Dynamics of optical forces and torques in plasmonic systems: a surface integral equation

A novel approach is introduced to determine the time evolution of optical forces and torques on arbitrary shape nanostructures by combining Maxwell's stress tensor with the surface integral equation method (SIE). Conventional time averaging of Maxwell’s stress tensor allows obtaining an elegant form in terms of surface currents for the force exerted on nanostructures. Unfortunately, the information about the time dependence of the force – which can be very important in ultrafast photonics experiments and in nano-manipulation applications – is lost in such an approach. To overcome this, we have developed a time-domain method based on the inverse Fourier transform of the frequency-domain SIE. The calculations in the frequency domain allow accurately taking into account the dispersion of the permittivity function of the system and the use of surface currents enables the rigorous treatment of intricate geometries for the scatterer. Furthermore, the integration of Maxwell’s stress tensor directly on the scatterer’s boundary significantly reduces the required computation time and increases the accuracy of the method. We show quite unusual sum frequency-like terms in the dynamics of the force appearing in Maxwell’s stress tensor, which normally vanish for the time-averaged force. To illustrate this effect, we study how the pulse duration influences the dynamics of optical force in the case of a rectangular shape and Gaussian pulses illuminating thin film at normal incidence. In the framework of the developed numerical method, we study the influence of the sum-frequency-like terms on the dynamics of optical forces in the case of a spherical scatterer.

Coherent Rayleigh time domain reflectometry

Xin Lu

Owing to the advantages of the optical fibre, such as lightweight, small size, low cost and immunity to electromagnetic interferences, the fibre sensors have been applied in diverse fields and the scientific research on fibre sensing keeps improving to meet new industrial demands. In this thesis, a novel distributed fibre sensor based on coherent Rayleigh scattering is investigated to realise best accuracy measurements. Coherent Rayleigh scattering is found to be essentially assimilated to a random walk process, thus its statistical properties are theoretically analysed based on the mature theory on random walk. Then the spectral characteristics of the Rayleigh backscattered light is investigated for different incident pulse shapes. The spectral distribution of the trace amplitude turns out to be identical to the spectrum of the incident pulse. Meanwhile, a theoretical model is proposed to describe the coherent Rayleigh trace in time and frequency domains. Experiments are carried out to validate the theoretical analysis and the proposed model. Then the statistical properties and the visibility of the coherent Rayleigh traces obtained by experiment and simulation are investigated for different detection bandwidth. The minimum 3 dB bandwidth required to resolve all features of a coherent Rayleigh intensity trace is determined to be 3 times larger than the bandwidth of the incident rectangular pulse. Then coherent Rayleigh scattering is applied to distributed sensing using the optical time domain reflectometry technique. The working principle is explained by the restorability of the Rayleigh trace in time domain and by a spectral shift in frequency domain. The temperature sensitivity is calculated over a wide range from 300 K down to 4.5 K based on the thermo-optic effect and the thermal expansion of the silica fibre. The experimental results in a standard fibre with acrylate coating and a fibre specially coated with ORMOCER® confirm the high temperature sensitivity (over 1 GHz/K) and demonstrate a temperature resolution in the milliKelvin range even under cryogenic conditions. The fibre with the ORMOCER® coating exhibits a higher temperature sensitivity at room temperature than the acrylate coated fibre due to the extra strain induced by the temperate change in the coating and a more stable sensing behaviour because the polymers forming the coating stabilise the mechanical and thermal responses of the fibre. The sensors based on coherent Rayleigh scattering are also applied to retrieve the phase birefringence distribution along polarisation-maintaining and standard single mode fibres. The minimum detectable birefringence reaches 3×10¿7 when the spatial resolution is 2 m and even smaller value can be resolved using longer optical pulses. In addition, this birefringence measurement is capable of distributed temperature and strain sensing in a PANDA and an elliptical-core polarisation-maintaining fibres since the birefringence is sensitive to environmental variations. The experimental results demonstrate that this method is one order of magnitude more sensitive than a fibre sensor based on Brillouin scattering. Moreover, discriminated temperature and strain sensing is realised by a combination of this birefringence measurement and a standard COTDR sensor. This thesis is concluded by a discussion of further research directions on coherent Rayleigh scattering based distributed sensing.

EPFL2016

Quantum optomechanics at room temperature

Hendrik Schütz

Why are classical theories often sufficient to describe the physics of our world even though everything around us is entirely composed of microscopic quantum systems? The boundary between these two fundamentally dissimilar theories remains an unsolved problem in modern physics. Position measurements of small objects allow us to probe the area where the classical approximation breaks down. In quantum mechanics, Heisenbergâs uncertainty principle dictates that any measurement of the position must be accompanied by measurement induced back-action---in this case manifested as an uncertainty in the momentum. In recent years, cavity optomechanics has become a powerful tool to perform precise position measurements and investigate their fundamental limitations. The utilization of optical micro-cavities greatly enhances the interaction between light and state-of-the-art nanomechanical oscillators. Therefore, quantum mechanical phenomena have been successfully observed in systems far beyond the microscopic world. In such a cavity optomechanical system, the fluctuations in the position of the oscillator are transduced onto the phase of the light, while fluctuations in the amplitude of the light disturb the momentum of the oscillator during the measurement. As a consequence, correlations are established between the amplitude and phase quadrature of the probe light. However, so far, observation of quantum effects has been limited exclusively to cryogenic experiments, and access to the quantum regime at room temperature has remained an elusive goal because the overwhelming amount of thermal motion masks the weak quantum effects. This thesis describes the engineering of a high-performance cavity optomechanical device and presents experimental results showing, for the first time, the broadband effects of quantum back-action at room temperature. The device strongly couples mechanical and optical modes of exceptionally high quality factors to provide a measurement sensitivity

\sim\!10^4

times below the requirement to resolve the zero-point fluctuations of the mechanical oscillator. The quantum back-action is then observed through the correlations created between the probe light and the motion of the nanomechanical oscillator. A so-called âvariational measurementâ, which detects the transmitted light in a homodyne detector tuned close to the amplitude quadrature, resolves the quantum noise due to these correlations at the level of 10% of the thermal noise over more than an octave of Fourier frequencies around mechanical resonance. Moreover, building on this result, an additional experiment demonstrates the ability to achieve quantum enhanced metrology. In this case, the generated quantum correlations are used to cancel quantum noise in the measurement record, which then leads to an improved relative signal-to-noise ratio in measurements of an external force. In conclusion, the successful observation of broadband quantum behavior on a macroscopic object at room temperature is an important milestone in the field of cavity optomechanics. Specifically, this result heralds the rise of optomechanical systems as a platform for quantum physics at room temperature and shows promise for generation of ponderomotive squeezing in room-temperature interferometers.

EPFL2017

Multipolar origin of electromagnetic transverse force resulting from TE/TM wave interference

Karim Achouri, Andrei Kiselev, Olivier Martin

In this paper, we aim at unveiling the underlying physical mechanism for transversal optical forces, appearing due to the simultaneous illumination of a spherical object with two plane waves possessing different polarizations. The appearance of such a transversal force is quite counterintuitive since it seems to contradict the law of momentum conservation. We consider the cases of perfect electric conductor (PEC) and silver spheres illuminated by two orthogonally polarized plane waves propagating obliquely with respect to each other. Interestingly, the Poynting vector in these cases acquires a nonzero component transverse to the plane of propagation. Since the momentum transfer is related to the energy transfer, or equivalently, to non-negligible Poynting vector pointed in a particular direction, an arbitrary object placed in such external field is expected to experience a transversal force. To cast light upon this peculiar effect, we use a surface integral equation method and, along with the Maxwell stress tensor formalism, find the optical force acting on various spheres. We observe this effect for PEC spheres of different sizes and find that they are indeed subject to such transversal force. We find an explanation for this phenomenon via interference effects between selected multipoles excited in the structure. With recently developed methods, we expand the optical force into contributing pairs of selected multipoles and show that, depending on the phase between each multipole pair, the sign and direction of the force can be controlled. We also compare the results for silver and PEC spheres and find that the transversal force magnitude in silver has higher values for more limited range of sphere radii, as compared to PEC.

2021