Nonlinear Polarization Spectroscopy Research Articles

Nonlinear polarization holography of nanoscale iridium films

Attosecond nonlinear polarization spectroscopy designates the subcycle-precise retrieval of the electric field of a femtosecond laser pulse together with the nonlinear polarization response that the laser pulse triggers in a sample. Here, we introduce a method that is all-optical and applicable to metal films. The method is called nonlinear polarization holography because it is based on the comparison of two time-domain holograms with and without a metal film on a substrate. The working principle can be understood as the time-domain analog of holographic interferometry, in which the comparison of two spatial holograms reveals changes in an object’s size and position with interferometric precision (i.e. to fractions of the wavelength). Analogously, nonlinear polarization holography provides subcycle precision (i.e. to fractions of the optical period). Nonlinear polarization holography is used here to retrieve the time-domain nonlinear response of a nanoscale iridium film. Using density matrix calculations it is shown that the knowledge of the nonlinear response with subcycle precision allows distinguishing excitation and relaxation mechanisms of low-energetic electrons.

Nonlinear polarization spectroscopy of a Rydberg state for laser stabilization.

We demonstrate modulation-free laser stabilization to an atomic Rydberg transition using nonlinear polarization spectroscopy. To stabilize a laser to the upper transition of a three-level ladder scheme, the techniques of standard polarization spectroscopy are adapted to use a narrow, nonlinear coherence feature. We obtain a subnatural linewidth dispersive signal that is directly suitable for laser frequency stabilization. We examine the effect of laser polarization on the dispersive line shape. This technique stabilizes the laser to an absolute frequency reference, can be used with numerous Rydberg levels, and eliminates laser modulation, which can enable high bandwidth feedback.

Line Narrowing of Excited-State Transitions in Nonlinear Polarization Spectroscopy: Application to Water-Soluble Chlorophyll-Binding Protein

The homogeneous linewidth of dye aggregates like photosynthetic light-harvesting complexes contains important information about energy transfer and relaxation times that is, however, masked by inhomogeneous broadening caused by static disorder. Whereas there exist line narrowing techniques for the study of low-energy exciton states, the homogeneous linewidth of the high-energy states is not so easy to decipher. Here we present a microscopic theory for nonlinear polarization spectroscopy in the frequency domain that contains a dynamic aggregate selection revealing the homogeneous linewidth of these states. The theory is applied to the water-soluble chlorophyll-binding protein for which the high-energy exciton state was predicted to exhibit a sub-100-fs lifetime.

Elucidation of structure–function relationships in plant major light-harvesting complex (LHC II) by nonlinear spectroscopy

Conventional linear and time-resolved spectroscopic techniques are often not appropriate to elucidate specific pigment-pigment interactions in light-harvesting pigment-protein complexes (LHCs). Nonlinear (laser-) spectroscopic techniques, including nonlinear polarization spectroscopy in the frequency domain (NLPF) as well as step-wise (resonant) and simultaneous (non-resonant) two-photon excitation spectroscopies may be advantageous in this regard. Nonlinear spectroscopies have been used to elucidate substructure(s) of very complex spectra, including analyses of strong excitonic couplings between chlorophylls and of interactions between (bacterio)chlorophylls and "optically dark" states of carotenoids in LHCs, including the major antenna complex of higher plants, LHC II. This article shortly reviews our previous study and outlines perspectives regarding the application of selected nonlinear laser-spectroscopic techniques to disentangle structure-function relationships in LHCs and other pigment-protein complexes.

Exciton Fine Structure and Spin Relaxation in Semiconductor Colloidal Quantum Dots

Quantum dots (QDs) have discrete quantum states isolated from the environment, making QDs well suited for quantum information processing. In semiconductor QDs, the electron spins can be coherently oriented by photoexcitation using circularly polarized light, creating optical orientation. The optically induced spin orientation could serve as a unit for data storage and processing. Carrier spin orientation is also envisioned to be a key component in a related, though parallel, field of semiconductor spintronics. However, the oriented spin population rapidly loses its coherence by interaction with the environment, thereby erasing the prepared information. Since long-lasting spin orientation is desirable in both areas of investigation, spin relaxation is the central focus of investigation for optimization of device performance. In this Account, we discuss a topic peripherally related to these emerging areas of investigation: exciton fine structure relaxation (EFSR). The radiationless transition occurring in the exciton fine structure not only highlights a novel aspect of QD exciton relaxation but also has implications for carrier spin relaxation in QDs. We focus on examining the EFSR in connection with optical spin orientation and subsequent ultrafast relaxation of electron and hole spin densities in the framework of the exciton fine structure basis. Despite its significance, the study of exciton fine structure in colloidal QDs has been hampered by the experimental challenge arising from inhomogeneous line broadening that obscures the details of closely spaced fine structure states in the frequency domain. In this Account, we show that spin relaxation occurring in the fine structure of CdSe QDs can be probed by a time-domain nonlinear polarization spectroscopy, circumventing the obstacles confronted in the frequency-domain spectroscopy. In particular, by combining polarization sequences of multiple optical pulses with the unique optical selection rules of semiconductors, fast energy relaxation among the QD exciton fine structure states is selectively measured. The measured exciton fine structure relaxation, which is a nanoscale analogue of molecular radiationless transitions, contains direct information on the relaxation of spin densities of electron and hole carriers, that is, spin relaxation in QDs. From the exciton fine structure relaxation rates measured for CdSe nanorods and complex-shaped nanocrystals using nonlinear polarization spectroscopy, we elucidated the implications of QD size and shape on the QD exciton properties as well, for example, size- and shape-scaling laws governing exciton spin flips and how an exciton is delocalized in a QD. We envision that the experimental development and the discoveries of QD exciton properties presented in this Account will inspire further studies toward revealing the characteristics of QD excitons and spin relaxation therein, for example, spin relaxation in QDs made of various materials with different electronic structures, spin relaxation under an external perturbation of QD electronic states using magnetic fields, and spin relaxation of separated electrons and holes in type-II QD heterostructures.

Influence of detergent concentration on aggregation and spectroscopic properties of light-harvesting complex II

Aggregation of photosynthetic light-harvesting complexes strongly influences their spectroscopic properties. Fluorescence yield and excited state lifetimes of the main light-harvesting complex (LHC II) of higher plants strongly depend on its aggregation state. Detergents are commonly used to solubilize membrane proteins and/or to circumvent their aggregation in aqueous environments. Nonlinear polarization spectroscopy in the frequency domain (NLPF) was performed with LHC II over a wide concentration range of the mild detergent n-dodecyl beta-D: -maltoside (beta-DM). Additionally, conventional absorption-, fluorescence- and circular dichroism-spectra were measured.The results indicate that: (i) conventional spectroscopic techniques are not well suited to investigate aggregation effects. NLPF provides a novel approach to overcome this problem: NLPF spectra display dramatic alterations upon even minor beta-DM concentration changes. (ii) Commonly used detergent concentrations (around or slightly above the critical micellar concentration) apparently do not lead to complete trimerization of LHC II. A long-wavelength species in the NLPF spectra (peaking at about 685 nm), indicative of residual aggregation, persists up to DM-concentrations of 0.06%. (iii) High-resolution NLPF spectra indicate the existence of a species with a considerably shortened excited state lifetime. (iv) No indication of denaturation was found even at the highest beta-DM concentrations used. (v) A specific change in interaction between certain chlorophyll(s) b and a xanthophyll molecule, probably neoxanthin, was detected upon aggregation as well as at higher beta-DM concentrations. The results are discussed with respect to the still elusive mechanism of nonradiative dissipation of excess excitation energy in the antenna system.

Reflective configuration in a system for nonlinear polarization spectroscopy

A reflective version of nonlinear polarization spectroscopy is analyzed in the most general form on the basis of a coordinate-free operator approach. A calculational algorithm is developed, and analytical expressions are obtained for the recorded signal in the case of arbitrary polarization of the pump excitation.

Polarization effects during the interaction of counterpropagating waves in isotropic resonant media

Using the method of nonlinear polarization spectroscopy in the oppositely propagating laser beams of different power at a wavelength of 3.39 µm, we study, both theoretically and experimentally, the mechanisms of the formation of dichroism and birefringence during the interaction of arbitrarily polarized fields of resonant radiation with gaseous methane under conditions of low pressure of the gas in the cell and saturation of the operating transition F 2 (2) by the field of a strong wave. Spectral forms of the polarization resonances of the test-wave intensity at the output of the polarization spectrometer are studied experimentally. Based on the vector theory of light interaction with matter, we present an exhaustive theoretical interpretation of the experimental data on the dependence of the polarization state of a weak wave on the intensity of a linearly or circularly polarized high-power counterpropagating wave. A new technique of measuring the induced nonlinear anisotropy for test-field waves and the macroscopic parameters of the resonant gas is developed.

Pigment-Pigment Interactions in PCP of Amphidinium carterae Investigated by Nonlinear Polarization Spectroscopy in the Frequency Domain

Peridinin-chlorophyll a-protein (PCP) is a unique antenna complex in dinoflagellates that employs peridinin (a carotenoid) as its main light-harvesting pigment. Strong excitonic interactions between peridinins, as well as between peridinins and chlorophylls (Chls) a, can be expected from the short intermolecular distances revealed by the crystal structure. Different experimental approaches of nonlinear polarization spectroscopy in the frequency domain (NLPF) were used to investigate the various interactions between pigments in PCP of Amphidinium carterae at room temperature. Lineshapes of NLPF spectra indicate strong excitonic interactions between the peridinin's optically allowed S 2 (1Bu +) states. A comprehensive subband analysis of the distinct NLPF spectral substructure in the peridinin region allows us to assign peridinin subbands to the two Chls a in PCP having different S 1-state lifetimes. Peridinin subbands at 487, 501, and 535 nm were assigned to the longer-lived Chl, whereas a peridinin subband peaking at 515 nm was detected in both clusters. Certain peridinin(s), obviously corresponding to the subband centered at 487 nm, show(s) specific (possibly Coulombic?) interaction between the optically dark S 1 ( 2 A g − ) and/or intramolecular charge-transfer (ICT) state and S 1 of Chl a. The NLPF spectrum, hence, indicates that this peridinin state is approximately isoenergetic or slightly above S 1 of Chl a. A global subband analysis of absorption and NLPF spectra reveals that the Chl a Q y-band consists of two subbands (peaking at 669 and 675 nm and having different lifetimes), confirmed by NLPF spectra recorded at high pump intensities. At the highest applied pump intensities an additional band centered at ≤660 nm appears, suggesting—together with the above results—an assignment to a low-dipole moment S 0 → S 1/ICT transition of peridinin.

Direct evidence for excitonically coupled chlorophylls a and b in LHC II of higher plants by nonlinear polarization spectroscopy in the frequency domain

Occurrence of excitonic interactions in light-harvesting complex II (LHC II) was investigated by nonlinear polarization spectroscopy in the frequency domain (NLPF) at room temperature. NLPF spectra were obtained upon probing in the chlorophyll (Chl) a/ b Soret region and pumping in the Q y region. The lowest energy Chl a absorbing at 678 nm is strongly excitonically coupled to Chl b.

Spectropolarimetric measurement of populations in degenerated atomic systems with metastable lower level during optical pumping in intense laser fields

The prospects for application of a nonlinear laser polarization spectroscopy for measuring the steady-state population characteristics in the degenerated atomic systems with metastable lower state during optical pumping in intense laser fields are considered. It is shown that a combination of the polarization spectroscopy of a probe field and the Faraday effect allows to measure immediately the population differences for “enriched ” and “depleted” Zeeman's sub-levels during the process of optical pumping as well as for optical coupled sub-levels of lower and upper states of the transition. The experimental results for the 1 s 5→2 p 2 (J=2→J=1) and 1 s 5→2 p 4 (J=2→J=2) Ne 20 transitions and their comparison are given.

Spectral substructure and excitonic interactions in the minor photosystem II antenna complex CP29 as revealed by nonlinear polarization spectroscopy in the frequency domain.

CP29 (the lhcb4 gene product), a minor photosystem II antenna complex, binds six chlorophyll (Chl) a, two Chl b, and two to three xanthophyll molecules. The Chl a/b Q(y) absorption band substructure of CP29 (purified from spinach) was investigated by nonlinear polarization spectroscopy in the frequency domain (NLPF) at room temperature. A set of NLPF spectra was obtained at 11 probe wavelengths. Seven probe wavelengths were located in the Q(y) spectral region (between 630 and 690 nm) and four in the Soret band (between 450 and 485 nm). Evaluation of the experimental data within the framework of global analysis leads to the following conclusions: (i) The dominant Chl a absorption (with a maximum at 674 nm) splits into (at least) three subbands (centered at 660, 670, and 681.5 nm). (ii) In the Chl b region two subbands can be identified with maxima located at 640 and 646 nm. (iii) The lowest energy Q(y) transition (peaking at 681.5 nm) is assigned to a Chl a which only weakly interacts with other Chl aor b molecules by incoherent Förster-type excitation energy transfer. (iv) Pronounced excitonic interaction exists between certain Chl a and Chl b molecules, which most likely form a Chl a/b heterodimer. The subbands centered at 640 and 670 nm constitute a strongly coupled Chl a/b pair. The findings of the study indicate that the currently favored view of spectral heterogeneity in CP29 being due essentially to pigment-protein interactions has to be revised.

Spectral substructure and excitonic interactions in the plant antenna complexes LHC II and CP29 as revealed by non-linear laser spectroscopy

Manifestation and extent of excitonic interactions in the plant light-harvesting complexes LHC II and CP29 have been investigated by nonlinear laser spectroscopy. Nonlinear absorption of 120 fs pulses indicates an increase in absorption cross section in the red wing of the Qy-band of LHC II as compared to monomeric chlorophyll (Chl) a in organic solution. Nonlinear polarization spectroscopy in the frequency domain (NLPF) exploiting also the intensity dependence of the NLPF signal reveals that in LHC II a species emitting at 682 nm is characterized by a 2.2(± 0.8)-fold larger dipole strength than that of monomeric Chl a. NLPF experiments indicate strong excitonic coupling between Chls a and b in CP29. Moreover, the lowest Qy-transition in CP29 (in contrast to LHC II) is assigned to a non-excitonically coupled Chl a. Stepwise two-photon excitation of Chls a and b in LHC II and CP29 with 100 fs pulses in the (red) Qy-region results in a weak blue fluorescence. The dependence of the spectral shape of the blue fluorescence on excitation wavelength together with a comparison to properties of Chls in solution are consistent with the existence of strongly excitonically coupled Chl a/b-heterodimers in LHC II. Implications of the results for the refinement of structural models and intra- as well as inter-antenna energy-transfer mechanisms are discussed.

Angular dependence of the signal in nonlinear polarization spectroscopy under conditions of noncollinear excitation by partially polarized radiation

The light-induced anisotropy tensor of initially isotropic media is calculated for the case of partially nominally polarized radiation. An analytic expression is obtained for the first time for the signal in a noncollinear nonlinear polarization spectroscopy system with a polarization-incoherent pump wave.

A new set-up for nonlinear polarization spectroscopy in the frequency domain: experimental examples and theoretical background

The advantages of a new experimental geometry for nonlinear polarization spectroscopy in the frequency domain (NLPF) are demonstrated. First, homogeneous illumination of the sample allows one to use lower pump intensities, so that the approach to describing the NLPF signal is valid. Secondly, there being fewer artefacts from competitive scattering processes results in distortionless NLPF spectra. These experimental improvements are illustrated by experimental results concerning pinacyanol.

Subband analysis of molecular electronic transitions by nonlinear polarization spectroscopy in the frequency domain

Nonlinear polarization spectroscopy in the frequency domain (NLPF) allows a detailed analysis of substructure behind a molecular optical absorption band. Using the χ(3)-approach here we deduce theoretically line shapes of NLPF spectra for systems with more than one optical transition, which contribute to linear as well as excited state absorption. It is possible from such line shapes to decide whether these transitions are independent from each other or belong to a common ground state. We demonstrate further how energy transfer and relaxation paths can be revealed and quantified by a global analysis of NLPF spectra. This is useful to get a comprehensive understanding of ultrafast processes in complex molecular and supermolecular systems.

Strong-field theory of nonlinear polarization spectroscopy Fundamentals and the two-level system

To formulate a description of nonlinear polarization spectroscopy that is valid over a wide range of pump intensities, we present an approach that considers any order of the external fields. The Liouville von Neumann equation governing the dynamics of a multilevel molecular system is made tractable by expansion of the particular density matrix into powers of the carrier waves of the external fields. For a two-level system a solution of the resulting set of equations to first order of the probe field but to all orders of pump field is demonstrated. A remarkable change in the homogeneously broadened line shape is obtained at high pump intensities, caused by the optical Stark effect among others. Improvements over results derived by other authors are discussed.

Experimental examination of the two-photon XX absorption models by a new type of polarization spectroscopy in CuCl

A new type of nonlinear polarization spectroscopy for precise measurements of the two-photon biexciton (XX) absorption in CuCl in the very weak excitation regime is developed. The spectral width, the two-photon absorption coefficient and the third-order nonlinear susceptibility associated with the two-photon XX resonance are evaluated. Experimental results are consistent with the theoretical ones evaluated from the new model of the two-photon XX absorption developed by Ivanov and Haug.

Nonlinear polarization spectroscopy in the frequency domain of light-harvesting complex II: absorption band substructure and exciton dynamics

Spectral substructure and ultrafast excitation dynamics have been investigated in the chlorophyll (Chl) a and b Qy region of isolated plant light-harvesting complex II (LHC II). We demonstrate the feasibility of Nonlinear Polarization Spectroscopy in the frequency domain, a novel photosynthesis research laser spectroscopic technique, to determine not only ultrafast population relaxation (T1) and dephasing (T2) times, but also to reveal the complex spectral substructure in the Qy band as well as the mode(s) of absorption band broadening at room temperature (RT). The study gives further direct evidence for the existence of up to now hypothetical "Chl forms". Of particular interest is the differentiated participation of the Chl forms in energy transfer in trimeric and aggregated LHC II. Limits for T2 are given in the range of a few ten fs. Inhomogeneous broadening does not exceed the homogeneous widths of the subbands at RT. The implications of the results for the energy transfer mechanisms in the antenna are discussed.

Mono‐ and bi‐excitonic states in j‐aggregates of pseudoisocyanine studied by nonlinear polarization and nonlinear absorption spectroscopy

The aggregation of pseudoisocyanine (PIC) is of interest because of the unusual properties of the aggretages which make them useful in photographic science as sensitizers as well as in photosynthesis research as model systems. Here, the room‐temperature substructure of the J‐absorption band of PIC–Cl aggregates is discussed on the basis of the results from nonlinear polarization spectroscopy in the frequency domain. The results are compared with the known substructure at 4.2 K.