Waveguide mode imaging and dispersion analysis with terahertz near-field microscopy

Propagation of terahertz waves in hollow metallic waveguides depends on the waveguide mode. Near-field scanning probe terahertz microscopy is applied to identify the mode structure and composition in dielectric-lined hollow metallic waveguides. Spatial profiles, relative amplitudes, and group velocities of three main waveguide modes are experimentally measured and matched to the HE11, HE12, and TE11 modes. The combination of near-field microscopy with terahertz time-resolved spectroscopy opens the possibility of waveguide mode characterization in the terahertz band

Terahertz s-SNOM with > λ/1000 resolution based on self-mixing in quantum cascade lasers

Near-field imaging techniques have great potential in many applications, ranging from the investigation of the optical properties of solid state and 2D materials to the excitation and direct retrieval of plasmonic resonant modes, to the mapping of carrier concentrations in semiconductor devices. Further to this, the capability of performing imaging with non-ionizing terahertz (THz) radiation on a subwavelength scale is of fundamental importance in biological applications and healthcare. The implementation of stable, compact solid state sources such as quantum cascade lasers (QCLs) in apertureless scanning near field optical microscopes (s-SNOM), instead of bulkier gas lasers, has been already reported with a resolution ≥ 1 μm [1] based on metallic tips. Here we report on the realization of an s-SNOM, based on tuning fork sensors [2], to maintain a constant sample/tip distance in tapping mode, and using quantum cascade lasers emitting around 3 THz as both source and detector in a self-mixing scheme [3]. The implementation of a fast and efficient feedback mechanism allowed the achievement of a spatial resolution lower than 100 nm, as shown in Fig. 1, thus achieving the record resolution with a QCL better than λ/1000. The self-mixing approach allows an extremely sensitive and fast detection scheme, which overcomes the slow response of traditional THz detectors, by monitoring the scattered signal fed back into the QCL cavity, modulating the power or the bias. In order to enhance the sensitivity of the whole apparatus, as well as the collection of the scattered light, silicon lenses have been attached to the QCLs with an antireflection parylene coating which was thick enough to strongly reduce the laser emission, but still allowed enough power for alignment. Figure 1 reports the topography a) and the THz voltage signal on the QCL b) of Au square features (top-left square corner) over a Si substrate, exhibiting an enhanced scattering. As the reference voltage used for subtraction from the QCL voltage was placed lower than the QCL voltage, the THz signal dropped on the Au square

Measurement of mechanical losses in the carbon nanotube black coating of silicon wafers

The successful detection of gravitational waves from astrophysical sources carried out by the laser interferometric detectors LIGO and Virgo have stimulated scientists to develop a new generation of more sensitive gravitational wave detectors. In the proposed upgrade called LIGO Voyager, silicon test masses will be cooled to cryogenic temperatures. To provide heat removal from the test masses when they absorb the laser light one can increase their thermal emissivity using a special black coating. We have studied mechanical losses in a carbon nanotube black coating deposited on silicon wafers. The additional thermal noise associated with mechanical loss in this coating was calculated using a value of the product of the coating Young's modulus and the coating mechanical loss angle determined from the measurements. It was found that at temperatures of about 123 K, the additional thermal noise of the LIGO Voyager test mass caused by the carbon nanotube black coating deposited on its barrel is less than the noise associated with the Acktar Black coating and is 20 times less than the noise due to the optical high reflective (HR) coating of the test mass

Near-field probes for sensitive detectorless near-field nanoscopy in the 2.0-4.6 THz range

Imaging and spectroscopy at terahertz (THz) frequencies have become key methods for fundamental studies across the physical sciences. With the emergence of nanoscale materials and devices, holding great promise for photonics, electronics, and communication technologies, the search for THz analysis at the nanoscale arises. Detectorless THz near-field nanoscopy emerged as a versatile method for hyperspectral mapping of light-matter interaction phenomena in bi-dimensional materials and systems. However, it is strongly limited by the weak scattering efficiencies of atomic force microscope (AFM) tips. Here, we experimentally evaluate the performance of unconventional AFM tip shapes to enhance the scattering efficiency, at three frequencies, namely, 2.0, 3.0, and 4.6 THz. The impact of tip geometry is corroborated by numerical simulations. The shorter shank length of the evaluated tips provides a very compelling alternative to commercial tips at frequencies &gt;2 THz.</p

Electrodynamics of Magnetars: Implications for the Persistent X-ray Emission and Spindown of the Soft Gamma Repeaters and Anomalous X-ray Pulsars

(ABBREVIATED) We consider the structure of neutron star magnetospheres threaded by large-scale electrical currents, and the effect of resonant Compton scattering by the charge carriers (both electrons and ions) on the emergent X-ray spectra and pulse profiles. In the magnetar model for the SGRs and AXPs, these currents are maintained by magnetic stresses acting deep inside the star. We construct self-similar, force-free equilibria of the current-carrying magnetosphere with a power-law dependence of magnetic field on radius, B ~ r^(-2-p), and show that a large-scale twist softens the radial dependence to p < 1. The spindown torque acting on the star is thereby increased in comparison with a vacuum dipole. We comment on the strength of the surface magnetic field in the SGR and AXP sources, and the implications of this model for the narrow measured distribution of spin periods. A magnetosphere with a strong twist, B_\phi/B_\theta = O(1) at the equator, has an optical depth ~ 1 to resonant cyclotron scattering, independent of frequency (radius), surface magnetic field strength, or charge/mass ratio of the scattering charge. When electrons and ions supply the current, the stellar surface is also heated by the impacting charges at a rate comparable to the observed X-ray output of the SGR and AXP sources, if B_{dipole} ~ 10^{14} G. Redistribution of the emerging X-ray flux at the ion and electron cyclotron resonances will significantly modify the emerging pulse profile and, through the Doppler effect, generate a non-thermal tail to the X-ray spectrum. The sudden change in the pulse profile of SGR 1900+14 after the 27 August 1998 giant flare is related to an enhanced optical depth to electron cyclotron scattering, resulting from a sudden twist imparted to the external magnetic field.Comment: 31 January 2002, minor revisions, new section 5.4.

Gamma Ray Bursts from the Evolved Galactic Nuclei

A new cosmological scenario for the origin of gamma ray bursts (GRBs) is proposed. In our scenario, a highly evolved central core in the dense galactic nucleus is formed containing a subsystem of compact stellar remnants (CSRs), such as neutron stars and black holes. Those subsystems result from the dynamical evolution of dense central stellar clusters in the galactic nuclei through merging of stars, thereby forming (as has been realized by many authors) the short-living massive stars and then CSRs. We estimate the rate of random CSR collisions in the evolved galactic nuclei by taking into account, similar to Quinlan & Shapiro (1987), the dissipative encounters of CSRs, mainly due to radiative losses of gravitational waves, which results in the formation of intermediate short-living binaries, with further coalescence of the companions to produce GRBs. We also consider how the possible presence of a central supermassive black hole, formed in a highly evolved galactic nucleus, influences the CSR binary formation. This scenario does not postulate ad hoc a required number of tight binary neutron stars in the galaxies. Instead, it gives, for the most realistic parameters of the evolved nuclei, the expected rate of GRBs consistent with the observed one, thereby explaining the GRB appearance in a natural way of the dynamical evolution of galactic nuclei. In addition, this scenario provides an opportunity for a cosmological GRB recurrence, previously considered to be a distinctive feature of GRBs of a local origin only. We also discuss some other observational tests of the proposed scenario.Comment: 25 pages, LATEX, uses aasms4.sty, accepted by Ap