Reducing thermal noise in future gravitational wave detectors by employing Khalili etalons

Reduction of thermal noise in dielectric mirror coatings is a key issue for the sensitivity improvement in the second and third generation interferometric gravitational wave detectors. Replacing an end mirror of the interferometer by an anti-resonant cavity (a so-called Khalili cavity) has been proposed to realize the reduction of the overall thermal noise level. In this article we show that the use of a Khalili etalon, which requires less hardware than a Khalili cavity, yields still a significant reduction of thermal noise. We identify the optimum distribution of coating layers on the front and rear surfaces of the etalon and compare the total noise budget with a conventional mirror. In addition we briefly discuss advantages and disadvantages of the Khalili etalon compared with the Khalili cavity in terms of technical aspects, such as interferometric length control and thermal lensing.     

Shot noise in semiclassical chaotic cavities

We construct a trajectory-based semiclassical theory of shot noise in clean chaotic cavities. In the universal regime of vanishing Ehrenfest time tau(E), we reproduce the random matrix theory result and show that the Fano factor is exponentially suppressed as tau(E) increases. We demonstrate how our theory preserves the unitarity of the scattering matrix even in the regime of finite tau(E). We discuss the range of validity of our semiclassical approach and point out subtleties relevant to the recent semiclassical treatment of shot noise in the universal regime by Braun et al. (cond-mat/0511292).     

Crescent waves in optical cavities

We theoretically and experimentally generate stationary crescent surface solitons pinged to the boundary of a microstructured vertical cavity surface emission laser by triggering the intrinsic cavity mode as a background potential. Instead of a direct transition from linear to nonlinear cavity modes, we demonstrate the existence of symmetry-breaking crescent waves without any analogs in the linear limit. Our results provide an alternative and general method to control lasing characteristics as well as to study optical surface waves.     

Effect of external noise on the optical thermal breakdown of semiconductor wafers

The optical thermal breakdown of a semiconductor wafer with a small Biot number under conditions of fluctuating incident light intensity is discussed. It is shown that external noise shifts the breakdown region toward somewhat higher values of the control parameter, and as the fluctuations grow this process leads to suppression of the critical point. Light intensities below the threshold noise can induce a nonequilibrium phase transition. Numerical estimates are given for germanium illuminated by a CO₂ laser. Zh. Tekh. Fiz. 67, 117–119 (September 1997)     

Probabilistic generation of entanglement in optical cavities

We propose to produce entanglement by measuring the reflection from an optical cavity. Conditioned on the detection of a reflected photon, pairs of atoms in the cavity are prepared in maximally entangled states. The success probability depends on the cavity parameters, but high quality entangled states may be produced with a high probability even for cavities of moderate quality.     

Dissipative preparation of entanglement in optical cavities

We propose a novel scheme for the preparation of a maximally entangled state of two atoms in an optical cavity. Starting from an arbitrary initial state, a singlet state is prepared as the unique fixed point of a dissipative quantum dynamical process. In our scheme, cavity decay is no longer undesirable, but plays an integral part in the dynamics. As a result, we get a qualitative improvement in the scaling of the fidelity with the cavity parameters. Our analysis indicates that dissipative state preparation is more than just a new conceptual approach, but can allow for significant improvement as compared to preparation protocols based on coherent unitary dynamics.     

Shot noise in chaotic cavities from action correlations

We consider universal shot noise in ballistic chaotic cavities from a semiclassical point of view and show that it is due to action correlations within certain groups of classical trajectories. Using quantum graphs as a model system, we sum these trajectories analytically and find agreement with random-matrix theory. Unlike all action correlations which have been considered before, the correlations relevant for shot noise involve four trajectories and do not depend on the presence of any symmetry.     

Shot noise by quantum scattering in chaotic cavities

We have experimentally studied shot noise of chaotic cavities defined by two quantum point contacts in series. The cavity noise is determined as (1/4)2e/I/ in agreement with theory and can be well distinguished from other contributions to noise generated at the contacts. Subsequently, we have found that cavity noise decreases if one of the contacts is further opened and reaches nearly zero for a highly asymmetric cavity. Heating inside the cavity due to electron-electron interaction can slightly enhance the noise of large cavities and is also discussed quantitatively.     

Controlling mode locking in optical ring cavities

Imperfections in the surface of intracavity elements of an optical ring resonator can scatter light from one mode into the counterpropagating mode. The phase-locking of the cavity modes induced by this backscattering is a well-known example that notoriously afflicts laser gyroscopes and similar active systems. We experimentally show how backscattering can be circumvented in a unidirectionally operated ring cavity either by an appropriate choice of the resonant cavity mode or by active feedback control. PACS 42.60.Da; 45.40.Cc; 42.79.Bh; 42.55.-f     

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.     

Tuning of optical resonances in asymmetric microtube cavities

We tune optical resonances in rolled-up SiO/SiO(2) microtube cavities by gradually modifying the tube structure through asymmetrical postdeposition of SiO(2). Spectral blueshifts followed by redshifts of the resonant modes are observed in a thin-walled microtube (tube-I), which is attributed to a competition between shape deformation and effective increase of tube wall thickness. In contrast, only a monotonic redshift is detected when asymmetrical deposition is performed on a thick-walled microtube (tube-II). Distinct wavelength-dependent tuning was revealed in both kinds of tubes. Numerical calculations based on perturbation theory are carried out to explain and confirm the experimental results.     

Enhanced excess noise in laser cavities with tilted mirrors

Enhancement of the transverse excess-noise factor is theoretically predicted in optical laser cavities with tilted end mirrors and nonuniform transverse loss. In particular, it is shown that in a loss-guided laser close to the plane-plane geometric instability boundary the excess-noise factor that is induced by mirror tilting may largely exceed that predicted for the aligned cavity. This excess noise is related to beam walk-off induced by mirror tilting, which makes possible transient amplification of noise in the transverse plane.     

Highly stable piezoelectrically tunable optical cavities

We have studied the effect on 2nd harmonic wavelength modulation spectroscopy of the use of integrating spheres as multipass gas cells. The gas lineshape becomes distorted at high concentrations, as a consequence of the exponential pathlength distribution of the sphere, introducing nonlinearity beyond that expected from the Beer–Lambert law. We have modelled this numerically for methane absorption at 1.651 μm, with gas concentrations in the range of 0–2.5 %vol in air. The results of this model compare well with experimental measurements. The nonlinearity for the 2fWMS measurements is larger than that for direct scan measurements; if this additional effect were not accounted for, the resulting error would be approximately 20 % of the reading at a concentration of 2.5 %vol methane.     

The effect of coherent optical phonon on thermal transport

Phonons are quantized lattice vibrations and the major heat carriers in most crystalline materials. We have been utilizing femtosecond phonon spectroscopy to excite and detect optical coherent phonons (CPs) in various materials. However, the impact of CPs to overall thermal transport is still unknown. In this study, we developed a small perturbation model in MD to simulate CPs and investigate the effects of CPs on thermal transport in Bi₂Te₃ at temperatures ranging from 20 to 325 K. The phonon frequency and lifetime predicted by our model agree very well with experimental results. It is found that the effective thermal transport estimated with the heat current autocorrelation function shows a great enhancement upon CP generation and extension, especially at low temperatures. Our results suggest that it is possible to manipulate thermal transport effectively with CPs.     

Modes of optical cavities containing selecting components

A model of the field mode structure in optical cavities containing selecting components is suggested. The cavity also contains field-attenuating or field-enhancing media where elements of the 4×4 symplectic detour matrix of the cavity become complex. The model makes it possible to determine the stability of the cavity, non-Hermiticity of higher modes, and the complex astigmatism of the eigenmode field. Conditions for unilateral (unidirectional) and bilateral (bidirectional) stability are formulated. A cavity showing unilateral stability at the first transverse mode is described.     

Test of Lorentz invariance using optical cavities

This paper indroduces the precision test of Lorentz invariance using ultra-stable and low-loss optical cavities. The effective-field theory widely adopted in the analysis of experimental data has been reviewed. The sensitivity of the cavity resonant frequency to the Lorentz-violating tensor field is discussed in detail. In addition, the polarization of the optical field has been added to the model, and our analysis shows that the frequency shift due to Lorentz violation is not sensitive to the polarization of the optical field.      

Semiclassical theory of conductance and noise in open chaotic cavities

Conductance and shot noise of an open cavity with diffusive boundary scattering are calculated within the Boltzmann-Langevin approach. In particular, conductance contains a nonuniversal geometric contribution, originating from the presence of open contacts. Subsequently, universal expressions for multiterminal conductance and noise, valid for all chaotic cavities, are obtained classically, based on the fact that the distribution function in the cavity depends only on energy, and using the principle of minimal correlations.