n-Type GaAs/AlGaAs heterostructure detector with a 3.2 THz threshold frequency

Terahertz detection using the free-carrier absorption requires a small internal work function of the order of a few millielectron volts. A threshold frequency of 3.2 THz (93 microm or approximately 13 meV work function) is demonstrated by using a 1 x 10(18) cm(-3) Si-doped GaAs emitter and an undoped Al(0.04)Ga(0.96)As barrier structure. The peak responsivity of 6.5 A/W, detectivity of 5.5 x 10(8) Jones, and quantum efficiency of 19% were obtained at 7.1 THz under a bias field of 0.7 kV/cm at 6 K, while the detector spectral response range spans from 3.2 to 30 THz.     

Development of a laser photothermoacoustic frequency-swept system for subsurface imaging: theory and experiment

In conventional biomedical photoacoustic imaging systems, a pulsed laser is used to generate time-of-flight acoustic information of the subsurface features. This paper reports the theoretical and experimental development of a new frequency-domain (FD) photo-thermo-acoustic (PTA) principle featuring frequency sweep (chirp) and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wavelength. PTA imaging is a promising new technique which is being developed to detect tumor masses in turbid biological tissue. Owing to the linear relationship between the depth of acoustic signal generation and the delay time of signal arrival to the transducer, information specific to a particular depth can be associated with a particular frequency in the chirp signal. Scanning laser modulation with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. Preliminary results performed on rubber samples and solid tissue phantoms indicate that the FD-PTA technique has the potential to be a reliable tool for biomedical depth-profilometric imaging.     

BCS theory of driven superconductivity

We study the impact of a time-dependent external driving of the lattice phonons in a minimal model of a BCS superconductor. Upon evaluating the driving-induced vertex corrections of the phonon-mediated electron-electron interaction, we show that parametric phonon driving can be used to elevate the critical temperature T_c, while a dipolar phonon drive has no effect. We provide simple analytic expressions for the enhancement factor of T_c. Furthermore, a mean-field analysis of a nonlinear phonon-phonon interaction also shows that phonon anharmonicities further amplify T_c. Our results hold universally for the large class of normal BCS superconductors.     

Compactons versus solitons

We investigate whether the recently proposed PT-symmetric extensions of generalized Korteweg-de Vries equations admit genuine soliton solutions besides compacton solitary waves. For models which admit stable compactons having a width which is independent of their amplitude and those which possess unstable compacton solutions the Painlevé test fails, such that no soliton solutions can be found. The Painlevé test is passed for models allowing for compacton solutions whose width is determined by their amplitude. Consequently, these models admit soliton solutions in addition to compactons and are integrable.     

Fluorescence tomography with simulated data based on the equation of radiative transfer

The quantification of a nonuniform quantum yield or fluorophore absorption distribution is of major interest in molecular imaging of biological tissue. We introduce what is believed to be the first fluorescence image reconstruction algorithm based on the equation of radiative transfer that recovers the spatial distribution of light-emitting fluorophores inside a highly scattering medium from measurements made on the surface of the medium. We obtain images of either the quantum yield or the fluorophore absorption.     

Fabrication of ridge waveguides in zinc-substituted lithium niobate by means of ion-beam enhanced etching

We present results on the fabrication and characterization of ridge waveguides in zinc-substituted lithium niobate. High-quality waveguides were fabricated by a combination of liquid-phase epitaxy and multiple applications of ion-beam enhanced etching. The two major demands on ridge waveguides, a very low side-wall roughness and a rectangle shape with side-wall angles close to 90 degrees , were realized simultaneously by using this technique. Single-mode waveguiding at a wavelength of 1064 nm was demonstrated with attenuation values of 0.9 dB/cm.     

Polymorphism of sorbitol

The polymorphism of sorbitol was investigated, confirming the existence of four anhydrous crystalline phases plus the hydrate. The crystallised melt (CM), the alpha form, and the gamma form were obtained via a dry route. The CM was confirmed to be a crystalline state with a spherulite morphology. The alpha form was obtained via direct conversion from the CM, in contrast to more complicated routes previously reported, and was found to have a very high crystallinity. Gamma crystals were obtained by seeding the melt at high temperature; however, crystallinity was clearly less than for alpha crystals.