Three-photon excitation ofp-quaterphenyl with a mode-locked femtosecond Ti:sapphire laser

We observed emission fromp-quaterphenyl (p-QT) at 360 nm when exposed to the focused light from a femtosecond (fs) Ti:sapphire laser at 850 nm. This wavelength is too long to allow two-photon excitation of p-QT. The emission intensity of p-QT was found to depend on the cube of the laser power at 850 nm, suggesting that excitation occurs due to a three-photon process. The same emission spectrum and single exponential decay times were observed for three-photon excitation at 850 nm as for two-photon excitation at 586 nm and for one-photon excitation at 283 nm. The same rotational correlation times were observed for one-, two-, and three-photon excitation, but higher time-zero anisotropies were observed for two- and three-photon excitation. The steady-state anisotropies for one-, two-, and three-photon excitation are precisely consistent with cos², cos⁴, and cos⁶ excitation photoselection, where is the angle between the electric field of the incident light and the absorption dipole. These experiments were performed with 3×10^–5 M solutions of p-QT. Use of such low concentrations was possible because p-QT displays one of the highest apparent cross sections we have observed to date for three-photon excitation. The spatial distribution of the excited fluorescence was less for three-photon excitation than for two-photon excitation of Coumarin 102 at the same 850-nm excitation wavelength. The high cross section, photostability, and clear cos⁶ photoselection of p-QT make it an ideal three-photon standard for spectroscopy and microscopy.     

Optimal spectral phase control of femtosecond laser-induced up-conversion luminescence in Sm3+:NaYF4 glass

The spectral phase of the femtosecond laser field is an important parameter that affects the up-conversion(UC)luminescence efficiency of dopant lanthanide ions.In this work,we report an experi-mental study on controlling the UC lmiiinescence efficiency in Sm^3+:NaYF4 glass by 800-nm femtosec-ond laser pulse shaping using spectral phase modulation.The optimal phase control strategy efficiently enhances or suppresses the UC luminescence intensity.Based on the laser-power dependence of the UC luminescence intensity and its comparison with the luminescence spectrum under direct 266-nm fem-tosecond lciser irradiation,we propose herein an excitation model combining non-resonant two-photon absorption with resonance-media ted three-photon absorption to explain the experimental observations.     

Nonresonant and Resonant Nonlinear Absorption of CdSe-Based Nanoplatelets

We present a comprehensive understanding of the nonlinear absorption characteristics of CdSe-based nanoplatelets(NPLs) synthesized by the solution-phase method and the colloidal atomic layer deposition approach through Z-scan techniques at 532 nm with picosecond pulses. The CdSe NPLs exhibit strong two-photon induced free carrier absorption(effective three-photon absorption) upon the nonresonant excitation, resulting in a remarkable optical limiting behavior with the limiting threshold of approximately 75 GW/cm~2. A nonlinear optical switching from saturable absorption(SA) to reverse saturable absorption(RSA) with increasing the laser intensity is observed when coating CdSe NPLs with a monolayer of CdS shell to realize the resonant absorption. The SA behavior originates from the ground state bleaching and the RSA behavior is attributed to the free carrier absorption.These findings explicitly demonstrate the potential applications of CdSe-based NPLs in nonlinear optoelectronics such as optical limiting devices, optical pulse compressors and optical switching devices.     

High sensitivity third-order autocorrelation measurement by intensity modulation and third harmonic detection

We demonstrate a novel (to our knowledge) method for high sensitivity measurement of pulsed light at the telecommunication wavelength of 1558 nm using a GaAsP photomultiplier tube. By modulating the input power and analyzing the harmonic components, high sensitivity third-order autocorrelation is obtained in the wavelength and power regimes where a mixture of two-photon absorption and three-photon absorption in the photomultiplier tube is present.     

Study of the 3s Rydberg state of pyrimidine by multiphoton ionization spectroscopy

The 3s Rydberg state of pyrimidine is seen as a two-photon resonant structure on its three-photon ionization spectrum. The vibrational structure is analyzed and compared with the VUV absorption and photoelectron spectra.     

基于弱非线性实现非破坏性测量两光子Bell态及三光子Greenberger-Horne-Zeilinger态

基于弱非线性及线性光学元件提出非破坏性测量两光子Bell态及三光子 Greenberger-Horne-Zeilinger (GHZ)态方案. 方案中, 首先应用光束分束器及交叉克尔非线性介质对两光子Bell态进行对称性分析, 进而结合控制非门提出三光子分析方案实现对八个三光子GHZ态完全且非破坏性区分. 关键词： Bell态测量 Greenberger-Horne-Zeilinger态测量 弱非线性 量子非破坏性测量     

Multicontrast nonlinear optical microscopy with a compact and rapid pulse shaper

Homodyne detection can dramatically enhance measurement sensitivity for weak signals. In nonlinear optical microscopy it can make accessible a range of novel, intrinsic, contrast like nonlinear absorption and nonlinear phase contrast. Here a compact and rapid pulse shaper is developed, implemented, and demonstrated for homodyne detection in nonlinear microscopy with high-repetition rate mode-locked femtosecond lasers. With this method we generate two-photon absorption (TPA) and self-phase modulation images of gold nanostars in biological samples. Simultaneous imaging of two-photon luminescence and TPA also enables us to produce two-photon quantum yield images.     

Double resonances upon three-photon ionization of the samarium atom

The manifestation of double resonances upon the three-photon ionization of the samarium atom has been studied in a frequency range of 17200–18400 cm⁻¹. Thirty peaks caused by double resonances have been found in the dependence of the yield of Sm⁺ ions on the laser radiation frequency. The majority of these peaks greatly exceed in amplitude the peaks associated with ordinary two-photon resonances. The influence of double resonances on the three-photon ionization has been theoretically analyzed using the density matrix approximation. The calculations show that the probability of the three-photon resonance ionization can increase by more than two orders of magnitude due to the additional one-photon population of an intermediate level. The double resonance slows down the rate of increase in the probability of ionization with an increase in the laser radiation intensity compared to ordinary two-photon resonance.     

咔唑类化合物的光谱特性与三光子激射

测量了一系列新合成的咔唑类化合物的光谱特性。光谱研究结果发现端基团的选择引起双光子吸收截面的巨大变化。用1.3 μm强fs激光激发其中一双光子吸收截面高达617.3 GM的分子溶液,其波长和时间分辨光谱特性表明此分子的高方向性光发射是三光子诱导激射。[1 GM(Gppert-Mayer)=10-50 cm4·s·(photon)-1]。     

Three-photon interference spectra in four-level atomic systems

We have considered the interference spectra that occur at the three-photon generated frequency arising from the interaction of three laser fields with a four-level atom, where two of the laser fields are on two-photon resonance with the three levels forming a “λ” scheme while the third laser operates between the second ground and the second excited state of the atom. At low intensities of all three laser fields, the overall intensity of the peak at the three-photon generated frequency, describing the spectrum of an electron in the second excited state, depends on the strength of the combined field of the two laser fields that are on two-photon resonance and it takes negative values. This indicates that light amplification without population inversion is likely to occur at the three-photon generated frequency. The combined field of the three laser fields induces multiphoton excitations near the three-photon generated frequency, whose peaks are characterized by linewidths which are much less than the natural linewidths of the atoms. These excitations describe absorption or stimulating emission processes depending on the values of the detunings of the laser fields. The derived results are graphically presented and discussed. Received: 24 January 2001 / Published online: 8 June 2001     

Three-Photon Absorption in InAs

Using 125 ps laser pulses, we observe three-photon absorption in room temperature InAs at a wavelength of 9.54 µm. This effect is readily identified by temperature-tuning the semiconductor bandgap through the three-photon absorption edge. A three-photon absorption coefficient of K₃ = 1 ± 0.6 × 10^-3 cm³ MW^-2 is extracted from non-linear absorption data obtained with an open-aperture Z-scan. Time-resolved measurement at high irradiance reveals an increase of absorption due to hot carriers. We also present an autocorrelation measurement of our CO₂ laser pulse that shows two complete optical free-induction-decay cycles.     

Pulse-length dependence of cellular response to intense near-infrared laser pulses in multiphoton microscopes

The influence of the pulse length, tau , of ultrashort laser pulses at 780 and 920 nm on cell vitality and cellular reproduction has been studied. A total of 2400 nonlabeled cells were exposed to a highly focused scanning beam from a mode-locked 80-MHz Ti:sapphire laser with 60-micros pixel dwell time. For the same pulse energy, destructive effects were more pronounced for shorter pulses. The damage behavior was found to follow approximately a P(2)/tau dependence (P , mean power), indicating that cell destruction is likely based on a two-photon excitation process rather than a one- or a three-photon event. Therefore, femtosecond as well as picosecond pulses provide approximately the same relative optical window for safe two-photon fluorescence microscopy.     

Optical Bell state and Greenberger-Horne-Zeilinger-state analyzers through the cavity input-output process

With the ancillary one-sided cavities each trapping an alkali atom, the schemes for the analyzers of two-photon Bell states and three-photon Greenberger-Horne-Zeilinger (GHZ) states are proposed, respectively. Moreover, all of two-photon Bell states and three-photon GHZ states can be nondestructively distinguished. The influence of atomic spontaneous emission and output coupling inefficiency are discussed.     

Nonlinear spectrum effect on the coherent control of molecular systems

This work demonstrates that the detuning of the fs-laser spectrum from the two-photon absorption band of organic materials can be used to reach further control of the two-photon absorption by pulse spectral phase manipulation. We investigate the coherent control of the two-photon absorption in imidazole-thiophene core compounds presenting distinct two-photon absorption spectra. The coherent control, performed using pulse phase shaping and genetic algorithm, exhibited different growth rates for each sample. Such distinct trends were explained by calculating the two-photon absorption probability considering the intrapulse interference mechanism, taking into account the two-photon absorption spectrum of the samples. Our results indicate that tuning the relative position between the nonlinear absorption and the pulse spectrum can be used as a novel strategy to optimize the two-photon absorption in broadband molecular systems.     

Three-photon-induced fluorescence of diphenylhexatriene in solvents and lipid bilayers

We observed the emission of l,6-diphenyl-l,3,5-hexatriene (DPH) when excited with the fundamental output of a fs Ti:sapphire laser at 860 nm. The emission spectra of DPH were identical to that observed for one-photon excitation at 287 nm. The dependence of the DPH emission intensity on laser power was cubic, indicating three-photon excitation of DPH at 860 nm. At a shorter wavelength of 810 nm, the dependence on laser power was quadratic, indicating a two-photon process. At an intermediate wavelength of 830 nm the mode of excitation was a mixture of two- and three-photon excitation. At 830 nm the anisotropy is no longer a molecular parameter, and the mode of excitation and anisotropy of DPH depends on laser power. Frequency-domain anisotropy decays of DPH in triacetin revealed the same rotational correlation times for two- and three-photon excitation. However, the time 0 anisotropy of DPH was larger for three-photon excitation than for two-photon excitation. Steady-state anisotropy data for DPH-labeled membranes revealed the same transition temperature for one- and three-photon excitation. These anisotropy data indicate that membrane heating was not significant with three-photon excitation and that three-photon excitation may thus be of practical usefulness in fluorescence spectroscopy and microscopy of membranes.     

Sm3+离子的三光子荧光现象

本文报导了1.06μm脉冲激光导致的Sm3+的三光子荧光现象,解释了观察到的4G5/2的荧光是6H5/2→6F9/2的单光子吸收经无辐射弛豫在6F1/2上集聚了较大的布居,然后由此经直接双光子吸收跃迁至6P3/2,再经无辐射弛豫到达4G5/2能级,形成的发光过程.     

Spontaneous Raman scattering upon resonant three-photon ionization of a samarium atom

The manifestation of spontaneous Raman scattering upon the three-photon ionization of a samarium atom has been investigated. The dependence of the Sm⁺ yield on the laser frequency, along with the peaks due to the two-photon excitation of bound states from different levels of the 4f ⁶6s ^{2 7} F ground term, exhibits a strong peak due to single-photon excitation of the 4f ⁵5d6s ^{2 7} D°₃ state from the 4f ⁶5d6s ⁹ H ₂ excited state. The 4f ⁶5d6s ⁹ H ₂ state was populated as a result of spontaneous Raman scattering. The large amplitude of the observed peak indicates a high efficiency of this process. The estimations performed indicate that the probability of resonant ionization through an excited state populated due to spontaneous Raman scattering can be comparable with the probability of three-photon ionization through the two-photon intermediate resonance. The necessary conditions for implementing this channel of resonant three-photon ionization are formulated.     

Pulse width measurement of high-order harmonics by autocorrelation

Pulse widths of the 7th and 9th harmonics of a Ti:sapphire (TiS) laser have been measured by an autocorrelation technique using two-photon and three-photon ionization of He and Ar as nonlinear phenomena. The pulse widths of the 7th and 9th harmonics were 47 fs and 27 fs, respectively, where the pulse width of the fundamental pulse was 34 fs. The pulse width of the 9th harmonic depends strongly on the peak intensity of the fundamental pulse. These results can be explained by quantum theory by taking into account the temporal and spatial distribution of the fundamental beam. Received: 15 June 1999 / Revised version: 8 October 1999 / Published online: 2 February 2000     

Two-photon absorption and two-photon induced absorption of liquid toluene at 347.15 nm

Pump pulse transmission and time-delayed probe pulse transmission measurements through liquid toluene were performed with linear and circular polarized second harmonic pulses of a mode-locked ruby laser system. Two-photon absorption and two-photon induced absorption are observed. The induced absorption anisotropy is investigated. A theory of two-photon absorption in isotropic media is presented. The two-photon absorption cross-section components, the effective excited-state absorption cross-section, the absorption anisotropy relaxation time and the excited singlet-state relaxation time are determined.     

Modeling of nonlinear responses in BK7 glass under irradiation of femtosecond laser pulses

In this paper, the nonlinear response in BK7 glass sample is investigated under irradiation of 200 fs laser pulses at 800 nm wavelength by using of transmission measurement method. Nonlinear transmission is observed for incident pulse energies above 37μJ. A theoretical model based on simultaneously absorption of one, two and three-photon absorption is presented. The contribution of free-carriers absorption is also included on the model. The differential equation is solved analytically. The experimental results and predictions of theoretical model are in good agreement. The values of three-photon absorption coefficient and free-carriers absorption cross-section are reported.