Challenges in thermal noise for 3rd generation of gravitational wave detectors

Various noise sources limit the sensitivity of current interferometric gravitational wave detectors, including seismic noise, thermal noise of the optical components and suspension elements and photon shot noise. Plans are in place for a suite of hardware upgrades which should increase the sensitivity of these detectors by reducing the various noise sources. With these designs for 2nd generation detectors mature, techniques for further improvement of detector sensitivity by a factor of approximately 10 are under study. A particular challenge is the reduction of the thermal noise associated with the interferometer mirrors and their suspensions. We review the current status of research on thermal noise in interferometric gravitational wave detectors. Aspects of possible techniques for use in future ‘3rd generation detectors’ such as cryogenics and diffractive optics are discussed.     

Δ<Subscript>33</Subscript> -medium mass modification and pion spectra

We study the π^± spectra obtained in 2, 4, 6 and 8A GeV Au - Au collisions within the thermal model. We find that the main features of the data can be well described after we include the pions from the decay of the Δ -resonance with medium mass modification.     

Anticipative control of switched queueing systems

The relevant dynamics of a queueing process can be anticipated by taking future arrivals into account. If the transport from one queue to another is associated with transportation delays, as it is typical for traffic or productions networks, future arrivals to a queue are known over some time horizon and, thus, can be used for an anticipative control of the corresponding flows. A queue is controlled by switching its outflow between “on” and “off” similar to green and red traffic lights, where switching to “on” requires a non-zero setup time. Due to the presence of both continuous and discrete state variables, the queueing process is described as a hybrid dynamical system. From this formulation, we derive one observable of fundamental importance: the green time required to clear the queue. This quantity allows to detect switching time points for serving platoons without delay, i.e., in a “green wave” manner. Moreover, we quantify the cost of delaying the start of a service period or its termination in terms of additional waiting time. Our findings may serve as a basis for strategic control decisions.     

Impurity nuclear physics

Using a germanium-detector array for hypernuclear γ spectroscopy (Hyperball), we measured B(E2) of the ⁷ _ΛLi hypernucleus and observed a significant shrinkage of the ⁶Li core induced by a Λ-particle. In this way, nuclear properties can be drastically changed by introducing a Λ-particle, which can be investigated by high-resolution hypernuclear γ spectroscopy. In the future neutron-rich hypernuclei will also be studied, where interesting modifications of nuclear structure by a Λ-particle are expected. Received: 1 May 2001 / Accepted: 4 December 2001     

Theoretical investigation of chirped mirrors in semiconductor lasers

This paper reports a theoretical design of chirped mirrors in 1.3-μm double-section semiconductor lasers to achieve high reflectivity and dispersion compensation over a broad bandwidth. Analytic expressions for reflectivity, group delay and group delay dispersion are derived. We use for the first time chirped air/semiconductor layer pairs as mirrors for higher-order dispersion compensation in semiconductor lasers. Our optimised calculations demonstrate that the broad-band mirrors designed consist of a total of only 12 air/semiconductor layers and achieve a reflectivity higher than 99.8%, a smooth group delay and almost stable dispersion in the laser cavity over a 100-nm bandwidth. Due to a high index contrast of both types of the layers, n _l = 1, n _h~ 3.5, a high-reflectivity bandwidth of > 700 nm is obtained in 1.3-μm semiconductor lasers. We also compare our results with that of a commercial simulation program and show a good agreement between them. As a conclusion, we assume from the theoretical results that air/semiconductor layer pairs with varying thicknesses used at one end of double-section semiconductor lasers can lead to femtosecond optical pulse generation using mode-locking techniques. An erratum to this article can be found at .     

Laser-induced desorption of Na-dimers

We find that Na-dimers are desorbed in a thermal process if rough Na surfaces are irradiated with pulsed laser light of λ=532 nm. In contrast, for light of λ=355 nm, Na₂ can be detached in a non-thermal reaction at low laser fluences. This is concluded from the kinetic energy distributions of the dimers determined by time-of-flight measurements using a second laser at λ=248 nm for photoionization. The transition from non-thermal to thermal desorption at large fluences of the laser light can also be identified. Received: 23 July 1996 / Accepted: 26 August 1996     

Optimization of TE–TM mode converters on X-cut, Y-propagation LiNbO3 used for PMD compensation

Two-phase and three-phase TE–TM mode converters for integrated optic polarization mode dispersion compensation are compared, and the latter are found to have a slightly better electro-optic efficiency. If a small differential group delay is needed, compensation performance can be drastically improved by a waveguide tilt in the YZ plane. Received: 16 May 2001 / Published online: 23 October 2001     

Calculation of production and decay of radioisotopes for future irradiation experiments and ion beam facilities

The design of future radioactive ion beam (RIB) facilites requires the forecast of radio isotope inventory after irradiation. At CERN – ISOLDE, we developed a software that estimates the activity of irradiated materials as a function of time dedicated to radioactive waste management. This tool can also be used for licensing procedures, planning of irradiation experiments and the estimation of yields.     

Thermal stability of the magnetic parameters of epitaxial iron garnet films subjected to planar radial stresses

The effect of external planar radial pressure on the thermal stability of the magnetic parameters of epitaxial iron garnet films is investigated in the temperature range 200–500 K for external mechanical stresses in the range 0–40 kgf/mm². It is shown that external planar radial pressure can be used to improve the thermal stability of these magnetic parameters by a factor of 1.5–2, and also to alter significantly the temperature interval of single-domain behavior for orientational phase transitions near a compensation point. Zh. Tekh. Fiz. 67, 114–116 (September 1997)     

Stochastic simulations on the cellular wave computers

The computational paradigm represented by Cellular Neural/nonlinear Networks (CNN) and the CNN Universal Machine (CNN-UM) as a Cellular Wave Computer, gives new perspectives for computational physics. Many numerical problems and simulations can be elegantly addressed on this fully parallelized and analogic architecture. Here we study the possibility of performing stochastic simulations on this chip. First a realistic random number generator is implemented on the CNN-UM, and then as an example the two-dimensional Ising model is studied by Monte Carlo type simulations. The results obtained on an experimental version of the CNN-UM with 128 ×128 cells are in good agreement with the results obtained on digital computers. Computational time measurements suggest that the developing trend of the CNN-UM chips — increasing the lattice size and the number of local logic memories — will assure an important advantage for the CNN-UM in the near future.     

Thermal lensing compensation principle for the ACIGA's High Optical Power Test Facility Test 1

Thermal lensing is becoming recognized as one of the dominant obstacles to the second generation of laser interferometric gravitational wave detectors. Very high optical power is required to circulate in the interferometer to reach the sensitivity goal, creating strong thermal induced wavefront distortion. These effects will be studied at the High Optical Power Test Facility in Gingin, Western Australia. In this paper, we present simulation results for the first test planned for the middle of 2004. This experiment will produce 5 kW of optical power circulating inside a Fabry–Perot cavity and will demonstrate large thermal lensing effects. Two compensation methods were investigated to offset the negative effect of thermal lensing on the cavity: a compensation plate within the arm cavity and adaptive laser mode matching. Advantages and disadvantages of both systems are discussed.     

Optimal investment horizons

In stochastic finance, one traditionally considers the return as a competitive measure of an asset, i.e., the profit generated by that asset after some fixed time span Δt, say one week or one year. This measures how well (or how bad) the asset performs over that given period of time. It has been established that the distribution of returns exhibits “fat tails” indicating that large returns occur more frequently than what is expected from standard Gaussian stochastic processes [1-3]. Instead of estimating this “fat tail” distribution of returns, we propose here an alternative approach, which is outlined by addressing the following question: What is the smallest time interval needed for an asset to cross a fixed return level of say 10%? For a particular asset, we refer to this time as the investment horizon and the corresponding distribution as the investment horizon distribution. This latter distribution complements that of returns and provides new and possibly crucial information for portfolio design and risk-management, as well as for pricing of more exotic options. By considering historical financial data, exemplified by the Dow Jones Industrial Average, we obtain a novel set of probability distributions for the investment horizons which can be used to estimate the optimal investment horizon for a stock or a future contract. Received 20 February 2002 Published online 25 June 2002     

Compensation of strong thermal lensing in high-optical-power cavities

In an experiment to simulate the conditions in high optical power advanced gravitational wave detectors, we show for the first time that the time evolution of strong thermal lenses follows the predicted infinite sum of exponentials (approximated by a double exponential), and that such lenses can be compensated using an intracavity compensation plate heated on its cylindrical surface. We show that high finesse approximately 1400 can be achieved in cavities with internal compensation plates, and that mode matching can be maintained. The experiment achieves a wave front distortion similar to that expected for the input test mass substrate in the Advanced Laser Interferometer Gravitational Wave Observatory, and shows that thermal compensation schemes are viable. It is also shown that the measurements allow a direct measurement of substrate optical absorption in the test mass and the compensation plate.     

Effects of size and surface anisotropy on thermal magnetization and hysteresis in the magnetic clusters

Based on Monte Carlo simulation, the spin configurations, thermal magnetization and hysteresis loops of the clusters coated by the surface shell with radial anisotropy are studied. Interestingly, a new multidomain containing a few of subdomains whose easy directions are along those of the configurational anisotropy, a magnetization curve in steps and a first order phase transition from the single domain to the multidomain in the thermal and field magnetization processes, are found, which is as a result of the interplay of the configurational anisotropy, the size effect, the surface anisotropy, the applied field and the thermal fluctuation. In this first order transition, we find a critical temperature, a critical surface anisotropy and a critical size. The simulated temperature dependence of the coercivity of the cluster with the surface anisotropy can be fitted by H_c (T)=H_c (0)(1-C_αT^α) with low value of α, which explains well the experimental results of the nanoparticles. Moreover, it is found that the hysteresis loops and coercivity are strongly affected by the cluster size and the thickness of the surface layer.     

Cold ignition of combustion-like waves of cryo-chemical reactions in solids

Fronts of weakly exothermal chemical reaction may propagate in solids at very low temperatures ( 4K≤T≤77K) thanks to a quite unusual mechanism, involving a feedback between the heat produced by the reaction and the disruption of the solid matrix. In this class of phenomena, the reaction may be induced by mechanical constraints, without a large elevation of temperature. On the basis of a simple phenomenological model, we investigate ignition of a propagating front by initially (i) disrupting a localized zone of the solid matrix, or by (ii) introducing a temperature jump, leading to a thermal shock with strong temperature gradients. In particular, we show that reaction can be initiated by disrupting only a very small fraction of the sample. Applications to the problem of initiation of solid explosives by friction or shocks is briefly discussed. Received 26 January 2001 and Received in final form 3 May 2001     

Ge diffusion into GaAs by pulsed laser irradiation

Ge diffusion into GaAs from thin evaporated layers as sources is reported. Irradiation with aQ-switched ruby laser gives rise ton-type diffused layers of a thickness from 240 to 710 Å. A strong compensation of the diffused layers, that cannot be removed by thermal annealing, was observed. From the present experimental results it can be inferred that the diffusion coefficient increases at the melting point by 5 to 6 orders of magnitude.     

Use of electron irradiation in studying thermal donors in silicon by the EPR method

The bistable thermal donors in Czochralski-grown silicon crystals were investigated by EPR spectroscopy and IR-absorption techniques. It is shown that using heat treatment at a temperature ≤400°C and appropriate conductivity compensation by irradiation with 3.5 MeV electrons, one can select from the total signal the EPR-signal associated with only one type, namely TDD2, of the thermal donors. Based on the model of a two-center core structure of the given complexes, an explanation of the EPR-spectroscopy data on the oxygen thermal donors in silicon is suggested. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 188–191, March–April, 2000.     

Van der Waals interaction between flux lines in high- superconductors

In anisotropic or layered superconductors thermal fluctuations as well as impurities induce a van der Waals (vdW) attraction between flux lines, as has recently been shown by Blatter and Geshkenbein in the thermal case [#!BlatterGeshkenbein!#] and by Mukherji and Nattermann in the disorder dominated case [#!NattermannMukherji!#]. This attraction together with the entropic or disorder induced repulsion has interesting consequences for the low field phase diagram. We present two derivations of the vdW attraction, one of which is based on an intuitive picture, the other one following from a systematic expansion of the free energy of two interacting flux lines. Both the thermal and the disorder dominated case are considered. In the thermal case in the absence of disorder, we use scaling arguments as well as a functional renormalization of the vortex-vortex interaction energy to calculate the effective Gibbs free energy on the scale of the mean flux line distance. We discuss the resulting low field phase diagram and make quantitative predictions for pure BiSCCO (Bi₂Sr₂CaCu₂O₈). In the case with impurities, the Gibbs free energy is calculated on the basis of scaling arguments, allowing for a semi-quantitative discussion of the low-field, low-temperature phase diagram in the presence of impurities. Received: 9 February 1998 / Accepted: 17 April 1998     

Squeezed thermal spin states of magnons in the ferromagnet

In this paper, we discuss squeezed thermal spin states of magnons that are described by the Heisenberg Hamiltonian in the ferromagnet, in which the magnon system possesses a new kind of quasiparticle, which we call ferromagnon, i.e. a “dressed” quasi-particle obtained from the magnons by a Bogoliubov-Valatin transformation . Generally, the mass and noise properties of ferromagnons possess potentially important and novel effects in condensed matter physics, which have extensive application in the fields of science and technology. Moreover, it is convenient to introduce the Holstein-Primakoff method, in order to take into account the nonlinear interaction among spin waves. At last we describe the quantum fluctuations of spin-components in the squeezed thermal spin states of magnons and their temperature-dependence. Below some temperature, the squeezed thermal spin states of ferromagnons show squeeze effect.     

Nonlinear excitations and electric transport in dissipative Morse-Toda lattices

We investigate the onset and maintenance of nonlinear soliton-like excitations in chains of atoms with Morse interactions at rather high densities, where the exponential repulsion dominates. First we discuss the atomic interactions and approximate the Morse potential by an effective Toda potential with adapted density-dependent parameters. Then we study several mechanisms to generate and stabilize the soliton-like excitations: (i) External forcing: we shake the masses periodically, mimicking a piezoelectric-like excitation, and delay subsequent damping by thermal excitation; (ii) heating, quenching and active friction: we heat up the system to a relatively high temperature Gaussian distribution, then quench to a low temperature, and subsequently stabilize by active friction. Finally, we assume that the atoms in the chain are ionized with free electrons able to move along the lattice. We show that the nonlinear soliton-like excitations running on the chain interact with the electrons. They influence their motion in the presence of an external field creating dynamic bound states (“solectrons”, etc.). We show that these bound states can move very fast and create extra current. The soliton-induced contribution to the current is constant, field-independent for a significant range of values when approaching the zero-field value.