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Abstract
Herein is analyzed how an electric field can induce a band gap shift in NiO films to generate an enhancement in their third-order optical nonlinearities. An electrochromic effect seems to be responsible for changes in absorbance and modification in off-resonance nonlinear refractive index. The optical Kerr effect was determined as the dominant physical mechanism emerging from the third-order optical susceptibility processes present in a nanosecond two-wave mixing configuration at 532 nm wavelength. Absence of any important multi-photonic absorption was validated by the constant trace of high-irradiance optical transmittance in single-beam mode. The inspection of nonlinear optical signals allowed us to propose an exclusive disjunctive logic gate assisted by an electrochromic effect in an optical Kerr gate. Asymmetric encryption by our XOR system with the influence of a switchable probe beam transmittance and electrical signals in the sample was studied. Immediate applications for developing multifunctional quantum systems driven by dynamic parameters in electrochromic and nonlinear optical materials were highlighted.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The progress in nonlinear optics has confronted a new dimension by including practical technology tools conducted by advanced materials and ultrafast functions. The potential of controlling quantum states in optoelectronic devices for collective manipulation of photons represents a remarkable topic of research [1]. Particular optical properties can be obtained by the coupling atoms in asymmetric complex systems [2]. Recent researches have demonstrated the functionality of atom-photon quantum switches [3]. Low-dimensional circuits can produce strong and controlled photon-photon interactions that enable single photons for quantum information processes [4]. Platforms based on the integration of nonlinear optics and electronics [5] open the possibility to develop all-optical commutation together to optoelectronic signal processing [6], high gain single-photon transistors [7] and electronic modules with photon control [8]. Photo-induced effects for triggering fully-reversible optical characteristics in molecular systems have been proposed for either blocking or transmitting photonic signals [9]. Electrical sensing driven by optical waves has been analyzed toward the fundamental limits of materials science [10]. Nonlinear systems featuring induced transparency could be useful for superconducting quantum circuits [11]. Future applications for encryption and ciphering by exploring novel many-body states of photons [12] or quantum networking with ultra-low switching energies is attractive [13].
In this direction, the electrical and optical characteristics exhibited by semiconductor materials like nickel oxide in thin film form has been studied for engineering energy transfer mechanisms dependent on nickel vacancies and hole concentration [14]. Variations in shape, thickness and optical absorbance can derive on generation or modification of electric currents travelling through NiO thin film samples under optical illumination [15]. Outstanding changes in the nanoscale refractive index exhibited by NiO have been observed [16]. NiO films with tunable optical and electrical properties have been employed in the synthesis of polarization-selectable components [17,18]. It is worth noting that temperature can induce colored states or high transparency in NiO [19]. Specifically, electrochromic (EC) functions provided by NiO materials are attractive due to their conductivity performance [20]. Current studies in diverse areas that correspond to EC materials have received a new interest for the design of advanced instruments [21]. Uses for EC materials can be extensively enumerated, such as, displays [22], smart windows [23], mirrors [24], e-skins [25], sunroofs [26] and textiles [27]. Advantages in reversible optical changes in EC materials [28] are promoted by oxidation and reduction driven by electric fields [29]. NiO can be considered between the most important p-type semiconductors with technological EC functions [30]. A crucial role in some important applications for developing electrochemical circuits [31] and electrical connectors [32] is undertaken by NiO materials. As a matter of fact, EC systems enable color and absorbance changes in response to an electric field [33]. Attractive applications of NiO, as electrochemical energy storage devices [34] and photo-induced energy transfer [35] have been optimized due to its chemical stability together to non-toxicity [36]. The excellent electrical and optical properties of NiO represent good promises for developing optoelectronic commutation devices [37].
Furthermore, regarding that nonlinear optical properties of Kerr-like materials are closely related to the resonant conditions governed by absorbance, it can be visualized the importance to further investigate the potential participation of an EC effect in nonlinear optical functions. With this motivation, this work systematically analyzes a vectorial two-wave mixing (TWM) in NiO samples featuring an EC effect. Slight changes in the band gap of the studied film generate significant changes in the Kerr transmittance exhibited by the NiO film. These results open the possibility for operating logic systems assisted by optical nonlinearities and EC phenomena; then in this work an asymmetric optoelectronic nonlinear encryption function was proposed.
2. Materials and methods
2.1 Synthesis of the NiO thin solid films
A spray pyrolysis processing route allowed us to synthetize nickel oxide thin films as previously described [38]. We used an ultrasonic nozzle (Sonaer) modulated at 130 kHz with the assistance of compressed air with 10 psi and 1 mL/min for the flow. We employed polyethylene glycol together to nickel acetate tetrahydrate 0.05M in a mix with a ratio of 1:5 deposited on indium tin oxide (ITO) substrates featuring 20Ω of sheet resistance. The temperature of substrate during the synthesis was 250 °C over a molten tin bath. The polyethylene glycol was used in order to improve the adhesion of the material to the substrate, it modifies the surface tension of the micro-droplets that are formed during the spraying process. The PEG influences the nucleation and growth processes and properties of the films. The electrical conductivity of the ITO strongly influences the switching time of the EC films; this is related to the number of ions transported from electrolyte to active layer. In order, to minimize the resistance effect of the ITO, substrates with high conductivities are usually chosen, especially for large-area applications; in our case.
NiO thin films with an average thickness close to 150 nm were characterized in our optical and electrical evaluations. Relevant impedance parameters exhibited by ITO substrates were considered in the experimental studies [39]. The morphology of the surface of the films was analyzed by using a FEG Quanta 3D FEI microscope to acquire Scanning electron microscopy (SEM) images. The thickness of the film was measured by Atomic Force Microscopy (AFM) observations recorded by a Park Auto Probe CP system. X-ray diffraction (XRD) measurements were conducted by a Panalytical EmpyreamTM system. An USB2000 + XR1-ES equipment was employed for the spectrophotometric analysis with the NiO samples immersed in a KOH electrolyte (0.5 M) to test the influence of the EC effect over the absorbance driven by direct current (DC) voltage.
2.2 Influence of EC effect in the nonlinear optical response exhibited by NiO films
In order to describe the contribution of the EC effect of the sample studied in their third-order nonlinear optical response, a TWM experiment assisted by a 5 V DC voltage in the film was conducted as schematized in Fig. 1. A Nd-YAG laser system, Continuum Model SL II-10, was selected to carry out the nonlinear optical exploration tested by pulses of 4 nanoseconds at 532 nm. A standard CS2 liquid sample in a quartz cuvette featuring 1 mm path length was used as a reference nonlinear optical material for the calibration of nonlinear experimental setup with |χ(3)| 1.9 ×10−12 esu [40]. The beams from the laser source emerge with vertical linear polarization in respect to the optical table, and we used a maximum 15 mJ of energy per pulse. A lens focused a spot size with diameter of 1 mm in the sample S. The estimated error bar in the experimental intensity recorded was ±10%.
Equivalent incident irradiances for the probe and pump rays were employed. A geometric angle conformed by the propagation vectors of the incident rays was settled in 30°. The direction of the electric field related to incident probe beam was fixed while it was rotated for the case of the pump beam by using a half-wave retarder in the experiment. Calcite polarizers where used to analyze any variation in the transmitted polarization of the beams by the TWM registered by a two PIN photodiodes PD.
2.3 OKG as a XOR optical logic gate for encryption functions
In order to propose the operation principle of a XOR logic gate, we employed a standard optical Kerr gate (OKG) assisted by a voltage signal that enables an EC effect in the NiO sample. The XOR logic gate can be represented by two inputs that produce an output with positive logic value if both binary input data are dissimilar; while a zero logic magnitude is returned for identical logic value for both inputs. In this work, the pump beam is considered as the plaintext while a voltage inducing an EC effect in the NiO film corresponds to the control. In our OKG, the direction of the electrical fields of the incident beams is linear, mutually parallel and fixed for the experiment. Initially, the polarizers were oriented to transmit the orthogonal component of the electric field related to the incident polarization behind the sample. The direction of the electric field of the probe beam was rotated in order to transmit light without the presence the pump beam and null transmittance when the pump beam is turned on. The EC effect amplifies the absorbance of the sample and automatically the probe irradiance decreases without the pump beam and increases under the action of the pump irradiance. The irradiance of the probe beam that is transmitted by this OKG system corresponds to the output of the XOR gate as it can be deduced from Table 1.
Table 1. True table of a XOR logic gate function obtained by our OKG assisted by EC effect in NiO.
From Table 1 can be seen that the transmitted probe beam in our OKG can exemplify the output of a conventional XOR. Regarding that a XOR logic gate can be employed as encryption function, the asymmetry in the encryptographic key for decryption can be obtained by switching the participation of the incident probe beam (idle) that initially is considered to be a positive logic value in the OKG.
3. Results and discussion
In order to verify any superficial distribution in the materials deposited to integrate the NiO film, SEM studies were undertaken and good homogeneity in the film surface was observed as it is shown in Fig. 2(a). Additionally, AFM studies were carried out in different sections of representative samples, and the analysis indicates a standard deviation (Rq) 16.0 nm and average deviation (Ra) 11.0 nm in the thickness of the film. AFM observations indicate a smooth spatial roughness, and uniform film surface featuring around 150 nm average thickness. A representative AFM micrograph is shown in Fig. 2(b).
Fig. 2. (a) SEM image of a representative surface exhibited by the NiO thin solid film (b) AFM micrograph showing a visualization of the surface features.
Figure 3(a) shows XRD data for as-deposited films onto glass substrates at 250 °C. The color appearance was brownish. The diffraction lines correspond to the structure of NiO (PDF file No. 04-0835). In Fig. 3(b) are plotted the spectrophotometric studies for the NiO film as grown and under the action of a 5 V voltage and the modification in the UV-vis spectrum by an electrical potential in the NiO sample can be observed. Figure 3(c) demonstrated by the Tauc plot an electrically induced shift in the bandgap of the NiO film. The morphology of characteristic defects on a typical NiO sample together to a high crystallinity degree of phase is assumed to be responsible for this condition of absorbance. Hydrogen evolution and oxidation of film material can be produce stronger absorbance changes. From the data plotted can be noticed the EC effect in the film as an increase in the absorbance together to a shift of almost 40 nm in the band gap from approximately 330 nm to almost 370 nm wavelength. It is remarkable the inhibition of the EC effect by the absence of voltage evaluated in 50 cycles for the data shown in Fig. 3.
Fig. 3. (a) Representative XRD data. (b) UV-VIS absorbance spectra of the NiO film with the influence of an EC effect. (c) Tauc plot showing a shift of the bandgap induced by the EC effect.
Regarding that the shift in the absorbance resonance must have implication in the resulting optical nonlinearities, we describe χ(3) for the optical frequency ω of high-intensity irradiation by considering a detuning of frequency Δ=ω- ω21, modeled by [41],
Equation (1) describes the evolution of the optical Kerr effect as the real part of χ(3) in a system with N two-level atoms per unit volume. The Planck constant is ħ, m represents the atomic dipole moment, 1/T1 corresponds to the population loss through radiative and non-radiative processes of the upper quantum level of excitation, and 1/T2 is associated with the dynamics of a two-level system with a characteristic rate of polarization loss. By Eq. (1) can be deduced that the shift in the band gap automatically derives in a contribution to the change of the multiphotonic processes of the nonlinear materials. In an off-resonant regime as it is our case, the detuning of the frequency can represent an induced decay due to phonons by the increase of the absorbance promoted by the EC effect. In Fig. 4 is illustrated that small modifications in Δ can generate a strong enhancement in χ(3) according to Eq. (1).
A refractive index n0 = 1.65 was measured by ellipsometry in our NiO films at 532 nm. It is possible to visualize the change in Kerr transmittance in Fig. 5(a) assisted by an EC effect using a numerical simulation of the intensity distribution through the propagation of a TWM in the sample described by the wave equation [40]. With this representation of the NiO sample in interaction with pump and probe beams at 532 nm is possible to graphically display in Fig. 5(b) an intensity-dependent index of refraction characterized as Δn in the range from 1×10−6 to 2.2×10−6 by the contribution of the EC effect and depending on the intensity variations.
Fig. 5. Numerical simulation for the NiO sample. (a) Optical irradiance in a TWM. (b) Range of variation exhibited by the index of refraction in the film.
The third-order optical properties were explored by single-beam transmitted beams and TWM experiments. In Fig. 6(a) is shown a constant transmittance with linear dependence on incident irradiance for nanosecond pulses at 532 nm. Absence of nonlinear optical absorption can be stated under this regime before reaching the ablation threshold of 1.12 J/ cm2 ± 4% for the nanosecond laser irradiation in single-shot mode. Figure 6(b) shows the probe beam transmitted by the sample with an evolution resulting from by the variation of the angle of polarization between both incident beams in the NiO sample.
Fig. 6. Nanosecond results at 532 nm for the NiO film. (a) Single-beam transmittance. (b) Evolution of the transmitted probe beam with dependence on the angle between planes of polarization of both incident beams in the TWM.
A straightforward comparison of the NiO film with a calibrated CS2 sample was conducted to estimate the magnitude of χ(3) using the best fitting of the wave equation with a nonlinear refractive index n2 [40]. The nonlinearities in the NiO assisted by the EC process doubles the nonlinear response with an enhancement from |χ(3)| = 2.25 × 10−10 esu to |χ(3)| = 4.05 × 10−10 esu. The optical Kerr effect for the all-optical experiment was characterized as n2=-2.18×10−12 cm2/W, and for experiment assisted by the EC effect n2=-3.93×10−12 cm2/W. The nonlinear parameters are in good agreement with the parameters evaluated in previously published NiO films evaluated by single-beam z-scan configurations [42].
We observed a monotonic increase in nonlinear optical transmittance by a systematic increment in the film thickness as it is shown in Fig. 7. However, the stronger n2 obtained in this work corresponded to the thinnest film with 150 nm thickness regarding the obtained n2=-1.78×10−12 cm2/W, n2=-1.65×10−12 cm2/W and n2=-1.52×10−12 cm2/W for films with thickness around 300 nm, 450 nm and 600 nm; respectively. In general, the optical contrast (difference in transmittance between the bleached and colored states) ΔT, and the switching time of the EC materials, depends on film thickness. However, the oxidation and reduction reactions (which happen with the insertion or desertion of ions, respectively, like in NiO) responsible for the EC phenomenon are carried out on the surface of the material. This causes grain size and porosity, among other characteristics, to play an important role. An intermediate thickness of a few hundred nanometers has been reported for maximum optical contrast [43,44] and faster color-switching speed.
A TWM can modify the index of refraction in a nonlinear sample and the transmitted polarization of interacting beams. An OKG can analyze the modification in the vectorial components of electric field associated to the incident beams while an EC effect induces absorption in the sample. With these considerations, Fig. 8 presents the experimental data of an XOR logic system controlled by using the interaction of the incident beams together to an EC effect in the proposed OKG. The switching of the transmitted probe beam transmittance as an output of an XOR system can be modulated by the pump intensity and by an electrical field applied to the EC sample.
Fig. 8. Chronogram describing the logic digital response of the data in the OKG experiment assisted by the EC effect in the NiO films. The input corresponds to the pump beam; while an electrical signal providing the EC effect represents the control. The participation of the EC effect changes the absorption condition with an influence on the polarization of the probe beam under the simultaneous action of the electric field and the pump beam.
Regarding that a XOR logic gate can represent an encryption function of a pump signal, the incident probe beam (idle) can be tuned for encoding asymmetry in the encryptographic key for decryption by the EC effect. We systematically studied a random key encryption in the OKG foe enabling the encryption of data. The third-order optical nonlinearities play a protagonist role in the performance of the binary system. Besides, the EC effect contributes to the nonlinearities exhibited by the OKG resulting from the modification of the band gap associated to the off-resonant nonlinear refractive index. The physical mechanisms responsible for optical nonlinearities are strongly dependent on wavelength and temporal regime of irradiation. The internal conversion of the space-charge electric field into a spatial modulation of the refractive index evolves with dependence on the electromagnetic field at the surface of the thin films [45]. Polarization-selectable effects in EC systems can have a crucial potential to integrate quantum information functions in different fields [46]. Diverse processes based on surface-to-volume ration of materials can be employed for energy transfer dynamics [47]. The EC characteristics exhibited by NiO can be envisioned as future candidates for designing alternative solutions for controlling optical and optoelectronic operations [48–50]. The scalar third-order optical effects exhibited by NiO are in good agreement with our parameters estimated by a vectorial TWM technique [51]. Dispersion derived by a Kerr effect with advantages for manipulation of quantum states [52] can be useful to promote EC systems. However, photonic decoherence emerging from multiphotonic processes ought to be considered to be tailored [53]. The potential of NiO samples assisted by multi-wave mixing experiments can be a base for generating energy transfer phenomena regulated by EC mechanisms. Our findings highlighted in this work can be mainly described by the study of the impact of the EC effect on the resulting synergistic evolution of absorption and polarization controlled by optical nonlinearities. The dominant refractive nonlinearities over the photonic absorption in the nanosecond nonlinearities can be applied in nanoscale platforms and nanophotonic systems.
4. Conclusions
The modification in the optical Kerr effect by the assistance of an EC phenomenon exhibited by NiO films was analyzed. The cubic nonlinearity |χ(3)| estimated ∼10−10 esu in NiO thin films can be represented by different physical mechanisms of nonlinear refractive index, and a detuning of the frequency can promote important changes for obtaining an enhancement that simultaneously can modulate the absorptive and refractive nonlinearities. The multi-photonic interactions dependent on off-resonance absorbance in the samples seem to be dependent on the electronic dynamics with a strong influence in the nonlinearity of refractive index. The evolution of the Kerr transmittance for different angles of polarization of two incident beams in an OKG was studied under the influence of voltage signals in propagation through the samples. An asymmetric encrypting function was proposed by a straightforward XOR logic gate with nonlinear optical waves as inputs and output, with the contribution of an EC effect in the ciphering and deciphering process. It is highlighted that non-reciprocal processes in TWM can derive in a selective enhancement of Kerr nonlinearities potentially promoted by an electronic voltage in one of the samples of the system with an enhancement from |χ(3)| = 2.25 × 10−10 esu to |χ(3)| = 4.05 × 10−10 esu, and from n2=-2.18×10−12 cm2/W, to n2=-3.93×10−12 cm2/W. Tailoring the electronically controlled band gap responsible for the enhancement in the optical Kerr effect in these NiO platforms represents a good alternative for tuning the steady-state and dynamic behavior in signal processing systems with optical and optoelectronic information.
Funding
Comisión de Operación y Fomento de Actividades Académicas, Instituto Politécnico Nacional (2022); Instituto Politécnico Nacional (SIP-2022); Centro de Investigación en Materiales Avanzados S.C. (CIMAV) Unidad Monterrey (2022); Consejo Nacional de Ciencia y Tecnología (2015-251201).
Acknowledgments
The authors kindly acknowledge the financial support from the Instituto Politécnico Nacional, Comisión de Operación y Fomento de Actividades Académicas del IPN, Universidad Politécnica del Bicentenario, Centro de Investigación en Materiales Avanzados S.C. (CIMAV), Unidad Monterrey and Consejo Nacional de Ciencia y Tecnología (CB-2015-251201).
Disclosures
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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