Lorentzian-type Spectra Research Articles

InGaAs Diodes for Terahertz Sensing-Effect of Molecular Beam Epitaxy Growth Conditions.

InGaAs-based bow-tie diodes for the terahertz (THz) range are found to be well suited for development of compact THz imaging systems. To further optimize design for sensitive and broadband THz detection, one of the major challenges remains: to understand the noise origin, influence of growth conditions and role of defects for device operation. We present a detailed study of photoreflectance, low-frequency noise characteristics and THz sensitivity of InGaAs bow-tie diodes. The diodes are fabricated from InGaAs wafers grown by molecular beam epitaxy (MBE) on semi-insulating InP substrate under different technological conditions. Photoreflectance spectra indicated the presence of strong built-in electric fields reaching up to 49 kV/cm. It was demonstrated that the spectral density of voltage fluctuations at room temperature was found to be proportional to 1/f, while at lower temperatures, 77–200 K, Lorentzian-type spectra dominate due to random telegraph signals caused by individual capture defects. Furthermore, varying bias voltage, we considered optimal conditions for device room temperature operation in the THz range with respect to signal-to-noise ratio. The THz detectors grown with beam equivalent pressure In/Ga ratio equal to 2.04 exhibit the minimal level of the low-frequency noise, while InGaAs layers grown with beam equivalent pressure In/Ga ratio equal to 2.06 are found to be well suited for fabrication of room temperature bow-tie THz detectors enabling sensitivity of 13 V/W and noise equivalent power (NEP) of 200 pW/√Hz at 0.6 THz due to strong built-in electric field effects.

Low-frequency noise characteristics of lamellar ferrielectric crystal CuInP2S6 at the phase transition

The detailed investigations of low-frequency noise characteristics of a ferrielectric CuInP2S6 crystal are presented at the vicinity of phase transition. The white noise spectra are observed at room temperature. Voltage noise spectra are characterized by 1/f noise above the phase transition temperature. Lorentzian-type spectra were applied to investigate the normalized voltage fluctuation density, in order to obtain a deeper insight into the phenomenon of 1/f noise in the CuInP2S6 crystal. The analysis of experimental data revealed that the source of the 1/f noise in the CuInP2S6 crystal is the carrier number fluctuation due to the recombination between defect levels. The obtained results indicate noise spectroscopy as being a reliable, sensitive, and non-destructive tool complementary to the other measurement techniques for the characterization of ferroelectric materials.

Analysis of electrical and optical fluctuations of light-emitting diodes by correlation method

A detailed analysis of electrical and optical uctuations of high power (3 W) light-emitting diodes (LEDs) is presented. A special attention was paid to the measurement and interpretation of simultaneous correlation coefcient between electrical and optical uctuations. The simultaneous correlation coefcient was measured not only over frequency range from 10 Hz to 20 kHz, but for the rst time in every one-octave frequency band. The presented technique not only permitted to determine the correlation coefcient dependence on frequency, but also to estimate what part to correlation coefcient is produced by low-frequency uctuations with 1=f , 1=f fi , and Lorentzian type spectra, respectively. From the obtained experimental and calculation results it is determined that the simultaneous correlated part of electrical and optical uctuations for these LEDs is related with the defect presence in the active layer or at its interfaces, while the uncorrelated part of electrical noise is caused by parallel leakage channel in other places, and the resistance of this channel is many times higher than that of pnn junction.

A modified mean normalization method to reduce the effect of peak overlap in two-dimensional correlation spectroscopy

Two-dimensional correlation spectroscopy (2D-COS) has been widely used to separate overlapped spectroscopic bands. However, band overlap may sometimes cause misleading results in the 2D-COS spectra, especially if one peak is embedded within another peak by the overlap. Based on the analysis of Lorentzian-type spectra, a new data normalization method has been proposed to overcome such overlap effect. The normalization factor is expressed as the square of the mean value divided by the peak value of each spectrum, F mean 2 / F max . This expression may be viewed as a modification to the mean normalization method originally put forward by Ozaki and co-workers. Computing simulation demonstrated that the new method works well with simulated dynamic spectra.

Effect of Oxygen Isotope Exchange on Ferroelectric Microregion in SrTiO3Studied by Raman Scattering

The ferroelectric microregion (FMR) in quantum paraelectric SrTiO 3 is studied by Raman scattering using an optical cryostat with a 3 He refrigerator. By the partial exchange of the oxygen atom for its isotope 18 O, broad FMR spectra change drastically to underdamped Lorentzian-type spectra with decreasing temperature below 10 K. The peak is assigned to the ferroelectric soft Eu mode. The intense and sharp spectra of the Eu mode, which are nominally Raman inactive, indicate that the symmetry is lowered to a non-centrosymmetric symmetry in FMR. Furthermore, the softening characteristic of the soft Eu mode in a low-temperature region implies that the symmetry is not necessarily ferroelectric. The local symmetry breaking is enhanced by the substitution of the isotope 18 O.

ESR study of new layered semiconductor Ga2Cr1.33Se5

The Ga2Cr1.33Se5 compound is a layered semiconductor and it can be visualised as layers of chromium ions with nearest-neighbour distances of 0.378nm within layers and interlayer distances are alternately of 0.626 and 0.923nm. The ESR measurements of a single crystal as a function of temperature were performed; a single Lorentzian-type spectrum was observed for T>20K. The observed angular anisotropy of the linewidth has been satisfactorily explained on the basis of the anisotropic exchange model. The peculiarities of ESR parameters (intensity, ΔB, Br) below T≅20K can be interpreted taking into account two mechanisms: (i) presence ferromagnetic, exchange interactions between in-plane chromium ions and antiferromagnetic interplane coupling which means magnetic dimensionality crossover from 2- to 3-d system; (ii) appearance of a spin disorder.

Saving Measurement Time in 13C NMR Spectroscopy

An algorithm to produce acceptably accurate Fourier transform NMR spectra using many fewer transients than commonly obtained is introduced and applied to 13C chemical shift and one-dimensional INADEQUATE Lorentzian type spectra. The algorithm supplies criteria to recognize when to stop transient collection at a time too early for an acceptable spectrum to be produced by Fourier signal processing but not by harmonic inversion signal processing.

Observation of distinct, temperature dependent flux noise near bicrystal grain boundaries in YBa 2Cu 3O 7− x films

The characteristics of the magnetic flux noise in high temperature superconducting thin-films of yttrium-barium-copper-oxide (YBa 2Cu 3O 7) in the vicinity of artificial grain boundaries have been studied by means of a low critical temperature superconducting quantum interference device (SQUID) magnetometer. The measured power spectrum of the flux noise typically shows 1/ f-behaviour at low frequencies. This is, in particular, evident near the transition temperature. When measuring the flux noise near an artificial grain boundary we observe in selected samples additional Lorentzian type spectra in narrow temperature ranges indicative for well-defined barriers for flux hopping. Such spectra are not observed when measuring away from the grain boundary. The distinct fluctuators are seen in the 60–80 K interval corresponding to barrier heights of order 0.1–0.4 eV. If present in Josephson junctions patterned across such artificial grain boundaries, they could lead to serious limitations in the junction performance in certain temperature ranges.

A model of 1/f noise spectrum generation in granular structures

A fluctuating spectrum was calculated and a Lorentzian type spectrum obtained, in a contacting spot between the tapered grain boundaries and 1/f type spectrum on the granular structure in low frequency. This is based on the spontaneous temperature fluctuation of the small conducting spots between the grains and thermal conduction in the granular structures.