Electrical manipulation of crystal symmetry for switching transverse acoustic phonons.

We experimentally explore the use of a novel device where lateral electric fields can be applied to break the translational symmetry within the isotropic plane and hence change the selection rules to allow normally forbidden transverse acoustic (TA) phonon generations. The ultrafast screening of the lateral electric field by the photocarriers relieves shear strain in the structure and switches on the propagating TA waves. The amplitude and on-state time of the TA mode can be modulated by the external field strength and size of the laterally biased region. The observed frequency shift with an external bias as well as the strong geometrical dependence confirm the role of the asymmetric potential distribution in electrically manipulating the crystal symmetry to control modal behavior of acoustic phonons.

Theory of coherent phonons in carbon nanotubes and graphene nanoribbons.

We survey our recent theoretical studies on the generation and detection of coherent radial breathing mode (RBM) phonons in single-walled carbon nanotubes and coherent radial breathing like mode (RBLM) phonons in graphene nanoribbons. We present a microscopic theory for the electronic states, phonon modes, optical matrix elements and electron-phonon interaction matrix elements that allows us to calculate the coherent phonon spectrum. An extended tight-binding (ETB) model has been used for the electronic structure and a valence force field (VFF) model has been used for the phonon modes. The coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on the photoexcited carrier density. We discuss the dependence of the coherent phonon spectrum on the nanotube chirality and type, and also on the graphene nanoribbon mod number and class (armchair versus zigzag). We compare these results with a simpler effective mass theory where reasonable agreement with the main features of the coherent phonon spectrum is found. In particular, the effective mass theory helps us to understand the initial phase of the coherent phonon oscillations for a given nanotube chirality and type. We compare these results to two different experiments for nanotubes: (i) micelle suspended tubes and (ii) aligned nanotube films. In the case of graphene nanoribbons, there are no experimental observations to date. We also discuss, based on the evaluation of the electron-phonon interaction matrix elements, the initial phase of the coherent phonon amplitude and its dependence on the chirality and type. Finally, we discuss previously unpublished results for coherent phonon amplitudes in zigzag nanoribbons obtained using an effective mass theory.

Chirality-selective excitation of coherent phonons in carbon nanotubes by femtosecond optical pulses.

Using predesigned trains of femtosecond optical pulses, we have selectively excited coherent phonons of the radial breathing mode of specific-chirality single-walled carbon nanotubes within an ensemble sample. By analyzing the initial phase of the phonon oscillations, we prove that the tube diameter initially increases in response to ultrafast photoexcitation. Furthermore, from excitation profiles, we demonstrate that an excitonic absorption peak of carbon nanotubes periodically oscillates as a function of time when the tube diameter undergoes coherent radial breathing mode oscillations.

Laparoscopic splenectomy for idiopathic thrombocytopenic purpura (ITP). A five-year experience.

BACKGROUND: Laparoscopic splenectomy (LS) has rapidly become the preferred surgical treatment for idiopathic thrombocytopenic purpura (ITP), but its long-term efficacy for this disorder is unproved. This report documents the author's 5-year experience with, and long-term follow-up of, LS for ITP. METHODS: Between September 1992 and September 1997, 30 patients with clinical ITP and intractable thrombocytopenia were referred as surgical candidates. Two of them (7%) were converted to open, and the other 28 underwent successful LS. The operative approach evolved from a supine lithotomy to right lateral decubitus position, and the harmonic scalpel became the primary dissection tool in the later part of the study. RESULTS: The 28 successful LS patients constituted the study group. Accessory spleens were identified and resected in six patients (21%). Surgical times and blood loss averaged 2.4 h and 170 cc, respectively. The typical hospital stay was 2 days. Initial reversal of thrombocytopenia and ultimate cessation of oral steroids was achieved in 25 of 28 patients (89%). There were no deaths, but two patients had major complications (bleeding and pneumonia). All but two patients experienced a return to full activity and/or employment by 3 weeks post-LS. In the three cases that failed LS, none had residual splenic tissue on subsequent radionuclide scan. Long-term follow-up (2-60 months) was obtained in 22 of 28 patients (79%). The only death (at 13 months) resulted from oncologic disease. Twenty-one patients had lasting clinical remission of ITP. A positive preoperative response to oral steroids was the best predictor of success. CONCLUSIONS: This 5-year experience with LS supports its use for the surgical treatment of ITP. The procedure is safe and efficacious, resulting in brief hospitalization, minimal recovery time, and excellent long-term results.

Renal flow response to contrast medium measured by video dilution technique.

The renal blood flow response following a sequence of small contrast medium injections was analysed in normal dogs and normal human subjects by video dilution technique. The results indicate that the video dilution technique offers an excellent opportunity in detecting abnormal flow responses in patients with renal hypertension.