The X-ray variability of AGN: power-spectrum and variance analysis of the Swift/BAT light curves

We study the X-ray power spectrum of Active Galactic Nuclei (AGN) to investigate whether Seyfert I and II power spectra are similar or not, whether the AGN variability depends on black hole mass and accretion rate, and to compare the AGN power spectra with the Galactic X-ray black hole binaries power-spectra. We used 14-195 keV band light curves from the 157th SWIFT/BAT hard X-ray survey and we computed the mean power spectrum and excess variance of AGN in narrow black-hole mass/luminosity bins. We fitted a power-law model to the AGN power spectra, and we investigated whether the power spectrum parameters and the excess variance depend on black hole mass, luminosity and accretion rate. The Seyfert I and Seyfert II power spectra are identical, in agreement with AGN unification models. The mean AGN X-ray power spectrum has the same, power-law like shape with a slope of -1 in all AGN, irrespective of their luminosity and BH mass. We do not detect any flattening to a slope of zero at frequencies as low as 10e-9 Hz. We detect an anti-correlation between the PSD amplitude and the accretion rate, similar to what has been seen in the past in the 2-10 keV band. This implies that the variability amplitude in AGN decreases with increasing accretion rate. The universal AGN power-spectrum is consistent with the mean, 2-9 keV band Cyg X-1 power spectrum in its soft state. The mean, low frequency AGN X-ray power spectrum is consistent with the extension of the mean, 0.01-25 Hz Cyg X-1 power spectrum to lower frequencies. The agreement between the AGN and the Cyg X-1 power spectrum (either in the soft or the hard state) over many decades in frequency indicates that the X-ray variability process is probably the same in all accreting objects, irrespective of the mass of the compact object. We plan to investigate this issue further in the near future.Comment: 13 pages, 10 figures. Accepted for publication in A&

Intersubband carrier scattering in n- and p-Si/SiGe quantum wells with diffuse interfaces

Scattering rate calculations in two-dimensional Si/Si1−xGex systems have typically been restricted to rectangular Ge profiles at interfaces between layers. Real interfaces however, may exhibit diffuse Ge profiles either by design or as a limitation of the growth process. It is shown here that alloy disorder scattering dramatically increases with Ge interdiffusion in (100) and (111) n-type quantum wells, but remains almost constant in (100) p-type heterostructures. It is also shown that smoothing of the confining potential leads to large changes in subband energies and scattering rates and a method is presented for calculating growth process tolerances

Frequency tunability and spectral control in terahertz quantum cascade lasers with phase-adjusted finite-defect-site photonic lattices

We report on the effect of finite-defect-site photonic lattices (PLs) on the spectral emission of terahertz frequency quantum cascade lasers, both theoretically and experimentally. A central π-phase adjusted defect incorporated in the PL is shown to favor emission selectively within the photonic bandgap. The effect of the duty cycle and the longitudinal position of such PLs is investigated, and used to demonstrate three distinct spectral behaviors: single mode emission from devices in the range 2.2−5 THz, with a side-mode suppression ratio of 40 dB and exhibiting continuous frequency tuning over >8 GHz; discrete tuning between two engineered emission modes separated by ~40 GHz; and, multiple-mode emission with an engineered frequency spacing between emission lines

Quantum Transmission Line Modelling and Experimental Investigation of the Output Characteristics of a Terahertz Quantum Cascade Laser

We describe a new approach to modelling the optoelectronic properties of a terahertz-frequency quantum cascade laser (THz QCL) based on a quantum transmission line modelling (Q-TLM) method. Parallel quantum cascade transmission line modelling units are employed to describe the dynamic optical processes in a nine-well THz QCL in both the time and frequency domains. The model is used to simulate the current-power characteristics of a QCL device and good agreement is found with experimental measurements, including an accurate prediction of the threshold current and emitted power. It is also confirmed that the Q-TLM model can accurately predict the Stark-induced blue shift of the emission spectrum of the THz QCL with increasing injection current. Furthermore, we establish the new Q-TLM model to describe the properties of a THz QCL device incorporating a photonic lattice patterned on the laser ridge, by linking the transmission line structure to each scattering module. The predicted effects of the lattice structure on the steady-state emission spectra of the THz QCL, including the side-mode suppression, are found to be in good agreement with experimental results. Our Q-TLM modelling approach is a promising tool for the future design of THz QCLs and analysis of their temporal and spectral behaviors

Discrete Vernier tuning in terahertz quantum cascade lasers using coupled cavities

Terahertz-frequency quantum cascade lasers (THz QCLs) are compact solid-state sources of coherent radiation in the 1–5 THz region of the electromagnetic spectrum . The emission spectra of THz QCLs typically exhibit multiple longitudinal modes characteristic of Fabry–Pérot (FP) cavities. However, widely-tunable (single-mode) THz QCLs would be ideally suited to many THz-sensing applications, such as trace gas detection, atmospheric observations , and security screening . Here we demonstrate discrete Vernier tuning using a simple two-section coupled-cavity geometry. A monolithic THz QCL ridge cavity was etched using focused ion beam milling to create two coupled FP cavities separated by an air gap. In this scheme, one of the two sections (the ‘lasing section’) is electrically driven above the lasing threshold, while the other is driven below threshold and acts as a ‘tuning section’. The lengths of the two sections and the air gap were designed such that the longitudinal FP modes of the respective sections coincide at a selected (‘resonant’) frequency. The dominant lasing mode of the coupled cavity occurs at this frequency owing to the reduction in threshold . A small perturbation to the frequency of the modes in either section of the device will detune the resonance, causing the dominant mode of the coupled-cavity to ‘hop’ to a different frequency, in a manner analogous to the Vernier effect. The longitudinal modes of the tuning section are controlled by perturbing its refractive index through current-induced heating

Discrete Vernier tuning with constant output power in terahertz quantum cascade lasers using coupled cavities

Terahertz-frequency quantum cascade lasers (THz QCLs) are compact solid-state sources of coherent radiation in the 1–5 THz region of the electromagnetic spectrum [1]. THz QCLs typically exhibit multiple longitudinal modes characteristic of Fabry–Pérot cavities. However, widely-tunable (single-mode) THz QCLs would be ideally suited to many THz- applications, such as atmospheric observations [2], and security screening [3]. Here we demonstrate discrete Vernier tuning using a simple two-section coupled-cavity geometry comprising of a ‘lasing section’, which is electrically driven above the lasing threshold, and a ‘tuning section’, which is driven below threshold. Our THz QCLs, based on a bound-to- continuum design [4], were processed into 150-μm-wide single–metal waveguides with lengths 4.5–4.8 mm. Devices were etched after packaging using a focused ion beam milling system to sculpt a14-μm–wide and 12-μm-deep air gap to form the two-section cavity [Fig. 1 (a)]. Devices were cooled in a continuous-flow helium cryostat and emission spectra measured using a Fourier-transform infrared spectrometer. The tuning section of the laser was heated below threshold using a train of 10-μs-long current pulses at a repetition rate of 8.21 kHz. The lasing section was driven with a single 500-ns-long pulses above threshold. Both the pulse trains were trigerred using a 600-Hz reference frequency. Discrete tuning with a blue shift in frequency was observed over bandwidths of 50 and 85 GHz from two devices with mode spacing of 15 GHz and 30 GHz respectively [Fig. 1 (b, c)]. A red shift in frequency over 30 GHz was also observed in device 2 by simply swapping the function of the lasing and tuning sections [Fig. 1 (c) Inset]. Negligible degradation in output power was observed with tuning current. R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, and F. Rossi, "Terahertz semiconductor-heterostructure laser," Nature 417, 156–159 (2002). P. H. Siegel, "Terahertz technology," Microw. Theory Tech. IEEE Trans. On 50, 910 –928 (2002). A. G. Davies, A. D. Burnett, W. Fan, E. H. Linfield, and J. E. Cunningham, "Terahertz spectroscopy of explosives and drugs," Mater. Today 11, 18 – 26 (2008). S. Barbieri, J. Alton, H. E. Beere, J. Fowler, E. H. Linfield, and D. A. Ritchie, "2.9 THz quantum cascade lasers operating up to 70 K in continuous wave," Appl. Phys. Lett. 85, 1674–1676 (2004)

Detector-free gas spectroscopy, with integrated frequency monitoring, through self-mixing in a terahertz quantum-cascade laser

Terahertz-frequency quantum cascade lasers (THz QCLs) have been used as compact, yet powerful THz radiation sources in a range of gas spectroscopy techniques, including both in situ active sensing and heterodyne radiometry. However, all such approaches require external THz instrumentation (detectors or mixers) in addition to the QCL, thus raising the system complexity and cost. A partial solution has recently been demonstrated, based on self-mixing interferometry (SMI) in a QCL, which occurs when radiation is fed back into the QCL from an external reflector. The resulting interference within the QCL perturbs the terminal voltage, and the absorption spectrum of a gas within the external cavity may be inferred from the amplitude of these perturbations. This both eliminates the need for an external THz detector or mixer, doubles the interaction-length for absorption spectroscopy, and the scanning speed can potentially be raised to the time-scale of the QCL lasing dynamics (~10 GHz). A limitation reported in the previous work is that the QCL emission frequency must be inferred from prior spectral measurements of the unperturbed laser, which introduces two principal problems: (1) additional THz instrumentation is still required, and (2) the system QCL frequency is itself perturbed by feedback effects, leading to apparent frequency shifts in the measured spectral lines. In this work, we demonstrate a technique to measure the QCL frequency directly by extending the external cavity length modulation to 400-mm using a motorised linear translation stage. By recording the QCL voltage modulation as a function of stage position, a full interferogram can be acquired, and a Fourier transform can then be used to determine the laser frequency and the amplitude of the transmitted signal. The QCL was shown to be tunable by adjusting the drive current over a 1.5-GHz bandwidth, around a centre frequency of 3.4052 THz. To demonstrate gas spectroscopy, a 1-m gas cell with TPX windows was filled with methanol vapour, and the transmitted QCL power was measured as a function of drive current through SMI analysis. Two absorption lines are clearly resolved. The technique was found to be accurate to partial methanol pressures of < 10 mTorr. In conclusion, we have demonstrated an accurate and low-cost THz gas spectroscopy technique based on self-mixing in a THz QCL, without the need for any external THz mixer or detector, or a priori calibration of the QCL emission frequency