Nonlinear Optical Correlator Research Articles

Nonlinear optical correlator in 4f configuration exploiting Kerr effect for optical processing and matrix multiplication

Nonlinear optical correlator in 4f configuration exploiting Kerr effect for optical processing and matrix multiplication

Bulk Photovoltaic Effect Along the Nonpolar Axis in Organic-Inorganic Hybrid Perovskites.

The origin of the bulk photovoltaic effect (BPVE) was considered as a built-in electric field formed by the macroscopic polarization of materials. Alternatively, the "shift current mechanism" has been gradually accepted as the more appropriate description of the BPVE. This mechanism implies that the photocurrent generated by the BPVE is a topological current featuring an ultrafast response and dissipation-less nature, which is very attractive for photodetector applications. Meanwhile, the origin of the BPVE in organic-inorganic hybrid perovskites (OIHPs) has not been discussed and is still widely accepted as the classical mechanism without any experimental evidence. Herein, we observed the BPVE along the nonpolar axis in OIHPs, which is inconsistent with the classical explanation. Furthermore, based on the nonlinear optical tensor correlation, we substantiated that the BPVE in OIHPs is originated in the shift current mechanism.

Bulk Photovoltaic Effect Along the Nonpolar Axis in Organic‐Inorganic Hybrid Perovskites

AbstractThe origin of the bulk photovoltaic effect (BPVE) was considered as a built‐in electric field formed by the macroscopic polarization of materials. Alternatively, the “shift current mechanism” has been gradually accepted as the more appropriate description of the BPVE. This mechanism implies that the photocurrent generated by the BPVE is a topological current featuring an ultrafast response and dissipation‐less nature, which is very attractive for photodetector applications. Meanwhile, the origin of the BPVE in organic–inorganic hybrid perovskites (OIHPs) has not been discussed and is still widely accepted as the classical mechanism without any experimental evidence. Herein, we observed the BPVE along the nonpolar axis in OIHPs, which is inconsistent with the classical explanation. Furthermore, based on the nonlinear optical tensor correlation, we substantiated that the BPVE in OIHPs is originated in the shift current mechanism.

Electronic dephasing in mixed quantum–classical molecular systems using the spin-boson model

Pure electronic dephasing is investigated using the spin-boson Hamiltonian in mixed quantum–classical environment. The spin-boson model used here is a composite system made up of a quantum subsystem, an electronic 2-level subsystem linearly coupled to harmonic vibrations, interacting with a classical bath. Experimental results for a multitude of molecular systems indicate that the zero-phonon line (ZPL) profile is determined by electronic dephasing, which is not accounted for in the multimode Brownian oscillator (MBO) model due to the unphysical contribution from the MBO bath modes to the ZPL profile. Mixed quantum–classical dynamics formalism of non-equilibrium systems is employed to assess the contribution of the bath modes to pure electronic dephasing by probing the ZPL profile when coupled to a classical bath in the mixed quantum–classical condensed systems. Pure electronic dephasing is discussed in the context of mixed quantum–classical dynamics formalism which starts with mixed quantum–classical Liouville equation in a mixed quantum–classical environment. It is noteworthy, however, that the fundamental difference between the fully quantum MBO model and the mixed quantum–classical Brownian oscillator, is that the zero-phonon line calculated by the former shows unphysical asymmetry on the low-energy side as it has not been observed in real systems, whereas the ZPL reported herein eliminates this asymmetry. A systematic approach using matrix mechanics is developed to treat this phenomenon. To this end, a closed-form expression of linear and nonlinear optical electronic transition dipole moment time correlation functions in a dissipative media are derived. Linear absorption spectra and 4-wave mixing signals at various temperatures showing a sound thermal broadening, temporal decay, and accurate pure dephasing further ratify the applicability and correctness of the mixed quantum–classical dynamics approach to spectroscopy and dynamics are computed.

Electronic dephasing of polyatomic molecules interacting with mixed quantum-classical media.

This paper offers an expedient, efficient, and unique treatment of multimode quantum subsystems (polyatomic molecules) interacting with a classical environment in which the time evolution of the coupling term is governed by the algebraic rules of statistical mechanics in mixed quantum-classical systems developed by Kapral and Nielsen [S. Nielsen, R. Kapral, and G. Ciccotti, J. Chem. Phys., 2001, 115, 5805]. This unique time evolution of the coupling term is neither quantal nor classical but rather something different that relies heavily on Wigner transform, thereby leading to non-Newtonian mechanics. As such, an argument is presented that the approach provided herein for treating polyatomic molecular systems in a mixed quantum-classical environment is new and different as opposed to the many other schemes of semiclassical dynamics that are normally employed to study such systems. The merits of expediency and efficiency of the herein mixed quantum-classical dynamics calculations emanate from avoiding using integrals for time evolutions, and, instead, employing matrix mechanics whereby LU decomposition and singular value decomposition (SVD) numerical techniques are utilized for diagonalization. An electronic 2-level subsystem interacting with a classical bath through the spin-boson model to render accurate pure electronic dephasing in multimode molecular systems by eliminating the unphysical asymmetry in the line shape of the zero-phonon line (ZPL) exhibited by other models is exploited. This work has a superior advantage over the single-mode spin-boson model, published previously, whereby a multitude of types of vibrational modes (slow, fast, or both) of the quantum subsystem may readily be handled using different spectral densities. The spin-boson model used here is a composite system made up of a quantum subsystem, i.e., a subsystem bilinearly coupled to a multidimensional harmonic oscillator (representing the intermediate quantum vibrational modes between the electronic subsystem and the bath), interacting with a classical bath, where the coupling term is governed by the mixed quantum-classical Liouville equation. A multidimensional coherent-state approach is employed to deal with the time evolution of the quantum subsystem. A closed-form expression of linear and nonlinear optical electronic transition dipole moment time correlation functions in mixed quantum-classical dissipative media is derived. Pure electronic dephasing is probed using the aforementioned approach. Linear absorption spectra and 4-wave mixing signals (e.g., photon echo and pump-probe) are calculated showing a reasonable thermal broadening, temporal decay, and accurate pure dephasing.

Nonlinear Optical Method of Determination the Chirp of Broadband Femtosecond Laser Pulse in IR-Range

A new method for determining the chirp of a femtosecond broadband IR laser pulse at a central wavelength of 2.5 μm, based on the generation of a spectral supercontinuum in the field of a spectrally limited femtosecond laser pulse, propagating in a single mode fiber based on chalcogenide glass, and subsequent noncollinear generation of radiation at summary frequency by spectrally limited and spectral broadened supercontinuum femtosecond pulses is proposed. The time dependences of the instantaneous frequency of a femtosecond broadband pulse, obtained both from the results of numerical integrations of the equation describing the process of spectral supercontinuum generation and from the results of two-dimensional distributions of dynamic spectrograms are presented. It is shown that the proposed method allows one to determine the chirp of a broadband femtosecond IR laser pulse, formed during the soliton propagation mode in a fiber, with a relative error of no more than 5%, but the chirp of a broadband femtosecond IR laser pulse, formed in the process of non-soliton propagation in a fiber, with a relative error of no more than 10%.The presented results can be used in the development of a nonlinear optical phase correlator for determination the phase and time profile of a femtosecond laser pulse in the mid-IR range.

Determination of the Subpicosecond Laser Pulse Chirp in the Middle IR Range Based on the Fourth Harmonic Noncollinear Generation

A new method is proposed to determine the subpicosecond laser pulse chirp in the middle IR range at the central wavelength of 10 μm, based on the generation of the second-harmonic pulses both by the bandwidth-limited and frequency-modulated subpicosecond pulses and the subsequent noncollinear generation of the fourth-harmonic radiation by the corresponding second-harmonic pulses. The time dependences are given of the instantaneous frequency of the frequency-modulated second-harmonic pulse at the central wavelength of 5 μm, generated in the field of the frequency-modulated subpicosecond IR pulse, propagating in the negative uniaxial AgGaS2 crystal along the direction of 61°36′ relative to the optical axis. These results can be used in designing a nonlinear optical phase correlator to determine the phase and time profile of the subpicosecond laser pulse in the middle IR range.

Optical information authentication via compressed sensing and double random phase encoding**This work was supported by the Fundamental Research Funds for the Central Universities (N162410002-4, N151904002), the National Natural Science Foundation of China (No. 61374178).

This paper presents a novel information authentication scheme via compressed sensing and double random phase encoding. Two alternative architectures have been investigated, in which significantly compressed data with micro percentage is sufficient for authentication. At the decoder end, a noise-like image with no leakage of the plaintext is recovered and subsequently authenticated using a nonlinear optical correlation approach. The authentication effectiveness, noise resistance and security performance of the proposed scheme have been experimentally validated.

Information authentication using sparse representation of double random phase encoding in fractional Fourier transform domain

This paper presents an information authentication scheme using sparse representation of double random phase encoding in fractional Fourier transform domain. Different from traditional cryptographic applications, only sparse version of the ciphertext is available at the receiver end. The decrypted image is unrecognizable and of no meaningful information of the plaintext, which can enhance the resistance against various attacks. The decrypted image can be authenticated using nonlinear optical correlation approach. Numerical simulations have been carried out, and the results prove the effectiveness and noise resistance of the proposed scheme.

Nonlinear optical phase correlator for determination of the phase of subpicosecond laser pulses in the mid-IR wavelength range

A method is proposed to determine the phase shift of the central frequency of the subpicosecond laser pulse in the mid-IR wavelength range. The method is based on the sum-frequency generation of radiation by two identical linearly polarized laser subpicosecond pumping pulses with the phase difference, propagating in GaSe crystals with a regular domain structure. It is shown that in the field of the subpicosecond IR laser pulse at the central wavelength of 9.6 µm propagating in the GaSe crystal with the regular domain structure with the 216 µm period, the simultaneous quasi-synchronous generation originates of the second, third and fourth harmonics of pumping pulses at the wavelengths 4.8 µm, 3.2 µm and 2.4 µm, respectively. The obtained results can be used to develop the nonlinear optical phase correlator for determination of the phase of subpicosecond laser pulse in the mid-IR wavelength range.

Security-enhanced phase encryption assisted by nonlinear optical correlation via sparse phase

We propose a method for security-enhanced phase encryption assisted by a nonlinear optical correlation via a sparse phase. Optical configurations are established based on a phase retrieval algorithm for embedding an input image and the secret data into phase-only masks. We found that when one or a few phase-only masks generated during data hiding are sparse, it is possible to integrate these sparse masks into those phase-only masks generated during the encoding of the input image. Synthesized phase-only masks are used for the recovery, and sparse distributions (i.e., binary maps) for generating the incomplete phase-only masks are considered as additional parameters for the recovery of secret data. It is difficult for unauthorized receivers to know that a useful phase has been sparsely distributed in the finally generated phase-only masks for secret-data recovery. Only when the secret data are correctly verified can the input image obtained with valid keys be claimed as targeted information.

Optical color-image verification using multiple-pinhole phase retrieval

We propose a multiple-pinhole phase retrieval algorithm to generate an intermediate modulated wavefront for optical color-image verification. Since an incomplete but modulated wavefront is extracted in the intermediate optical path, the reconstructed objects will contain noise-like signals and do not visually render specimen information. Nonlinear optical correlation is further conducted to verify the recovered objects. The performance of the setup parameters and robustness of the proposed method are also illustrated. It is found that pinhole patterns placed in the intermediate optical path not only serve as the constraints for the developed phase retrieval algorithm, but also can provide a channel to modulate noise-like images obtained in the object plane during the recovery.

Real-time two-dimensional imaging in scattering media by use of a femtosecond Cr^4+:forsterite laser

An original femtosecond Cr(4+):forsterite laser source associated with a nonlinear optical correlator was used for imaging through scattering media with 1220-nm light. The system, which operates as an ultrafast optical gate by sum-frequency generation in a nonlinear crystal, was able to detect the light reflected from a resolution chart hidden in a turbid medium, at an attenuation of as much as 15 mean free paths. When the object was illuminated with a collimated beam, real-time two-dimensional images were obtained, with a maximum transverse resolution of ~20 microm.

Pattern recognition with variable discrimination capability by dual non-linear optical correlation

The discrimination capability requirements of pattern recognition systems may vary from one given purpose to another. In this work a recognition system with variable and selective discrimination capability is obtained by applying a dual non-linear correlation (DNC) model to a joint-transform correlator. DNC is obtained by means of two non-linear operators that are applied to both the reference and input channels. A particular DNC is given by the values taken by two real control parameters that determine the non-linear operators. In comparison with conventional filtering methods, an increased and variable discrimination capability is achieved by varying the parameters values. Thus, variable tolerances are introduced in the recognition process. Specifically, tolerances to slight shape variations and intensity variations of the objects (alphabetic characters) are analysed in this work. Ranges for the two control parameters are found in each case in order to achieve either an increase or a relaxation in the system's discrimination capability. The developed application is extended to colour pattern recognition by multichannel correlation. In this case, four further applications with selective discrimination capability are developed: pattern recognition with high discrimination capability for shape variations and some tolerance to colour variations and, vice versa, pattern recognition with high discrimination capability for colour variations and some tolerance to slight shape variations; pattern recognition with high discrimination for both shape and colour, and, finally, a tolerance to slight variations in both shape and colour.

Application of LCTV to nonlinear speckle correlator

The intensity and phase modulation characteristics of a liquid crystal television (LCTV) and its diffraction efficiency as a spatial light modulator are investigated. A nonlinear optical correlator using the phase modulation of an LCTV and the hard clipping property of an Optic RAM detector is proposed. The velocity measurement of a light scattering object is conducted by a real-time speckle clipping correlator based on the proposed method.

Hot carrier relaxation processes and nonequilibrium phonon effect in multiple quantum well structures

We have used a picosecond nonlinear optical correlation technique to investigate the relaxation processes of hot nonequilibrium carriers in multiple quantum well structures. The average energy loss time constant π avg of the hot electrons was measured. By solving the average energy loss rate equation of carriers and the hot phonon rate equation, taking both the phonon emission and reabsorption into account, we show that π avg is the sum of the electron-phonon interaction time constant and the nonequilibrium phonon decay time, assuming only a single mode of phonons are involved. It is found that the temperature will be higher and/or the energy loss rate of the electron system will be slower as the quantum well width decreases, or the excitation density increases, or the degree of the lattice mismatch between the well layer and the barrier layer increases in the case of strained quantum wells.

Synthetic discriminant function-based binary nonlinear optical correlator

We describe a binary nonlinear optical correlator that uses a synthetic discrimination function (SDF) at the input plane to store the reference images. The SDF is displayed at the input plane side by side the input scene. The input scene, the SDF, and their joint power spectrum are thresholded to only two values. Thus, binary spatial light modulators can be used to implement the correlator. The correlation performance of the SDF-based binary nonlinear optical correlator is compared to that of the classical SDF-based correlator in the areas of correlation peak intensity, peak-to-sidelobe ratio, signal-to-noise ratio, and correlation width. We show that the SDF-based binary nonlinear optical correlator performs reasonably well. Furthermore, a single binary SLM can be used to read in sequentially the binarized input signals and the binarized joint power spectrum which results in a reduction in cost, size, and complexity of the system.

Statistical properties and stabilization of a picosecond phosphate-glass laser with 2 Hz repetition frequency

Detailed investigations were made of the statistical relationships between the energy and time parameters of a picosecond phosphate-glass laser operating at a repetition frequency of 2 Hz. This was done using an original measurement system based on a nonlinear-optical correlator, amplitude analyzers, and computers. Optimization of the gain saturation and bleaching of a two-component medium ensured a reproducibility of better than 0.95 in ultrashort pulse generation. The pump energy was reduced to 260 J and the coefficient representing fluctuations of the picosecond pulse energy was lowered to 0.08. The average pulse duration in a train was 8.5 psec. Stimulated Raman scattering produced pulses of τp = 2.6 psec duration. The duration of a single pulse selected from the initial part of a train was 6.2 psec. The spectral Q factor was Δυτp = 0.5, indicating the absence of phase modulation. The high stability of the laser made it possible to use its second harmonic in pumping an optical parametric oscillator in which a crystal of α-HIO3 was used inside the resonator. The energy conversion efficiency was up to 10% for Δυτp = 0.7.