Ultrafast Mid-infrared Spectroscopy Research Articles

Suppressed electronic contribution in thermal conductivity of Ge2Sb2Se4Te

Integrated nanophotonics is an emerging research direction that has attracted great interests for technologies ranging from classical to quantum computing. One of the key-components in the development of nanophotonic circuits is the phase-change unit that undergoes a solid-state phase transformation upon thermal excitation. The quaternary alloy, Ge2Sb2Se4Te, is one of the most promising material candidates for application in photonic circuits due to its broadband transparency and large optical contrast in the infrared spectrum. Here, we investigate the thermal properties of Ge2Sb2Se4Te and show that upon substituting tellurium with selenium, the thermal transport transitions from an electron dominated to a phonon dominated regime. By implementing an ultrafast mid-infrared pump-probe spectroscopy technique that allows for direct monitoring of electronic and vibrational energy carrier lifetimes in these materials, we find that this reduction in thermal conductivity is a result of a drastic change in electronic lifetimes of Ge2Sb2Se4Te, leading to a transition from an electron-dominated to a phonon-dominated thermal transport mechanism upon selenium substitution. In addition to thermal conductivity measurements, we provide an extensive study on the thermophysical properties of Ge2Sb2Se4Te thin films such as thermal boundary conductance, specific heat, and sound speed from room temperature to 400 °C across varying thicknesses.

Compact OPCPA system seeded by a Cr:ZnS laser for generating tunable femtosecond pulses in the MWIR.

A compact mid-wavelength infrared (MWIR) optical parametric chirped pulse amplification (OPCPA) system generates sub-150 fs pulses at wavelengths from 5.4 to 6.8 µm with >400µJ energy at a 1 kHz repetition rate. A femtosecond Cr:ZnS master oscillator emitting 40 fs pulses at 2.4 µm seeds both a Ho:YLF regenerative amplifier and a two-stage OPCPA based on ZnGeP2 crystals. The 2.05 µm few-picosecond pump pulses from the Ho:YLF amplifier have an energy of 13.4 mJ. Seed pulses for the OPCPA are generated by soliton self-frequency shifting in a fluoride fiber and are tunable between 2.8 and 3.25 µm with a sub-100 fs duration and few-nanojoule energy. The intense MWIR pulses hold strong potential for applications in ultrafast mid-infrared nonlinear optics and spectroscopy.

Design of high-efficiency diffractive optical elements towards ultrafast mid-infrared time-stretched imaging and spectroscopy

Ultrafast time stretch imaging offers unprecedented imaging speed and enables new discoveries in scientific research and engineering. One challenge in exploiting time stretch imaging in mid-infrared is the lack of high-quality diffractive optical elements (DOEs), which encode the image information into mid-infrared optical spectrum. This work reports the design and optimization of mid-infrared DOE with high diffraction-efficiency, broad bandwidth and large field of view. Using various typical materials with their refractive indices ranging from 1.32 to 4.06 in mid-infrared band, diffraction efficiencies of single-layer and double-layer DOEs have been studied in different wavelength bands with different field of views. More importantly, by replacing the air gap of double-layer DOE with carefully selected optical materials, one optimized triple-layer DOE, with efficiency higher than 95% in the whole mid-infrared window and field of view greater than , is designed and analyzed. This new DOE device holds great potential in ultrafast mid-infrared time stretch imaging and spectroscopy.

Using molecular vibrations to probe exciton delocalization in films of perylene diimides with ultrafast mid-IR spectroscopy.

Ultrafast vibrational spectroscopy in the mid-infrared was used to directly probe the delocalization of excitons in two different perylenediimide (PDI) derivatives that are predicted to preclude the formation of excimers, which can act as trap sites for excited state energy in organic semiconductors. We identified vibrational modes within the conjugated C-C stretch modes of PDI molecules whose frequencies reported the interactions of molecules within delocalized excitonic states. The vibrational linewidths of these modes, which we call intermolecular coordinate coupled (ICC) modes, provided a direct probe of the extent of exciton delocalization among the PDI molecules, which was confirmed using X-ray diffraction and electro-absorption spectroscopy. We show that a slip-stacked geometry among the PDI molecules in their crystals promotes delocalized charge-transfer (CT) excitons, while localized Frenkel excitons tend to form in crystals with helical, columnar stacking geometries. Because all molecules possess vibrational modes, the use of ultrafast mid-infrared spectroscopy to measure ICC vibrational modes offers a new approach to examine exciton delocalization in a variety of small molecule electron acceptors for optoelectronic and organic photovoltaic applications.

1s-intraexcitonic dynamics in monolayer MoS2 probed by ultrafast mid-infrared spectroscopy.

The 1s exciton—the ground state of a bound electron-hole pair—is central to understanding the photoresponse of monolayer transition metal dichalcogenides. Above the 1s exciton, recent visible and near-infrared investigations have revealed that the excited excitons are much richer, exhibiting a series of Rydberg-like states. A natural question is then how the internal excitonic transitions are interrelated on photoexcitation. Accessing these intraexcitonic transitions, however, demands a fundamentally different experimental tool capable of probing optical transitions from 1s ‘bright' to np ‘dark' states. Here we employ ultrafast mid-infrared spectroscopy to explore the 1s intraexcitonic transitions in monolayer MoS2. We observed twofold 1s→3p intraexcitonic transitions within the A and B excitons and 1s→2p transition between the A and B excitons. Our results revealed that it takes about 0.7 ps for the 1s A exciton to reach quasi-equilibrium; a characteristic time that is associated with a rapid population transfer from the 1s B exciton, providing rich characteristics of many-body exciton dynamics in two-dimensional materials.

The photochemical route to octahedral iron(V). Primary processes and quantum yields from ultrafast mid-infrared spectroscopy.

Recently, the complex cation [(cyclam-ac)Fe(III)(N3)](+) has been used in solid matrices under cryogenic conditions as a photochemical precursor for an octahedral iron nitride containing the metal at the remarkably high oxidation state +5. Here, we study the photochemical primary events of this complex cation in liquid solution at room temperature using femtosecond time-resolved mid-infrared (fs-MIR) spectroscopy as well as step-scan Fourier-transform infrared spectroscopy, both of which were carried out with variable-wavelength excitation. In stark contrast to the cryomatrix experiments, a photooxidized product cannot be detected in liquid solution when the complex is excited through its putative LMCT band in the visible region. Instead, only a redox-neutral dissociation of azide anions is seen under these conditions. However, clear evidence is found for the formation of the highly oxidized iron nitride product when the photolysis is carried out in liquid solution with UV light. Yet, the photooxidation must compete with photoreductive Fe-N bond cleavage leading to azide radicals and an iron(II) complex. Both, redox-neutral and photoreductive Fe-N bond breakage as well as photooxidative N-N bond breakage occur on a time scale well below a few hundred femtoseconds. The majority of fragments suffer from geminate recombination back to the parent complex on a time scale of 10 ps. Upper limits of the primary quantum yield for photooxidation are derived from the fs-MIR data, which increase with increasing energy of the photolysis photon.

Terahertz optical properties of multilayer graphene: Experimental observation of strong dependence on stacking arrangements and misorientation angles

The optical conductivity of monolayer and multilayer graphene in the terahertz spectral region is experimentally measured using terahertz time-domain spectroscopy. The stacking arrangement and the misorientation angle of each sample are determined by Raman spectroscopy. The chemical potential of each sample is measured using ultrafast midinfrared pump-probe spectroscopy to be 63 or 64 meV for all samples. The intraband scattering rate can be obtained by fitting the measured data with theoretical models. Other physical parameters, including carrier density, dc conductivity, and carrier mobility, of each sample can also be deduced from the theoretical fitting. The fitting results show the existence of misoriented or AA-stacked layers with an interaction energy of ${\ensuremath{\alpha}}_{1}=217\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ in our multilayer samples. Here we show that the scattering rate strongly depends on the stacking arrangement of the sample. High scattering rates and high optical conductivity are associated with AA-stacked samples, while lower ones are associated with misoriented multilayer graphene. This implies that the THz optoelectronic properties of multilayer graphene can be tuned by purposefully misorienting layers or employing different stacking schemes.

Ultrafast mid-infrared spectroscopy by chirped pulse upconversion in 1800-1000cm^−1 region

Broadband femtosecond mid-infrared pulses can be converted into the visible spectral region by chirped pulse upconversion. We report here the upconversion of pump probe transient signals in the frequency region below 1800cm(-1), using the nonlinear optical crystal AgGaGeS4, realizing an important expansion of the application range of this method. Experiments were demonstrated with a slab of GaAs, in which the upconverted signals cover a window of 120cm(-1), with 1.5cm(-1) resolution. In experiments on the BLUF photoreceptor Slr1694, signals below 1 milliOD were well resolved after baseline correction. Possibilities for further optimization of the method are discussed. We conclude that this method is an attractive alternative for the traditional MCT arrays used in most mid-infrared pump probe experiments.

Primary Reactions of Bacteriophytochrome Observed with Ultrafast Mid-Infrared Spectroscopy

Phytochromes are red-light photoreceptor proteins that regulate a variety of responses and cellular processes in plants, bacteria, and fungi. The phytochrome light activation mechanism involves isomerization around the C(15)═C(16) double bond of an open-chain tetrapyrrole chromophore, resulting in a flip of its D-ring. In an important recent development, bacteriophytochrome (Bph) has been engineered for use as a fluorescent marker in mammalian tissues. Bphs covalently bind a biliverdin (BV) chromophore, naturally abundant in mammalian cells. Here, we report an ultrafast time-resolved mid-infrared spectroscopic study on the Pr state of two highly related Bphs from Rps. palustris , RpBphP2 (P2) and RpBphP3 (P3) with distinct photoconversion and fluorescence properties. We observed that the BV excited state of P2 decays in 58 ps, while the BV excited state of P3 decays in 362 ps. By combining ultrafast mid-IR spectroscopy with FTIR spectroscopy on P2 and P3 wild type and mutant proteins, we demonstrate that the hydrogen bond strength at the ring D carbonyl of the BV chromophore is significantly stronger in P3 as compared to P2. This result is consistent with the X-ray structures of Bph, which indicate one hydrogen bond from a conserved histidine to the BV ring D carbonyl for classical bacteriophytochromes such as P2, and one or two additional hydrogen bonds from a serine and a lysine side chain to the BV ring D carbonyl for P3. We conclude that the hydrogen-bond strength at BV ring D is a key determinant of excited-state lifetime and fluorescence quantum yield. Excited-state decay is followed by the formation of a primary intermediate that does not decay on the nanosecond time scale of the experiment, which shows a narrow absorption band at ∼1540 cm(-1). Possible origins of this product band are discussed. This work may aid in rational structure- and mechanism-based conversion of BPh into an efficient near-IR fluorescent marker.

Spectroscopic Measurement of Interlayer Screening in Multilayer Epitaxial Graphene

The substrate-induced charge-density profile in carbon face epitaxial graphene is determined using nondegenerate ultrafast midinfrared pump-probe spectroscopy. Distinct zero crossings in the differential transmission spectra are used to identify the Fermi levels of layers within the multilayer stack. Probing within the transmission window of the SiC substrate, we find the Fermi levels of the first four heavily doped layers to be, respectively, 360, 215, 140, and 93 meV above the Dirac point. The charge screening length is determined to be one graphene layer, in good agreement with theoretical predictions.

Primary Reactions of the LOV2 Domain of Phototropin Studied with Ultrafast Mid-Infrared Spectroscopy and Quantum Chemistry

Phototropins, major blue-light receptors in plants, are sensitive to blue light through a pair of flavin mononucleotide (FMN)-binding light oxygen and voltage (LOV) domains, LOV1 and LOV2. LOV2 undergoes a photocycle involving light-driven covalent adduct formation between a conserved cysteine and the FMN C(4a) atom. Here, the primary reactions of Avena sativa phototropin 1 LOV2 (AsLOV2) were studied using ultrafast mid-infrared spectroscopy and quantum chemistry. The singlet excited state (S1) evolves into the triplet state (T1) with a lifetime of 1.5 ns at a yield of ∼50%. The infrared signature of S1 is characterized by absorption bands at 1657 cm−1, 1495–1415 cm−1, and 1375 cm−1. The T1 state shows infrared bands at 1657 cm−1, 1645 cm−1, 1491–1438 cm−1, and 1390 cm−1. For both electronic states, these bands are assigned principally to C=O, C=N, C-C, and C-N stretch modes. The overall downshifting of C=O and C=N bond stretch modes is consistent with an overall bond-order decrease of the conjugated isoalloxazine system upon a π-π∗ transition. The configuration interaction singles (CIS) method was used to calculate the vibrational spectra of the S1 and T1 excited ππ∗ states, as well as respective electronic energies, structural parameters, electronic dipole moments, and intrinsic force constants. The harmonic frequencies of S1 and T1, as calculated by the CIS method, are in satisfactory agreement with the evident band positions and intensities. On the other hand, CIS calculations of a T1 cation that was protonated at the N(5) site did not reproduce the experimental FMN T1 spectrum. We conclude that the FMN T1 state remains nonprotonated on a nanosecond timescale, which rules out an ionic mechanism for covalent adduct formation involving cysteine-N(5) proton transfer on this timescale. Finally, we observed a heterogeneous population of singly and doubly H-bonded FMN C(4)=O conformers in the dark state, with stretch frequencies at 1714 cm−1 and 1694 cm−1, respectively.

Identification of the First Steps in Charge Separation in Bacterial Photosynthetic Reaction Centers of Rhodobacter sphaeroides by Ultrafast Mid-Infrared Spectroscopy: Electron Transfer and Protein Dynamics

Time-resolved visible pump/mid-infrared (mid-IR) probe spectroscopy in the region between 1600 and 1800 cm −1 was used to investigate electron transfer, radical pair relaxation, and protein relaxation at room temperature in the Rhodobacter sphaeroides reaction center (RC). Wild-type RCs both with and without the quinone electron acceptor Q A, were excited at 600 nm (nonselective excitation), 800 nm (direct excitation of the monomeric bacteriochlorophyll (BChl) cofactors), and 860 nm (direct excitation of the dimer of primary donor ( P) BChls ( P L/ P M)). The region between 1600 and 1800 cm −1 encompasses absorption changes associated with carbonyl (C O) stretch vibrational modes of the cofactors and protein. After photoexcitation of the RC the primary electron donor P excited singlet state ( P*) decayed on a timescale of 3.7 ps to the state P + B L − (where B L is the accessory BChl electron acceptor). This is the first report of the mid-IR absorption spectrum of P + B L − ; the difference spectrum indicates that the 9-keto C O stretch of B L is located around 1670–1680 cm −1. After subsequent electron transfer to the bacteriopheophytin H L in ∼1 ps, the state P + H L − was formed. A sequential analysis and simultaneous target analysis of the data showed a relaxation of the P + H L − radical pair on the ∼20 ps timescale, accompanied by a change in the relative ratio of the P L + and P M + bands and by a minor change in the band amplitude at 1640 cm −1 that may be tentatively ascribed to the response of an amide C O to the radical pair formation. We conclude that the drop in free energy associated with the relaxation of P + H L − , is due to an increased localization of the electron hole on the P L half of the dimer and a further consequence is a reduction in the electrical field causing the Stark shift of one or more amide C O oscillators.

Probing nanoscale inhomogeneities in transition metal oxides with ultrafast mid-infrared spectroscopy

Transition metal oxides are a rich area of investigation for experimentalists and theorists alike due to their coupling of multiple degrees of freedom with similar interaction strengths. This complexity leads to intrinsic phase inhomogeneities that are believed to play a significant role in the fascinating phenomena observed in these systems. We use ultrafast mid-infrared spectroscopy to probe quasiparticle dynamics in the colossal magnetoresistive oxides, Nd 0.5Sr 0.5MnO 3 and Tl 2Mn 2O 7. Our results demonstrate for the first time that ultrafast spectroscopy is sensitive to the presence of nanoscale phase inhomogeneities, strongly indicating the universality of phase coexistence in complex transition metal oxides.

Phase inhomogeneities in the charge-orbital-ordered manganiteNd0.5Sr0.5MnO3revealed through polaron dynamics

Ultrafast midinfrared spectroscopy is used to probe dynamics in the intermediate bandwidth manganite ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$. In the majority paramagnetic and ferromagnetic phases, the early time dynamics are consistent with the excitation and subsequent redressing of uncorrelated lattice polarons, with longer time dynamics related to spin-lattice thermalization. These polaron excitations reveal the intrinsically inhomogeneous nature of these phases. At lower temperatures we observe ultrafast melting of charge-orbital order, liberating quasiparticles that subsequently relax into bound polaronic states on a subpicosecond time scale. The temperature-dependent amplitude of the polaron excitations scales with the volume fraction of the CE phase. Thus, polaron dynamics, as measured using ultrafast spectroscopy, serve as a sensitive probe of phase inhomogeneity.

Ultrafast Chemistry of Nanoenergetic Materials Studied by Time-Resolved Infrared Spectroscopy: Aluminum Nanoparticles in Teflon

Ultrafast mid-infrared (IR) spectroscopy is used to monitor chemical reactions initiated by flash-heating a nanoenergetic material consisting of 30 nm diameter Al nanoparticle fuel in a TeflonAF oxidizer. TeflonAF is a copolymer of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (dioxole), so Al can react with several different moieties. Transitions associated with CF2 stretching of TFE or CFO stretching or CF3 stretching of dioxole were monitored. The reactions of Al with CFO occurred with time constant (k1)-1 = 50 (±20 ps); reactions of Al with CF2 or CF3 were more than 10 times slower, (k2)-1 = 0.7 (±0.05 ns). An interesting frequency oscillation is seen in the 1148 cm-1 band, where the peak frequency undergoes a time-dependent shift from 1148 to 1155 cm-1 and then back to 1148 cm-1. Due to the coincidence of CFO and CF2 stretching transitions, this band in the copolymer represents an amalgamated vibration with amplitude on both TFE and dioxole. As concentration is varie...

Formation of the Early Photoproduct Lumi-R of Cyanobacterial Phytochrome Cph1 Observed by Ultrafast Mid-Infrared Spectroscopy

The photoreactions of the Pr ground state of cyanobacterial phytochrome Cph1 from Synechocystis PCC 6803 have been investigated by picosecond time-resolved mid-infrared spectroscopy at ambient temperature. With femtosecond excitation of the Pr state at 640 nm, the photoisomerized Lumi-R product state is generated with kinetics and associated difference spectra indicative of vibrational cooling with tau(1) = 3 ps time constant and excited state decay with tau(1) = 3 ps, tau(2) = 14 ps, and tau(3) = 134 ps time constants. The Lumi-R state is characterized by downshifted absorption of three C=C modes assigned to C(15)=C(16), C(4)=C(6), and a delocalized C=C mode, in addition to the downshifted C(19)=O mode. The Lumi-R minus Pr difference spectrum is indicative of global restructuring of the chromophore on the ultrafast timescale, which is discussed in light of C(15) Z/E photoisomerization in addition to changes near C(5), which could be low bond order torsional angle changes.

Coherent vibrational dynamics of intermolecular hydrogen bonds in acetic aciddimers studied by ultrafast mid-infrared spectroscopy

Ultrafast vibrational motions and the underlying microscopic couplings inhydrogen-bonded cyclic dimers of acetic acid are studied by pump–probe andphoton echo experiments in the mid-infrared. Upon femtosecond excitationof the O–H stretching mode, we demonstrate coherent nuclear motionsalong the in-plane dimer stretching mode which persist for picoseconds.The anharmonic coupling of the O–H stretching and the low-frequencymode is isolated in the nonlinear vibrational response, whereas othercouplings are suppressed. Three-pulse photon echo experiments demonstrate adephasing of O–H stretching excitations on a femtosecond timescale andquantum beats due to the anharmonically coupled dimer stretching mode.

Hole relaxation in p-type InxGa1-xAs/AlyGa1-yAs quantum wells observed by ultrafast midinfrared spectroscopy.

We have performed midinfrared pump-probe transmission measurements in p-type strained ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As/${\mathrm{Al}}_{0.5}$${\mathrm{Ga}}_{0.5}$As quantum wells at room temperature. The laser source was a midinfrared free-electron laser generating subpicosecond pulses and tuned on resonance between the heavy-hole and the light-hole subbands. The relaxation time is \ensuremath{\sim}1 ps, which is shorter than for electrons in the conduction band. Interactions between the optical phonons and holes account for the measured relaxation.