Multiphoton Absorption Research Articles

Frequency‐Upconverted Stimulated Emission by Up to Six‐Photon Excitation from Highly Extended Spiro‐Fused Ladder‐Type Oligo(p‐phenylene)s

Frequency-upconverted fluorescence and stimulated emission induced by multiphoton absorption (MPA) have attracted much interest. As compared with low-order MPA processes, the construction of high-order MPA processes is highly desirable and rather attractive, yet remains a formidable challenge due to its inherent low transition probability. We report the observation of the first experimental frequency-upconverted fluorescence and stimulated emission by simultaneous six-photon excitation in an organic molecular system. The well-designed organic conjugated system based on cross-shaped spiro-fused ladder-type oligo(p-phenylene)s (SpL-z, z=1-3) manifests reasonably high MPA cross-sections and brilliant luminescence emission simultaneously. The six-photon absorption cross-section of SpL-3 with an extended π-conjugation was evaluated as 8.67×10-169 cm12 s5 photon-5 . Exceptionally efficient 2- to 6-photon excited stimulated emission was achieved under near-infrared laser excitation.

Highly efficient THz generation by optical rectification of mid-IR pulses in DAST

We report on efficient THz generation in DAST by optical rectification of intense mid-IR pulses centered at (i) 3.9 μm and (ii) its second harmonic at 1.95 μm. Suppression of multi-photon absorption shifts the onset of saturation of the THz conversion efficiency to pump energy densities, which are almost an order of magnitude higher as compared to conventional pump schemes at 1.5 μm. Despite strong linear absorption at 3.9 μm, DAST exhibits a high optical-to-THz conversion efficiency, which we attribute to resonantly enhanced nonlinearity and advantageous phase matching of the THz phase velocity and group velocity of the driving pulse. At 1.95 μm, we find that low linear and multi-photon absorption in combination with cascaded optical rectification lead to record optical-to-THz conversion efficiencies approaching 6%. The observed high sensitivity of the THz generation to the parameters of the mid-IR driving pulses motivates an in-depth study of the underlying interplay of nonlinear wavelength- and intensity-dependent effects.

Single pulse development of chromium-deposited imprint micro/nano patterns of photo-cured crosslinked resin using a femtosecond pulsed laser

Exposure to a single pulse generated from a femtosecond pulsed laser for a duration of 100 fs at a wavelength of 800 nm and a laser fluence of 0.31 J cm−2 resulted in chromium (Cr) deposition on photocured insoluble imprint resin patterns to be ablated selectively; however, Cr deposition on synthetic quartz surfaces was retained. Single-pulse development of Cr-deposited imprint micro/nano patterns occurred at the same laser fluence, independent of the presence of a laser absorbing dye. Laser ablation of the cured resin near the Cr deposition was confirmed by atomic force microscopy. Depth profile analyses confirmed that Cr species were partially embedded in the cured resin underneath the layer of Cr. These results suggest that the mechanism of laser ablation was due to the rapid thermal expansion of resin near the embedded Cr species and deposited Cr layer rather than the multi-photon absorption of the cured resin.

Two novel thiophene derivatives based on chalcone skeleton:a systematic study of ultrafast nonlinear absorption and excited-state dynamics

Two thiophene derivatives(TH-17 and TH-19) were synthesized and the ultra-fast nonlinear optical (NLO) performance was investigated using z-scan technique. Both compounds exhibit NLO response of reverse saturated absorption(RSA) in wide wavelength range (560 nm–800 nm). Intrinsic multi-photon absorption is considered to cause performance. The mechanism of two-photon absorption(TPA) and TPA induced excited-state absorption (TPA-ESA) was introduced to explain the progress. Several primary nonlinear coefficients, including the two-photon absorption(TPA) coefficients and TPA cross section, were calculated. TH-19 displays superior NLO response than TH-17 at all wavelengths due to better electron delocalization and the TPA coefficient in 10−12(m/w) order of magnitude at 560 nm is obtained. The NLO performance is also better than several anthracene derivatives and chalcone derivatives with similar molecular structure reported recently. The substituted terminal thiophene is proved to improve the NLO properties due to the enhancement of π-electron delocalization. Moreover, the excited-state absorption was investigated via femtosecond transient absorption experiment. Intense excited-state absorption at the region of 550 nm–750 nm is observed according to the transient absorption(TA) spectrum. The kinetic processes are analyzed and four relaxation processes(TH-19) of 0.25 ps, 4.5 ps, 188ps and 2.43ns are obtained. The enhanced NLO response and changing region of excited-state absorption indicates the fine-tuned performance of NLO response via the change of terminal substituents.

Dissipative soliton dynamics and switching in split ring resonator based metamaterial with multi-photon absorption and diffusion

Compound dissipative soliton (DS) is obtained theoretically in Split Ring Resonator based metamaterial having higher order nonlinearities, multi-photon absorption and diffusion. Effects of both the multi-photon absorption and diffusion on the DS are found to be detrimental however can be compensated by a suitable external gain to stabilize the DS. The interaction dynamics of two DSs is studied for different phase difference and initial separation. The phase controlled soliton switching is demonstrated. Notably, a weak solitonic beam is demonstrated to control a strong one. The DSs are robust against the randomness of the metamaterial. The outcome of this investigation may be useful to design experiments that employ DS in higher order nonlinear metamaterials.

Near-infrared two-photon absorption upconversion of PbS/CdS quantum dots prepared by cation exchange method

Taking advantage of multi-photon absorption, the semiconductor quantum dots show great potential in the field of bio-logical fluorescence imaging. In this paper, PbS/CdS core-shell quantum dots with the band alignment adjustable from type-I to quasi-type II are prepared through the cation exchange method. The core-shell quantum dots exhibit bright broad-band visible/near-infrared upconversion emission upon the excitation of 1064 nm pulse laser, and the core-shell QDs hold a decay lifetime of hundreds of nanoseconds. These characteristics enable the present PbS/CdS core-shell quantum dots to be a promising candidate for biomedical fluorescence imaging.

Realization of flexible and parallel laser direct writing by multifocal spot modulation.

In this investigation, we propose a strip segmentation phase (SSP) method for a spatial light modulator (SLM) to generate independent multifocal spots when the beam passes through a high numerical aperture (NA) lens. With the SSP method, multifocal spots can be generated with each spot independently, flexibly and uniformly distributed. The performance of the SSP method is first validated with numerical simulation. Then, by applying the modulation method with SLM and importing the beams into an inverted fluorescence microscopy system with a high-NA lens, the spot distribution and their shapes can be observed by fluorescent image. The fluorescent image exhibits high uniformity and high consistency with the aforementioned numerical simulations. Finally, we dynamically load a series of phase maps on SLM to realize continuous and independent spot movement in a multifocal array. By laser direct writing on photoresist, a complex NWU-shape structure can be realized flexibly with multi-task fabrication capability. The SSP method can significantly improve the efficiency and flexibility of laser direct writing. It is also compatible with most recent techniques, e.g., multiphoton absorption, stimulated emission depletion and photo-induced depolymerization etc., to realize parallel super-resolution imaging and fabrications.

Cavity molecular dynamics simulations of vibrational polariton-enhanced molecular nonlinear absorption

Recent experiments have observed that the chemical and photophysical properties of molecules can be modified inside an optical Fabry-Pérot microcavity under collective vibrational strong coupling (VSC) conditions, and such modification is currently not well understood by theory. In an effort to understand the origin of such cavity-induced phenomena, some recent studies have focused on the effect of the cavity environment on the nonlinear optical response of the molecular subsystem. Here, we use a recently proposed protocol for classical cavity molecular dynamics simulations to numerically investigate the linear and the nonlinear response of liquid carbon dioxide under such VSC conditions following an optical pulse excitation. We find that applying a strong pulse of excitation to the lower hybrid light-matter state, i.e., the lower polariton (LP), can lead to an overall molecular nonlinear absorption that is enhanced by up to two orders of magnitude relative to the excitation outside the cavity. This polariton-enhanced multiphoton absorption also causes an ultrashort LP lifetime (0.2 ps) under strong illumination. Unlike usual polariton relaxation processes-whereby polaritonic energy transfers directly to the manifold of singly excited vibrational dark states-under the present mechanism, the LP transfers energy directly to the manifold of higher vibrationally excited dark states; these highly excited dark states subsequently relax to the manifold of singly excited states with a lifetime of tens of ps. Because the present mechanism is generic in nature, we expect these numerical predictions to be experimentally observed in different molecular systems and in cavities with different volumes.

Nanoparticle Assembling Dynamics Induced by Pulsed Optical Force.

Femtosecond (fs) laser trapping dynamics is summarized for silica, hydrophobically modified silica, and polystyrene nanoparticles (NPs) in aqueous solution, highlighting their distinct optical trapping dynamics under CW laser. Mutually repulsive silica nanoparticles are tightly confined under fs laser compared to CW laser trapping and, upon increasing laser power, they are ejected from the focus as an assembly. Hydrophobically modified silica and polystyrene (PS) NPs are sequentially ejected just like a stream or ablated, giving bubbles. The ejection and bubbling take place with the direction perpendicular to laser polarization and its direction is randomly switched from one to the other. These characteristic features are interpreted from the viewpoint of single assembly formation of NPs at an asymmetric position in the optical potential. Temporal change in optical forces map is prepared for a single PS NP by calculating scattering, gradient, and temporal forces. The relative contribution of the forces changes with the volume increase of the assembly and, when the pushing force along the trapping pulse propagation overcome the gradient in the focal plane, the assembly undergoes the ejection. Further fs multiphoton absorption is induced for the larger assembly leading to bubble generation. The assembling, ejection, and bubbling dynamics of NPs are characteristic features of pulsed optical force and are considered as a new platform for developing new material fabrication method.

Pulsed Laser Deposition of Tio2 Nanostructures for Verify the Linear and Non-Linear Optical Characteristics

The present work aims to achieve pulsed laser deposition ofTiO2 nanostructures and investigate their nonlinear properties using z-scan technique.The second harmonic Q-switched Nd: YAG laser at repetition rate of 1Hz and wavelength of 532 nm with three different laser fluencies in the range of 0.77-1.1 J/cm2 was utilized to irradiate the TiO2 target. The products of laser-induced plasma were characterized by utilizing UV-Vis absorption spectroscopy, x-ray diffraction (XRD), atomic force Microscope (AFM),and Fourier transform infrared (FTIR). A reasonable agreement was found among the data obtained usingX-Ray diffraction, UV-Vis and Raman spectroscopy. The XRD results showed that the prepared TiO2 thin films were all crystallite structure with no impurity peaks of other elements. Also, their peak intensities were increased with increasing the ablating laser fluency. AFM measurements indicated that,during pulsed laser deposition, as the laser fluency was increased, the average diameter of the prepared TiO2 nanoparticles (TiO2 NPs)was decreased from 86 to 57 nm, althoughthe differences were increased with the increase in the laser fluency. The multiphoton absorption was investigated using ultra-fast femtosecond laser with the z-scan method.The impact of thickness of the prepared films on the non-linear absorption coefficient was studied as well.

Free-carrier-assisted mid-infrared microcavity soliton generation

Multi-photon absorption (MPA) and free-carrier (FC) effects, usually considered to be detrimental to microcomb generation by introducing strong nonlinear loss, also offer opportunities to overwhelm the thermal-optic effect by modifying the refractive index. Here, we derive the theoretical expression of solitons expanded with MPA and FC effects, accompanied by numerical simulations to reveal the dynamics and mechanism for capturing steady mid-infrared solitons in both Si and Ge microcavities. It is found that with increased FC lifetime, the intensity-dependent MPA underlies non-monotonous variation of nonlinear detuning which enables soliton generation. More interestingly, proper control on the FC lifetime admits bidirectional switching of soliton states (i.e., both decreasing and increasing the number of solitons) as well as self-starting solitons, yielding different technique routes toward microcavity solitons. This research could contribute to a better understanding of nonlinear behaviors influenced by FC effects and find practical applications by releasing the demand on precise control of laser sources, which is especially meaningful for the mid-infrared region.

Laser dynamics in nonlinear transparent media with electron plasma generation: effects of electron–hole radiative recombinations

The performance of optical devices manufactured via laser micromachining on nonlinear transparent materials usually relies on three main factors, which are the characteristic laser parameters (i.e. the laser power, pulse duration and pulse repetition rate), the characteristic properties of host materials (e.g. their chromatic dispersions, optical nonlinearities or self-focusing features) and the relative importance of physical processes such as the avalanche impact ionization, multiphoton ionization and electron–hole radiative recombination processes. These factors act in conjunction to impose the regime of laser operation; in particular, their competition determines the appropriate laser operation regime. In this work a theoretical study is proposed to explore the effects of the competition between multiphoton absorption, plasma ionization and electron–hole radiative recombination processes on the laser dynamics in transparent materials with Kerr nonlinearity. The study rests on a model consisting of a K-order nonlinear complex Ginzburg–Landau equation, coupled to a first-order equation describing time variation of the electron plasma density. An analysis of the stability of continuous waves, following the modulational instability approach, reveals that the combination of multiphoton absorption and electron–hole radiative recombination processes can be detrimental or favorable to continuous-wave operation, depending on the group-velocity dispersion of the host medium. Numerical simulations of the model equations in the fully nonlinear regime reveal the existence of pulse trains, the amplitudes of which are enhanced by the radiative recombination processes. Numerical results for the density of the induced electron plasma feature two distinct regimes of time evolution, depending on the strength of the electron–hole radiative recombination processes.

Mid-infrared nonlinear optical performances of Ge-Sb-S chalcogenide glasses

We report a systematical investigation on the mid-infrared nonlinear performances of Ge-Sb-S glasses. Laser damage threshold (Ith) of Ge-Sb-S glasses was measured under femtosecond pulsed laser incidence ranging between 1.55-3.6 μm. It is found that the Ith has the maximum value at stoichiometric composition. Moreover, the relationship between the refractive index refractive (n0) and nonlinear refractive indices (n2) was obtained, following the semi-empirical Miller’s rule. The n2 shows a nonlinear decay with the increase of wavelength. The multi-photon (up to 7-photon) absorption coefficients of Ge-Sb-S glasses were characterized. The composition Ge25Sb10S65 with high Ith was selected as the core of the designed fiber. A compatible composition Ge25Sb8S67 was chosen as the cladding glass. A 10 μm-diameter-core fiber was made via rod-in-tube method. By pumping a 10-cm-long fiber at 4.8 μm with 170 fs (100 kHz) pulses, we achieved a supercontinuum covering the 3–8 μm spectral range. It indicates that Ge-Sb-S glass family is a type of environment-friendly host materials for mid-infrared nonlinear applications.

Highly sensitive and selective detection of Alprenolol using upconversion nanoparticles functionalized with amphiphilic conjugated polythiophene

Upconversion nanoparticles (UCNPs) are exclusive photoluminescent (PL) materials with the ability to convert low-energy excitation into high-energy emission via multiphoton absorption processes. In this research, a novel amphiphilic conjugated polythiophene was prepared to act as a functionalizing agent for NaLuF4:Yb3+/Er3+ UCNPs. The UCNPs, which were coated with amphiphilic polythiophene, were characterized by using FTIR, TGA, SEM, TEM, UV, and PL. This material was used as an optical sensor to detect Alprenolol, which is widely used for the treatment of high blood pressure. The developed sensor displays good analytical performance including a low limit of detection (LOD = 0.22 nM) and good linearity in the range of 0.5 nM to 75 μM. Furthermore, this sensor was applied successfully to determine the level of Alprenolol in human urine and serum samples with good recoveries of 97–104%.

Multiphoton and plasma absorption measurements in CaF2 and UV fused silica at 473 nm

Transmission versus intensity in 100 μm CaF2 and UV fused silica samples at a wavelength of 473 nm for different durations of the initial pulse was studied. Fitting the experimental data with a model based on the solution of the nonlinear Schrödinger equation and taking into account multiphoton absorption and absorption by the plasma, as well as the Kerr nonlinearity, diffraction and dispersion of the medium, made it possible to obtain the values of multiphoton (k-number of photons) σk and plasma absorption (inverse bremsstrahlung) σ cross sections. The obtained values are σ 4 = 2.5 × 10−117 cm8 s3, σ = 3.6 × 10−18 cm2 for CaF2 and σ 4 = 4.4 × 10−116 cm8 s3, σ = 3 × 10−18 cm2 for UV fused silica.

Comparison of carrier-recombination in Ga(As,Bi)/Ga(N,As)-type-II quantum wells and W-type heterostructures

The realization of efficient semiconductor lasers on GaAs substrates operating at 1.55 μm and beyond remains a technological challenge. As a potential solution, epitaxial heterostructures with type-II band alignment are currently discussed as an active region. Each individual layer in such heterostructures features a comparably large bandgap energy; therefore, spurious effects in laser operation such as reabsorption, multi-photon absorption, or Auger scattering are expected to be suppressed. The actual laser operation occurs across the internal interfaces as the electron and hole wave functions have their extrema in adjacent layers. Hence, a large wave-function overlap is key for efficient recombination. A direct comparison of symmetric and asymmetric Ga(N,As)/Ga(As,Bi) type-II quantum well heterostructures reveals that the symmetry of the layer arrangement drastically influences the charge-carrier recombination: disorder in the Ga(As,Bi) layer has more prominent effects for the asymmetric configuration compared to the symmetric one. The temperature dependence of the emission energy is mainly influenced by the Ga(N,As)-electron layers, while the temperature dependence of the full width at half maximum and the excitation dependence of the emission energy are dominated by the Ga(As,Bi)-hole layers. Photoluminescence excitation spectroscopy reveals the corresponding carrier-relaxation paths to the type-II transition.

Visible Luminescence of Y2SiO5:Tb3+ Powders Excited by Simultaneous Absorption of up to Five Photons in the Short Wavelength IR Band

Efficient infrared-to-visible upconversion luminescence excited by multiphoton absorption was systematically studied in yttrium silicate (Y2SiO5) powders doped with trivalent terbium ions (Tb3+). T...

Laser induced broad band white emission from transparent Cr4+:YAG ceramics: Origin of broadband emission

Laser-induced white (light) emission was observed from transparent Cr4+:YAG ceramics irradiated with a focused continuous wave beam of light from an infrared laser diode. The laser-induced white emission is detected only on the surface of the sample and is not observed in volume. It is found that the intensity of the emission increases exponentially with the laser power density above the threshold. The impact of broadband emission on the power of the transmitted laser beam through the sample was measured. The disappearance of broadband emission due to displacement of the laser beam or an increase in ambient pressure leads to a decrease in the power of transmitted laser beam. Origins of the laser-induced white light emission along with its characteristic features are discussed in terms of multiphoton absorption, intervalence charge transfer and ionic space charge models.

Semiconducting ZnO＆WS2 heterojunction composite films: Fabrication, characterization and ultrafast nonlinear properties

In this work, a series of WS2, ZnO base films and ZnO&WS2 (ZnO@WS2 and WS2@ZnO) heterojunction composite films were synthesized by the single/two-step Radio Frequency Magnetron Sputtering. Interestingly, it is found that structure, morphology and growth mechanism of the samples depends on the regulation different experimental parameters. The result of morphology characterization demonstrated that the standing WS2 with different morphology depend on growth time and the multi-directional growth of the ZnO based film. And using the WS2 base films promoted the growth of ZnO sheet-like morphology. The result of X-ray diffraction characterization showed that the location and the shape of the characteristic peaks existed in the ZnO&WS2 heterojunction composite films are analogous to ZnO (002) and WS2 (002)/(101) alone. Furthermore, the ultrafast nonlinear optical behavior and dynamics of the samples were studied. The “W” and “M” type image of open-aperture Z-scan curve were analyzed. From the results, the samples showed multiphoton absorption induced the ground state or excited state saturated absorption or reverse saturated absorption behavior. And the two-photon absorption and three-photon absorption coefficients of ZnO@WS2 heterojunction composite films reached 1.93 × 10−9 m/W, −2.22 × 10−23 m3/W2, respectively. The bi-exponential decay of the positive signal assigned to the pump-induced deep-level or excited state absorption. Moreover, these findings indicate that the excitation wavelength increases and the detection wave band widen. The study shows that the carrier lifetime of ZnO@WS2 heterojunction composite films is significantly higher than that of the WS2 base films. ZnO&WS2 heterojunction composite films will be promising candidates in nonlinear optical material.

Nonlinear Optical Response of a WS2 Monolayer at Room Temperature upon Multicolor Laser Excitation.

Currently, the nonlinear optical properties of 2D materials are attracting the attention of an ever-increasing number of research groups due to their large potential for applications in a broad range of scientific disciplines. Here, we investigate the interplay between nonlinear photoluminescence (PL) and several degenerate and nondegenerate nonlinear optical processes of a WS2 monolayer at room temperature. We illuminate the sample using two femtosecond laser pulses at frequencies ω1 and ω2 with photon energies below the optical bandgap. As a result, the sample emits light that shows characteristic spectral peaks of the second-harmonic generation, sum-frequency generation, and four-wave mixing. In addition, we find that both resonant and off-resonant nonlinear excitation via frequency mixing contributes to the (nonlinear) PL emission at the A-exciton frequency. The PL exhibits a clear correlation with the observed nonlinear effects, which we attribute to the generation of excitons via degenerate and nondegenerate multiphoton absorption. Our work illustrates a further step toward understanding the fundamental relation between parametric and nonparametric nondegenerate optical mechanisms in transition-metal dichalcogenides. In turn, such understanding has great potential to expand the range of applicability of nonlinear optical processes of 2D materials in different fields of science and technology, where nonlinear mechanisms are typically limited to degenerate processes.