Excited State Absorption Transition Research Articles

Using two-dimensional photon echo spectroscopy to probe the fine structure of the ground state biexciton of CdSe nanocrystals

The origin of the fine structure of the ground state single- and biexciton of CdSe nanocrystals is reviewed, along with the theoretical framework used to describe these states. Calculations were performed to determine the transition dipole moments of optically allowed transitions from the single- to biexciton fine structure states. Two-dimensional photon echo spectroscopy measurements for a sample of CdSe nanocrystals are reported. The two-dimensional electronic spectrum at a population time of 0 fs is analyzed using a simulation based on k.p theory predictions of the exciton and biexciton manifolds of states. The analysis suggests that a particular excited state absorption transition from the single- to biexciton fine structure dominates the 2D spectra. These excited state absorptions are clearly resolved in 2D spectra and the method therefore has promise for gaining clearer insights into quantum dot spectroscopy.

Double-quantum two-dimensional electronic spectroscopy of a three-level system: Experiments and simulations

Double-quantum coherence two-dimensional (2Q2D) electronic spectroscopy is utilized to probe the dynamic fluctuations of electronic states in a solvated molecule at approximately twice the energy of the ground state bleach transition. The 2Q2D spectrum gives insight into the energetic position and spectral fluctuations (system-bath interaction) of the probed excited states. Combining it with single-quantum two-dimensional (1Q2D) electronic spectroscopy enables one to determine the strength of the excited state absorption transition and the relative detuning of electronic states, as well as the dynamics of the single-quantum coherence. To investigate the correlation of spectral fluctuations in different electronically excited states, we have carried out experiments on a solvated dye (Rhodamine 6G) with 23 fs pulses centered at the maximum of the linear absorption spectrum. The 2Q2D spectrum reveals three peaks of alternating signs with the major negative peak located at higher frequencies along the emission axis compared to the single positive peak. The 1Q2D spectrum, on the other hand, shows a negative peak stemming from excited state absorption at lower frequencies along the emission axis. Analysis of the signal in the homogeneous limit fails to account for this observation as well as the number of peaks in the 2Q2D spectrum. Employing a three-level model in which all time correlations of the third-order response function are accounted for via second-order cumulant expansion gives good agreement with both the 1Q2D and 2Q2D data. Furthermore, the analysis shows that the fluctuations of the probed electronic states are highly correlated, reflecting the modulation by a common nuclear bath and similarities in the nature of the electronic transitions.

Weakly chirped pulses in frequency resolved coherent spectroscopy

The role of weakly chirped pulses (time bandwidth product, DeltanuDeltatau<0.61) on three-pulse photon echo signals has been systematically studied. Pulses with varying chirp were characterized with frequency resolved optical gating (FROG) and used to measure spectrally resolved three-pulse photon echoes of a dye in solution. The weakly chirped pulses give rise to markedly different echo signals for population times below approximately 100 fs. The chirped pulses can decrease or enhance spectral signatures of an excited state absorption transition in the echo signal. Furthermore, the observed dephasing dynamics depend on the phase of the electric fields. Simulations based on a three-level model and the electric fields retrieved from the FROG traces give a good agreement for photon echo experiments with both transform limited and chirped pulses. The simulations also allow for a numerical investigation of effects of chirp in two-dimensional spectroscopy. For a two-level system, the chirped pulses result in nonelliptical two-dimensional spectra that can erroneously be interpreted as spectral heterogeneity with frequency dependent dephasing dynamics. Furthermore, chirped pulses can give rise to "false" cross peaks when strong vibrational modes are involved in the system-bath interaction.

Characteristics of intrinsic optical bistability and parameter optimization in Tm3+/Yb3+ codoped laser crystal

Intrinsic optical bistability （IOB） in Tm3+/Yb3+ codoped laser crystal under 650 nm laser excitation is theoretically studied. Based on the theory of nonlinear coupled rate equations, we numerically analyze IOB evolutions of the populations at Tm3+/Yb3+ energy levels, and the influence of system parameters on IOB and the dynamic operation of bistable hysteresis loop. The simulation result shows the fluorescence IOB in visible-infrared spectra of Tm3+/Yb3+ codoped laser crystal under photon avalanche pumping scheme. It is found that IOB phenomena can be largely enhanced by optimizing the Tm3+/Yb3+ concentration ratio, choosing low phonon-energy crystal host and using crystal cooling technology, and also by tuning the pump laser to match the resonant frequency of Tm3+ excited-state absorption transition. Through tuning the pump laser wavelength and adjusting the rate of change in pump intensity, the controllable IOB switching can be achieved.

Resonant enhancement of refractive index in transition element doped crystals via coherent control of excited state absorption

Index of refraction characterizes the linear response of a medium and hence it is strongly enhanced near atomic resonance. However, it is accompanied by either resonant absorption or gain preventing the use of high index. We show that these problems may be overcome using coherent control of excited state absorption (ESA), i.e. absorption from the populated excited electronic state. The idea is to compensate gain at the lower operating transition via partially suppressed ESA at the frequency of the probe field tuned to the maximum refractive index of the ESA transition. It results in resonant enhancement of refractive index without absorption in the absence of any neighboring amplification regions.

Initial state-resolved excited state absorption spectroscopy of ZBLAN:Ho3+ glass

Phase-sensitive and frequency-resolved detection techniques are used for the initial state-resolved excited state absorption (ESA) measurements in ZBLAN:Ho3+ glass. Both experimental techniques were applied simultaneously in a broad spectral range (550–1750 nm) for the first time. Estimated results are compared and discussed in detail. A simple kinetic model, used for qualitative considerations, is presented and successfully compared with the experimental data. The measured spectra will be useful for identifying new up-conversion excitation channels in the considered system, where ESA transitions originating from several excited levels are observed.

Pump excited state absorption in holmium-doped fluoride glass

The primary excited state absorption processes relating to the I65→I75 3μm laser transition in singly Ho3+-doped fluoride glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the I65 and I75 energy levels established the occurrence of two excited state absorption transitions from these energy levels that compete with previously described energy transfer upconversion processes. The I75→I45 excited state absorption transition has peak cross sections at 1216nm (σesa=2.8×10−21cm2), 1174nm (σesa=1×10−21cm2), and 1134nm (σesa=7.4×10−22cm2) which have a strong overlap with the I85→I65 ground state absorption. On the other hand, it was established that the excited state absorption transition I65→S25 had a weak overlap with ground state absorption. Using numerical solution of the rate equations, we show that Ho3+-doped fluoride fiber lasers employing pumping at 1100nm rely on excited state absorption from the lowest excited state of Ho3+ to maintain a population inversion and that energy transfer upconversion processes compete detrimentally with the excited state absorption processes in concentrated Ho3+-doped fluoride glass.

Spectroscopic properties of Er3+ in Sc:LiNbO3 crystal

The results of Er3+ ion spectroscopic analysis in Sc:LiNbO3 crystals were reported. The line strengths from the ground state to the excited state were evaluated from the measured unpolarized absorption spectrum and analyzed by using standard Judd–Ofelt theory. For Sc(3 mol. %):Er (1 mol. %):LiNbO3 crystal, the obtained intensity parameters are: Ω2=3.72×10-20 cm2, Ω4=1.07×10-20 cm2, and Ω6=0.98×10-20 cm2. The fluorescence spectra and microsecond time-resolved spectra were investigated in the visible region. The excited state absorption transition strengths at 800 nm excitation were evaluated based on Judd–Ofelt theory. The results obtained here were compared to results from other research on Er:LiNbO3 crystals.

An analysis of erbium excited state absorption in silicon-rich silica

Erbium-doped silicon-rich silica is a material that has generated great interest, as it holds considerable promise for practical silicon-based optical sources. There are numerous reports of sensitization of erbium 1.5 μm luminescence by silicon nanoclusters (Si nc), and it is now accepted that this process increases the effective absorption cross-section of Er ions by three orders of magnitude. There have been reports of optical gain in a planar waveguide pumped at wavelengths away from Er absorption bands. However, no lasing action has yet been achieved. In this paper, we discuss a key interaction—excited state absorption (ESA)—which has been neglected in this material. Typically, Si nc responsible for sensitization have a broad photoluminescence peak centred around 800 nm. Moreover, spectral hole-burning studies suggest that resonant energy transfer occurs from the Si to the Er in this wavelength region. However, this energy transfer route may be problematic, due to an ESA transition known from early work on erbium-doped fibre amplifiers (EDFAs). Excitation around 800 nm causes the Er metastable state to be excited to higher-level states. Such transitions serve no useful purpose, but constitute an energy drain: EDFAs pumped at 800 nm require an order of magnitude more pump power to achieve the same gain as those pumped at 980 nm. We have conducted an analysis of the erbium-doped silicon-rich silica system, incorporating ESA in the model. We compare the results of our analysis with reported experimental data and extract the Si nc–Er excited state energy transfer coefficient, yielding a value of 1×10 −15 cm 3/s. This is comparable to the Si nc–Er ground state transfer coefficient, confirming that ESA is potentially significant.

Photoionization energies of Cr 3+-doped LiNbO 3

We report on a study of intra-ion and electron transfer transitions in Cr 3+-doped LiNbO 3. The energies of the meta-stable emitting levels of Cr 3+ on both Li and the Nb sites are determined by standard optical spectroscopy techniques. Photoconductive measurements are used to determine the ionization energies of both species of Cr 3+ ions. From these results, energy level diagrams are created which give the energies of the optically active levels with respect to the host valence and conduction bands. The emitting 4T 2 level of Cr 3+ on a Li site and on a Nb site is found to lie 1.5 and 1.2 eV below the conduction band bottom, respectively. These results lead to the suggestion that the lack of laser action found in LiNbO 3:Cr 3+ may be due to strong excited state absorption transitions from the meta-stable 4T 2 level to the host conduction band.

Energy transfer processes andHo3+:I55level population dynamics in chalcohalide glasses

Population dynamics of ${\mathrm{Ho}}^{3+}$: $^{5}\mathrm{I}_{5}$ levels doped in chalcohalide glasses were investigated through the analysis of energy transfer processes. Excitation migration $(^{5}\mathrm{I}_{5},^{5}\mathrm{I}_{8}\ensuremath{\rightarrow}^{5}\mathrm{I}_{8},^{5}\mathrm{I}_{5})$ predicted from the Dexter model dominated over the cross relaxation $(^{5}\mathrm{I}_{5},^{5}\mathrm{I}_{8}\ensuremath{\rightarrow}^{5}\mathrm{I}_{7},^{5}\mathrm{I}_{7})$. Depopulation of the $^{5}\mathrm{I}_{5}$ level due to upconversion was suppressed when the $890\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ excitation source was used since this pump wavelength significantly depressed the excited-state absorption transition from the $^{5}\mathrm{I}_{5}$ level. The dominant energy transfer channel leading to the depopulation of the $^{5}\mathrm{I}_{5}$ level is the energy transfer upconversion process between ${\mathrm{Ho}}^{3+}$ ions.

Excited-State Upconversion of Pr(0.5)Yb(1.5):ZBLAN underTwo-Colour Excitation

We report the excited-state upconversion in Pr(0.5)Yb(1.5):ZBLANunder two-colour excitation of 960 nm laser and xenon lamp light. Threeobvious upconversion excitation peaks around 856.0, 804.2 and 787.1 nmwere observed and the involved mechanism has been explained. Themeasured upconversion excitation peak 856.0 nm corresponds to the sumof theoretical values 852 nm and 866 nm owing to the1G4(Pr3+)→ 1I6(Pr3+) and1G4(Pr3+)→ 3P1(Pr3+) excited stateabsorption transitions. The measured 804.2 and 787.1 nm upconversionexcitation peaks originate from the excited state absorption transitions3H6(Pr3+)→ 1D2(Pr3+) and1G4(Pr3+)→ 3P2(Pr3+), respectively. Theexcited state absorption upconversion 1G4(Pr3+)→1I6(Pr3+) is strongbecause of its large oscillator strength f = 23.040×10−6.

Phase-sensitive detection of excited-state absorption transitions in Yb3+-codoped, Ho3+-doped YLiF4

This paper presents a method for the separation of excited-state absorption transitions from different initial levels by phase-sensitive detection by use of the double-modulation technique. This technique is also applicable for the distinction of spectrally overlapping transitions, and in connection with Judd-Ofelt calculations it allows the excited-state absorption cross sections to be determined. With this method, the 5I_7 --&gt; (5S_2, 5F_4), 5I_6 --&gt; (5G_6, 5F_1), 5I_6 --&gt; 5F_2, and 5I_6 --&gt; 5F_3 transitions in the spectral range between 720 and 860 nm in Ho3+-doped YLiF4 were measured and their cross sections determined. The method can be used for any similar system.

The excited state upconversion of Pr(0.5):ZBLAN under two-color excitation*

This paper reports the excited-state upconversion of Pr(0.5):ZBLAN glass under two-color excitation. It is found that the fluorescence of upconversion-emiss ion spectrum is the same as that of common-emission spectrum. It is found also that there are three obvious peaks on upconversionexcitation spectrum under twocolor excitation, which corresponds to the 788.5nm 1G4→3P2,850.5nm 1G4→1I6,and 805.0nm 3H6→1D2 excited state absorpt ion transitions res pectively. The large 850.5nm peak of upconversion-excitation spectrum results from the large 1G4(Pr3+ )→1I6(Pr3+) (Pr3+) oscil lator strength f=23.04×10-6. It illustrates the excited state absorp tion upconve rsion from 1G4 level, especially the 1G4(Pr3+)→1I6(Pr3+) upconversion process is large. It results in the upcon version luminescence of Pr(0.5):ZBLAN under two-color excitation.

Experimental determination of the orientation of excited state transition dipoles in tetrapyrroles with different molecular symmetries

So far, the orientations of transition dipole moments in tetrapyrroles have been investigated mainly by fluorescence anisotropy spectroscopy that allows the determination of the orientations of S 0–S n absorption transition dipoles with respect to a fixed emission transition dipole. However, little is known about the orientation of excited state absorption (ESA) transition dipole moments. Transient absorption spectroscopy (TAS) with polarized probing permits access to these transitions. We present photoinduced fluorescence and TAS anisotropy data of three tetrapyrroles of different molecular symmetries. The orientation of the S 0–S n and S 1–S n transition dipoles were extracted from these data. A distinctive influence of the molecular symmetry was found for tetra( t-butyl)-phthalocyanine and its zinc complex due to a doubly degenerate S 1 state in the latter. For a reduced porphyrin, pheophorbide-a, the higher asymmetry of the molecule influences the orientation of the electronic transition dipole moment only slightly.

Crystal field effects on the optical absorption and luminescence properties of Ni2+-doped chlorides and bromides: crossover in the emitting higher excited state.

Single crystals of CsCdCl3, CsCdBr3, CsMgBr3, and MgBr2 doped with 0.1/5% Ni2+ were grown by the Bridgman technique and studied by variable-temperature optical absorption and luminescence spectroscopies. At cryogenic temperatures all these systems are dual emitters; i.e., they emit light from two distinct, thermally nonequilibrated excited states. The emitting higher excited state is 1T2g in Ni2+:CsCdCl3 and Ni2+:CsCdBr3 and 1A1g in Ni2+:CsMgBr3 and Ni2+:MgBr2. This crossover manifests itself in a change from red broad-band to yellow sharp-line luminescence, and it is rationalized on the basis of crystal field theory. Temperature-dependent luminescence as well as two-color pump and probe experiments reveal that in Ni2+:CsMgBr3 and Ni2+:MgBr2 the 1T2g state lies only about 70 and 170 cm-1, respectively, above 1A1g. The effect of crystal field strength on thermally activated nonradiative multiphonon relaxation processes in the bromides is examined for both 1A1g/1T2g higher excited state and 3T2g first excited-state emission. Two-color excited-state excitation experiments are used to monitor Ni2+ excited-state absorption transitions originating from 3T2g.

Sequence lasing in a gain-switched Yb3+,Er(3+)-doped silica double-clad fiber laser.

Experimental results relating to the gain-switched operation of a double-clad Yb3+,Er(3+)-doped silica fiber laser that is pulse pumped with the output from a flash-lamp-pumped Ti:sapphire laser are presented. For all the configurations of the fiber laser that we studied, the 2F5/2-->2F7/2 laser transition of the Yb3+ ion lased prior to laser emission from the 4I13/2-->4I15/2 transition of the Er3+ ion. To the best of our knowledge, this is the first reported operation of sequence lasing in the Yb3+,Er(3+)-codoped system. This succession of laser pulses deduced from the measurements of this investigation is a consequence of both the short intense pump pulse and the short 900-nm wavelength of the pump that does not overlap with any important excited-state absorption transitions. We believe that the predominant interionic interaction during the course of the pump pulse is the double-energy transfer to the Er3+ ion acting twice from the 2F5/2 energy level of the Yb3+ donor ion. A maximum total output of 1.65 mJ is obtained (1.38 mJ from the 2F5/2-->2F7/2 transition of Yb3+ and 0.27 mJ from the 4I13/2-->4I15/2 transition of Er3+) from a nonoptimized configuration of the fiber laser. The wavelength of the output from the fiber laser was measured to vary approximately linearly with fiber length from 1040 to 1046 nm for the Yb(3+)-based laser and 1535 to 1541 nm for the Er(3+)-based laser.

Luminescence Upconversion Under High Pressure in Ni 2+ Doped CsCdCl 3

The near-infrared to visible upconversion properties of Ni 2+ doped CsCdCl 3 are studied as a function of hydrostatic pressure. At 15 K and ambient pressure, near-infrared excitation of Ni 2+ around 810 nm leads to upconversion luminescence centered around 610 nm. Due to the increase of the ligand field strength with increasing pressure, the emission band moves to higher energies with a rate of 19 cm m 1 /kbar, and at 46 kbar it is centered around 585 nm. Thus the upconversion luminescence undergoes a color change from red to yellow in the 1 bar-46 kbar pressure range. The 15 K pressure-dependent upconversion excitation spectra reveal a slight red-shift of m 4 cm m 1 /kbar of the upconversion relevant 3 T 2g M 1 T 2g excited state absorption (ESA) transition, and this is related to a reduction of the Racah B and C parameters, caused by an increasing degree of covalency in the Ni 2+ -Cl m interaction towards higher pressures. Thus, with upconversion luminescence excitation spectroscopy we can obtain information about the pressure-dependence of an ESA transition. The pressure-induced intensity redistributions within this excitation spectrum are indicative of an increasing spectral overlap of 3 T 2g M 1 T 2g ESA with 3 A 2g M 3 T 1g ground state absorption, potentially leading to more efficient upconversion at higher pressures than at ambient pressure.

Excited-state-absorption in low concentrated Er : YAG crystals for pulsed and cw pumping

In this paper we discuss the upconversion processes responsible for violet luminescence from the erbium levels 2P 3/2 (transition 2P 3/2→ 4I 13/2) and 2H2 9/2 ( 2H2 9/2→ 4I 15/2), excited at room temperature in low concentrated Er : YAG crystals with pulsed (at 532 nm) as well with cw (at 488 nm) lasers. Besides the general need for new spectroscopic data concerning laser materials, the interest for this study is generated by the fact that energy levels of very different quantum efficiencies (∼0.25 for 2P 3/2 and ∼0.0004 for 2H2 9/2) can produce, in favorable pumping conditions, fluorescence spectra of comparable intensities. Thus, though for 532 nm pulse excitation (pump transition 4I 15/2→ 4S 3/2), the intensity of the lines originating from 2P 3/2 is much stronger than the lines starting from 2H2 9/2, in accord with the corresponding quantum efficiencies, cw pumping at 488 nm (pump transition 4I 15/2→ 4F 7/2) produces luminescence spectra of comparable intensities. In order to explain the relative intensities of the observed luminescence spectra, various two-step upconversion mechanisms (excited state absorption (ESA) at low erbium concentrations) were proposed. We found that for cw excitation, an analysis of the energetic resonance between ESA transitions and pumping quanta was necessary. For this, we used a simple model to simulate the ESA spectra, in which both initial and final states were approximated with sums of Lorentzian functions and the multiplet to multiplet transition probabilities were estimated with the Judd–Ofelt theory. The choice of the linewidths in our model was based on the analysis of the room temperature absorption and fluorescence spectra and on the possible resonances with the phonons. A fairly good agreement between the observed intensity ratio of the 2P 3/2→ 4I 13/2 and 2H2 9/2→ 4I 15/2 luminescence spectra and that predicted by our model was obtained.

In situ measurement of absorption and emission cross sections in Er/sup 3+/-doped waveguides for transitions involving thermalized states

An application to transitions involving thermalized states of a procedure based on the McCumber theory to determine emission and absorption cross sections in waveguides from transversal fluorescence spectra is presented. The procedure is tested in Er-doped Ti:LiNbO/sub 3/ waveguides to provide accurate cross-section values for the recently attractive 550-nm band. We also study the excited-state absorption transition around 860 nm and present, to our knowledge, the first published cross-section values for this transition.