ultrabroadband time-resolved spectroscopy in novel types ...2016)-ultra-broadband...

Ultrabroadband time-resolved spectroscopyin novel types of condensed matterCHIH-WEI LUO,1,2,* YU-TING WANG,1 ATSUSHI YABUSHITA,1 AND TAKAYOSHI KOBAYASHI1,3,4

1Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan2Taiwan Consortium of Emergent Crystalline Materials, Ministry of Science and Technology, Taipei 10601, Taiwan3Advanced Ultrafast Laser Research Center and Department of Engineering Science, The University of Electro-Communications,Chofu, Tokyo 182-8585, Japan4JST, CREST, 5 Sanbancho, Chiyoda-Ku, Tokyo 102-0075, Japan*Corresponding author: [email protected]

Received 11 September 2015; revised 2 December 2015; accepted 3 December 2015 (Doc. ID 249942); published 14 January 2016

In condensed matter physics, quasi-particle correlations are crucial to understanding a material’s properties. For ex-ample, strong interaction between electrons with metal-like electron configuration produces strongly correlatedinsulators rather than conductors, which band theory would predict. Therefore, it is important to determine theinteraction strength between different degrees of freedom, e.g., electron, phonon, and spin. Time-resolved spectros-copy is a powerful technique for observing energy transfer between quasi-particles and determining the interactionstrength. Ultrashort-pulse light sources with extremely broadband spectra have extended exploration of ultrafast dy-namics in various materials. Here, novel types of condensed-phase matter are presented to show how several key issuesregarding these materials can be resolved by broadband ultrafast time-resolved spectroscopy. © 2016 Optical Society of

America

OCIS codes: (300.6500) Spectroscopy, time-resolved; (320.7150) Ultrafast spectroscopy; (320.7130) Ultrafast processes in condensed

matter, including semiconductors; (320.7160) Ultrafast technology.

http://dx.doi.org/10.1364/OPTICA.3.000082

1. INTRODUCTION

To understand condensed materials as demonstrated by bandtheory, one could imagine that electrons behave as an extendedplane wave. This theory derives an energy band structure for elec-trons in a periodic lattice of atoms, and electrons in the materialmay be described as having or not having a bandgap [1,2]. In thisdescription, electrons in the material can be considered as being ina “sea” of the averaged motion of the other quasi-particles. Thisapproach of nearly free particles is valid in some well-understoodmaterials, such as most metals, as the interaction strength betweenquasi-particles is negligible compared with their kinetic energy.However, strong correlation between the quasi-particles leads toa new type of behavior in some important materials. Such mate-rials are difficult to describe theoretically because strong inter-actions between quasi-particles cause phenomena that cannot bepredicted by studying the behavior of individual particles alone,and these interactions play a major role in determining the prop-erties of such systems. The seemingly simple material NiO, as thetypical example of metal–insulator transitions, would be expectedto be a good conductor with a partially filled 3D band [3].However, the strong Coulomb repulsion between electrons makesNiO an insulator. Therefore, this type of strongly correlatedmaterial cannot be understood using a free-electron-like scenario.In addition to the metal–insulator transitions just mentioned

[3,4], there are numerous physical properties arising from theeffects of strong correlations, e.g., high-T c superconductivity[5], colossal magnetoresistance [6], heavy fermions [7], multifer-roics [8], and low-dimensional phenomena [9]. Accordingly, thecrucial correlations between quasi-particles can be responsible forsignificant characteristics of some materials and so it is extremelyimportant to discover the underlying interactions among quasi-particles in these materials.

Because interactions among quasi-particles are known to playan important role in understanding condensed matter, experi-mental techniques that can unambiguously clarify these inter-actions are needed. Studies have demonstrated the ability ofnumerous methods to indirectly estimate correlation amongquasi-particles by measuring certain related physical characteris-tics, such as the carrier mobility [10], Shubnikov–de Haas oscil-lations [11], the thermoelectric power [12], the Burstein–Mossshift [13], the Raman shift [14], and Faraday rotation [15]. Asan example, electron–electron interaction is usually studied bytransport measurements. However, the contribution of electron–electron interaction to the resistivity can be observed only at lowtemperatures because the electrical resistance is primarily domi-nated by electron–phonon scattering above the Debye tem-perature. On the other hand, the electron–phonon interactionstrength can be determined from the phonon linewidths obtained

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Review Article Vol. 3, No. 1 / January 2016 / Optica 82

http://dx.doi.org/10.1364/OPTICA.3.000082

through Raman or neutron scattering, which are easily influencedby selection rules and inhomogeneous broadening. Moreover, thequasi-particle interactions obtained by the techniques just men-tioned are derived mainly from stationary experiments.

Ultrafast spectroscopy is one of the desired techniques that en-able direct observation of transient interactions among quasi-particles. With transient spectroscopy, the so-called pump–probemeasurement, we can monitor energy transfer among quasi-particles and even specify the interaction strength, as shown inFig. 1. Taking advantage of developments in the area of ultrashortpulses in recent decades [16–20], ultrafast optical spectroscopycan provide the required time resolution for studying ultrafast pri-mary phenomena on the characteristic time scales of electron,phonon, and spin dynamics [21–33], that is, in the range offemtoseconds (10−15 s), picoseconds (10−12 s), and nanoseconds(10−9 s). Advanced progress in pulsed lasers has also extended ul-trafast spectroscopy from the visible region to the mid-infrared(MIR) and ultraviolet (UV) regions using nonlinear techniquessuch as optical parametric amplification, sum and differencefrequency generation, and four-wave mixing [34–36].

2. BROADBAND TIME-RESOLVEDSPECTROSCOPY

A. Development

Compared with monochromatic detection, the use of spectralbroadband probes for optical measurements provides the abilityto record responses at various wavelengths simultaneously and ob-tain much broader insight into the underlying physics. In 1964,optical broadband detection was first used by Jones and Stoicheff[37]. In that influential work, a continuum light source was gen-erated by incident maser radiation on liquid toluene to study theinduced Raman absorption of liquid benzene. The sample wasirradiated simultaneously with a monochromatic excitation lightof frequency ν0 and a continuum probe light. The excited atomsand molecules change their energy states by �hνM and absorbfrequencies at the Stokes frequency, ν0 − νM , and anti-Stokes fre-quency, ν0 � νM , from the continuum probe light. This break-through in broad spectrum generation enabled the first Ramanabsorption spectrum measurement.

Seven years later, a broadband pulsed light source wasapplied to transient absorption spectroscopy for studying

nonradiative relaxation processes in excited molecules [38,39].Photoisomerization on a time scale of picoseconds was observedin 3-3′diethyloxadicarbocyanine iodide by broadband detectionin the visible region [39]. In 1979, pioneering work by Shanket al., who used a time-resolved white-light continuum pulseprobe to study GaAs thin films, opened a new era for dynamicstudies in solid-state physics [40]. In this period, broadband probepulses were generated by focusing the 750 nm pulses from aNd:YAG amplifier into a cell containing water. The spectral rangeof interest, 785–835 nm, was selected using filters. By taking ad-vantage of broadband detection, the entire relaxation process ofband filling and bandgap renormalization was clearly observedwithin 0.5 ps. In addition, the relaxation processes of excited car-riers in the heavy, light, and split-off hole bands were simultane-ously observed by broadband detection [41,42]. Furthermore, thedynamics of magnetoexcitons in GaAs quantum wells were stud-ied using a spin-resolved broadband probe [43]. Circularly polar-ized pump and probe beams were used to resolve excitonicinteractions with regard to angular momentum states. By usingthe broadband probe, interactions between magnetoexcitons gen-erated by the pump pulses at various angular states were unam-biguously revealed. Magnetoexcitons with identical spins repeleach other and cause a blueshift, whereas those with oppositespins attract each other, causing a redshift.

This brief review shows how the evolution of broadband time-resolved spectroscopy was driven by the development of lightsources. In the 1970s, most continuum light sources were gen-erated by self-phase modulation (SPM) and stimulated Ramanscattering by focusing a colliding pulse mode-locked dye laseron a medium [44–46] or fiber [47], or generated by the fluores-cence from a scintillator dye [48]. In the 1990s, femtosecond lightsources were significantly improved with the development ofsolid-state laser materials [49], e.g., a sapphire crystal (Al2O3)doped with titanium ions (Ti:sapphire) and the chirp-pulse am-plification technique [50]. Solid-state lasers have allowed opticalparametric conversion to extend the spectral range of femtosecondpulses to the UV [35], visible [34], and IR [51,52] regions.A novel method, the noncollinear optical parametric amplifier(NOPA), was proposed to provide a broad spectrum with asub-10-fs pulse width [53,54] or even a sub-5-fs pulse width[19,20,55], and the pulse width has recently even reached2.4 fs [56].

These advanced broadband light sources with ultrashort pulseduration have been applied to various research areas, includingultrafast chemical reactions, photoisomerization, biophysics, andsolid-state materials. By using their extremely high time resolu-tion, the relaxation processes during trans–cis isomerization inthe retinal chromophore of bacteriorhodopsin have been revealedby studying the real-time vibrational dynamics [57]. The environ-mentally affected vibrational photoisomerization processes ofpush–pull substituted azobenzene dye have been disclosed [58].Broadband detection has enabled demonstration of the energytransfer channels and efficiencies in photosynthetic light harvest-ing [59]. The pathways for exciton fine structure relaxation inCdSe nanorods have also been revealed [60]. Furthermore, theelectron–phonon interaction strength in high-T c superconduc-tors was unambiguously determined recently [61].

Time-resolved spectroscopy provides a versatile and effectivetool for studying the dynamics of photoexcited carriers in realtime. Moreover, light sources with a broad spectrum and

Fig. 1. Schematic representation of a system including electron, pho-non, and spin degrees of freedom.


high time resolution enable the unambiguous discovery of thedynamics of energy transfer among quasi-particles both exten-sively and correctly. According to the measured energy transferrate, the interplay among electron, phonon, spin, and orbital mea-surements in various materials or at the interfaces between dis-similar functional materials can be clearly revealed. Therefore,we can further understand some previously baffling issues in crys-talline and dissimilar functional materials.

B. Femtosecond Light Sources

1. Narrowband Optical Parametric Amplifier

A regenerative amplifier (RGA) seeded with a Ti:sapphire laseroscillator served as a light source for a homemade optical para-metric amplifier (OPA). The type-I (e → o� o) β-BBO (bariumborate, BaB2O4)-based OPA was pumped by the second har-monic of the RGA (wavelength, 400 nm; repetition rate, 5 kHz)to obtain output pulses in the visible range of 500–700 nm.

The second harmonic of the RGA was generated by 800 nmpulses focused on a BBO crystal. The 400 nm beam was separatedinto two copies; one served as the pump beam of the parametricinteraction process and the other was used to obtain a white-lightcontinuum as the signal beam. The white-light continuum wasproduced by focusing the 400 nm pulses on a sapphire disk toinduce SPM. Then the white-light continuum passed througha prism pair to remove the fundamental light (400 nm). Boththe signal (the white-light continuum) and pump (400 nm)beams were focused on a BBO crystal to realize a NOPA. Thephase-matching condition is satisfied in the visible broadband re-gion from 500 to 700 nm, so the wavelength of the amplifiedsignal beam can be selected by adjusting the delay between thepump pulse and the linearly chirped visible seed pulse. As a result,the output wavelength of the signal beam can be tuned from500 to 700 nm continuously (Fig. 2).

It is noteworthy that the constructed narrowband OPA systemis based on information on the carrier envelope phase structure inthe NOPA. The noncollinear geometry in the OPA process isused to amplify the broad visible tuning range. For the presentpurposes of the narrowband OPA, the seed pulse is positivelychirped with further insertion of the prism. Therefore, theOPA spectrum can be linearly adjusted to have a single colorby changing the delay between the pump pulse and seed pulse.Additionally, the gain of the OPA was saturated to avoid variationin its output caused by the fluence of the RGA pulses.

2. Broadband Optical Parametric Amplifier

For the broadband OPA, we also used the noncollinear configu-ration, which is the same as that used in the narrowband OPA.However, the signal beam, namely, the femtosecond continuum,is generated by the SPM of an 800 nm pulse focused on a 2 mmsapphire plate. To amplify the femtosecond visible broadbandcontinuum in the full bandwidth, the signal pulses are precom-pressed using a pair of ultrabroadband chirped mirrors. The signalbeam is then noncollinearly overlapped with the pump pulse andfocused on a BBO crystal with the noncollinear angle of α � 3.7°to fulfill the broadband phase-matching condition [19,55].

Finally, both the pulse front-tilted pump beam and the non-collinearly incident signal beam are focused on a BBO crystal.The signal beam is then amplified through the OPA process. Thepulse energy is 86 nJ after the first-stage amplification. Bothpump and signal beams are reflected back to the BBO crystalby concave mirrors in the confocal configuration for the second-stage amplification. The resulting amplified output pulse energy is144 nJ, which is not affected by the fluence of the RGA pulsesowing to the gain saturation of the OPA.

By using a total pulse compressor, the pulse width was com-pressed to ∼9 fs. The output pulse energy after compression is40 nJ. Amplitude and phase characterization of the compressedbroadband OPA pulses was verified by a second-harmonic gen-eration frequency-resolved optical gating (SHG FROG) in a verythin BBO crystal (5 μm). The corresponding spectrum and themeasured SHG FROG are shown in Fig. 3; it exhibits a widevisible spectrum with a nearly constant phase.

C. Pump–Probe Spectroscopy

As shown in Fig. 4, the general idea of the pump–probe techniqueis that responses from a sample induced by a pump pulse are in-vestigated by detecting the changes in the reflectivity (ΔR) ortransmissivity (ΔT ) of probe pulses as a function of the delay timebetween pump and probe pulses. However, the difference in ab-sorbance should be obtained indirectly from ΔR and ΔT . Theabsorbance of the target material without excitation, i.e., beforea pump pulse arrives, is

A � log10

�I 0 − RT

�: (1)

On the other hand, the absorbance after excitation is

A 0 � log10

�I0 − �R � ΔR�

T � ΔT

�; (2)

Fig. 2. Schematic diagram of BBO-based OPA. The OPA waspumped by 400 nm pulses, and its tunable range is 480–700 nm witha pulse duration of 35 fs. Inset: normalized output spectra of thewavelength-tunable OPA, ranging from 500 to 700 nm.

(a) (b)

Fig. 3. (a) Broadband OPA output spectrum, which covers almostthe entire visible range. (b) Measured second-harmonic generationfrequency-resolved optical gating (SHG FROG) trace of the outputbroadband OPA pulses.


where I o is the intensity of the probe pulse. T and R are the trans-mitted and reflected intensities, respectively, of the probe pulsefrom the sample. Therefore, the difference in absorbance,ΔA � A 0 − A, can be derived as follows:

ΔA � log10

�I 0 − �R � ΔR�

T � ΔT

�− log10

�I 0 − RT

�

� log10I 0 − �R � ΔR��1� ΔT

T

��I 0 − R�

: (3)

The femtosecond time evolutions are derived by delaying therelative arrival times of the pump and probe pulses using amechanical delay line. The fluence of the probe beam is usuallymuch weaker than that of the pump beam to avoid a second ex-citation in the samples. Further, the polarizations of the pumpand probe pulses are set perpendicular to each other to avoidinterference between the pump and probe beams [62].

1. Fast-Scan Techniques

In pump–probe measurements, traditional scanning methodscollect data step by step, which is time consuming and easily in-fluenced by the ultrashort pulse laser’s instability. The instabilityof the light sources thus hinders precise determination of elec-tronic decay dynamics and may introduce systematic errors.This makes it difficult to obtain reproducible and reliable exper-imental data. However, a fast-scan pump–probe spectroscopicsystem that can complete a single scan in 5 s has been developed[63]. The rapid scan system is described in detail in [63].

In the fast-scan method, the signal (ΔR or ΔT ) is collectedwhile the delay time is scanned rapidly in 500 steps across the

scanning range. A fast-scan stage with a total accessible pulse delayrange of 15 ps is controlled by an external voltage generated by adigital/analog converter. At each delay point, the signal is ob-tained in 10 ms and stored in the memory of the lock-in amplifier.Averaged values of the data are collected for several hundred scans.This method provides a good signal-to-noise ratio and avoids theeffects of laser fluctuations, including instability of the laser out-put power and pulse width.

2. Broadband Detection Techniques

To detect the relatively weak signal (on the order of 10−4 or less)in pump–probe measurements at multiple probe wavelengths,a multichannel lock-in amplifier developed by our group wasused for time-resolved spectroscopy. The avalanche photodiodes(APDs) were commercial products optimized for UV to visiblelight detection.

As shown in Fig. 5, the probe pulse was dispersed by a poly-chromator into a 96-branch fiber bundle whose other endwas separated into 96 fiber branches and connected to APDs.Therefore, the time-resolved absorption differences at 96 probewavelengths were simultaneously detected at the photodiodes.The detected signals were sent to a multichannel lock-in ampli-fier to be spectrally resolved for simultaneous detection of tinychanges in the probe intensity over the entire spectral region.

3. ULTRAFAST DYNAMICS IN NOVELCONDENSED MATTER

A. Spin-Valley Coupled Polarization in Monolayer MoS2

The discovery of graphene initiated a new era for two-dimensional(2D) materials in condensed matter physics. In particular, muchattention has been focused on single-layer semiconducting mate-rials. For example, the transition metal dichalcogenide MoS2 ex-hibits unique physical, optical, and electrical properties correlatedwith its atomic layered structure. MoS2 is a 2D material consist-ing of a horizontal single layer of molybdenum stacked verticallybetween two single layers of sulfur, as shown in the inset of Fig. 6.The sulfur layers are held together by weak van der Waals forces,allowing MoS2 sheets to be easily separated. Unlike pristine gra-phene, which does not have a bandgap for applications, MoS2possesses an indirect bandgap of 1.2 eV in bulk form, similarto that of silicon, and a direct bandgap of 1.8 eV as an atomicallythin monolayer [64,65]. Moreover, the inherent coupling be-tween the valley and spin in monolayer MoS2 provides a note-worthy characteristic for spintronics [66], valleytronics [67],

(a)

(b)

Fig. 4. (a) Schematic diagram of the pump–probe technique. Timedelay (Δt) between pump and probe pulses can be controlled by amechanical delay line. (b) Fundamental principle of pump–probe spec-troscopy. Time-dependent refractive index changes n�t� of the sampleinduced by pump pulses can be observed by detecting the intensityvariations of probe pulses.

Fig. 5. Schematic diagram of the multichannel lock-in amplifier forthe broadband pump–probe measurement system. APDs, avalanchephotodiodes.


and semiconductor devices [9,68]. For example, monolayerMoS2exhibits a high channel mobility (200 cm2 V−1 s−1) and currentON/OFF ratio (1 × 108) when it is used as the channel materialin a field-effect transistor [68].

Highly polarized luminescence in monolayer MoS2 has beenobserved with resonant excitation. Furthermore, the valley–spinlifetime was previously predicted to be larger than 1 ns [69].However, a time-resolved study of the polarized photolumines-cence (PL) demonstrated that the carrier spin flip has a time scaleof several picoseconds, which is limited by the time resolution ofthe time-resolved PL measurement system [70]. Mai et al. furtherobserved that the polarized exciton A decays within only severalhundred femtoseconds according to optical pump–probe mea-surements [71]. This controversial situation was resolved by aconclusive study of the full dynamics and physical nature of po-larized excitons in monolayer MoS2, including the spin–valleycoupling [72].

The absorption spectrum of monolayer MoS2 in Fig. 6 clearlyshows A (1.89 eV) and B (2.04 eV) excitonic transitions, whichindicate the splitting of the valence band at the K valley due tospin–orbit coupling [64,65]. The pump pulse was generated byan OPA (as demonstrated in Section 2.B.1), and the excitationenergy was set to be resonant with either exciton A or B. A probepulse with a visible broadband spectrum was produced by SPM ofan RGA pulse in a sapphire plate. To distinguish the nonequiva-lent K and K 0 valleys, the polarizations of the pump and probebeams were adjusted to be circular by broadband quarter-waveplates. The pump (probe) beam was focused on the sample in aspot with an area of 1.3 × 10−4 cm2 (0.7 × 10−4 cm2) and a pulseenergy of 40 μJ (3 μJ). The time resolution of the measuring sys-tem was estimated to be 30 fs. The transient absorbance changesof the probe pulses induced by the pump pulses were detected bya CCD camera at all probe wavelengths simultaneously. The sam-ple was mounted inside a cryostat to control the environmentaltemperature of the samples.

Figure 7 shows a 2D display of the photon-energy and time-resolved transient absorbance difference ΔA�ω; t� at 78 K. Forthe measurements, the polarizations of the broadband probepulses were adjusted to be σ� and σ−, whereas the pump pulses

were set to σ� circular polarization and 1.89 eV to resonate withexciton A. For both the σ� and σ− probes, the time-resolved spec-tra exhibited negative ΔA in the spectral band of excitons Aand B. The negative ΔA signal could be caused by stimulatedemission from the excited state and/or photobleaching due todepletion of the ground state and population of the excited state.The lifetime of photobleaching is usually much longer than thatof stimulated emission because stimulated emission occurs onlywithin the lifetime of the excited state whereas photobleachingremains until the ground state is fully repopulated.

Obviously, ΔA is significantly probe polarization dependentwithin 100 fs, as shown in Fig. 7(b). The nonlinear optical re-sponse after 100 fs is independent of the angular momentum ofthe initially excited distribution, which indicates that the initialpolarization distribution relaxes to some quasi-equilibrium statesin 100 fs. Following fast spin-polarization relaxation, the peaks intheΔA spectrum show a blueshift before 10 ps and a redshift after10 ps, as shown by the red line in Fig. 7(a). We note that thepump-induced response at the K 0 valley is observed even whenexciton A at the K valley is excited by the σ� pump in contrastto cases of excitons coupled to pump and probe pulses, which donot share common states. This unexpected phenomenon could beexplained by various possible mechanisms, e.g., dark excitons gen-erated by pump pulses [71], weakening of the excitonic bindingenergy [43], or dielectric screening from the excited excitons [43](see Supplement 1).

The measured time-resolved traces of ΔA�ω; t� were fittedusing the sum of three exponential functions and a constant term,as follows:

ΔA�ω; t� � ΔAspin�ω�e−t

τspin � ΔAexciton�ω�e−t

τexciton

� ΔAcarrier�ω�e−t

τcarrier � ΔAe−h�ω�: (4)

The fitting results are shown in Fig. 8. For the σ� probe, the timeconstants τspin, τexciton, and τcarrier are 55� 7 fs, 1.02� 0.22 ps,and 26.32� 5.41 ps, respectively. For the σ− probe, they are

Fig. 6. Spectra of 1.89 eV pump pulse (red), 2.01 eV pump pulse(orange), broadband visible probe pulse (gray), and the stationary absorp-tion of monolayer MoS2 at room temperature (blue). Inset: lattice struc-ture of MoS2 in out-of-plane direction. Adapted from [72].

(a) (b)

Fig. 7. (a) Transient absorbance difference (ΔA) induced by excitationusing σ� circularly polarized pump pulses with a photon energy of1.89 eV and probed by σ� circularly polarized pulses at 78 K. Blackcurves are contours for ΔA � zero. Red line indicates expected valuesof transition A as a function of the time delays and probe photon en-ergies. (b) Probe delay time traces of ΔA at various probe photon ener-gies. Red and blue lines represent σ� and σ− probes, respectively, andhorizontal green lines show ΔA � 0. Adapted from [72].


http://www.osapublishing.org/optica/viewmedia.cfm?URI=optica-3-1-82&seq=1

63� 42 fs, 0.96� 0.49 ps, and 25.72� 8.61 ps, respectively.Because of the small signal amplitudes at photon energies of∼1.93 eV and ∼2.08 eV, the fitting error is large. A comparisonof the spectra of the σ� and σ− probes in Fig. 8(a) reveals thatΔAexciton, ΔAcarrier, and ΔAe−h—but not ΔAspin—exhibit similardependences on the probe photon energy. Thus, these three re-laxation processes occur regardless of the initial polarization dis-tribution within 100 fs. However, ΔAspin is completely differentfor the σ� and σ− probes in that it depends on the relative polar-izations between the probe and pump beams. For the σ� pumpand σ� probe, ΔAspin is negative and possesses larger amplitudethan for the σ� pump and σ− probe. This implies that the po-larized exciton A at the K valley excited by the σ� pump pulsesleads to intense photobleaching and stimulated emission onlywhen the probe pulses have the same circular polarization as thepump pulses and the probe photon energy overlaps the band ofexciton A. Moreover, the spectral shape of ΔA fits well with ex-citonic transition A, which indicates that the valley polarization isefficiently excited at the high-symmetry K point [73]. On theother hand, the σ− probe pulses generate exciton A with oppositespin polarization at the K 0 valley. The presence of excitons withopposite polarization leads to generation of biexcitons, which arethe origin of induced absorption (ΔA > 0) at ∼1.87 eV [71,74],as shown in the right panel of Fig. 8(a).

After the excitons are dissociated to become free carriers inhighly excited states, the exciton peak exhibits a redshift, as shownby the red line in Fig. 7(a). This shift is attributed to intravalleyscattering of free carriers, in which electrons relax to the bottom ofthe conduction band and holes relax to the top of the valenceband. The ΔAcarrier spectra show the sum of bleaching at thetransition energy peaks and the induced absorption of a broadconduction band. Thus, the intermediate relaxation time τcarrier ∼25 ps was assigned to the intraband transition of free carriers. Thedecay time of ΔAe−h is found to be too long to be determinedin the present work. The constant term ΔAe−h represents onlybleaching behavior, which can be attributed to electron–hole re-combination in the direct band. The recombination time wasestimated to be ∼300 ps in a previous study [75].

The present study completely elucidates the fairly comprehen-sive ultrafast dynamics of spin-polarized excitons in monolayerMoS2, as schematically shown in Fig. 9. Owing to the high tem-poral resolution and visible broadband detection, the time con-stants for the 60 fs spin-polarized exciton decay, 1 ps excitondissociation (intervalley scattering), and 25 ps hot carrier relaxa-tion (intravalley scattering) are clearly identified. Moreover, sub-stantial intervalley scattering strongly diminished the spin–valleycoupled polarization under off-resonant excitation. These resultsprovide a complete understanding of spin–valley coupled polari-zation anisotropy and carrier dynamics of atomic-layer MoS2,which can further help us to develop ultrafast multilevel logicgates.

B. Effect of Annealing on the Performance of P3HT:PCBM Solar Cells

The demand for renewable energy sources has stimulated progressin the development of efficient photovoltaic devices, and organicsolar cell research has achieved several critical milestones in recentdecades. Replacing traditional inorganic semiconductor-based so-lar cells, organic solar cells have become established as a futurephotovoltaic technology because of their advantages of cost-effective production, large area, light weight, and flexibility[76,77]. The highest reported power-conversion efficiency to dateis 10.8% [78], whereas it barely reached 1% in the first reportedpolymer solar cell [79]. In the past couple of years, the polymer–fullerene heterojunction has dominated organic solar cell research[80,81]. For standard bulk polymer–fullerene heterojunctionsystems, the polymer poly(3-hexylthiophene) (P3HT) as theelectron donor and the fullerene [6,6]-phenyl-C61-butyric acid

(a)

(b)

Fig. 8. Triple exponential fitting results of time-resolved ΔA data ex-cited by 2.01 eV and σ� pump pulse at 78 K. Left column, σ� probe;right column, σ− probe. (a) ΔA spectra, (b) time constant of each com-ponent. Dotted lines indicate estimated values. Adapted from [72].

Fig. 9. Schematic diagram of the relaxation processes in monolayerMoS2. Adapted from [72].


methyl ester (PCBM) as the electron acceptor are typicallyblended to create a composite material that has been demon-strated to exhibit effective device performance [81].

In solar cell devices, an anode and cathode are necessary tocollect separated charges. The anode is made of tin-doped indiumoxide (ITO) coated with a layer of poly-ethylene-dioxythiophene:polystyrene-sulfonic acid (PEDOT:PSS). ITO is one of the mostextensively used electrode materials because of its high electricalconductivity and optical transparency. The transparent, water-soluble PEDOT:PSS is used to smooth the rough ITO surfaceand further effectively collect the separated holes into the elec-trode because it has a higher work function. A metal layer(e.g., aluminum) serves as the cathode.

For a P3HT:PCBM device, as shown in the inset of Fig. 10,ITO-coated glass substrates were used as the anode; theywere modified by spin-coating with ∼40 nm thick conductivePEDOT:PSS followed by baking at 150°C for 30 min. P3HTwas blended with PCBM at a weight ratio of 2.5% and dissolvedin 1,2-dichlorobenzene. The active layer was thermally annealedat 190°C for 10 min in a nitrogen-filled glove box before (pre-annealing) or after (post-annealing) deposition of the aluminum.The cathode, which had a 100 nm Al layer, was thermally evapo-rated onto the polymer film at a base pressure of 7.5 × 10−9 Pa toform an active area of 0.06 cm2. The current density–voltage(J-V ) characteristics for the devices were recorded under lightillumination using standard solar irradiation of 100 mW∕cm2

with a xenon lamp as the light source and a computer-controlledvoltage–current source meter.

Figure 10 shows the current density–voltage (J-V ) character-istics of a device with the structure ITO/PEDOT:PSS/P3HT:PCBM/Al and different thermal annealing processes. As expected,the device fabricated using a preannealing process exhibits poorperformance characteristics and its power conversion efficiency isonly 1.63%. The other device, prepared with a post-annealingprocess, demonstrates better performance and its power conver-sion efficiency is 2.88%. In terms of device performance, both the

open-circuit voltage (V OC) and short-circuit current (JSC) areimproved by the post-annealing process. Some crucial studies[82–85] have pointed out the reasons for the higher performancein post-annealed devices. However, the microscopic viewpoint ofhigh-performance devices has yet to be considered. Here, the fun-damental carrier dynamics directly correlated to the efficiency ofcharge transport in solar cell devices are studied by ultrafast spec-troscopy [86]. The stationary absorbance spectra of pre- and post-annealed devices in the visible range show that the absorbanceincreases rapidly at photon energies greater than ∼1.9 eV (seeSupplement 1), which demonstrates that the polymer has a wideabsorption band [80]. The spectra of both pre- and post-annealeddevices exhibit the π − π� transition of P3HT at 2.05 and 2.23 eV[87,88].

Time-resolved spectroscopy using sub-10-fs visible pulsesfrom a broadband OPA (as demonstrated in Section 2.B.2) wasperformed at room temperature. The output pulses of a broad-band OPA were separated into pump and probe pulses. Thefluences of the pump and probe pulses at the sample were2.7 mJ∕cm2 and 0.3 mJ∕cm2, respectively. The changes in thesample induced by a pump pulse were obtained by detectingthe change in absorption (ΔA) of probe pulses as a function ofprobe delay time. The femtosecond time evolutions were derivedby delaying the relative arrival times of the pump and probe pulsesrapidly by a fast-scan stage (Section 2.C.1). The probe pulse wasdispersed using a polychromator into a 96 branch fiber bundle,the other end of which was separated into 96 fiber branchesand connected to APDs with a spectral resolution of 2.56 nm,i.e., ∼10 meV (Section 2.C.2). Therefore, the time-resolved ab-sorption differences at 96 probe wavelengths were simultaneouslydetected at the photodiodes. The detected signals were sent to amultichannel lock-in amplifier to be spectrally resolved for simul-taneous detection of low-intensity signals over the entire spectralregion.

The time- and photon-energy-resolved transient absorptiondifference, ΔA�ω; t�, of the pre- and post-annealed deviceswas measured using the pump–probe technique. Figures 11(a)and 11(c) show 2D plots of the ΔA spectra as functions of timeand photon energy. The ΔA spectrum is positive at photon en-ergies of less than ∼1.98 eV, which is attributed to the inducedabsorption for transitions from the first excited state to higherstates. The negativeΔA at photon energies greater than ∼1.98 eVis due to stimulated emission from the excited state and photo-bleaching due to ground state depletion. The two peaks at 2.05and 2.23 eV represent the π − π� transition in P3HT. Figure 12illustrates the relaxation processes of the P3HT:PCBM blend,which are excited by pump pulses with a photon energy of>1.9 eV (the absorption gap energy) [89,90]. In the compositesamples, it is estimated that more than 60% of the incident pho-tons are absorbed by the polymer [91]. Therefore, the sample ex-cited by the pump pulses generates excited electron–hole pairs,primarily in P3HT molecules. The excited electrons at the lowestunoccupied molecular orbital (LUMO) of P3HT are transferredto the LUMO of PCBM, and the holes remain in the P3HT toform a bounded polaron pair (BPP) with the excited electrons.The time constant for this interfacial charge transfer is measuredas ∼90 fs [90,92]. The generated BPP then relaxes to the groundstate via the parallel processes of dissociation into separated polar-ons, trapping by defect states, and recombination. The timeconstants for dissociation into the separated polarons and

Fig. 10. Current density–voltage (J-V ) characteristics of solar cells ofITO/PEDOT:PSS/P3HT:PCBM/Al with pre- and post-annealing proc-esses. Inset: device architecture of a bulk heterojunction solar cell device.The Al layer is the cathode. The active layer is a semiconducting polymer/fullerene blend. ITO coated with PEDOT:PSS serves as the anode.These layers are deposited on a glass substrate. Adapted with permissionfrom [86]. Copyright (2015) American Chemical Society.



defect trapping are reported to be ∼0.95 and ∼2.8 ps [90],respectively.

According to the scenario just described, the real-time tracesfor ΔA�ω; t� are expressed by the equation

ΔA�t� � ACTe− tτCT � ASP

�−e−

tτCT � e−

tτSP

�

� Atrap

�−e−

tτCT � e−

tτtrap

�� ARecomb; (5)

where the suffixes CT, SP, trap, and Recomb correspond tocharge transfer, separated polarons (dissociated BPP), trappedBPP, and carrier recombination, respectively. The time constantfor carrier recombination is beyond the measurement range in thisstudy. For the post-annealed device, the time constants τCT, τSP,and τtrap are ∼0.13, ∼0.68, and ∼8.48 ps, respectively. For thepreannealed device, the time constants τCT, τSP, and τtrap are∼0.13, ∼0.54, and ∼2.6 ps, respectively. The relaxation processesfor the BPP, especially for trapping by defect states (τtrap),

apparently have a longer lifetime in the post-annealed device.This implies that the excited carriers in the EA

LUMO state have alonger lifetime and so are more likely to be dissociated into photo-carriers which further produce a photocurrent. Accordingly, thislonger lifetime of the excited carriers in the post-annealed devicemay explain the increase in the JSC value. However, in realitythere are several relaxation channels for excited carriers in theEALUMO state, such as dissociation into separate polarons, trapping

by defect states, and recombination. Most excited carriers in theEALUMO state are trapped by defect states or recombine with op-

posite charges without contributing to the photocurrent, and thenthese excited carriers certainly do not increase the value of JSCeven though they have a longer lifetime in the EA

LUMO state.Consequently, it is necessary to determine how many excited car-riers in the EA

LUMO state relax through each channel, an issue thatis still unresolved.

It should be emphasized that pump–probe spectroscopy withhigh time and photon energy resolution can be further used toshow the relative amount of photoexcited carriers relaxed througheach of the relaxation processes in the ED

LUMO state (see Fig. 12),which is the key to understanding any improvement in deviceperformance. The percentage of carriers relaxed through everychannel is calculated using the coefficients ACT, ASP, Atrap, andARecomb. In addition, the percentage of each component in theregion of 1.98–2.13 eV for stimulated emission can be furtherestimated (Fig. 13). The percentage of charge transfer increasesby 4.5% for the post-annealed devices. This demonstrates thatinterfacial charge transfer from an electron donor (P3HT) to anelectron acceptor (PCBM) in the post-annealed devices is moreefficient than that in the preannealed devices. There are 1.8%more separated polarons in the post-annealed devices than inthe preannealed devices, but there is 6.4% less recombinationin the post-annealed devices. Consequently, more charges aretransferred from the electron donor (P3HT) to the electron ac-ceptor (PCBM). More separated polarons and less recombinationmean that there are more effective free carriers, which produces alarger short-circuit current (JSC) in the post-annealed devices, asshown in Fig. 10.

4. CONCLUSION AND PERSPECTIVES

In conclusion, in this short review article we describe thefascinating relationships between the energy, spin, and valley

Fig. 11. (a), (c) Two-dimensional plots of transient absorptiondifference ΔA�ω; t�. (b), (d) ΔA�ω� spectra at various time delays forpreannealed and post-annealed P3HT:PCBM devices in (a) and (c), re-spectively. Adapted with permission from [86]. Copyright (2015)American Chemical Society.

Fig. 12. Schematic representation of ultrafast carrier dynamics afterphotoexcitation. ED

LUMO, the LUMO of the electron donor; EDHOMO,

the highest occupied molecular orbital (HOMO) of the electron donor;EALUMO, the LUMO of the electron acceptor; EA

HOMO, the HOMO of theelectron acceptor. In this study, the electron donor and electron acceptorare P3HT and PCBM, respectively. τ is the time constant for the relax-ation processes. Adapted with permission from [86]. Copyright (2015)American Chemical Society.

Fig. 13. Average percentages of each of the relaxation processes shownin Fig. 12 at 1.98–2.13 eV. Adapted with permission from [86].Copyright (2015) American Chemical Society.


in monolayer MoS2. By using these degrees of freedom, opticallydriven logic gates can be realized. A two-level logic gate can beoperated by sequential excitation with circularly polarized2.01 eV (resonant with exciton B) and 1.98 eV (resonant withexciton A) pulses at room temperature. According to our time-resolved studies, the nonequilibrium population between the Kand K 0 valley lasts for ∼1 ps in monolayer MoS2, making it anexcellent candidate material for ultrafast optical control. For ap-plication to high-rate optical pulse control, the problem of accu-mulation of the remnant coherence after the control pulse alwaysexists. Thus, the following pulse to control the succeeding stepmust wait for decoherence of the target, which limits the band-width of optical spin control devices. Additionally, analyses ofeach relaxation process in P3HT:PCBM solar cells show thatthere are increases in the charge transfer and the number of sep-arated polarons and a decrease in the amount of recombinationbetween excited carriers, which is one of the physical mechanismsresponsible for enhanced performance after a post-annealingprocess. These findings are consistent with observations of theannealing-dependent surface morphology and vertical distribu-tion of P3HT:PCBM blends, which provides key informationfor the design of high-performance solar cells.

These important results and conclusions indicate that ultra-broadband time-resolved spectroscopy provides a powerful meansof studying the interactions between quasi-particles, which are thebasis for composing a material. Using broadband OPA, we canobserve several energy levels simultaneously with extremely hightime resolution and study the correlations among them. Thisprovides much clearer physical insight into the interactions ofquasi-particles in several novel types of condensed matter. Thedevelopment of ultrabroadband light sources continues. For ex-ample, the generation of ultrabroadband MIR coherent lightusing four-wave difference-frequency generation from two-colorfemtosecond pulses in gases has been demonstrated [93]. Thistype of ultrabroadband light source extending to the MIR regionas well as the terahertz region is desirable and extremely importantfor investigating the detailed ultrafast dynamics in solids, as thebandgap energy or number of optical transitions have resonanceenergies in this frequency region.

Funding. Japan Science and Technology Corporation (JST)(JST Strategic Basic Research Programs); Ministry of Scienceand Technology, Taiwan (MOST) (101-2112-M-009-016-MY2,102-2112-M-009-006-MY3, 103-2119-M-009-004-MY3, 103-2628-M-009-002-MY3); Ministry of Education (MoE).

Acknowledgment. We thank Prof. K. H. Wu, Prof. L. J.Li, Prof. M. H. Chen, Dr. C. H. Chen, C. T. Lin, J. J. Fang,and C. J. Chang for their help with sample preparations anddiscussions. Financial support was provided by Japan Scienceand Technology Corporation (JST) through the JST StrategicBasic Research Programs and by a grant from the Ministry ofEducation (MoE) through the Aiming for the Top University(ATU) Program at National Chiao Tung University.

See Supplement 1 for supporting content.

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