Fernande Fotsa-Ngaffo

2024Article de revue

Localization behavior in a thresholdless PT symmetric electrical transmission line

Fernande Fotsa-Ngaffo, Senghor Tagouegni, Emmanuel Fendzi-Donfack, A. Kenfack-Jiotsa, William Stello Tchouatcheu Tchamabe

Physica B Condensed Matter

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2023Article de revue

Construction of abundant solitons in a coupled nonlinear pendulum lattice through two discrete distinct techniques

Emmanuel Fendzi-Donfack, Marcial Baduidana, Fernande Fotsa-Ngaffo, A. Kenfack-Jiotsa

Results in Physics

In this paper, we deal with two distinct discrete techniques named the discrete Tanh method and the differential-difference Jacobi elliptic functions sub-ODE method to extract abundant soliton solutions for a coupled nonlinear pendulum chains. According to the two straightforward schemes’, we construct various solutions likewise, kink, anti-kink, bright, dark, singular-bright solitary wave, traveling compacton, traveling gray soliton-like, periodic soliton-like haven’t reported on the most recently studied mechanical model. We extract the sought solutions with less approximation (introduction of a discrete wave transformation) and display some graphical representations showing well known, novel shapes discovered in a coupled nonlinear pendulum lattice. More often, the tediousness computations of discrete techniques lead some researchers to changing a nonlinear differential-difference equation into a nonlinear ordinary differential equation throughout the semi-discrete approximation before introducing a wave transformation. Nonetheless, the semi-discrete approximation involve a Taylor bounded expansion’s such as the threshold order depends on the ability and skills of researchers to get solutions through available methods and those that they can perform. Hence, while an algorithm can be lead us to avoiding approximation (eventual source of errors), tediousness analytical computations, we perform by using it, in addition of software availability. And therefore, we establish as is the case in the present work various soliton-like solutions by means of two direct discrete techniques. The effect of some physical parameters are displayed on the shaped through 2D and 3D graphical simulations. The used methods (tanh, sn, cn, dn Jacobi elliptic functions approaches) previously applied on other physical models (discrete electrical transmission lattices) are presented here in manner to be replicable for exploring more soliton solutions in different nonlinear discrete physical models.

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2021PrépublicationAccès ouvert

Non-Hermitian Electronics Multipods of Electromagnetically Induced Transparency (EIT) and Absorption (EIA)

Senghor Tagouegni, Fernande Fotsa-Ngaffo, A. Kenfack-Jiotsa

Research Square

Abstract We study a non-Hermitian electronic dimers system based on an imaginary resistor (Z) in a (N+2) level atomic multi-pod configuration. Non-Hermitian systems depend on a gain/loss parameter and are specifically marked by a degeneracy exhibited at an exceptional point (EP) separating different phases of complex modes dynamics. Interestingly, the structural characterization and the dispersive properties reveal a broad range of strong coupling where the interplay between the control and the probe field induce a simultaneous EIT, EIA and ATS. Here, by identifying the underlying physical mechanisms, we show that multiple windows of transparency can be strongly enhanced by the incorporation of several dimers in the multipod network. On the other hand, if the pumping field is resonant in the weak regime, multiple EIT and EIA windows result in the number of dimers. Remarkably, the proposed system embedded a multiple coupling mechanism whose modulation induces a couplingless point whereby the energy cross. At this point EIT and related phenomena vanish.

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2020Article de revueAccès ouvert

Energy transport and Anderson-like localization in non-Hermitian electrical transmission line

Senghor Tagouegni, Fernande Fotsa-Ngaffo, Stéphane Boris Tabeu, A. Kenfack-Jiotsa

Physica Scripta

Abstract We investigate on the right and left-handed (RH/LH) balanced gain and loss non-Hermitian electrical transmission lines (ETL) modeled using an imaginary resistor. The hamiltonian of each system is successfully derived in the framework of the tight-binding theory. We discuss the underlying symmetries and calculate the breaking thresholds of the Parity time (PT) and Anti PT (APT) symmetry phase transitions. Moreover, the modes dynamic characterization reveal the existence of critical points beyond which operation requires thresholdless transition and the eigen modes are either real or complex. We also present a self-consistent theory to investigate on their scattering properties. In particular, we demonstrate that the Anderson-like localized modes mostly emerge in the broken phase where eigen modes are complex with the localization length proportional to the cell numbers. Importantly, the mode localization is most likely to occur in structures consisting of a large cells number for RH ETL while LH ETL will confined modes even for infinitesimal small cell numbers. These results unveil embryonic applications in cryptography, switching and system control.

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2017Article de revue

Thresholdless characterization in space and time reflection symmetry electronic dimers

Fernande Fotsa-Ngaffo, Stéphane Boris Tabeu, Senghor Tagouegni, A. Kenfack-Jiotsa

Journal of the Optical Society of America B

We develop a general mutual inductive M, inductive L, and capacitive C couplings parity time symmetry (MLC PTS) electronic dimer model. We successfully show that the dimers separately coupled with M, L, or ML can be made thresholdless by adding a capacitance C in parallel. Thus, sufficient conditions for thresholdless transitions were determined. The scattering properties of our model reveal the suppression of unidirectional invisibility from the gain input when the Hermitian line coupling exceeds a critical point. Likewise, the scattering system becomes unidirectional PTS. Remarkably, under the thresholdless conditions, lasing modes appear for imaginary values of wavenumbers corresponding to either real or purely imaginary values of the gain/loss modulation. This latter demonstrates the ability to control prohibited waves in the linear regime.

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2016Article de revueAccès ouvert

Non-Hermitian trimers: PT-symmetry versus pseudo-Hermiticity

Sergey V. Suchkov, Fernande Fotsa-Ngaffo, A. Kenfack-Jiotsa, Arnaud D Tikeng, Timoléon Crépin Kofané, Yuri S. Kivshar et 1 autre(s)

New Journal of Physics

We study a structure composed of three coupled waveguides with gain and loss, a non-Hermitian trimer. We demonstrate that the mode spectrum can be entirely real if the waveguides are placed in a special order and at certain distances between each other. Such structures generally lack a spatial symmetry, in contrast to parity-time symmetric trimers which are known to feature a real spectrum. We also determine a threshold for wave amplification and analyse the scattering properties of such non-conservative systems embedded into a chain of conservative waveguides.

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2008Autre

Reactive pulsed laser ablation deposition (RPLAD) of indium tin oxide (ITO), titanium dioxide (TiO2) thin films and gold (AU) nanoparticles for dye sensitised solar cells (DSSC) applications

Fernande Fotsa-Ngaffo

University of the Witwatersrand, Johannesburg Institutional Repository on DSpace (University of the Witwatersrand, Johannesburg)

ABSTRACT\nThe focus of this work was the study possible ways to improve the efficiency of solar cells. To this end, the main aim was to investigate the deposition process of Indium Tin Oxide (ITO), Titanium Dioxide (TiO2), multi-layers ITO/TiO2 on quartz SiO2 substrates under different conditions (oxygen pressure, laser fluence and wavelength, and temperature) and later gold nanoparticles by the Reactive Pulsed Laser Ablation Deposition (RPLAD) technique. It was intended to investigate their electrical structural and optical properties under selected conditions for possible application to Dye Sensitised Solar Cells (DSSC).\nUnder optimised conditions, maximum deposition rates of 12nm/min for ITO and 21nm/min for TiO2 thin films were achieved. Rutherford Backscattering Spectrometry (RBS) with 2MeV He+ ions was used to measure the films thickness. Uniform thicknesses over a large area were found to be about 400nm and 800nm for ITO and TiO2 films, respectively. Crystalline properties were studied via x-ray diffraction and Raman spectroscopy. X-ray Diffraction (XRD) analysis revealed that the ITO films are highly orientated nanocrystals with their a-axis normal to the glass substrate surface. The average particle size of the precipitated nanocrystals was calculated to be 10-15nm.\nThe structure of the films was characterised via Atomic Force Microscopy (AFM) imaging of the top surface of the film. The films have a rough surface with average roughness of 26-30nm. Pores were observed with a density of 144 and 125 pores/mm2 and average size of 150 and 110nm for ITO films deposited at 200 and 400°C, respectively. TiO2 films deposited on the prepared ITO films were less crystalline. Annealing was performed at 300 and 500°C for 3 consecutive hours and the XRD results show that the transformation of TiO2 film into anatase phase was almost complete with a crystal size of ~ 6-7nm.\nScanning Transmission Electron Microscopy (STEM) of the surfaces was also performed. The TiO2 films deposited onto the prepared ITO films present a \nrelatively high pore size with an average pore diameter of ~ 40nm and excellent uniformity. It is interesting to note that the pores are randomly arranged. The random arrangement of the pores network may actually be beneficial for producing a uniform electrode. In addition, STEM cross-sectional analysis of the films showed a columnar structure but no evidence of voids in the structure. The large surface area produced suggests applications in DSSC.\nThe electrical properties of the films were investigated and an estimation of resistivity and Hall mobility was made. Low values of resistivity and high values of mobility were observed for ITO films. The resistivity of the film increases with increasing thickness while it decreases when increasing the deposition temperature. The lowest value was found to be 1.5x10-6Ωm for ITO films deposited at 400°C. Hall mobility was found to increase with substrate temperature. In this investigation, the highest Hall mobility at room temperature was estimated to be 22.3cm2/Vs under ambient O2 pressure (PO2) of 1Pa and 52.1 and 51.3cm2/Vs for films deposited at 200 and 400°C, respectively. But the best ITO film was deposited at 200°C, since this film combines good resistivity, good Hall mobility and good transmittance.\nUV-VIS-IR transmission spectra were recorded on a Perkin Elmer Lambda 900. From the transmission data, the energy gap as well as the optical constant was estimated. A high transmission for ITO films in the visible (Vis) range was observed which was above 88% for films produced at room temperature and above 95% for those deposited at 200°C. The transmission for the films produced in oxygen was about 90% above 400nm, whereas it lies between 70 and 80% for films produced in rare gases. An increase in the band gap was observed by increasing the oxygen pressure and substrate temperature for ITO films. Increasing the quartz SiO2 substrate temperature from room temperature to 400 °C resulted in an increase of the transmission of TiO2 films, mostly in the Visible Near Infrared (Vis-NIR) from about 70% to 92%. After annealing at 500°C for 3 consecutive hours, the transmission of TiO2 film further sharply decreases toward shorter wavelengths. \nAnalysis of the transmittance curve of TiO2/Au shows a decrease of about 6% of the transmission in the Ultraviolet Visible (UV-Vis) range.\nOptical absorption edge analysis showed that the optical density could be used to detect the film growth conditions and to correlate the film structure and the absorption edge. The TiO2 films deposited present a direct band gap at 3.51eV and 3.37eV for TiO2 as deposited and after annealing, respectively, while the indirect band gap was found to be 3.55eV and 3.26eV for TiO2 films as deposited and after annealing, respectively. There was a shift of about 0.1eV between as deposited ITO monolayer films and ITO/TiO2 bilayers deposited at 200°C. A small shift towards shorter wavelengths has been observed for multilayer ITO/TiO2/Au. In this case, the increase of Eg was ascribed to a reduction of the oxygen vacancies with increasing substrate temperature at which the ITO film was deposited.\nThe change in the shape of the fundamental absorption edge is considered to reflect the variation of density and the short range structural modifications undetected by structural characterisations. Enlargement of band-gap energies of semiconductors may be advantageous when used in DSSC to suppress the charge recombination between the reduced electrolytes and the photo-excited holes in the valence band of TiO2 substrates and enhance the open-circuit potential of the cell. When ITO/TiO2 bilayers were annealed before depositing Au, the gap energy remained constant.

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2023PrépublicationAccès ouvert

Experimental observation of real spectra in Parity-Time symmetric ZRC dimers with positive and negative frequencies

Stéphane Boris Tabeu, Fernande Fotsa-Ngaffo, Kazuhiro Shouno, A. Kenfack-Jiotsa

Research Square

<title>Abstract</title> We present in this work the first experimental observation of oscillations in Parity-Time symmetric ZRC electronic dimers. The system obtained is of first order ordinary differential equation due to the use of imaginary resistors. The coupled cells must share the same type of frequency: positive or negative. We observed the real and imaginary parts of the voltage across the components of a ZRC cell. Exceptional points are well identified. This work may be very useful in the generation of new type of oscillators. It can also be used in the design of new optoelectronic devices for major applications in the transport of information and the mimics of two-level systems for quantum computing.

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2023PrépublicationAccès ouvert

Observation of Hermitian and Non-Hermitian Diabolic Points and Exceptional Rings in Parity-Time symmetric ZRC and RLC Dimers

Stéphane Boris Tabeu, Fernande Fotsa-Ngaffo, A. Kenfack-Jiotsa

Research Square

Abstract We present the observation of diabolic points in Hermitian and non-Hermitian electronics dimers. The condition of unbreakable Parity-time symmetry is established for both PT-symmetric ZRC and RLC dimers. We show how appears non-Hermitian degeneracy points in the spectrum and how they are protected against a Hermitian perturbation. When a non- Hermitian perturbation is added in the setup, the non-Hermitian diabolic point (NHDP) turns into a ring of exceptional points as in some Dirac and Weyl semimetals. Some experimental simulations of oscillations around these particular points in LTspice are in perfect accordance with the one predicted analytically and numerically. This work opens a gold road for investigations on topological electrical circuits for robust transport of information at room temperature.

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2023Article de revue

PT -symmetric electronic dimer without gain material

Senghor Tagouegni, Fernande Fotsa-Ngaffo, A. Kenfack-Jiotsa

Communications in Theoretical Physics

Abstract In this paper, a way of building an electronic Parity Time ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic"></mml:mi> <mml:mi mathvariant="italic"></mml:mi> </mml:math> )-symmetric dimer without gain material is presented. This is achieved by capacitively coupling a pair of LZC circuits, each combining an inductance L , an imaginary resistance Z and a positive/negative capacitance C . We derive the effective Hamiltonian of the system, which commutes with the joint <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic"></mml:mi> <mml:mi mathvariant="italic"></mml:mi> </mml:math> operator. The eigenspectrum displays spontaneous breaking points, where the system undergoes a transition from real to complex values. The transition points are imposed by the range value of the coupling thanks to the use of a negative capacitance. Temporal charge solutions and energy propagation are also analytically and numerically investigated, and the results are compatible. In the exact phase, these quantities oscillate, whereas in the broken phase, oscillations disappear, giving place to amplification. Our results pave the way to innovative <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic"></mml:mi> <mml:mi mathvariant="italic"></mml:mi> </mml:math> -symmetric circuits. Applications could include, among others, optics, metamaterials, photonics and sensitive detection.

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2021Article de revue

Pseudo-Hermitian trimer: Asymmetric resonances and exceptional points

Arnaud D Tikeng-Manfouo, Fernande Fotsa-Ngaffo, Timoléon Crépin Kofané

Physica Scripta

Abstract A pseudo-Hermitian (PH) system made up of three coupled waveguides (trimer) with an overall balanced gain and loss is investigated in a parity time (PT)-symmetric-like regime. Remarkably, the pseudo hermiticity parameter controls the asymmetric spatial field modulation between left and right propagating waves whereby, a complex coupling arises that induces an additional source of non-Hermiticity, which in the system acts as a relative gain/loss Hellmann-Feynman correction. In this situation, we show that, transmissionless backscatterings are observed for the PH-trimer in the non PT broken phase. Interestingly, the scattering properties of PH-trimer result in multiple lasing states with different mechanisms. In particular, we show that in the PT-symmetric case, the lasing modes associated to the coherent perfect-absorption (CPA), occur from the spectral singularity induced by the Hermitian chain coupling and give rise to unidirectional invisibility in the direction of incident wave. On the contrary, additional emerging lasing modes in a PH regime result from the scattering maxima resonance. We demonstrate that this specific cases suppress the CPA-laser phenomenon and rather exhibit exceptional points characterized by reflectionless but also the points of transmissionless. Numerical beam propagations realized to emphasize the results are in good agreement with the scattering investigation until gain/loss parameter is around it’s value giving resonance.

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2020PrépublicationAccès ouvert

Experimental observation of real spectra in Parity-Time symmetric ZRC dimers with positive and negative frequencies

Stéphane Boris Tabeu, Fernande Fotsa-Ngaffo, Senghor Tagouegni, Kazuhiro Shouno, A. Kenfack-Jiotsa

arXiv (Cornell University)

We present in this work the first experimental observation of oscillations in Parity-Time symmetric ZRC dimers. The system obtained is of first order ordinary differential equation due to the use of imaginary resistors. The coupled cells must share the same type of frequency: positive or negative. We observed the real and imaginary parts of the voltage across the components of a ZRC cell. Exceptional points are well identified. This work may be very useful in the generation of new type of oscillators. It can also be used in the design of new optoelectronic devices for major applications in the transport of information and the mimics of two-level systems for quantum computing.

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2020PrépublicationAccès ouvert

Observation of Hermitian and Non-Hermitian Diabolic Points and Exceptional Rings in Parity-Time symmetric ZRC and RLC Dimers

Stéphane Boris Tabeu, Fernande Fotsa-Ngaffo, A. Kenfack-Jiotsa

arXiv (Cornell University)

We present the observation of diabolic points in Hermitian and non-Hermitian electronics dimers. The condition of unbreakable Parity-time symmetry is established for both PT-symmetric ZRC and RLC dimers. We show how appears non-Hermitian degeneracy points in the spectrum and how they are protected against a Hermitian perturbation. When a non- Hermitian perturbation is added in the setup, the non-Hermitian diabolic point (NHDP) turns into a ring of exceptional points as in some Dirac and Weyl semimetals. Some experimental simulations of oscillations around these particular points in LTspice are in perfect accordance with the one predicted analytically and numerically. This work opens a gold road for investigations on topological electrical circuits for robust transport of information at room temperature.

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2019Article de revue

Non-Hermitian Hamiltonian of two-level systems in complex quaternionic space: An introduction in electronics

Stéphane Boris Tabeu, Fernande Fotsa-Ngaffo, A. Kenfack-Jiotsa

Europhysics Letters (EPL)

An imaginary resistor ( Z ) based electronic dimer is used to describe the gyroscopic and resistive coupled two-level systems in quaternionic space. We successfully used the quaternionic coefficients to characterize the different classes of non-Hermitian systems: the pseudo-Hermitian and anti-Hermitian, both having exceptional points (EPs) separating the exact and the broken phases. Remarkably, the EPs conical dynamic is fully described to follow a hyperbolic/parabolic shape in the case of parity-time symmetry (PTS)/anti-PTS, respectively. Interestingly, our results demonstrated that the gyroscopic coupling mechanism allows identical non-dissipative but non-oscillatory systems with negative capacitor to exhibit PTS-like behavior.

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Fernande Fotsa-Ngaffo

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