Short-pulse Laser Excitation Research Articles

Charge Carrier Dynamics at the Perovskite Interface with Self-Assembled Monolayers.

Self-assembled monolayers (SAMs) deposited on the hole-collecting electrodes of p-i-n perovskite solar cells effectively replace bulky hole transporting layers. However, the mechanism by which monolayers control the electronic processes and how they depend on the properties of the monolayer molecules remain poorly understood. In this study, we developed a simplified perovskite solar cell imitator with blocked electron extraction to investigate the photocurrent dynamics between the perovskite and the hole-collecting ITO electrode. We investigated the photoluminescence and photovoltage dynamics under short laser pulse excitation and addressed the influence of bulky and monomolecular hole transport layers. Our findings reveal that the photovoltage dynamics is significantly affected by the properties of the transport and perovskite layers, which in turn depend on the methods of sample preparation and exploration. Photocurrent dynamics is determined by several processes, including charge carrier displacement in the local electric field, hole transport to ITO, trapping of holes in interface trap states, and electron-hole recombination at the interface. We propose a model that takes into account molecular dipole moments and their ionization potentials to partially explain the different influences of different monolayers on the hole extraction and interfacial recombination rates. Additionally, the photovoltage dynamics also strongly depends on the illumination of the sample and shows memory effects that persist over minutes and hours and are attributed to the redistribution of ions.

Broadband THz Sources from Gases to Liquids

Matters are generally classified within four states: solid, liquid, gas, and plasma. Three of the four states of matter (solid, gas, and plasma) have been used for THz wave generation with short laser pulse excitation for decades, including the recent vigorous development of THz photonics in gases (air plasma). However, the demonstration of THz generation from liquids was conspicuously absent. It is well known that water, the most common liquid, is a strong absorber in the far infrared range. Therefore, liquid water has historically been sworn off as a source for THz radiation. Recently, broadband THz wave generation from a flowing liquid target has been experimentally demonstrated through laser-induced microplasma. The liquid target as the THz source presents unique properties. Specifically, liquids have the comparable material density to that of solids, meaning that laser pulses over a certain area will interact with three orders more molecules than an equivalent cross-section of gases. In contrast with solid targets, the fluidity of liquid allows every laser pulse to interact with a fresh area on the target, meaning that material damage or degradation is not an issue with the high-repetition rate intense laser pulses. These make liquids very promising candidates for the investigation of high-energy-density plasma, as well as the possibility of being the next generation of THz sources.

Super-quadratic upconversion luminescence among lanthanide ions.

We explore the arguably most fundamental aspect of energy-transfer upconversion (ETU), namely the dependence of upconversion luminescence from a higher-energy level, following ETU excitation from a metastable lower-energy level, on direct luminescence from that metastable level. We investigate ETU among neighboring Nd3+ ions in single crystals of GdVO4 and LaSc3(BO3)4 with different doping concentrations by measuring, after short-pulse laser excitation with different pump energies, the infrared luminescence decay from the metastable 4F3/2 level and the yellow upconversion luminescence decay from the 4G7/2 level. We observe a highly super-quadratic dependence of upconversion on direct luminescence intensity. We conclude that the commonly assumed quadratic law of ETU, as proposed by Grant's model and frequently employed in rate-equation simulations, is inadequate to the description of ETU processes. Whereas Zubenko's model, which considers a finite migration rate, provides significantly better fits to the experimental luminescence-decay curves, also this model cannot accurately reproduce the measured decay curves, partly because it does not take the non-homogeneous distribution of active ions into account.

Electron-lattice energy relaxation in laser-excited thin-film Au-insulator heterostructures studied by ultrafast MeV electron diffraction.

We apply time-resolved MeV electron diffraction to study the electron-lattice energy relaxation in thin film Au-insulator heterostructures. Through precise measurements of the transient Debye-Waller-factor, the mean-square atomic displacement is directly determined, which allows to quantitatively follow the temporal evolution of the lattice temperature after short pulse laser excitation. Data obtained over an extended range of laser fluences reveal an increased relaxation rate when the film thickness is reduced or the Au-film is capped with an additional insulator top-layer. This behavior is attributed to a cross-interfacial coupling of excited electrons in the Au film to phonons in the adjacent insulator layer(s). Analysis of the data using the two-temperature-model taking explicitly into account the additional energy loss at the interface(s) allows to deduce the relative strength of the two relaxation channels.

Numerical Detector Theory for the Longitudinal Momentum Distribution of the Electron in Strong Field Ionization.

The lack of analytical solutions for the exit momentum in the laser-driven tunneling theory is a well-recognized problem in strong field physics. Theoretical studies of electron momentum distributions in the neighborhood of the tunneling exit depend heavily on adhoc assumptions. In this Letter, we apply a new numerical method to study the exiting electron's longitudinal momentum distribution under intense short-pulse laser excitation. We present the first realizations of the dynamic behavior of an electron near the so-called tunneling exit region without adopting a tunneling approximation.

Emerging Low-Dimensional Materials for Nonlinear Optics and Ultrafast Photonics.

Low-dimensional (LD) materials demonstrate intriguing optical properties, which lead to applications in diverse fields, such as photonics, biomedicine and energy. Due to modulation of electronic structure by the reduced structural dimensionality, LD versions of metal, semiconductor and topological insulators (TIs) at the same time bear distinct nonlinear optical (NLO) properties as compared with their bulk counterparts. Their interaction with short pulse laser excitation exhibits a strong nonlinear character manifested by NLO absorption, giving rise to optical limiting or saturated absorption associated with excited state absorption and Pauli blocking in different materials. In particular, the saturable absorption of these emerging LD materials including two-dimensional semiconductors as well as colloidal TI nanoparticles has recently been utilized for Q-switching and mode-locking ultra-short pulse generation across the visible, near infrared and middle infrared wavelength regions. Beside the large operation bandwidth, these ultrafast photonics applications are especially benefit from the high recovery rate as well as the facile processibility of these LD materials. The prominent NLO response of these LD materials have also provided new avenues for the development of novel NLO and photonics devices for all-optical control as well as optical circuits beyond ultrafast lasers.

Intensity-dependent transient photocurrent of organic bulk heterojunction solar cells

An understanding of the behaviors of photo-generated charge carriers (CCs) has a crucial meaning for establishing a reliable model to describe the operating concept of photovoltaic devices. One of the most-widely used techniques to characterize transport behavior of CCs is a transient photocurrent measurement using a short laser pulse excitation. However, conventional transient photocurrent measurements, often referred to as the time-of-flight method, have a drawback due to the demand for relatively thicker active layers. This is mainly to observe a clear transit time for the CCs; however, some uncertainties can arise when the properties of a thicker active layer are adapted to the properties of conventionally thin active layer of an optimized device configuration. Therefore, we studied two models, one based on a polythiophene derivative and the other based on a narrow band-gap polymer derivative of an organic bulk heterojunction system, by using a transient photocurrent method with a conventional active layer thickness. This comparative study has shown clearly different transient photocurrent behaviors between the two systems when the excitation intensity, as well as the applied electric field, is varied.

Vibronic Enhancement of Algae Light Harvesting

Summary Plants, algae, and photosynthetic bacteria use surprisingly sophisticated optimizations at the quantum mechanical level to harvest the sun's energy. The observation of coherence phenomena within light-harvesting complexes after short laser-pulse excitation has inspired advances in our understanding of light-harvesting optimization, highlighting the interplay of electronic excitations and vibrations. However, it remains unclear how these vibronic effects change or optimize the function of light-harvesting complexes—in other words, what is the design principle we could learn? Here, we use two-dimensional electronic spectroscopy to quantify the vibronic mixing among the light-absorbing molecules of a light-harvesting complex from cryptophyte algae. These data reveal a striking reallocation of absorption strength that, in turn, provides a robust increase in the rate of energy transfer of up to 3.5-fold. The realization of how absorption-strength redistribution, induced by vibronic coupling, provides a multiplicative increase in the rate of energy funneling establishes a bioinspired design principle for optimal light-harvesting systems.

Spectral tuning via multi-phonon-assisted stokes and anti-stokes excitations in LaF3: Tm3+ nanoparticles

We present a facile and highly effective method to tailor upconversion (UC) emission from LaF3: Tm3+ nanoparticles (NPs) by adjusting ambient temperature from 20 K to 400 K accompanied with the pulse laser excitation. Spectral tuning mechanism controlled by ambient temperature at pulse laser excitation is revealed, and a mechanism based on the modification on multi-phonon relaxation rates for the rapid population of intermediate level 3H4 and multi-phonon-assisted excited state absorption is proposed. Based on multi-phonon relaxation theory and time-resolved photoluminescence studies, it is reasonable that UC luminescence under short-pulse laser excitation mainly originates from the ions at/near the surface of NPs. These exciting findings in ambient temperature accompanied with the short-pulse excitation dependent UC selectivity offer a general approach to tailoring lanthanide related UC emissions, which will benefit multicolor displays and imaging.

Luminescence selective output characteristics tuned by laser pulse width in Tm3+ doped NaYF4 nanorods

The variations in material composition, phase and structure can provide a useful tool for tuning emission colour, but the controlling of the emission colour in a material, with a composition fixed, remains to be a daunting challenge. In this work, we systematically study the luminescence selective output characteristics of Tm3+ doped NaYF4 nanorods, and also the dependences of fluerecence output on pulse duration, excitation wavelength, pump power, and ambient temperature. The results show that the color of output light is strongly dependent on laser pulse duration compared with other factors. The temperature dependent luminescence of the nanorods shows very different behaviors with short-pulse laser excitation from those of continuous wave (CW) laser. When the pulse laser at 656 nm is employed, the emission spectra from NaYF4:0.5 mol% Tm3+ nanorods at the different temperatures are dominated by near-infrared (NIR) luminescence about 800 nm accompanied with weak blue luminescence, giving rise to nearly spectrally-pure NIR emissions at 20 K. When the pulse laser is replaced by CW laser, blue double emissions at 453 and 478 nm with the same order of magnitude of NIR luminescence can be clearly detected at room temperature. The key mechanism responsible for colour-tunable emission can be explained in terms of the population process of luminescence level, in which the different luminescence level populations need different time intervals. Considering excited-state absorption (ESA) for a particular 1D2 energy level, there needs an extra step of 3F2, 33H4 multiphonon nonradiation relaxation process to populate the 3H4 state and subsequently pump its 1D2 state for blue emission. Therefore, the pulse width should be longer than nonradiation relaxation time of 3F2, 33H4 to comply with the ESA, while the nonradiation relaxation time can further be tuned by controlling ambient temperature. We show that the variation of the excitation power leads to interesting change in the upconversion (UC) decay curve. We focus our attention on the excitation wavelength dependences of 3H4 and 1D2 emission lifetimes in order to validate the population mechanism of luminescence level. We demonstrate that the 3H4 luminescence time depends on excitation wavelength, while 1D2 emission lifetime nearly keeps constant when varying the excitation wavelength. Based on multi-phonon relaxation theory and time-resolved photoluminescence studies, it is indicated that the UC luminescence under short-pulse laser excitation mainly originates from the ions at/near the surface, while downconversion is mainly from the ions in the core for NaYF4:Tm3+ nanorods. The single-band NIR luminescence output by changing the pulse width and excitation wavelength provides an insight into the controlling of the population processes of luminescent levels and offers a versatile approach to tuning the spectral output.

Synthesis, characterization and luminescence properties of Eu3+-doped hydroxyapatite nanocrystal and the thermal treatment effects

In this work, we present the synthesis, characterization and the luminescence properties of Ca10(PO4)6(OH)2 (hydroxyapatite/HAp) nanocrystals doped with europium trivalent ions. The most important processes that lead to europium emissions in the visible region were identified. Eu:HAp nanopowder excited at 394nm (or 460nm) exhibits several emissions: (i) weak emissions at 579nm, 592nm and 616nm due to the 5D0→7F0, 5D0→7F1 and 5D0→7F2 transitions, respectively, with europium ion occupying site I in hydroxyapatite structure and (ii) strong emissions due to the 5D0→7F0 (574nm), 5D0→7F1 (602nm) and 5D0→7F2 (610–630nm) transitions, when Eu3+ is occupying site II. The emission spectrum and the time-resolved luminescence analysis showed that the HAp nanocrystals (nanopowder) thermally treated at temperature (T) between 500 and 800°C have a change in the initial Eu3+ site distribution of 100 % of Eu3+ at site I to a more stable one where the majority of europium ions are at site II: 30% remains at site I and 70% migrates to site II. In addition, an enhancement of the Eu3+ emission intensity is observed due to the increasing crystallite size. A time-resolved luminescence investigation using a short pulse laser excitation at 460nm was employed to measure the luminescence decays and to determine the most important mechanisms involved in the deexcitation process of 5D0 excited state of Eu3+, where it is seen a fast (2.9μs) energy transfer from Eu3+- site I (donor) to Eu3+- site II (acceptor) in the thermally treated nanopowders with T>500°C. The initial presence of 100% of Eu3+ at site I in the synthesized nanocrystals is gradually modified by the thermal treatments with temperatures above 500°C by thermal activation of Ca2+ vacancy (the charge compensator) diffusion through the HAp lattice, which propitiates the Ca2+- vacancies and Eu3+ ions to exchange positions in the lattice. By this thermal activated mechanism, Eu3+ ion migrates through the lattice until get the final distribution of 30% at site I and 70% at site II. As a result, the complete description of the Eu3+ (5D0) decay and the energy transfer process from Eu3+ (site I)→Eu3+ (site II) were proposed.

Short-pulse laser excitation of quartz: experiments and modelling of transient optical properties and ablation

Time-resolved optical studies using spectral interferometry and reflectance measurements of ultrafast laser excitation of quartz have been performed and combined with subsequent atomic force microscopy imaging of the laser-formed holes. A comprehensive investigation of these transient properties of the dielectric has been analysed in view of a single theoretical model describing the laser–material interaction in a multirate equation model including light propagation. Simulations demonstrate that self-trapped excitons in quartz significantly change the optical properties immediately after excitation.

Comparing ultrafast surface and bulk heating using time-resolved electron diffraction

From measurements of the transient Debye-Waller effect in Bismuth, we determine the buildup time of the random atomic motion resulting from the electronic relaxation after short pulse laser excitation. The surface sensitive reflection high energy electron diffraction and transmission electron diffraction yield a time constant of about 12 ps and 3 ps, respectively. The different energy transfer rates indicate relatively weak coupling between bulk and surface vibrational modes.

Modeling short-pulse laser excitation of dielectric materials

A theoretical description of ultrashort-pulse laser excitation of dielectric materials based on strong-field excitation in the Keldysh picture combined with a multiple-rate-equation model for the electronic excitation including collisional processes is presented. The model includes light attenuation in a self-consistent manner and changing optical properties described in a Drude picture. The model can be used to calculate the electronic excitation as a function of time and depth, and from these quantities the time-dependent optical parameters as well as the ablation depth can be derived. The simulations provide insight into the excitation and propagation dynamics of short-pulse excitation and show that at increasing fluence the excitation becomes localized near the material surface and gives rise to strong modifications of the optical properties of the material.

Time-domain geometrical localization of point-like fluorescence inclusions in turbid media with early photon arrival times

We introduce a novel approach for localizing a plurality of discrete point-like fluorescent inclusions embedded in a thick turbid medium using time-domain measurements. The approach uses early photon information contained in measured time-of-flight distributions originating from fluorescence emission. Fluorescence time point-spread functions (FTPSFs) are acquired with ultrafast time-correlated single photon counting after short pulse laser excitation. Early photon arrival times are extracted from the FTPSFs obtained from several source-detector positions. Each source-detector measurement allows defining a geometrical locus where an inclusion is to be found. These loci take the form of ovals in 2D or ovoids in 3D. From these loci a map can be built, with the maxima thereof corresponding to positions of inclusions. This geometrical approach is supported by Monte Carlo simulations performed for biological tissue-like media with embedded fluorescent inclusions. To validate the approach, several experiments are conducted with a homogeneous phantom mimicking tissue optical properties. In the experiments, inclusions filled with indocyanine green are embedded in the phantom and the fluorescence response to a short pulse of excitation laser is recorded. With our approach, several inclusions can be localized with low millimeter positional error. Our results support the approach as an accurate, efficient, and fast method for localizing fluorescent inclusions embedded in highly turbid media mimicking biological tissues. Further Monte Carlo simulations on a realistic mouse model show the feasibility of the technique for small animal imaging.

A multi-view time-domain non-contact diffuse optical tomography scanner with dual wavelength detection for intrinsic and fluorescence small animal imaging

We present a non-contact diffuse optical tomography (DOT) scanner with multi-view detection (over 360°) for localizing fluorescent markers in scattering and absorbing media, in particular small animals. It relies on time-domain detection after short pulse laser excitation. Ultrafast time-correlated single photon counting and photomultiplier tubes are used for time-domain measurements. For light collection, seven free-space optics non-contact dual wavelength detection channels comprising 14 detectors overall are placed around the subject, allowing the measurement of time point-spread functions at both excitation and fluorescence wavelengths. The scanner is endowed with a stereo camera pair for measuring the outer shape of the subject in 3D. Surface and DOT measurements are acquired simultaneously with the same laser beam. The hardware and software architecture of the scanner are discussed. Phantoms are used to validate the instrument. Results on the localization of fluorescent point-like inclusions immersed in a scattering and absorbing object are presented. The localization algorithm relies on distance ranging based on the measurement of early photons arrival times at different positions around the subject. This requires exquisite timing accuracy from the scanner. Further exploiting this capability, we show results on the effect of a scattering hetereogenity on the arrival time of early photons. These results demonstrate that our scanner provides all that is necessary for reconstructing images of small animals using full tomographic reconstruction algorithms, which will be the next step. Through its free-space optics design and the short pulse laser used, our scanner shows unprecedented timing resolution compared to other multi-view time-domain scanners.

Broadband measurements of the transient optical complex dielectric function of a nanoparticle/polymer composite upon ultrafast excitation

We determined experimentally the complex transient optical dielectric function of a well-characterized polyelectrolyte/gold-nanoparticle composite system over a broad spectral range upon short pulse laser excitation by simultaneously measuring the time-dependent reflectance and transmittance of white light pulses with femtosecond pump-probe spectroscopy. We extracted directly the ultrafast changes in the real and imaginary parts of the effective dielectric function, ${\ensuremath{\epsilon}}_{r}^{\text{eff}}(\ensuremath{\omega},t)$ and ${\ensuremath{\epsilon}}_{i}^{\text{eff}}(\ensuremath{\omega},t)$, from the experiment. This complete experimental set of information on the time-dependent complex dielectric function challenges theories modeling the transient dielectric function of gold particles and the effective medium.

Carrier transfer from wetting layer to quantum dots studied by cw-resolved and time-resolved photoluminescence in CdSe/ZnSe quantum dot system

We have studied the cw-resolved and time-resolved photoluminescence (PL) spectra of the CdSe quantum dots (QDs), where PL peaks from the “wetting layer” and from the QDs are observed. The temporal behavior of the PL spectra after a short-pulse laser excitation provides evidence of carrier transfer from the wetting layer to the QDs for sufficiently low excitation power and sufficiently high temperature. The dependences of the cw-PL spectra and of PL lifetimes on excitation power are explained by the saturation of the carrier concentration in the QDs as the excitation power increases.

Absorption and emission spectroscopic characterization of platinum-octaethyl-porphyrin (PtOEP)

The absorption and emission spectroscopic behaviour of the platinum complexed porphyrin PtOEP (platinum-octaethyl-porphyrin) is studied at room temperature. Liquid solutions, doped films, and a neat film are investigated. The absorption cross-section spectra including singlet–triplet absorption, the triplet–singlet stimulated emission cross-section spectra, the phosphorescence quantum distributions, the phosphorescence quantum yields, and the phosphorescence signal decays are determined. In the neat film a red-shifted phosphorescent excimer emission is observed. In diluted solid solution (polystyrene and dicarbazole-biphenyl films) as well as in de-aerated liquid solutions (tetrahydrofuran, toluene, chloroform) high phosphorescence quantum yields are obtained. In air-saturated liquid solutions, the phosphorescence efficiency is reduced by oxygen quenching. At intense short-pulse laser excitation the phosphorescence lifetime is shortened by triplet–triplet annihilation. No amplification of spontaneous emission was observed.

Electronic and Molecular Structure of Photoexcited [RuII(bpy)3]2+Probed by Picosecond X-ray Absorption Spectroscopy

L(2,3) X-ray absorption spectra of aqueous [Ru(II)(bpy)3]2+ have been recorded in its ground and excited states, 50 ps after short pulse laser excitation. Significant changes in both the XANES (X-ray Near-Edge Absorption Structure) and the EXAFS (Extended X-ray Absorption Fine Structure) regions of the excited state complex are detected. The XANES line shapes have been quantitatively simulated using a crystal field multiplet code in trigonal symmetry. In addition, spectral changes in the EXAFS region of both ground and excited states are analyzed in order to extract structural parameters of their corresponding molecular structures. We obtain a Ru-N bond contraction by approximately 0.03 angstroms in the excited-state complex, as compared to the ground-state compound. This contraction results from electrostatic and polarization contributions, limited by steric constraints on the bpy ligands.