Role of doubled-frequency absorption in two-photon response of the Si MSM structure

The physical mechanism of two-photon response was studied in this paper by measuring characteristics of the two-photon response of the Si metal–semiconductor–metal (MSM) structure sample. The two-photon response includes two-photon absorption (TPA) and doubled-frequency absorption (DFA). An experiment was designed to measure the photocurrent dependence on incident light power, the dependence of the photoelectric signal on the applied voltage and the relationship between the photoelectric current and the light-spot position. The experimental fact that two-photon response of the silicon sample is relative to the applied electric field shows that DFA is the main physical mechanism of two-photon response and establishes the foundation for fabricating high-sensitivity two-photon response Si photodetector.     

Fluorescence upconversion properties of a class of improved pyridinium dyes induced by two-photon absorption

We report the fluorescence upconversion properties of a class of improved pyridinium toluene-p-sulfonates having donor–π–acceptor (D–π–A) structure under two-photon excitation at 1064 nm. The experimental results show that both the two-photon excited (TPE) fluorescence lifetime and the two-photon pumped (TPP) energy upconversion efficiency were increased with the enhancement of electron-donating capability of the donor in the molecule. It is also indicated that an overlong alkyl group tends to result in a weakened molecular conjugation, leading to a decreased two-photon absorption (TPA) cross section. By choosing the donor, we can obtain a longest fluorescence lifetime of 837 ps, a highest energy upconversion efficiency of 6.1%, and a maximum TPA cross-section of 8.74×10⁻⁴⁸ cm⁴ s/photon in these dyes.     

Two-photon and two-photon-assisted slow light

We show that light pulses propagating in two-photon absorbing systems may present time delays like slow light produced via coherent population oscillations in one-photon interactions. Two regimes are numerically studied for a simplified two-level system: (a) a light pulse at frequency ω/2 undergoes two-photon absorption (TPA) and is delayed by the absorbing system (two-photon slow light) and (b) a light pulse at frequency ω is delayed in a system prepared by TPA of a light pulse at frequency ω/2 (two-photon-assisted slow light). The study carried out in solutions of dyes and dendrites shows significant delays, low distortion, and good transmission for easily reachable experimental conditions. The working principle can be applied to other media and can be used in telecommunications technology.     

The aggregation effects on the two-photon absorption properties of para-nitroaniline polymers by hydrogen-bond interactions

This paper has theoretically designed a series of aggregate polymers on the basis of several para-nitroaniline monomers by hydrogen-bond interactions. At the level of time-dependent hybrid density functional theory, it has optimized their geometrical structures and studied their two-photon absorption (TPA) properties by using analytical response theory. The calculated results exhibit that the aggregation effects not only bring out the considerable red shift of the excited energies but also greatly enhance the TPA intensities of the aggregate polymers in comparison with the para-nitroaniline monomer. The aggregate configurations also have an important influence on the TPA abilities of the polymers; the trimer has the largest TPA cross section. The electron transitions between the molecular orbits involving the strong TPA excitations of the trimer are depicted to illuminate the relationship between the intermolecular charge transfer and the TPA property.     

Theoretical studies on the one- and two-photon absorption properties of azulenylporphyrins and azulene-fused porphyrins

The electronic structures, one-photon absorption (OPA) and two-photon absorption (TPA) properties of the azulenylporphyrins and azulene-fused porphyrins have been comparatively studied by using DFT/B3LYP/6-31G(d) and the ZINDO/SDCI method. With the number of azulenyl groups increasing, the OPA wavelengths of all molecules are red-shifted in 400--600~nm and the two-photon absorption cross section is gradually enlarged. The azulene-fused structures facilitate an expanding conjugated area and increasing TPA cross section. The origin of TPA properties of studied compounds is studied with a two-level model. In summary, the azulene-fused porphyrins exhibit strong two-photon absorption.     

两个新合成化合物EPVPC和OPVPC非线性光学特性的理论研究

在杂化密度泛函理论（DFT/B3LYP）的水平上，研究了最新实验室合成的两个化合物分子9-乙烷基-3-{2-[4-2-吡啶-4-乙烯基苯]-乙烯基}-9氢咔唑（EPVPC）和9-十八烷基-3-{2-[4-2-吡啶-4-乙烯基苯]-乙烯基}-9氢咔唑（OPVPC）的非线性光学特性。理论结果与现有的实验测量结果符合得的较好。利用扩展的少态模型方法计算了分子的双光子吸收截面，结果表明三态模型可以很好地描述它们的最大双光子吸收截面。数值模拟显示这两个化合物都具有较大的双光子吸收截面，并且，在低频范围内，OPVPC分子比EPVPC分子显示出较强的双光子吸收特性。     

Microfabrication of gold dots in SiO2/TiO2 glass films by two-photon absorption

Photogeneration of Au nanoparticles in SiO₂/TiO₂ glass films was carried out by two-photon absorption with a femtosecond pulse laser. Exquisite microdot-arrays of Au with micrometer spatial resolution were achieved by scanning of the focused laser beam. These structures were constructed in SiO₂/TiO₂ glass films by a sol–gel method. The sol–gel method demonstrated that Au dots microarray are fabricated at any position by two-photon absorption in the glass. The results show the utility of a two-photon absorption technique in the fabrication of complicated patterns with metal particles.     

Z-scan analytical theory for material with saturable absorption and two-photon absorption

By using the Adomian decomposition method, we present the transmitted optical intensity of a nonlinear medium with the concurrence of saturable absorption (SA) and two-photon absorption (TPA) processes. We obtain the analytical expression of the open-aperture Z-scan and investigate the interplay between SA and TPA in the Z-scan trace. Through Z-scan measurements at different levels of laser intensities, both the saturable intensity and TPA coefficient could be obtained quickly yet unambiguously.     

Theory of two-photon photothermal deflection spectroscopy in stationary and flowing media for arbitrary optical pulse length

A general theory of pulsed two-photon photothermal deflection spectroscopy (PTDS) is presented. We find that there are significant enough differences in the amplitude and temporal evolution of PTDS signals between the results of the single- and two-photon theories that if one tries to interpret two-photon data with single-photon theory, the extracted values may be considerably in error. Our theory is sufficiently general that it incorporates both stationary and flowing media and considers optical pulses of arbitrary length. Moreover, the temporal profile of the optical pulse is explicitly taken into account. The two-photon absorption coefficient is explicitly expressed in terms of oscillator strengths and Clebsch–Gordan coefficients, and the Doppler width for both co-propagating and counter-propagating beams is taken into account. Although the theory is primarily developed for atomic and molecular vapors, it can easily be adapted for condensed matter by expressing the absorption coefficient in terms of the properties of the liquid or solid under investigation. PACS 82.80.Kq; 42.62.Fi; 39.30.+w     

9,10-Anthracene-centered oligo(p-phenyleneethynylene)s with end-capping didecylamines exhibiting enhanced two-photon fluorescence action cross-section

This study reports the synthesis and the two-photon absorption (TPA) properties of a series of 9,10-anthracene-centred oligo(p-phenyleneethynylene)s with end-capping didecylamine. The results show that both the maximal TPA cross-sections (δ_max) and the fluorescence quantum yields (Φ) of the oligomers increase with the extension of phenylethynyl bridges. Interestingly, the Φδ_max per repeating unit to is obviously enhanced by extending linearly conjugation size of the oligomers. Moreover, these oligomers all show the maximal two-photon absorption cross-sections at 800 nm and emit strong green to yellowish-green two-photon excited fluorescence. These materials may find applications in which two-photon excited fluorescence and photo-induced electron transfer are utilized.     

Responsive mechanism and molecular design of di-2-picolylamine-based two-photon fluorescent probes for zinc ions

The properties of one-photon absorption(OPA), emission and two-photon absorption(TPA) of a di-2-picolylaminebased zinc ion sensor are investigated by employing the density functional theory in combination with response functions.The responsive mechanism is explored. It is found that the calculated OPA and TPA properties are quite consistent with experimental data. Because the intra-molecular charge transfer(ICT) increases upon zinc ion binding, the TPA intensity is enhanced dramatically. According to the model sensor, we design a series of zinc ion probes which differ by conjugation center, acceptor and donor moieties. The properties of OPA, emission and TPA of the designed molecules are calculated at the same computational level. Our results demonstrate that the OPA and emission wavelengths of the designed probes have large red-shifts after zinc ions have been bound. Comparing with the model sensor, the TPA intensities of the designed probes are enhanced significantly and the absorption positions are red-shifted to longer wavelength range. Furthermore, the TPA intensity can be improved greatly upon zinc ion binding due to the increased ICT mechanism. These compounds are potential excellent candidates for two-photon fluorescent zinc ion probes.     

4,4''-二甲氨基二苯乙烯双光子吸收理论研究--溶剂效应

对最近实验室合成的分子材料 4 ,4 ' 二甲氨基二苯乙烯的双光子吸收特性在从头计算的基础上进行了理论研究。理论模型是建立在密度泛函理论的基础上的。利用含时的密度泛函理论来计算分子的非线性光学性质 ,而溶剂效应则通过自洽响应场方法的极化连续模型来模拟。计算结果表明 ,三态模型可以很好地给出该分子在低激发态范围内的双光子吸收截面。随着溶剂极性的增加 ,单光子波长红移 ,双光子吸收截面增加。双光子吸收截面的大小和实验给出的结果符合得较好。     

Polarization-dependent carrier dynamics in a passive InAs/InP quantum dot waveguide

We investigate carrier dynamics in a passive InAs/InP quantum dot (QD) waveguide using 255 fs optical pulses at a central wavelength of 1568 nm. We observe strong anisotropy of absorption saturation for different polarizations. Pump-probe measurements indicate the presence of carrier relaxation dynamics on a timescale in the order of tens of picoseconds due to cascaded relaxation of carriers generated by two-photon absorption (TPA) from the bulk region to the QDs via the wetting layer. These relaxation timescales are much longer than in QD amplifiers. Our observations are supported by a rate-equation model which includes TPA, showing good agreement with the pump-probe measurements.     

Transparency due to dipole–dipole interaction in photonic crystals doped with nanoparticles

In this paper, we have studied the two-photon absorption process in photonic crystals doped with four-level nanoparticles. It is considered that all the levels of the particles are interacting with the photonic crystal. The decay linewidths of all the four levels are included in the calculations of two-photon absorption process. The effect of the dipole–dipole interaction has also been included in the formulation. Numerical simulations have been performed for the two-photon absorption spectrum on an isotropic photonic crystal. It is found that the system switches between the transparent and the nontransparent states due to the dipole–dipole interaction. The phenomenon of switching depends on the linewidths of these states.     

Two-photon induced optical-power limiting and upconverted superradiance properties of a new organic dye HEASPI

A new organic dye trans-4-[p-(N-ethyl-N-hydroxyethylamino)styryl]-N-methylpyridinium iodide (abbreviated as HEASPI hereafter) with large two-photon absorption (TPA) cross section and excellent upconverted superradiance properties was synthesized in our group recently. The TPA cross section was measured to be σ₂′=7.0×10⁻⁴⁸ cm⁴ s/photon by using an open aperture Z-scan system. Linear absorption, single-photon induced fluorescence, two-photon induced fluorescence and two-photon pumped (TPP) upconverted superradiance properties were systematically studied. The highest net upconversion efficiency from the absorbed pump energy to the output upconverted superradiance energy is as high as 19.6% at the pump energy of 2.07 mJ from a mode-locked Nd : YAG picosecond laser. The dye solution also shows a clear optical power limiting effect.     

有机染料HEASPS在不同波长时的激发态吸收和双光子吸收性质研究

提出了有机染料反式 4 [4′ (N 羟乙基 N 乙基胺基 )苯乙烯基 ] N 甲基吡啶对甲苯磺酸盐 (简称HEASPS)非线性吸收的三态模型。认为在较短波长 (<1 0 0 0nm)时 ,双光子吸收和激发态吸收同时存在。成功地解释了该染料双光子吸收峰相对线性吸收峰的两倍的明显蓝移 ,以及最高上转换效率波长相对最强吸收波长的明显红移。提出了一种新的计算方法 ,利用不同波长的非线性透过率和不同波长的上转换效率 ,求出了该染料的双光子吸收截面和激发态吸收系数。     

Two-photon absorption properties of D-A-D type chromophores containing tetraphenylethylene and triphenylamine moieties: a close look at the effects of the strength,position and number of donors

The two-photon absorption (TPA) properties of a series of D-A-D type chromophores containing tetraphenylethylene (TPE) and triphenylamine moieties have been investigated employing the density functional response theory in combination with the polarisable continuum model. Special emphases are placed on the effects of the strength, position and number of donors on TPA. Our calculations demonstrate that the TPA intensity enhances with the increase of the electron-donating ability of the substituents on the triphenylamine unit, which is consistent with experimental observations. It is shown that the position of donors has an important effect on TPA. The donor on the side position of TPE unit decreases the TPA cross section, thereby more donors could not necessarily produce stronger TPA. The cis-trans isomerism effect is also explored and it is found that all the cis structures have lower TPA intensities than those of corresponding trans forms. By considering a different number of donors on TPE and triphenylamine unit respectively, it is interesting to find that the structure with one terminal donor on TPE and two donors on triphenylamine unit has the largest TPA cross section. The few-state model and the natural bond orbital charge analysis are used to explore the reason behind it.     

Two-photon absorption and laser photolysis of trans-stilbene substitutes

The measured the two-photon absorption (TPA) cross sections of ethanol solution of trans-stilbene and its two substitutes excited by nanosecond radiation of the 2nd harmonic of a Nd:YAG laser (532 nm) are presented. To determine the TPA cross section, a method based on measurements of photochemical decomposition of molecules of the examined compounds having low photostability is suggested. The measured value of the TPA cross section of trans-stilbene in ethanol is found to be 25 GM, and the TPA cross sections of the trans-stilbene substitutes are found to be 60 and 70 GM. The TPA cross section of trans-stilbene substitutes obtained in the present paper is compared with the experimental and theoretical data obtained by other authors. The results of investigations demonstrate that the photochemical decomposition method can be successfully used to determine experimentally the two-photon absorption cross section of the compounds unstable under photoexcitation, including photochromatic molecules, biological objects, and radical photoinitiators.     

All-optical logic gates based on two-photon absorption in semiconductor optical amplifiers

When the two-photon absorption of a high intensity pump beam takes place in a semiconductor optical amplifier there is an associated fast phase change of a weak probe signal. A scheme to realize fast all-optical XOR logic function using two-photon absorption induced phase change has been analyzed. Rate equations for semiconductor optical amplifiers, for input data signals with high intensity, configured in the form of a Mach–Zehnder interferometer has been solved. The input intensities are high enough so that the two-photon induced phase change is larger than the regular gain induced phase change. The model shows that both XOR operation and pseudo-random binary sequence generation at 250 Gb/s with good signal to noise ratio is feasible.     

Efficient two-photon absorption of CdSe-CdS/ZnS core-multishell quantum dots under the excitation of near-infrared femtosecond pulsed laser

We report that two-photon absorption (TPA) properties of semiconductor CdSe-core CdS/ZnS-multishell quantum dots (QDs) in toluene under excitation of femtosecond laser at 800 nm. The results show efficient TPA process and large TPA cross section of three types of size QDs, which is 1900, 5710, and 16060 GM (1 GM = 10⁻⁵⁰ cm⁴ s photon⁻¹), respectively. TPA cross section dramatically increases with increased core size, showing a strong size-dependence effect. Furthermore, two-photon excitation (TPE) fluorescence intensity not only depends on TPA capacity, but also relies on improved quantum yield resulting from passivation of QD surface by different coated monolayers (MLs). These facts in combination with the narrow fluorescence bandwidth make these QDs as promising probes for multicolor two-photon microscopy.