Two-step Two-photon Absorption Research Articles

(Invited) Nanowires from Dilute Nitride and Dilute Bismide Alloys for Nanophotonics

GaAs-based nanowires (NWs) have a great potential for applications in photonics and optoelectronics, partly owing to flexibility in band structure engineering via alloying, strain and structural polymorphism. Among promising GaAs-based materials for device applications are highly mismatched GaNAs and GaAsBi alloys. The giant bowing in the bandgap energy characteristic for these materials allows easy tuning of emission wavelengths, while other modifications of their electronic structure promise to improve performance of infrared lasers and solar cells. In this talk we will review our recent results related to the electronic properties of novel GaNAs and GaAsBi NWs and provide several examples highlighting their advantages.First of all, we will show that a GaNAs-based mutishell NW can be used as an efficient coherent photon source, where the NW acts both as a miniaturized optical resonator and as a photonic gain medium. Its wavelength can be efficiently tuned by minor changes in N composition, e.g. down to 1 µm with only 2.5 % of nitrogen [1]. The lasing mode is found to arise from the fundamental HE11a mode of the Fabry-Perot cavity from a single NW, exhibiting optical polarization along the NW axis. Moreover, due to superior nonlinear properties of GaNAs, self-frequency conversion of the stimulated emission through second harmonic generation and sum-frequency generation is observed, providing coherent light emission in the cyan-green range.Secondly, we will demonstrate that alloying of GaAs with N and/or Bi leads to the formation of self-assembled quantum dots (QDs) embedded in NWs and, therefore, is an efficient way to fabricate hybrid QD-NW structures with high optical quality. In the case of dilute nitrides, optically active and highly localized QD states are formed due to local fluctuations in N composition [2], whereas in dilute bismides they are caused by Bi segregation at twin planes [3]. Such QDs act as single photon emitters, promising for application in quantum communication technologies.And finally, we will show that favorable band alignment at the interface between a dilute nitride alloy and parental N-free material leads to very efficient energy upconversion due to two-step two-photon absorption in the related NW heterostructures, with the efficiency of this process being among the highest reported in semiconductor nanostructures. Specifically, we show [4] that, in radial GaAs(P)/GaNAs(P) core/shell NW heterostructures, the upconversion efficiency increases by 500 times as compared with that of the constituent materials, even under an excitation power as low as 100 mW/cm2 that is comparable to the one-sun illumination. The upconversion efficiency can be further improved by 8 times through engineering the electric-field distribution of the excitation light inside the NWs so that light absorption is maximized within the desired region of the heterostructure.

Influence of bovine serum albumin (BSA) on photodynamic properties of meso-tetrakis(sulfonatophenyl) porphyrin (TPPS4) for picosecond pulse trains

Due to the special interest and unique advantage of bovine serum albumin among various proteins, we carried out researches into effects of bovine serum albumin on photophysical characters of meso-tetrakis(sulfonatophenyl) porphyrin. The laser source we chose is picosecond pulse trains containing 20 subpulses of 70 picoseconds width separating by 13 nanoseconds at 532 nanometers. The theoretical analysis was carried out with a generalized five-level model representing the molecular populating structure. The two-dimensional paraxial field combined with the rate equations was solved by Crank-Nicholson numerical method. The nonlinear optical absorption mechanism in porphyrin is mainly two-step two-photon absorption. We found meso-tetrakis(sulfonatophenyl) porphyrin always shows prominent optical limiting effects binding with or without bovine serum albumin. In the presence of bovine serum albumin, the photophysical parameters of porphyrin get certain influence and cause slight weakening of optical limiting for porphyrin.

Optical properties of aminophenoxy substituted phthalocyanines and covalent linkage conjugates to cadmium telluride quantum dots interacting with picosecond pulse trains

We investigated nonlinear optical dynamics of asymmetric benzothiazole substituted phthalocyanines and their covalent attachments to glutathione functionalized cadmium telluride quantum dots (CdTe QDs) interaction with picosecond pulse trains. A pulse train with 50 subpulses of 100 picosecond width which was separated by 12 nanosecond spacing was employed as the laser source. A generalized five-level model was adopted to retrench the studied materials. In our simulation, we solved two-dimensional paraxial field equation entangled with the rate equations by Crank–Nicholson numerical method. Interacting with long pulse trains, the principal absorption channel in substituted phthalocyanines and their covalent attachments is the two-step two-photon absorption (TPA). All materials show excellent optical limiting behaviours and the results indicated that the covalent attachments of phthalocyanines to CdTe QDs are much better than phthalocyanines with long pulse trains.

Unraveling relation between nonlinear absorption and structure of push pull ornamented anthracenyl chalcone derivatives

Using green chemical method a series of fifteen systematically substituted 3-(9-substituted anthracen-10-yl)-1-(4-phenyl substituted)prop-2-en-1-one derivatives having high nonlinear absorption coefficient have been synthesized. The nonlinear absorption properties of these compounds were studied using nanosecond pulses at 532 nm wavelength. All 15 derivatives are found to show high nonlinear absorption in the range of 35–640 cm/GW. Among these the derivatives with –NO2 as one of the substitution group show higher nonlinear absorption. The origin of high nonlinear absorption with low linear absorption in the compounds has been attributed to two-step two-photon absorption process. We have shown that the energy bands of these compounds are distributed such that for excitation at 532 nm they have strong nonlinear absorption. These properties of the reported compounds make them a potential candidate for biological imaging and other applications which are based on nonlinear absorption.

Optical dynamics of aminophenoxy substituted phthalocyanines and the covalent attachments to cadmium telluride quantum dots for picosecond pulse trains

Optical interaction of aminophenoxy substituted phthalocyanines with picosecond pulse trains were studied in this work. Additionally, the covalent attachments of phthalocyanines to glutathione (GSH) functionalized cadmium telluride quantum dots (CdTe QDs) were also discussed. A five-level model was applied to simplify the materials. The pulse train has 50 subpulses with 100 ps width for each one and 12 ns spacing. We simulated the dynamical interaction by solving two-dimensional paraxial filed equation coupled with the rate equations based on Crank–Nicholson numerical method. The two-step two-photon absorption (TPA) is the fundamental channel of optical absorption for long pulse trains. Our results indicated that the covalent attachments of phthalocyanines to CdTe QDs show better optical limiting effects than phthalocyanines at the same densities. The cross-sections of one-photon absorption and lifetimes of lowest triplet state as well as the intersystem crossing play important roles in optical limiting effects.

Analysis of the intermediate-band absorption properties of type-II GaSb/GaAs quantum-dot photovoltaics

Quantum-dot (QD) intermediate-band (IB) materials are regarded as promising candidates for high-efficiency photovoltaics. The sequential two-step two-photon absorption processes that take place in these materials have been proposed to develop high-efficiency solar cells and infrared (IR) photodetectors. In this work, we experimentally and theoretically study the interrelation of the absorptivity with transitions of carriers to and from the IB in type-II GaSb/GaAs QD devices. Our devices exhibit three optical band gaps with: ${E}_{\mathrm{L}}=0.49\phantom{\rule{0.16em}{0ex}}\mathrm{eV},{E}_{\mathrm{H}}=1.02\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, and ${E}_{\mathrm{G}}=1.52\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, with the IB located 0.49 eV above the valence band. These values are well supported by semiempirical calculations of the QDs electronic structure. Through intensity-dependent two-photon photocurrent experiments, we are able to vary the filling state of the IB, thus modifying the absorptivity of the transitions to and from this band. By filling the IB with holes via $E=1.32\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ or $E=1.93\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ monochromatic illumination, we demonstrate an increase in the ${E}_{\mathrm{L}}$-related absorptivity of more than two orders of magnitude and a decrease in the ${E}_{\mathrm{H}}$-related absorptivity of one order of magnitude. The antisymmetrical evolution of those absorptivities is quantitatively explained by a photoinduced shift of the quasi-Fermi level of the IB. Furthermore, we report the observation of a two-photon photovoltage, i.e., the contribution of subband gap two-photon absorption to increase the open-circuit voltage of solar cells. We find that the generation of the two-photon photovoltage is related, in general, to the production of a two-photon photocurrent. However, while photons with energy close to ${E}_{\mathrm{L}}$ participate in the production of the two-photon photocurrent, they are not effective in the production of a two-photon photovoltage. We also report the responsivity of GaSb/GaAs QD devices performing as optically triggered photodetectors. These devices exhibit an amplification factor of almost 400 in the IR spectral region. This high value is achieved by minimizing---via doping---the absorptivity in the IR range of the QDs under equilibrium conditions.

Laser-driven isomerization of HCN → HNC: the importance of rotational excitation.

We report a time-dependent quantum wave packet theory, which is employed to interpret the isomerization dynamics of HCN molecules induced by an intense picosecond infrared laser field. Considering the molecular rotational degrees of the freedom, the wave functions are expanded in terms of molecular rotational bases. Our full-dimensional quantum model includes the full Coriolis coupling in the molecular kinetic energy Hamiltonian and dipole approximation in interaction terms. The numerical results show that the field-induced molecule rotational excitation plays an important role in the isomerization dynamical process. Some phenomena appear such as two-step two-photon absorption and highly oscillatory structure in rotational state distributions. The centrifugal sudden (CS) approximation calculation is also carried out and compared in this work; it is shown that the Coriolis couplings may lead to a significant decrease in the isomerization rate but highly enhanced molecular rotational excitation.

Energy upconversion in GaP/GaNP core/shell nanowires for enhanced near-infrared light harvesting.

Semiconductor nanowires (NWs) have recently gained increasing interest due to their great potential for photovoltaics. A novel material system based on GaNP NWs is considered to be highly suitable for applications in efficient multi-junction and intermediate band solar cells. This work shows that though the bandgap energies of GaN(x)P(1-x) alloys lie within the visible spectral range (i.e., within 540-650 nm for the currently achievable x < 3%), coaxial GaNP NWs grown on Si substrates can also harvest infrared light utilizing energy upconversion. This energy upconversion can be monitored via anti-Stokes near-band-edge photoluminescence (PL) from GaNP, visible even from a single NW. The dominant process responsible for this effect is identified as being due to two-step two-photon absorption (TS-TPA) via a deep level lying at about 1.28 eV above the valence band, based on the measured dependences of the anti-Stokes PL on excitation power and wavelength. The formation of the defect participating in the TS-TPA process is concluded to be promoted by nitrogen incorporation. The revealed defect-mediated TS-TPA process can boost efficiency of harvesting solar energy in GaNP NWs, beneficial for applications of this novel material system in third-generation photovoltaic devices.

Characteristics of intermediate state related to anti-Stokes luminescence of ZnO single crystals

Anti-Stokes luminescence from ZnO is supposed to be a two-step two-photon absorption process with an intermediate state. The intermediate state is assumed to be a localized state with two different excited and relaxed states. One of the localized states is believed to be the well-known 2.4 eV green luminescence; the other is difficult to observe experimentally. We found an interesting 2.25 eV deep luminescence from ZnO, which has been shown to relate to anti-Stokes luminescence. The 2.25 eV yellow luminescence was observable only below the band gap excitation and through a time-resolved observation after the excitation light was turned off. The intermediate states were found to be a photo-excited donor-acceptor pair and its lattice relaxation state. The characteristics and the role of the intermediate state of ZnO related to the anti-Stokes luminescence are discussed.

Identification of strong one-step two-photon absorption in a cascadethree-level system

The ultrafast one-step and two-step two-photon absorption processes in a cascade three-level systeminduced by a strong femtosecond laser pulse is studied. The strong change of the population in the two-photonabsorption state at the zero points of the first derivative of the population in the real intermediate state showsstraightforwardly in time domain the footprint of the coherent one-step two-photon absorption.

Turning ZnO into an Efficient Energy Upconversion Material by Defect Engineering

Photon upconversion materials are attractive for a wide range of applications from medicine, biology, to photonics. Among them, ZnO is of particular interest owing to its outstanding combination of materials and physical properties. Though energy upconversion has been demonstrated in ZnO, the exact physical mechanism is still unknown, preventing control of the processes. Here, defects formed in bulk and nanostructured ZnO synthesized using standard growth techniques play a key role in promoting efficient energy upconversion via two‐step two‐photon absorption (TS‐TPA). From photoluminescence excitation of the anti‐Stokes emissions, the threshold energy of the TS‐TPA process is determined as being 2.10–2.14 eV in all studied ZnO materials irrespective of the employed growth techniques. This photo‐electron paramagnetic resonance studies show that this threshold closely matches the ionization energy of the zinc vacancy (a common grown‐in intrinsic defect in ZnO), thereby identifying the zinc vacancy as being the dominant defect responsible for the observed efficient energy upconversion. The upconversion is found to persist even at a low excitation density, making it attractive for photonic and photovoltaic applications.

Control of hot-carrier relaxation for realizing ideal quantum-dot intermediate-band solar cells

For intermediate-band solar cells, the broad absorption spectrum of quantum dots (QDs) offers a favorable conversion efficiency, and photocurrent generation via efficient two-step two-photon-absorption (TS-TPA) in QDs is essential for realizing high-performance solar cells. In the last decade, many works were dedicated to improve the TS-TPA efficiency by modifying the QD itself, however, the obtained results are far from the requirements for practical applications. To reveal the mechanisms behind the low TS-TPA efficiency in QDs, we report here on two- and three-beam photocurrent measurements of InAs quantum structures embedded in AlGaAs. Comparison of two- and three-beam photocurrent spectra obtained by subbandgap excitation reveals that the QD TS-TPA efficiency is improved significantly by suppressing the relaxation of hot TS-TPA carriers to unoccupied shallow InAs quantum structure states.

Efficient upconversion of photoluminescence via two-photon absorption in bulk and nanorod ZnO

Efficient upconversion of photoluminescence from donor-bound excitons is revealed in bulk and nanorod ZnO. Based on excitation power-dependent PL measurements performed with different energies of excitation photons, two-photon absorption (TPA) and two-step TPA (TS-TPA) processes are concluded to be responsible for the upconversion. The TS-TPA process is found to occur via a defect/impurity (or defects/impurities) with an energy level (or levels) lying within 1.14–1.56 eV from one of the band edges, without involving photon recycling. One of the possible defect candidates could be VZn. A sharp energy threshold, different from that for the corresponding one-photon absorption, is observed for the TPA process and is explained in terms of selection rules for the involved optical transitions.

Up-conversion luminescence of CdTe and CdTe/CdS quantum dots excited by femtosecond laser of low intensity

We studied the steady-state and time-resolved luminescence properties of CdTe and CdTe/CdS core/shell quantum dots by one-and two-photon excitation with a femtosecond laser of low intensity. The 800 nm excitation causes a blue shift of the emission peak compared with 400 nm laser excitation. Near-quadratic laser power dependence of the up-conversion intensity and biexponential decay kinetics are observed. It is found that upconversion luminescence is composed of a photoinduced trapping and a band edge excitonic state. The blue shift of the emission peak is caused by the relative change in luminescence intensity between excitonic and trapping states. Two-step two-photon absorption involving the surface as intermediate states has been proposed for the mechanism of up-conversion luminescence of CdTe/CdS quantum dots.

Anti-Stokes photoluminescence in ZnO microcrystal

Low temperature (10K) strong anti-Stokes photoluminescence (ASPL) of ZnO microcrystal excited by low power cw 532nm laser is reported here. Energy upconversion of 1.1eV is obtained in our experiment with no conventional nonlinear effect. Through the study of the normal photoluminescence and temperature dependence of ASPL we conclude that the green band luminescence in ZnO is related to deep donor to valance band transition. Using the two-step two-photon absorption model, we provide a plausible mechanism leading to the ASPL phenomenon in our experiment.

Photoluminescence upconversion in GaInNP∕GaAs heterostructures grown by gas source molecular beam epitaxy

Properties of photoluminescence (PL) upconversion (PLU) in GaInNP∕GaAs heterostructures are studied in detail by employing a number of optical spectroscopies. Based on excitation power dependent and temperature dependent PL measurements, the upconverted PL from GaInNP under optical excitation below its band gap is attributed to radiative transitions involving spatially separated localized electron-hole pairs, which is of a similar origin as the near-band-gap emission detected under optical excitation above the GaInNP band gap. The PLU process is shown to be largely promoted by increasing N content in the GaInNP alloys, due to a N-induced change in the band alignment at the GaInNP∕GaAs heterointerface from the type I in the N-free structure to the type II in the samples with N compositions exceeding 0.5%. A possible mechanism for the energy upconversion is discussed in terms of two-step two-photon absorption. The photon recycling effect is shown to be important for the structures with N=1%.

Interplay of One- and Two-Step Channels in Electrovibrational Two-Photon Absorption

We present a theory of two-photon absorption that addresses the formation of spectral shapes taking the vibrational degrees of freedom into account. The theory is used to rationalize the observed differences between the spectral shapes of one- and two-photon absorption. We find that the main cause of these differences is that the two-step and coherent two-photon spectral bands are different even considering a single final state. Our formalism is applied to the N101 molecule (p-nitro-p'-diphenylamine stilbene), which was recently studied experimentally. Simulations show that the two-step two-photon electrovibrational absorption results in a blue shift of the absorption spectrum in agreement with the measurements.

Spectral profiles of two-photon absorption: Coherent versus two-step two-photon absorption

ABSTRACTWe present a theory of two-photon absorption in solutions which addresses the formation of spectral shapes taking account of the vibrational degrees of freedom. The theory is used to rationalize observed differences between spectral shapes of one- and two-photon absorption. We elaborate on two underlying causes, one trivial and one non-trivial, behind these differences. The first refers simply to the fact that the set of excited electronic states constituting the spectra will have different relative cross sections for one-and two- photon absorption. The second reason is that the two-step and coherent two-photon absorption processes are competing, making the one-and two-photon spectral bands different even considering a single final state. The theory is applied to the N-101 molecule [di-phenyl-amino-nitro-stilbene] which was recently studied experimentally in the paper [ T.-C. Lin, G.S. He, P.N. Prasad, and L.-S. Tan, J. Mater. Chem., 14, 982, 2004.]

Photoluminescence upconversion in colloidal CdTe quantum dots

Photoluminescence ~PL! up-conversion, the observation of luminescence with energies higher than those of the excitation photons, has been widely observed in the bulk 1 and heterostructures 2‐5 of semiconductors, quantum wells, 6 porous silicon, 7 self-assembled quantum dots ~QDs!, 8 and colloidal QDs. 9,10 It was proposed that the existence of intermediate states with energies resonant with or lower than those of the excitation photons is a prerequisite for these upconverted photoluminescence ~UCPL! processes. Carriers must be excited to these intermediate states by the first excitation photon and further to the higher energy states through various underlying mechanisms such as Auger excitation, 7 two-step two-photon absorption ~TS TPA!, 1‐3,5,6 or the thermal effect. 9,10 Most of these intermediate states are ‘‘observable,’’ 2‐7 i.e., in addition to the UCPL emission, one can also see the normal PL emission from these states with energies lower than those of the excitation photons. However, in some cases of self-assembled and colloidal QDs, these intermediate states can hardly be observed since carriers cannot efficiently populate them by the above-band-gap excitation of normal PL. 8 In a way, UCPL can provide a unique method for detecting the existence of these ‘‘defect’’ states in semiconductor quantum nanostructures and investigating their properties, which are usually different from those of the band-gap states. Semiconductor colloidal QDs are characterized by a large surface-to-volume ratio due to their small particle sizes, as well as the surface complexity originating from the passivation ligands used to neutralize the surface dangling bonds. Surface states have long been invoked to be responsible for some complex phenomena that could not be explained by using the concept of band-gap states only. For example, the almost universal occurrence of a biexponential distribution in the radiative lifetime strongly implies the existence of surface states in colloidal QDs. 11,12 In this sense, time-resolved measurements can provide one, but only indirect, way to probe the surface states in colloidal QDs, and consequently the existence of them is still controversial. 13 However, if one can observe a UCPL signal from colloidal QDs, this problem will be easily resolved since the demand of intermediate states can only be fulfilled by the existence of surface states. Recently, UCPL signals were observed in colloidal InP, 9 CdSe, 9,10 and CdTe ~Ref. 10! QDs, which have provided a strong proof of the existence of surface states in colloidal QDs and opened up a new opportunity for the systematic study of these surface states. Here, we report on our optical studies of efficient UCPL in colloidal CdTe QDs with an energy gain of as high as 360 meV. Compared with the normal PL, the UCPL signal shows a redshift of about 80 meV in its peak energy, while the excitation spectroscopy of the UCPL ~UCPLE! measurement at this peak energy showed a broad excitation band with energies below those of the band gap. Time-resolved measurements were utilized to study the time-resolved dynamics of this UCPL, whose radiative lifetime now becomes nearly twice as long as that of the normal PL. Finally this UCPL is attributed to the recombination of carriers from the band-gap states and surface states mainly through a thermal excitation process.

Low-temperature anti-Stokes photoluminescence in CdSe/ZnSe nanostructures

Intense anti-Stokes photoluminescence was observed at low temperatures in CdSe/ZnSe nanostructures with separate CdSe inserts in a ZnSe matrix; the nominal thickness of these inserts amounted to 1.5 and 0.6 monolayers. It is shown that the intensity of an anti-Stokes band excited by the photons of energies considerably lower than the band’s peak is quadratic in the excitation power; in the case of resonance excitation, a weaker dependence is obtained. A mechanism behind the excitation of anti-Stokes photoluminescence is suggested on the basis of nonlinear two-step two-photon absorption via the deep states of the defect centers including cation vacancies localized at the barrier-nanoisland interface.