Nonlinear photoionization of transparent solids: a nonperturbative theory obeying selection rules

We provide a nonperturbative theory for photoionization of transparent solids. By applying a particular steepest-descent method, we derive analytical expressions for the photoionization rate within the two-band structure model, which consistently account for the $selection$ $rules$ related to the parity of the number of absorbed photons ($odd$ or $even$). We demonstrate the crucial role of the interference of the transition amplitudes (saddle-points), which in the semi-classical limit, can be interpreted in terms of interfering quantum trajectories. Keldysh's foundational work of laser physics [Sov. Phys. JETP 20, 1307 (1965)] disregarded this interference, resulting in the violation of $selection$ $rules$. We provide an improved Keldysh photoionization theory and show its excellent agreement with measurements for the frequency dependence of the two-photon absorption and nonlinear refractive index coefficients in dielectrics

Argon and other defects in amorphous SiO2 coatings for gravitational-wave detectors

Amorphous SiO2 thin films are one of the two components of the highly reflective mirror coatings of gravitational-wave detectors. For this study, layers of amorphous SiO2 on crystalline Si substrates were produced by ion-beam sputtering (IBS), using accelerated neutralized argon ions as sputtering particles, as is the case for the actual mirror coatings of gravitational-wave detectors. The aim of this study is to investigate the possible presence of various defects in the materials in order to improve the coating quality. We provide evidence that, due to the synthesis method, about 0.2 wt.% of Ar is present in the coatings, and it can be released by means of thermal treatments, starting around 400 degrees C. The time and temperature to obtain the total release of Ar increases with the coating thickness; for a thickness of 100 nm, all argon is released below 600 degrees C, while an isotherm of one hour at 900 degrees C is necessary for a coating 5 mu m thick. Besides the Ar atoms left from the synthesis, other defects, such as Si clusters and silicon dangling bonds, are present in the coatings. The concentration of both of them is strongly reduced by thermal treatments either in vacuum or in air. The overall thickness of the coating is slightly increased after thermal treatments, as witnessed by the change of the period of interference fringes

Vibrational and structural properties of P2O5P_2O_5 glass: Advances from a combined modeling approach

We present experimental measurements and ab initio simulations of the crystalline and amorphous phases of $P_2O_5$. The calculated Raman, infrared, and vibrational density of states (VDOS) spectra are in excellent agreement with experimental measurements and contain the signatures of all the peculiar local structures of the amorphous phase, namely, bridging and nonbridging (double-bonded or terminal) oxygens and tetrahedral $PO_4$ units associated with $Q^2$, $Q^3$, and $Q^4$ species ($Q^n$ denotes the various types of $PO_4$ tetrahedra, with $n$ being the number of bridging oxygen atoms that connect the tetrahedra to the rest of the network). In order to reveal the internal structure of the vibrational spectrum, the characteristics of vibrational modes in different frequency ranges are investigated using a mode-projection approach at different symmetries based on the $T_d$ symmetry group. In particular, the VDOS spectrum in the range from $∼ 600$ to $870$ $cm^-$$^1$ is dominated by bending ($F_2$$_b$) motions related to bridging oxygen and phosphorus ($∼ 800$ $cm^-$$^1$ band) atoms, while the high-frequency doublet zone ($∼ 870 – 1250$ $cm^-$$^1$ is associated mostly with the asymmetric (($F_2$$_s$) and symmetric ($A_1$) stretching modes, and most prominent peak around $1400$ $cm^-$$^1$ (exp. $1380$ $cm^-$$^1$) is mainly due to asymmetric stretching vibrations supported by double-bonded oxygen atoms. The lower-frequency range below $600$ $cm^-$$^1$ is shown to arise from a mixture of bending ($E$ and ($F_2$$_b$) and rotation ($F_1$) modes. The scissors bending ($E$) and rotation ($F_1$) modes are well localized below $600$ $cm^-$$^1$, whereas the ($F_2$$_b$ bending modes spread further into the range $∼ 600 – 870$ $cm^-$$^1$. The projections of the eigenmodes onto $Q^2$, $Q^3$, and $Q^4$ species yield well-defined contributions at frequencies in striking correspondence with the positions of the Raman and infrared bands

Accurate thermal conductivities from optimally short molecular dynamics simulations

The evaluation of transport coefficients in extended systems, such as thermal conductivity or shear viscosity, is known to require impractically long simulations, thus calling for a paradigm shift that would allow to deploy state-of-the-art quantum simulation methods. We introduce a new method to compute these coefficients from optimally short molecular dynamics simulations, based on the Green-Kubo theory of linear response and the cepstral analysis of time series. Information from the full sample power spectrum of the relevant current for a single and relatively short trajectory is leveraged to evaluate and optimally reduce the noise affecting its zero-frequency value, whose expectation is proportional to the corresponding conductivity. Our method is unbiased and consistent, in that both the resulting bias and statistical error can be made arbitrarily small in the long-time limit. A simple data-analysis protocol is proposed and validated with the calculation of thermal conductivities in the paradigmatic cases of elemental and molecular fluids (liquid Ar and H2O) and of crystalline and glassy solids (MgO and a-SiO2). We find that simulation times of one to a few hundred picoseconds are sufficient in these systems to achieve an accuracy of the order of 10% on the estimated thermal conductivities

Advanced Virgo Plus: Future Perspectives

While completing the commissioning phase to prepare the Virgo interferometer for the next joint Observation Run (O4), the Virgo collaboration is also finalizing the design of the next upgrades to the detector to be employed in the following Observation Run (O5). The major upgrade will concern decreasing the thermal noise limit, which will imply using very large test masses and increased laser beam size. But this will not be the only upgrade to be implemented in the break between the O4 and O5 observation runs to increase the Virgo detector strain sensitivity. The paper will cover the challenges linked to this upgrade and implications on the detector's reach and observational potential, reflecting the talk given at 12th Cosmic Ray International Seminar - CRIS 2022 held in September 2022 in Napoli

The Advanced Virgo+ status

The gravitational wave detector Advanced Virgo+ is currently in the commissioning phase in view of the fourth Observing Run (O4). The major upgrades with respect to the Advanced Virgo configuration are the implementation of an additional recycling cavity, the Signal Recycling cavity (SRC), at the output of the interferometer to broaden the sensitivity band and the Frequency Dependent Squeezing (FDS) to reduce quantum noise at all frequencies. The main difference of the Advanced Virgo + detector with respect to the LIGO detectors is the presence of marginally stable recycling cavities, with respect to the stable recycling cavities present in the LIGO detectors, which increases the difficulties in controlling the interferometer in presence of defects (both thermal and cold defects). This work will focus on the interferometer commissioning, highlighting the control challenges to maintain the detector in the working point which maximizes the sensitivity and the duty cycle for scientific data taking

Calibration of advanced Virgo and reconstruction of the detector strain h( t) during the observing run O3

The three advanced Virgo and LIGO gravitational wave detectors participated to the third observing run (O3) between 1 April 2019 15:00 UTC and 27 March 2020 17:00 UTC, leading to several gravitational wave detections per month. This paper describes the advanced Virgo detector calibration and the reconstruction of the detector strain h(t) during O3, as well as the estimation of the associated uncertainties. For the first time, the photon calibration technique as been used as reference for Virgo calibration, which allowed to cross-calibrate the strain amplitude of the Virgo and LIGO detectors. The previous reference, so-called free swinging Michelson technique, has still been used but as an independent cross-check. h(t) reconstruction and noise subtraction were processed online, with good enough quality to prevent the need for offline reprocessing, except for the two last weeks of September 2019. The uncertainties for the reconstructed h(t) strain, estimated in this paper in a 20-2000 Hz frequency band, are frequency independent: 5% in amplitude, 35 mrad in phase and 10 μs in timing, with the exception of larger uncertainties around 50 Hz

Frequency-Dependent Squeezed Vacuum Source for the Advanced Virgo Gravitational-Wave Detector

In this Letter, we present the design and performance of the frequency-dependent squeezed vacuum source that will be used for the broadband quantum noise reduction of the Advanced Virgo Plus gravitational-wave detector in the upcoming observation run. The frequency-dependent squeezed field is generated by a phase rotation of a frequency-independent squeezed state through a 285 m long, high-finesse, near-detuned optical resonator. With about 8.5 dB of generated squeezing, up to 5.6 dB of quantum noise suppression has been measured at high frequency while close to the filter cavity resonance frequency, the intracavity losses limit this value to about 2 dB. Frequency-dependent squeezing is produced with a rotation frequency stability of about 6 Hz rms, which is maintained over the long term. The achieved results fulfill the frequency dependent squeezed vacuum source requirements for Advanced Virgo Plus. With the current squeezing source, considering also the estimated squeezing degradation induced by the interferometer, we expect a reduction of the quantum shot noise and radiation pressure noise of up to 4.5 dB and 2 dB, respectively

Frequency-Dependent Squeezed Vacuum Source for the Advanced Virgo Gravitational-Wave Detector

In this Letter, we present the design and performance of the frequency-dependent squeezed vacuum source that will be used for the broadband quantum noise reduction of the Advanced Virgo Plus gravitational-wave detector in the upcoming observation run. The frequency-dependent squeezed field is generated by a phase rotation of a frequency-independent squeezed state through a 285 m long, high-finesse, near-detuned optical resonator. With about 8.5 dB of generated squeezing, up to 5.6 dB of quantum noise suppression has been measured at high frequency while close to the filter cavity resonance frequency, the intracavity losses limit this value to about 2 dB. Frequency-dependent squeezing is produced with a rotation frequency stability of about 6 Hz rms, which is maintained over the long term. The achieved results fulfill the frequency dependent squeezed vacuum source requirements for Advanced Virgo Plus. With the current squeezing source, considering also the estimated squeezing degradation induced by the interferometer, we expect a reduction of the quantum shot noise and radiation pressure noise of up to 4.5 dB and 2 dB, respectively