the current standard of care in SABR. A patient-specific approach should be taken based on tumour size and surrounding anatomy.

physics.med-ph↗

arXiv2020

Third order optical nonlinearity of three dimensional massless Dirac fermions

We present analytic expressions for the electronic contributions to the linear conductivity $σ^{(1)}_{3d}(ω)$ and the third order optical conductivity $σ^{(3)}_{3d}(ω_1,ω_2,ω_3)$ of three dimensional massless Dirac fermions, the quasi-particles relevant for the low energy excitation of topological Dirac semimetals and Weyl semimetals. Although there is no gap for massless Dirac fermions, a finite chemical potential $μ$ can lead to an effective gap parameter, which plays an important role in the qualitative features of interband optical transitions. For gapless linear dispersion in three dimension, the imaginary part of the linear conductivity diverges as a logarithmic function of the cutoff energy, while the real part is linear with photon frequency $ω$ as $\hbarω>2|μ|$. The third order conductivity exhibits features very similar to those of two dimensional Dirac fermions, i.e., graphene, but with the amplitude for a single Dirac cone generally two orders of magnitude smaller in three dimension than in two dimension. There are many resonances associated with the chemical potential induced gap parameters, and divergences associated with the intraband transitions. The details of the third order conductivity are discussed for third harmonic generation, the Kerr effect and two-photon carrier injection, parametric frequency conversion, and two-color coherent current injection. Although the expressions we derive are limited to the clean limit at zero temperature, the generalization to include phenomenological relaxation processes at finite temperature is straightforward and is presented.

cond-mat.mtrl-sci↗

arXiv2018

Intraband divergences in third order optical response of 2D systems

The existence of large nonlinear optical coefficients is one of the preconditions for using nonlinear optical materials in nonlinear optical devices. For a crystal, such large coefficients can be achieved by matching photon energies with resonant energies between different bands, and so the details of the crystal band structure play an important role. Here we demonstrate that large third-order nonlinearities can also be generally obtained by a different strategy: As any of the incident frequencies or the sum of any two or three frequencies approaches zero, the doped or excited populations of electronic states lead to divergent contributions in the induced current density. We refer to these as intraband divergences, by analogy with the behavior of Drude conductivity in linear response. Physically, such resonant processes can be associated with a combination of inraband and interband optical transitions. Current-induced second order nonlinearity, coherent current injection, and jerk currents are all related to such divergences, and we find similar divergences in degenerate four wave mixing and cross-phase modulation under certain conditions. These divergences are limited by intraband relaxation parameters, and lead to a large optical response from a high quality sample; we find they are very robust with respect to variations in the details of the band structure. To clearly track all of these effects, we analyze gapped graphene, describing the electrons as massive Dirac fermions; under the relaxation time approximation, we derive analytic expressions for the third order conductivities, and identify the divergences that arise in describing the associated nonlinear phenomena.

cond-mat.mtrl-sci↗