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<jats:p>Owing to the importance of excited state dynamical relaxation, the excited state intramolecular proton transfer (ESIPT) mechanism for a novel compound containing dual hydrogen bond (abbreviated as “1-enol”) is studied in this work. Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) method, the experimental electronic spectra can be reproduced for 1-enol compound. We first verify the formation of dual intramolecular hydrogen bonds, and then confirm that the dual hydrogen bond should be strengthened in the first excited state. The photo-excitation process is analyzed by using frontier molecular orbital (HOMO and LUMO) for 1-enol compound. The obvious intramolecular charge transfer (ICT) provides the driving force to effectively facilitate the ESIPT process in the S<jats:sub>1</jats:sub> state. Exploration of the constructed S<jats:sub>0</jats:sub>-state and S<jats:sub>1</jats:sub>-state potential energy surface (PES) reveals that only the excited state intramolecular single proton transfer occurs for 1-enol system, which makes up for the deficiencies in previous experiment.</jats:p>

<jats:p>According to the frequency-domain theory, we investigate the asymmetric structure of above-threshold ionization (ATI) spectrum of an atom in two-color elliptically polarized (EP) laser fields. When both laser fields are linearly polarized (LP), the spectrum shows that the multi-plateau structure is symmetric about the emitted angle of electron at <jats:italic>π</jats:italic>/2, while the spectrum becomes asymmetric and shifts rightwards with the increase of the EP degree of the IR laser field. Since the total ATI process is regarded as including direct ATI and the rescattering ATI, we analyze the spectrum structure of direct ATI and rescattering ATI separately. Using the saddle-point approximation, we find that for direct ATI, the fringes on the spectrum are mainly attributed to the fact that the ionization probability becomes very small when the direction of emitted electrons is perpendicular to the direction of the XUV laser polarization; while for the rescattering ATI, the interference fringes on the spectrum mainly come from the superposition of the waist structures on the spectra of all sub-channels.</jats:p>

<jats:p>The polarization properties of high-order harmonic generation (HHG) in the two-color circularly polarized laser fields are investigated by numerically solving the two-dimensional time-dependent Schrödinger equation. By adding a wavelength of 1600-nm right-circular-polarized field to an 800-nm left-circular-polarized field, HHG is simulated from a real model of neon atom with <jats:italic>p</jats:italic> orbital, but not from a hydrogen-like atom model with <jats:italic>s</jats:italic> orbital. The orders of 3<jats:italic>n</jats:italic>+1 can be selected while the orders of 3<jats:italic>n</jats:italic>+2 are suppressed by adjusting the intensities of the two pulses. The physical mechanism is analyzed by time–frequency analysis and semiclassical model.</jats:p>

<jats:p>We theoretically investigate the high-order harmonic generation (HHG) of helium atom driven by bichromatic counter-rotating circularly polarized laser fields. By changing the intensity ratio of the two driving laser fields, the spectral chirality of the HHG can be controlled. As the intensity ratio increases, the spectral chirality will change from positive- to negative-value around a large intensity ratio of the two driving fields when the total laser intensity keeps unchanged. However, the sign of the spectral chirality can be changed from positive to negative around a small intensity ratio of the two driving fields when the total laser intensity changes. At this time, we can effectively control the helicity of the harmonic spectrum and the polarization of the resulting attosecond pulses by adjusting the intensity ratio of the two driving laser fields. As the intensity ratio and the total intensity of the driving laser fields increase, the relative intensity of either the left-circularly or right-circularly polarized harmonic can be enhanced. The attosecond pulses can evolve from being elliptical to near linear correspondingly.</jats:p>

<jats:p>We present a quantum secure imaging (QSI) scheme based on the phase encoding and weak+vacuum decoy-state BB84 protocol of quantum key distribution (QKD). It allows us to implement a computational ghost imaging (CGI) system with more simplified equipment and reconstructed algorithm by using a digital micro-mirror device (DMD) to preset the specific spatial distribution of the light intensity. What is more, the quantum bit error rate (QBER) and the secure key rate analytical functions of QKD are used to see through the intercept-resend jamming attacks and ensure the authenticity of the imaging information. In the experiment, we obtained the image of the object quickly and efficiently by measuring the signal photon counts with a single-photon detector (SPD), and achieved a secure key rate of 571.0 bps and a secure QBER of 3.99%, which is well below the lower bound of QBER of 14.51%. Besides, our imaging system uses a laser with invisible wavelength of 1550 nm, whose intensity is as low as single-photon, that can realize weak-light imaging and is immune to the stray light or air turbulence, thus it will become a better choice for quantum security radar against intercept-resend jamming attacks.</jats:p>

<jats:p>The second-order interference of two independent photons with different spectra in a Shih–Alley/Hong–Ou–Mandel interferometer is studied in Feynman’s path integral theory. There is a second-order interference pattern for photons with different spectra if the photons are indistinguishable for the employed detection system. The conditions to observe the second-order temporal beating with photons of different spectra are analyzed. The influence of the response time of the detection system on the observed second-order interference pattern is also discussed. It is a direct result of that measurement in quantum mechanics is dependent on the employed measuring apparatus. The results are helpful to understand the physics of two-photon interference in different schemes.</jats:p>

<jats:p>A polymer waveguide Y-branch power splitter with loss compensation is proposed based on NaYF<jats:sub>4</jats:sub>:Er<jats:sup>3+</jats:sup>, Yb<jats:sup>3+</jats:sup> nanocrystals prepared by a high temperature thermal decomposition method. The Y-branch power splitter is designed as a structure of embedded waveguide, and its core material is nanocrystals-doped SU-8. The insertion loss of the device is ∼15 dB. For an input signal power of 0.05 mW and a pump power of 267.7 mW, the two branches with 5.81-dB and 5.41-dB loss compensations at 1530 nm are achieved respectively. A polymer waveguide Y-branch power splitter with loss compensation has an important research significance.</jats:p>

<jats:p>By adjusting the waveguide length ratio, we study the extraordinary characteristics of electromagnetic waves propagating in one-dimensional (1D) parity-time-symmetric (<jats:inline-formula> <jats:tex-math><?CDATA ${PT}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic">PT</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_104208_ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>-symmetric) two-segment-connected triangular optical waveguide networks with perfect and broken integer waveguide length ratios respectively. It is found that the number and the corresponding frequencies of the extremum spontaneous <jats:inline-formula> <jats:tex-math><?CDATA ${PT}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic">PT</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_104208_ieqn4.gif" xlink:type="simple" /> </jats:inline-formula>-symmetric breaking points are dependent on the waveguide length ratio. Near the extremum breaking points, ultrastrong extraordinary transmissions are created and the maximal can arrive at, respectively, <jats:inline-formula> <jats:tex-math><?CDATA $2.4079\times {10}^{14}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2.4079</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>14</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_104208_ieqn5.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math><?CDATA $4.3555\times {10}^{13}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>4.3555</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>13</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_104208_ieqn6.gif" xlink:type="simple" /> </jats:inline-formula> in both kinds of networks. However, bidirectional invisibility can only be produced by the networks with broken integer waveguide length ratio, whose mechanism is explained in detail from the perspective of photonic band structure. The findings of this work can be useful optical characteristic control in the fabrication of <jats:inline-formula> <jats:tex-math><?CDATA ${PT}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic">PT</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_104208_ieqn7.gif" xlink:type="simple" /> </jats:inline-formula>-symmetric optical waveguide networks, which possesses great potential in designing optical amplifiers, optical energy saver devices, and special optical filters.</jats:p>

<jats:p>The extinction ratio and insertion loss of spatial light modulator are subject to the material problem, thus limiting its applications. One reflection-type silicon-based spatial light modulator with high reflective materials outside the Fabry–Perot cavity is demonstrated in this paper. The reflectivity values of the outside-cavity materials with different film layer numbers are simulated. The reflectivity values of 6-pair Ta<jats:sub>2</jats:sub>O<jats:sub>5</jats:sub>/SiO<jats:sub>2</jats:sub> films at 1550 nm are experimentally verified to be as high as 99.9%. The surfaces of 6-pair Ta<jats:sub>2</jats:sub>O<jats:sub>5</jats:sub>/SiO<jats:sub>2</jats:sub> films are smooth: their root-mean-square roughness values are as small as 0.53 nm. The insertion loss of the device at 1550 nm is only 1.2 dB. The high extinction ratio of the device at 1550 nm and 11 V is achieved to be 29.7 dB. The spatial light modulator has a high extinction ratio and low insertion loss for applications.</jats:p>

<jats:p>Dithering optimization techniques can be divided into the phase-optimized technique and the intensity-optimized technique. The problem with the former is the poor sensitivity to various defocusing amounts, and the problem with the latter is that it cannot enhance phase quality directly nor efficiently. In this paper, we present a multi-objective optimization framework for three-dimensional (3D) measurement by utilizing binary defocusing technique. Moreover, a binary patch optimization technique is used to solve the time-consuming issue of genetic algorithm. It is demonstrated that the presented technique consistently obtains significant phase performance improvement under various defocusing amounts.</jats:p>