Multiphoton Absorption Processes Research Articles

Experimental observation of self-imaging in SMF-28 optical fibers.

Spatial self-imaging, consisting of the periodic replication of the optical transverse beam profile along the propagation direction, can be achieved in guided wave systems when all excited modes interfere in phase. We exploited material defects photoluminescence for directly visualizing self-imaging in a few-mode, nominal singlemode SMF-28 optical fiber. Visible luminescence was excited by intense femtosecond infrared pulses via multiphoton absorption processes. Our method permits us to determine the mode propagation constants and the cutoff wavelength of transverse fiber modes.

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%.

High enhancement factor in low-power unipolar discharge arc assisted laser induced plasma spectroscopy

The enhanced laser induced breakdown spectroscopy is highly desirable for the quantitatively detection with low limit of detection (LOD) and high accuracy. Higher enhancement factor is always obtained from discharge assisted laser-induced breakdown spectroscopy with enough higher energy injection in comparison to the normally standard laser induced breakdown spectroscopy (LIBS), which means dangerous with high voltage and elevated power. Here, a low power (10 W) injected unipolar discharge arc is proven to greatly enhance the LIBS with a large enhancement factor up to 13.8 when the discharged electric arc followed by a laser induced plasma are simultaneously applied on a silicon wafer, which is comparative or larger than those results with high-power (kW) and high voltage (kV) being applied. Furthermore, the enhanced technique is applied on an actual qualitative detection of cadmium in soil. The lines of Cd II (214.44 nm) from soil is enhanced as compared to a LIBS measurement. Accordingly, the LOD of the typical emission line is reduced to 23.5 ppm by the unipolar discharge arc assisted laser induced breakdown spectroscopy (UD-LIBS), which is 2.45-fold lower than that from the normal LIBS. The pre-ionization of the environment and the surface by the modulated discharge arc to directly make the Inverse Bremsstrahlung (IB) absorption with higher absorption coefficient dominate the initial stage of plasma formation rather than the multi-photon absorption process, thus make both electron density and the temperature of plasma raising are the main mechanism of spectral enhancement, which is further proved by the evolution dynamics images of two plasma overlapping processes under an in-situ intensified charge-coupled device (ICCD) based photographic system. The simple, portable, commercial highly enhancement way is greatly benefit to the actual applications.

Donor-Acceptor Type Reduced Graphene-Oxide and a Tin-Selenide Nanohybrid With Broad and Ultrafast Optical Limiting Properties

The nonlinear absorption properties of RGO have been studied extensively but the optical limiting (OL) performance of RGO was always confined to visible light. In this study, by anchoring SnSe nanosheets onto the surface of RGO, the SnSe/RGO nanohybrid shows a broader reverse saturable absorption (RSA), ranging from 400 nm to 800 nm, and enhanced nonlinear optical (NLO) response. The improvement of the NLO absorption response is attributed to a multiphoton-absorption process and electron-transfer effect in this artificially constructed donor-acceptor system. Pump-probe experiments suggest a response time of approximately 1.7-8 picoseconds for SnSe/RGO nanohybrid.

High-power picosecond deep-UV source via group velocity matched frequency conversion

Powerful ultrashort pulses in the deep-ultraviolet (deep-UV) are beneficial for diverse applications from fundamental science to industrial materials processing. However, reaching high powers via conventional approaches is challenging due to three central issues: dispersion, multiphoton absorption, and optical damage. Here, we simultaneously overcome these issues with a novel fifth-harmonic generation architecture optimized for group velocity matching. We use tilted pulse fronts, including a noncollinear geometry in the final sum-frequency generation stage. This enables lower intensities and longer crystals, thereby favoring the birefringently phase matched χ ( 2 ) process over higher-order multiphoton absorption processes. Moreover, we demonstrate low-loss cascaded χ ( 2 ) -based spatiotemporal flattening of the input pulses, which enhances the uniformity of the conversion efficiency throughout the beam profile. Through these techniques, we realize a picosecond deep-UV generation source at 206 nm with record-high 2.5 W average output power and a repetition rate of 100 kHz. This result paves the way for a new era of high-power ultrafast deep-UV lasers.

Giant Optical Nonlinearities of Photonic Minibands in Metal–Dielectric Multilayers

The giant nonlinear optical responses of photonic minibands of (Ag/SiO2)4 metal–dielectric multilayers are reported using high intense femtosecond laser pulses. Ag and SiO2 alternative stack of layers form a series of coupled Fabry–Pérot resonators (Ag–SiO2–Ag) and the cavity modes are split into transmission minibands in the metal reflective spectral region. The strong saturation of two‐photon absorption associated with multiphoton absorption (MPA) is observed at photonic miniband minimum (≈700 nm), whereas MPA is the strong dominant nonlinearity at peak maximum (≈725 nm). The metal‐cavity induced manifold enhancement (>103 times) in the Ag intrinsic nonlinearity follows wide miniband spectral (700–765 nm) trend. Results are discussed with respect to single (Ag/SiO2)1 and bare Ag thin film results. The broad origin of giant nonlinearity is ascribed to the strong local cavity field enhanced metal interband transitions initiated by multiphoton absorption processes. The strong confinement of optical field and energy dissipation within these metal–dielectric coupled resonators are further evidenced by transfer matrix method simulations. The giant multiphoton absorptive type nonlinearities with very high laser‐damage threshold (≈50 GW cm−2) from these composite transparent metals are potentially beneficial for novel nonlinear optical limiters, optical Bragg coatings, and saturable optical absorbers for extremely high‐power ultrafast lasers operation.

A quantum model for photoemission from metal surfaces and its comparison with the three-step model and Fowler–DuBridge model

This paper studies a quantum mechanical model for photoemission from a metal surface due to the excitation of laser electric fields, which was developed by solving the time-dependent Schrödinger equation exactly. The quantum model includes the effects of laser fields (wavelength and intensity), properties of metals (Fermi energy and work function including Schottky effect), and the applied dc field on the cathode surface. Shorter wavelength lasers can induce more photoemission from electron initial energy levels further below the Fermi level and, therefore, yield larger quantum efficiency (QE). The dc field increases QE, but it is found to have a greater impact on lasers with wavelengths close to the threshold (i.e., the corresponding photon energy is the same as the cathode work function) than on shorter wavelength lasers. The quantum model is compared with existing classical models, i.e., the three-step model, the Fowler–DuBridge model, and the Monte Carlo simulation based on the three-step model. Even though with very different settings and assumptions, it is found that the scaling of QE of the quantum model agrees well with other models for low intensity laser fields. When the laser field increases, QE increases with the laser field strength in the longer laser wavelength range due to the increased contributions from multiphoton absorption processes.

Interaction of (CCl4)n clusters with nanosecond and picosecond laser pulses at discrete wavelengths spanning from IR to UV region: A comparative study using TOFMS

Using time-of-flight mass spectrometer (TOFMS), photochemistry of carbon tetrachloride clusters ((CCl4)n) has been investigated upon interaction with nanosecond and picosecond Nd:YAG laser pulses (266, 355, 532 and 1064 nm) spanning over intensity range of ∼1.2 × 109 to 7.9 × 1012 W/cm2. At all the laser wavelengths upon interaction of (CCl4)n clusters with laser pulse, multiphoton excitation leads to excessive fragmentation of cluster constituents, which are subsequently ionized by multiphoton absorption process. Depending on laser wavelength and pulse duration, at 266 and 355 nm these clusters exhibit dominant ion signal corresponding to singly charged fragment ions i.e. Cl+, CCl+, CCl2+ and CCl3+ in the mass spectra. In addition, for studies carried out at 266 nm using picosecond laser pulses minor ion signal corresponding to C2Cl+ and C2Cl2+ were also observed. Generation of these ions has been ascribed to intra-cluster radical chemistry resulting in generation of neutral precursors, which subsequently are multiphoton ionized. At 532 and 1064 nm, dominant ion signal corresponding to multiply charged atomic ions of carbon (up to C4+/C5+) and chlorine (up to C11+) have been observed. Though multiphoton absorption is the primary process driving the photochemistry of (CCl4)n clusters, observation of multiply charged atomic ions at 532 and 1064 nm has been ascribed to dominant role played by quasi-free electrons which are confined within the ionized cluster, in efficient coupling of laser energy at longer laser wavelengths via inverse Bremsstrahlung process.

Thermally Activated Upconversion Near-Infrared Photoluminescence from Carbon Dots Synthesized via Microwave Assisted Exfoliation.

Upconversion near-infrared (NIR) fluorescent carbon dots (CDs) are important for imaging applications. Herein, thermally activated upconversion photoluminescence (UCPL) in the NIR region, with an emission peak at 784 nm, which appears under 808 nm continuous-wave laser excitation, are realized in the NIR absorbing/emissive CDs (NIR-CDs). The NIR-CDs are synthesized by microwave-assisted exfoliation of red emissive CDs in dimethylformamide, and feature single or few-layered graphene-like cores. This structure provides an enhanced contact area of the graphene-like plates in the core with the electron-acceptor carbonyl groups in dimethylformamide, which contributes to the main NIR absorption band peaked at 724 nm and a tail band in 800-850 nm. Temperature-dependent photoluminescence spectra and transient absorption spectra confirm that the UCPL of NIR-CDs is due to the thermally activated electron transitions in the excited state, rather than the multiphoton absorption process. Temperature dependent upconversion NIR luminescence imaging is demonstrated for NIR-CDs embedded in a polyvinyl pyrrolidone film, and the NIR upconversion luminescence imaging in vivo using NIR-CDs in a mouse model is accomplished.

Field emission microscopy pattern of a single-crystal diamond needle under ultrafast laser illumination

We report herein on the spatial beam properties of a field emission electron source based on a single-crystal diamond needle illuminated by ultrashort light pulses. We show that the increasing of the laser intensity strongly modifies the emission pattern, leading to the emergence of a new emission region at high peak power. This region is situated on the opposite side of the diamond needle to the one irradiated by the laser. By spatially-resolved energy spectrometry, we prove that the electrons emitted from this region are governed by a multi-photon absorption process. The occurrence of this emission pattern can be explained by accounting for the inhomogeneous distribution of the optical field enhancement and the laser absorption induced by light diffraction within the nanometric needle. The numerical simulations performed on a real sub-wavelength tip confirm this localization of the optical field enhancement and reveal that the electrons trajectories match the spatial beam distribution evidenced experimentally. This work underlines the need to closely monitor the surface roughness of the field emitter as well as the laser illumination conditions to finely control its emission pattern.

Z-scan theoretical model for simultaneous n- and (n+1)-photon absorption

We present a theoretical analysis of multi-photon absorption processes. We find an approximate closed-form expression of the normalized transmittance for a thin sample with two successive nonlinear absorption processes at a time ([Formula: see text]- and [Formula: see text]-photon absorption) in the presence of residual linear absorption. Our approach allows for simultaneous identification and evaluation of the nonlinear absorption coefficients and Rayleigh lengths of the incident elliptic Gaussian beam by fitting the open [Formula: see text]-scan traces.

Wavelength Dependence of Poly(<i>p</i>-hydroxystyrene) Ablation by Mid-Infrared Free-Electron Laser

Ablation of polymer thin films for resists was studied using a mid-Infrared (IR) Free-Electron Laser (FEL). Irradiation fluence of the IR light in the mid-IR region (5.6 to 8.0 μm) to a thin film of poly(p-hydroxystyrene) (PHOST) on a silicon wafer increased to cause PHOST ablation. The ablated spot size was smaller than the irradiated area calculated by wave optics, because the ablation is a multiphoton absorption process. The ablation threshold energy was determined using the irradiation area calculated by the wave optics. The threshold energy depended upon the film thickness of PHOST and irradiation wavelength of FEL. The threshold energy of thin PHOST film with 61 nm increased at the wavelength of vibrational modes of phenyl ring. It associates the durability of phenyl ring to photon decomposition.

Understanding Tunneling Ionization of Atoms in Laser Fields using the Principle of Multiphoton Absorption**Supported by the National Natural Science Foundation of China under Grant Nos 11725417 and 11575027, the Joint Fund of the National Natural Science Foundation of China and the China Academy of Engineering Physics under Grant No U1730449, and the Science Challenge Project under Grant No TZ2018005.

The elaborate energy and momentum spectra of ionized electrons from atoms in laser fields suggest that the ionization dynamics described by tunneling theory should be modified. Although great efforts have been carried out within semiclassical models, there are few discussions describing the multiphoton absorption process within a quantum framework. Comparing the results obtained with the time-dependent Schrödinger equation (TDSE) and the Keldysh–Faisal–Reiss (KFR) theory, we study the nonperturbative effects of ionization dynamics beyond the KFR theory. The difference in momentum spectra between multiphoton and tunneling regimes is understood in a unified picture with virtual multiphoton absorption processes. For the multiphoton regime, the momentum spectra can be obtained by coherent interference of each periodic contribution. However, the interference of multiphoton absorption peaks will result in a complex structure of virtual multiphoton bands in the tunneling regime. It is shown that the virtual spectra will be almost continuous in the tunneling regime instead of the discrete levels found in the multiphoton regime. Finally, with a model combining the TDSE and the KFR theory, we try to understand the different effects of virtual multiphoton processes on ionization dynamics.

Analytical model for a single multiphoton absorption process

We present a theoretical analysis of a single [Formula: see text]-photon absorption process. We found an approximate analytical but closed-form expression of the normalized transmittance for a thin nonlinear material under an elliptic Gaussian incident beam. The obtained expression can be used to fit the experimental [Formula: see text]-scan traces, allowing the determination of material’s multiphoton absorption coefficient and Rayleigh lengths of the input elliptic Gaussian beam.

Coherent metamaterial absorption of two-photon states with 40% efficiency

Multi-photon absorption processes have a nonlinear dependence on the amplitude of the incident optical field i.e. the number of photons. However, multi-photon absorption is generally weak and multi-photon events occur with extremely low probability. Consequently, it is extremely challenging to engineer quantum nonlinear devices that operate at the single photon level and the majority of quantum technologies have to rely on single photon interactions. Here, we demonstrate experimentally and theoretically that exploiting coherent absorption of N = 2 N00N states makes it possible to enhance the number of two-photon states that are absorbed. An absorbing metasurface placed inside a Sagnac-style interferometer into which we inject an N = 2 N00N state, exhibits two-photon absorption with 40.5% efficiency, close to the theoretical maximum. This high probability of simultaneous absorption of two photons holds the promise for applications in fields that require multi-photon upconversion but are hindered by high peak intensities.

Two-photon and three-photon absorption in ZnO nanocrystals embedded in Al2O3 matrix influenced by defect states.

The broadband nonlinear absorption in ZnO nanocrystals embedded in Al2O3 matrix was investigated by Z-scan and pump-probe techniques from 400nm to 800nm. The effective two-photon absorption and three-photon absorption coefficients were determined to be ∼1.1×103 cm/GW at 400nm and ∼1.1×10-1 cm3/GW2 at 800nm, respectively, which are at least two orders of magnitude greater than that in ZnO bulk crystal. It may be attributed to the defect-states-mediated multiphoton absorption process, which was proofed by comparison experiments with different densities of interfacial defect states. The corresponding lifetimes for the intraband relaxation, defect-states trapping, and interband recombination processes were measured by femtosecond transient absorption measurements as τ1 ∼ 1 ps, τ2 ∼ 13 ps, and τ3 ∼ 350 ps, respectively.

Probing Multiphoton Photophysics Using Two-Beam Action Spectroscopy.

Multiphoton absorption (MPA) is an enabling technology for many applications. However, due to the low probability of MPA processes, their accurate characterization remains a challenge. Here we introduce a new technique, two-beam constant emission intensity (2-BCEIn) spectroscopy, that offers substantial advantages over other existing methods that use the generation of optical emission for the characterization of absorptive nonlinearities. We use 2-BCEIn to study nonlinear absorption in solutions of crystal violet lactone (CVL) over a range of excitation wavelengths in which the dominant nonlinear absorption process transitions from two-photon absorption (750 nm) to three-photon absorption (830 nm). At an excitation wavelength of 800 nm, both two-photon absorption and three-photon absorption contribute substantially to the nonlinear fluorescence excitation (NFE) signal, although the dynamic range of the NFE data is not sufficient to quantify the contributions of each process. 2-BCEIn spectroscopy enables the direct measurement of the local exponent at each emission intensity. 2-BCEIn measurements made at several different emission intensities demonstrate unambiguously that the nonlinear excitation of CVL at 800 nm cannot be described solely as the sum of a two-photon process and a three-photon process. A kinetic model that includes intrapulse excited-state absorption reproduces the features of the 2-BCEIn measurements and enables the determination of the ratio of the three-photon absorption cross section to the two-photon absorption cross section. Such information cannot easily be extracted from conventional NFE measurements. These results demonstrate the power and versatility of two-beam action spectroscopies for elucidating the complex photophysics of multiphoton absorption processes.

Lanthanide-doped KGd3F10 nanocrystals embedded glass ceramics: Self-crystallization, optical properties and temperature sensing

Lanthanide-doped transparent bulk glass ceramics containing cubic KGd3F10 nanocrystals were successfully fabricated via self-crystallization and subsequent heat-treatment. Elemental mapping and Eu3+ optical spectroscopy evidenced the incorporation of lanthanide dopants into low-phonon-energy KGd3F10 crystalline lattice. As a consequence, bright red(green) photoluminescence was achieved for the Ce/Eu(Tb) co-doped glass ceramics via energy transfer of Ce3+→Gd3+ →Eu3+(Tb3+), and intense tunable upconversion emissions were realized for the Yb/Ln (Ln = Er, Ho, Tm) co-doped glass ceramics via a multi-photon absorption process. Furthermore, temperature-dependent optical performance of Yb/Er and Yb/Tm co-doped glass ceramics was systematically studied to explore their possible application as optical thermometric media. Impressively, fluorescence intensity ratios of Er3+2H11/2/4S3/2 thermally coupled states as well as Tm3+3F2,3/1G4 non-thermally coupled ones were evidenced to be applicable as temperature probes.

Photodissociation Spectroscopy of Cold Protonated Synephrine: Surprising Differences between IR-UV Hole-Burning and IR Photodissociation Spectroscopy of the O-H and N-H Modes.

We report the UV and IR photofragmentation spectroscopies of protonated synephrine in a cryogenically cooled Paul trap. Single (UV or IR) and double (UV-UV and IR-UV) resonance spectroscopies have been performed and compared to quantum chemistry calculations, allowing the assignment of the lowest-energy conformer with two rotamers depending on the orientation of the phenol hydroxyl (OH) group. The IR-UV hole burning spectrum exhibits the four expected vibrational modes in the 3 μm region, i.e., the phenol OH, Cβ-OH, and two NH2+ stretches. The striking difference is that, among these modes, only the free phenol OH mode is active through IRPD. The protonated amino group acts as a proton donor in the internal hydrogen bond and displays large frequency shifts upon isomerization expected during the multiphoton absorption process, leading to the so-called IRMPD transparency. More interestingly, while the Cβ-OH is a proton acceptor group with moderate frequency shift for the different conformations, this mode is still inactive through IRPD.

Simulating resonance-mediated two-photon absorption enhancement in rare-earth ions by a rectangle phase modulation**Project supported by the National Natural Science Foundation of China (Grant No. 11474096), the Science and Technology Commission of Shanghai Municipality, China (Grant Nos. 14JC1401500, 17ZR146900, and 16520721200), and the Higher Education Key Program of He’nan Province of China (Grant No. 17A140025).

Improving the up-conversion luminescence efficiency of rare-earth ions via the multi-photon absorption process is crucial in several related application areas. In this work, we theoretically propose a feasible scheme to enhance the resonance-mediated two-photon absorption in Er3+ ions by shaping the femtosecond laser field with a rectangle phase modulation. Our theoretical results show that the resonance-mediated two-photon absorption can be decomposed into the on-resonant and near-resonant parts, and the on-resonant part mainly comes from the contribution of laser central frequency components, while the near-resonant part mainly results from the excitation of low and high laser frequency components. So, the rectangle phase modulation can induce a constructive interference between the two parts by properly designing the modulation depth and width, and finally realizes the resonance-mediated two-photon absorption enhancement. Moreover, our results also show that the enhancement efficiency of resonance-mediated two-photon absorption depends on the laser pulse width (or laser spectral bandwidth), final state transition frequency, and intermediate and final state absorption bandwidths. The enhancement efficiency modulation can be attributed to the relative weight manipulation of on-resonant and near-resonant two-photon absorption in the whole excitation process. This study presents a clear physical insight for the quantum control of resonance-mediated two-photon absorption in the rare-earth ions, and there will be an important significance for improving the up-conversion luminescence efficiency of rare-earth ions.