Characterisation of multi-layered structures using a vector-based gradient descent algorithm at terahertz frequencies.

Material characterisation and imaging applications using terahertz radiation have gained interest in the past few years due to their enormous potential for industrial applications. The availability of fast terahertz spectrometers or multi-pixel terahertz cameras has accelerated research in this domain. In this work, we present a novel vector-based implementation of the gradient descent algorithm to fit the measured transmission and reflection coefficients of multilayered objects to a scattering parameter-based model, without requiring any analytical formulation of the error function. We thereby extract thicknesses and refractive indices of the layers within a maximum 2% error margin. Using the precise thickness estimates, we further image a 50 nm-thick Siemens star deposited on a silicon substrate using wavelengths larger than 300 µm. The vector-based algorithm heuristically finds the error minimum where the optimisation problem cannot be analytically formulated, which can be utilised also for applications outside the terahertz domain.

Visualizing nanometric structures with sub-millimeter waves.

The resolution along the propagation direction of far field imagers can be much smaller than the wavelength by exploiting coherent interference phenomena. We demonstrate a height profile precision as low as 31 nm using wavelengths between 0.375 mm and 0.5 mm (corresponding to 0.6 THz-0.8 THz) by evaluating the Fabry-Pérot oscillations within surface-structured samples. We prove the extreme precision by visualizing structures with a height of only 49 nm, corresponding to 1:7500 to 1:10000 vacuum wavelengths, a height difference usually only accessible to near field measurement techniques at this wavelength range. At the same time, the approach can determine thicknesses in the centimeter range, surpassing the dynamic range of any near field measurement system by orders of magnitude. The measurement technique combined with a Hilbert-transform approach yields the (optical) thickness extracted from the relative phase without any extraordinary wavelength stabilization.

Single-spin resonance in a van der Waals embedded paramagnetic defect.

A plethora of single-photon emitters have been identified in the atomic layers of two-dimensional van der Waals materials1-8. Here, we report on a set of isolated optical emitters embedded in hexagonal boron nitride that exhibit optically detected magnetic resonance. The defect spins show an isotropic ge-factor of ~2 and zero-field splitting below 10 MHz. The photokinetics of one type of defect is compatible with ground-state electron-spin paramagnetism. The narrow and inhomogeneously broadened magnetic resonance spectrum differs significantly from the known spectra of in-plane defects. We determined a hyperfine coupling of ~10 MHz. Its angular dependence indicates an unpaired, out-of-plane delocalized π-orbital electron, probably originating from substitutional impurity atoms. We extracted spin-lattice relaxation times T1 of 13-17 µs with estimated spin coherence times T2 of less than 1 µs. Our results provide further insight into the structure, composition and dynamics of single optically active spin defects in hexagonal boron nitride.

Coherent electrical readout of defect spins in silicon carbide by photo-ionization at ambient conditions.

Quantum technology relies on proper hardware, enabling coherent quantum state control as well as efficient quantum state readout. In this regard, wide-bandgap semiconductors are an emerging material platform with scalable wafer fabrication methods, hosting several promising spin-active point defects. Conventional readout protocols for defect spins rely on fluorescence detection and are limited by a low photon collection efficiency. Here, we demonstrate a photo-electrical detection technique for electron spins of silicon vacancy ensembles in the 4H polytype of silicon carbide (SiC). Further, we show coherent spin state control, proving that this electrical readout technique enables detection of coherent spin motion. Our readout works at ambient conditions, while other electrical readout approaches are often limited to low temperatures or high magnetic fields. Considering the excellent maturity of SiC electronics with the outstanding coherence properties of SiC defects, the approach presented here holds promises for scalability of future SiC quantum devices.

Angle of incidence averaging in reflectance measurements with optical microscopes for studying layered two-dimensional materials.

Reflectance spectrum measured using an optical microscope with a large numerical aperture objective lens is shown to get modified. The change is most prominent when there are optical interference related features in the spectrum. This modification is shown to arise primarily due to the wide range of angles of incidence involved in the measurement and a simple formulation is provided to correct for this in simulations. The importance of such analysis is brought out through a reflectance contrast spectroscopy based study for identifying mono-layer and bi-layer graphene and MoS2.

Note: improved sensitivity of photoreflectance measurements with a combination of dual detection and electronic compensation.

A setup is described for performing photoreflectance (PR) measurements using two photodetectors, wherein the photodetectors need not be identical. The second detector monitors the photoluminescence and scattered pump laser background signal in real time. It is then eliminated from the measured PR signal using electronic circuits that compensate for amplitude and phase differences between the background signals from the two detectors. The technique overcomes the adverse effect of short-term fluctuations in the pump laser intensity. The signal-to-noise ratio is shown to improve significantly, enabling measurement of weak PR signals.

Epidemic of hand, foot and mouth disease in West Bengal, India in August, 2007: a multicentric study.

BACKGROUND: Hand, foot, and mouth disease (HFMD) is caused mostly by Coxsackievirus A16 (CA16) and enterovirus 71 (EV71). Epidemic of HFMD has occurred in India only once in Kerala in 2003. We report here a recent outbreak of HFMD in three districts of West Bengal, India. MATERIALS AND METHODS: A case detection system developed with 1) three private clinics in three districts; two at Howrah and one at Hooghly, 2) Pediatrics Department of two medical colleges in Kolkata, 3) 12 practioners of these three districts with 4) a central referral center at Department of Dermatology, NRS Medical College, Kolkata where all cases from this system were confirmed by a single observer. Pediatric Dermatology unit of the Institute of Child Health, Kolkata was another independent unit. RESULTS: A total of 38 cases of HFMD were reported till 08.10.07. Age group ranged from 12 months to 12 years (mean 40.76 months, SD 29.49). Males were slightly higher than females (M:F - 21:17). Disease was distributed mostly over buttocks, knees, hands, feet - both dorsum and palmar or the plantar surface and the oral mucosa. Highest severity noted over the buttocks and the knee. Healing time for skin lesions was 6-13 days (mean 9.13 days, SD 1.93). Oral lesions were found in 33 (86.8%) cases. CONCLUSION: This outbreak far away from the initial one confirmed regular outsourcing of the virus with possibilities of future epidemics. Also the fact that EV71 induced epidemic is on rise in this part of globe is alarming for India. We hope this early report will be of help for strategic planning for a better management of the disease and prevention of dreaded neurological complications in India.