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With the development of laser technology, the application scope of nondiffracting beams, such as Bessel beams, Mathieu beams, cosine beams, and parabolic beams, which remain invariant along their propagation, continues to expand. During its propagation, the main lobes of these beams tend to bend towards off-axis position, which is called self-accelerating (or self-bending) property. A Bessel-like beam with self-acceleration can realize the propagation of the main lobe along a curved trajectory while maintaining the non-diffraction. Because of the above property, Bessel-like beams have been utilized in various areas such as guiding particles along arbitrarily curved trajectories, self-accelerating beams in nonlinear medium, plasma guidance, and laser-assisted guiding of electric discharges around objects. In this paper, we propose a method of bending the trajectory of Bessel-like beams by using a magnetic fluid deformable mirror (MFDM) instead of traditional spatial light modulator (SLM) and Pancharatnam-Berry (PB) phase manipulation. The MFDM provides a method without pixelation, where all parameters can be rapidly modified for fine-tuning. Furthermore, compared with the conventional deformable mirror, the MFDM has the advantages of a continuous and smooth mirror surface, large shape deformation, low manufacture cost, easy extension, and large inter-actuator stroke. Therefore, it is easy for the MFDM to generate the ideal shape of an axicon. Firstly, according to geometric analysis, the asymmetrical mirror profile to produce a self-accelerating Bessel-like optical beam is proposed. The proposed mirror profile can be used to compensate for the difference in optical path length for each annular slice of the axicon. If a collimated Gaussian beam is incident on the mirror combining the axicon and the asymmetrical mirror profiles, which can obtain Bessel-like beams with arbitrarily curved trajectories. Secondly, the resultant of the self-accelerating Bessel-like beams along parabolic trajectories is validated by the simulation in MATLAB. Finally, a prototype of MFDM consisting of the dual-layer arrays of miniature electromagnetic coils, a Maxwell coil and the magnetic fluid filled in a circular container is fabricated for the experiment. The experimental results show that the Bessel-like beams propagate along the parabolic trajectories, with the MFDM used, and the accuracy of the curved trajectories is verified. The proposed method in this paper opens a new experimental way to the study of Bessel-like beam; the theoretical approach can also be generalized mathematically for other non-paraxial beam propagation.

As a key component of the adaptive optics (AO) system,wavefront corrector plays a crucial role in determining the performance of the AO system.At present,the typical wavefront correctors,including solid deformable mirrors and liquid crystal spatial light modulators,have the common drawbacks of high cost of per actuator channel,and the relatively low stroke deflection (normally less than 50 m) due to the limitation of material and manufacturing technology.In the face of the growing demand for deformable mirrors with large stroke,low power dissipation and low cost,the magnetic fluid based deformable mirror (MFDM) is proposed in this paper.The magnetic fluid has the characteristic of the fluidity of liquid and can be magnetized by an external magnetic field.Therefore,the surface deflection of the MFDM can be controlled by the surrounding magnetic field generated by an array of electromagnetic coils located underneath the magnetic fluid layer.Compared with the conventional deformable mirrors,the MFDM has the advantages of a continuous and smooth mirror surface,large shape deformation,low manufacture cost,and easy extension.The surface dynamics model of MFDM with a circular geometry has been studied previously in the literature.In the present paper, considering the possible applications in the wavefront control of rectangular laser beams,we study the MFDM with a rectangular array of actuators. Firstly,based on the governing equations of the magnetic fluid,derived from the principles of conservation of fluid mass and magnetic field,the dynamics model of surface deflection of the rectangular MFDM is analyzed in Cartesian coordinates under the boundary condition of magnetic field and the kinematic conditions of magnetic fluid.The analytical solutions of the surface movement of the mirror subject to the applied currents in the electromagnetic coils are obtained by properly separating the variables with truncated model numbers.Secondly,based on the derived analytical model, the optimal design procedure for the structure and parameters of the MFDM to obtain the required performance,i.e. the largest stroke and inter-actuator stroke of the mirror,as well as the coupling coefficient of the influence function, is presented.The resulting surface response performance of the designed MFDM is validated by the co-simulation in MATLAB,COMSOL Multiphysics and Tracepro software.Finally,a prototype of square MFDM consisting of the square array of miniature electromagnetic coils,a Maxwell coil and the magnetic fluid filled in a rectangular container is fabricated for experimental evaluation.The experimental results of the surface response of the mirror subject to two adjacent active coils are first presented to validate the stroke performance and linear characteristics of the MFDM. A parabolic surface shape is then further produced in the AO setup system with the MFDM subject to the array of coils driven by the currents calculated from the analytical model.The experimental results verify the accuracy of the established dynamics model and show that the proposed MFDM can be used to effectively control the wavefront of laser beam.

Voice conversion is the process to modify a speech signal of one speaker (source) to sound like an intended speaker (target) without changing the language content. This technology has several applications, such as personalized speech synthesis and speech to singing, and a possible malicious application to fool a voice biometric system, also called a speaker verification system. This thesis focuses on techniques to improve voice conversion performance and the use of voice conversion technology to attack speaker verification systems. The study is important as the robustness of the conversion function will affect the performance of voice conversion directly. To this end, the thesis first proposes a method to improve the voice conversion function by benefiting from nonparallel data of background speakers. The strategy is to decompose a speech spectral vector into a phonetic component and a speaker-specific component, which are modeled by a factor analysis model. The nonparallel data are used to estimate the phonetic component and the factor loadings. The speaker-specific component can then be represented by a low-dimensional set of variables via factor loadings, and hence allow for more robust modeling under sparse training data condition. The experimental results show that the proposed method outperforms the conventional Gaussian mixture model (GMM) based method considerably when there are limited parallel training data. The second contribution of the thesis focuses on implementing the conversion function by directly modelling the high-dimensional spectral features using exemplars found in the training data. In this approach, each speech segment is reconstructed as a weighted linear combination of a set of basis exemplars with residual compensation. An exemplar is defined as a speech segment spanning multiple frames extracted from training data. The value of the linear combination weights is constrained to be nonnegative, and most of them are restricted to have a value close to zero. Experiments are conducted to compare the proposed method with a large set of baseline approaches. It is observed that the proposed method can achieve similar performance to the state-of-the-art GMM based and dynamic kernel partial least square based voice conversion methods. The experiments also confirm its flexibility when the amount of training data is varied. The third contribution focuses on the use of voice conversion technology to attack current state-of-the-art speaker verification system with the purpose to identify the weak links of the verification algorithms. Recently, speaker verification technology has been advanced significantly and has led to mass market adoption, such as in smartphones for user authentication. A major concern when deploying speaker verification technology is whether a system is still robust against spoofing attacks. Unfortunately, voice conversion has become one of the most easily accessible techniques to carry out spoofing attacks, and hence presents a threat to speaker verification systems. To address the above concern, this thesis examines the vulnerabilities of nine current state-of-the-art speaker verification systems in the face of voice conversion spoofing attacks. Doctor of Philosophy (SCE)

In a semi-infinite crystal, the periodic potential is destroyed at the surface, and the electronic wave functions exponentially decay from the surface to both sides. Such localized electronic states in the vicinity of the surface are known as Tamm surface states. In analogy to the electronic Tamm states, in recent years, optical Tamm states have been found at the surface of the truncated photonic crystal composed of two kinds of dielectrics. Very recently, novel types of optical Tamm states including backward Tamm states in which the phase velocity and the group velocity of optical waves are in the opposite direction have been discovered in the photonic structures containing metamaterials. In fact, the concepts in electronic field and photonic field can inspire each other. Many unique phenomena in photonic systems can also be mapped to the electronic systems. In this paper, we study the novel types of electronic Tamm states in electronic systems, inspired by the novel types of optical Tamm states in photonic structures. #br#At first, comparing Maxwell equations with Schrodinger equations, one can see a correspondence between the parameters in electromagnetic system and the parameters in the electronic system. In particular, Hg1-xCdxTe semiconductors with special electronic band structures can realize various electronic materials in analogy to the optical metamaterials with various values of permittivity and permeability. By tuning the parameter x of Hg1-xCdxTe, we obtain a variety of metamaterial-like electronic materials, in analogy to the single-negative metamaterials, the double-negative metamaterials and the near-zero-index metamaterials in optical systems. Then, inspired by the one-dimensional heterostructures with metamaterials that generate optical Tamm states, we design a one-dimensional electronic heterostructure consisting of Hg0.847Cd0.153Te and CdTe/HgTe superlattice. When Hg0.847Cd0.153Te is analogous to the double-negative metamaterial, we find the backward electronic Tamm states in which the phase velocity and the group velocity of electronic waves are in the opposite directions. When Hg0.847Cd0.153Te is analogous to the near-zero-index metamaterial, we find a novel electronic Tamm states in which the amplitude of the electronic probability decays very slowly in Hg0.847Cd0.153Te. The discovery of these new types of electronic Tamm states enlarges our knowledge of electronic surface states.