Coherent Transient Spectroscopy Research Articles

High-resolution phase-sensitive sum frequency generation spectroscopy by time-domain ptychography

We demonstrate that time-domain ptychography, when applied to a set of broadband vibrational sum frequency spectra, reconstructs amplitude and phase of the vibrational free induction decay from an interfacial sample with a resolution that is independent of up-converting pulse bandwidth and spectrometer resolution. These important improvements require no modifications to most standard homodyne setups, and the method is applicable to other coherent homodyne spectroscopies such as coherent anti-Stokes Raman spectroscopy and transient grating spectroscopy.

Coherent transient spectroscopy with continuous wave quantum cascade lasers

A high power continuous wave quantum cascade laser operating around 1900 cm(-1) has been used to conduct Lamb dip spectroscopy on a low pressure sample of NO. The widths of the Lamb dips indicate that the laser linewidth is 800 ± 60 kHz and the power sufficient to induce significant population transfer of up to 35%. While the Lamb dip signals are symmetric at low laser chirp rates, they become increasingly asymmetric as the chirp rate increases, further confirming the significant degree of population transfer. In addition rapid passage structure on the Lamb dip signal is observed after the weak probe beam is swept through the line center. This structure is sensitive to both the probe chirp rate and the underlying hyperfine structure of the rovibrational transition, and is accurately modeled using the optical Bloch equations.

Generalizedn-Photon Resonant2n-Wave Mixing in an (n+1)-Level System with Phase-Conjugate Geometry

A generalized scheme for phase-conjugate resonant 2n-wave mixing, which has a high efficiency and is easy for phase matching, is proposed. As a new type of coherent laser spectroscopy this approach can be employed for studying highly excited atomic states or states with a high angular momentum. To demonstrate its feasibility we have studied the doubly excited autoionizing Rydberg states of Ba by phase-conjugate six-wave mixing, and have furthermore achieved eight-wave mixing in Na. This method may find wide application in related areas such as coherent transient spectroscopy, Autler-Townes spectroscopy and electromagnetically induced transparency. In particular, it may provide new insights into the nature of highly excited states.

Infrared transient grating and photon echo spectroscopy of oxygen vibrational modes in amorphous silicon thin films

We report on picosecond coherent transient spectroscopy on the asymmetric stretching mode in $a\ensuremath{-}{\mathrm{SiO}}_{0.1}$ thin films using the Dutch free-electron laser FELIX. A fast 10-K lifetime of 2.8 ps is obtained using laser-induced transient grating spectroscopy. Its thermal behavior suggests relaxation into two accepting modes possibly via the symmetric stretch mode of the Si-O-Si complex. Two-pulse photon echo measurements reveal phase dynamics with both an excitation density and temperature dependence, suggesting the presence of nonequilibrium Si-Si phonons. The temperature-dependent component of the pure dephasing may be attributable to two-phonon elastic scattering. From the probe pulse diffraction efficiency, a value of the nonlinear refractive index has been determined to be ${n}_{2}=3.73\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\mathrm{cm}}^{2}/\mathrm{GW}.$

High-resolution laser spectroscopy of rare-earth doped insulators: a personal perspective

I offer some reflections on the past three decades of high-resolution spectroscopy of rare-earth ions in solids which was ushered in by the development of tunable lasers in the mid 1970s. A brief review is given of some of the accomplishments in the area of spectral hole-burning and coherent transient spectroscopy, emphasizing work with which the author has been associated. Spectral hole-burning has been characterized by a richness of mechanisms. These include population storage in nuclear-spin and electron-spin Zeeman sub-levels, hyperfine and superhyperfine levels and metastable optical levels with corresponding hole lifetimes from many hours to microseconds. In addition, persistent hole-burning has been seen in disordered materials and in those showing photo-ionization or photo-chemistry following excitation into zero-phonon lines. This has made hole-burning a generally useful technique for the measurement of magnetic and electric dipole moments, hyperfine interactions, spin relaxation and thermally induced line-broadening. Photon-echoes have proven to be the prime source of coherence-time information and coherence times as long as several milliseconds corresponding to optical resonance widths of less than 100 Hz have been reported. Tables summarizing these results and providing references to original work are included.

Transit-time effects in coherent transient spectroscopy.

We have developed a general technique for obtaining analytical expressions for coherent transient signals arising from atoms interacting with two or more laser pulses having Gaussian spatial profiles. After deriving the Maxwell-Bloch equations for short laser pulses incident on an ensemble of three-level atoms, we specialize to photon echo spectroscopy. As an example, the stimulated photon echo signal is calculated as a function of the time delay between pulses. Our exact results are compared with both the lowest-order perturbative limit and a calculation in which transit time is treated phenomenologically. We conclude by commenting on the extension of our technique to atoms undergoing velocity-changing collisions. \textcopyright{} 1996 The American Physical Society.

Theory of coherent transient spectroscopy in molecular aggregates: The effects of interacting excitons

Using a density matrix formalism we derive a general expression for the resonant response of an ensemble of molecular aggregates to two optical pulses. The aggregates, which can be of arbitrary geometry, are described by a Frenkel exciton Hamiltonian including exciton–exciton interactions. A hierarchy of resonant coherent transient signals is generated having wave vectors sk1+(1−s)k2, where ki is the wave vector of the ith pulse and s is an integer. General expressions for the differential pump–probe absorption spectrum D(ω;tD) (s=0) and the photon echo Iecho(tD) (s=−1) are presented for arbitrary pulse intensities. Applications are made to a linear chain of interacting Frenkel excitons with nearest neighbor exciton–exciton interaction A and exciton transfer J. Analytical expressions for D(ω;tD) and Iecho(tD) are obtained which are valid to third order in the aggregate-field interaction. The influence of the optically active red (blue) biexciton which detaches from the bottom (top) of the two-exciton band when A<−2‖J‖ (A≳2‖J‖) is investigated. Biexcitons appear as extra peaks in D(ω;tD) and as oscillations in Iecho(tD). Finite size effects are responsible for quantum beat oscillations in D(ω;tD) and long time oscillations in the echo decay. Quantum beats arise predominantly from the difference between the lowest one-exciton frequencies. Our theory successfully describes the salient spectral features in the pump–probe spectra of J-aggregates.

Sublevel spectroscopy of alkali atoms in superfluid helium

We report the results of optical pumping and optical detection of magnetic resonance of alkali atoms (Cs and Rb) in superfluid helium. The magnetic resonances between the ground-state Zeeman sublevels and hyperfine levels are observed through monitoring theD1 fluorescence by means of the optical-rf double resonance technique. Although the ground stateg values in superfluid helium are the same as in vacuum within the experimental error, the hyperfine constant of the ground state of the Cs atom in superfluid helium is found to be slightly larger than in vacuum. Coherent transient spectroscopy is also performed.

Femtosecond coherent transient infrared spectroscopy of reaction centers from Rhodobacter sphaeroides.

Protein and cofactor vibrational dynamics associated with photoexcitation and charge separation in the photosynthetic reaction center were investigated with femto-second (300-400 fs) time-resolved infrared (1560-1960 cm-1) spectroscopy. The experiments are in the coherent transient limit where the quantum uncertainty principle governs the evolution of the protein vibrational changes. No significant protein relaxation accompanies charge separation, although the electric field resulting from charge separation modifies the polypeptide carbonyl spectra. The potential energy surfaces of the "special pair" P and the photoexcited singlet state P* and environmental perturbations on them are similar as judged from coherence transfer measurements. The vibrational dephasing time of P* modes in this region is 600 fs. A subpicosecond transient at 1665 cm-1 was found to have the kinetics expected for a sequential electron transfer process. Kinetic signatures of all other transient intermediates, P, P*, and P+, participating in the primary steps of photosynthesis were identified in the difference infrared spectra.

Coherent excitonic effects in quantum wells

Coherent transient spectroscopy of semiconductor nanostructures has become an active field of research in recent years. Most such experiments are analyzed in terms of a simple two-level model in which the effects of interaction and induced fields are ignored. We discuss two very different cases where inclusion of these generally-neglected effects leads to large and important modification in the temporal response of the system. In the first case, interactions between excitons strongly modify the temporal response of four-wave-mixing (IWM) signals from quantum wells. This is the temporal analog of well-known effects in the spectral domain, such as bleaching of the exciton absorption and bandgap renormalization. In the second case, the shape of a low-intensity pulse is severely distorted after transmission through a typical quantum well sample. Interferometric measurements show that reradiation of induced polarization is responsible for the distortion. The distortion depends strongly on the dephasing time and has important implications for all time-resolved measurements

Coherent transient spectroscopy of excitons in GaAs-AlGaAs quantum wells

Coherent transient optical spectroscopy techniques provide a means of studying the relaxation of electronic excitations in solids. Using these techniques the authors examine the relaxation of excitons in GaAs-AlGaAs multiple quantum wells at low temperature. Localization of excitons due to interface disorder results in inhomogeneous broadening of the absorption spectrum and relaxation due to migration between localization sites. The temperature dependence and energy dependence of the relaxation indicate the presence of complex scattering mechanisms. The dependence of the optical response on the polarization of the incident excitation fields is shown to result in a dramatic change of the dephasing and broadening characteristics of the optical response. This response is interpreted in terms of the presence of both localized and delocalized excitons. Mechanisms for the polarization dependence are discussed. >

Cw photon echo: Theory and observations.

A novel coherent transient spectroscopy, cw photon echo, is described. A medium is exposed to broadband cw laser radiation and then to a pulsed laser, resulting in a continuous emission signal from the medium. The temporal profile of this signal is studied using full density-matrix theory for the cases of a two-level system and a multilevel system. It is shown that such signal decays with a homogeneous dephasing time ${\mathit{T}}_{2}$ and can have a quantum-beat-type modulation, reproducing in a single shot the conventional two-pulse photon echo plot versus first-to-second pulse separation. Experimental demonstrations are given for the $^{7}$${\mathit{F}}_{0}$${\mathrm{\ensuremath{-}}}^{5}$${\mathit{D}}_{0}$ transition of ${\mathrm{Eu}}^{3+}$ and the $^{3}$${\mathit{H}}_{4}$${\mathrm{\ensuremath{-}}}^{1}$${\mathit{D}}_{2}$ transition of ${\mathrm{Pr}}^{3+}$; both ions are doped in ${\mathrm{YAlO}}_{3}$. The Eu ions exhibit a single exponential decay with a slight modulation due to the ground-state hyperfine splittings. The temperature dependence of the signal is also studied. The Pr ions show a very deep modulation pattern, which provides excellent agreement with the independent two pulse echo measurement. A numerical simulation based on the modulation theory also agrees with the observed decay curves and the nuclear axis configuration in this crystal is discussed.

Ultrahigh-time-resolution coherent transient spectroscopy with incoherent light

A new method of picosecond and femtosecond transient spectroscopy yielding information about ultrafast relaxation processes without requiring ultrashort light pulses is presented. It is based on the present theoretical analysis of the resonant degenerate four-wave-mixing process excited by two temporally incoherent light beams with wave vectors ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{1}$ and ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{2}$ which are originated from a single beam at frequency $\ensuremath{\omega}$ but have mutual time delay $\ensuremath{\tau}$. Under the assumption that the incoherent light field has Gaussian random complex amplitude and that the resonant material consists of the usual two-level atoms, the statistically averaged intensity of the output light field with ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{3}=2{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{2}\ensuremath{-}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{1}$ at $\ensuremath{\omega}$ is calculated as a function of $\ensuremath{\tau}$. Even with the light having a much longer duration than both ${T}_{1}$ (the longitudinal relaxation time) and ${T}_{2}$ (the transverse relaxation time), the correlation trace, i.e., output intensity versus $\ensuremath{\tau}$, represents a decay profile determined mainly by ${T}_{2}$ for both homogeneously and inhomogeneously broadened transitions, as long as the light correlation time ${\ensuremath{\tau}}_{c}$ is much shorter than the relaxation times. The correlation trace does not always represent a single-exponential decay but is sometimes slightly deformed by the ${T}_{1}$ effect. However, it does not cause a significant error in the determination of ${T}_{2}$. Moreover, as $\frac{{T}_{2}}{{T}_{1}}\ensuremath{\rightarrow}0$, the trace becomes a single-exponential decay curve determined only by ${T}_{2}$. The feature of the results obtained by the present method is similar to that obtained by the conventional coherent transient spectroscopy with short pulses, such as the photon echo. The time resolution in the present method, however, is limited only by ${\ensuremath{\tau}}_{c}$ much shorter than the light duration. By regarding the incoherent light as a series of random ultrashort pulses, the present four-wave-mixing process is also interpreted as the ensemble of numerous transient four-wave-mixing processes caused by various combinations of these pulses.

A Novel Method of Stabilizing Frequency-Modulated Microwave for Coherent Transient Spectroscopy

A method of stabilizing the microwave frequency modulated for the observation of transient molecular responses has been developed. Good long-term stability of the mean values of the modulated microwave frequencies is achieved; the stability is 3×10-9/day for an NEC 10V13 klystron and 10-8/day for OKI 24V11, 25V12, and 47V12 klystrons. This stabilizer has operated successfully in measuring the relaxation rate constants of weak rotational transitions of OCS and HCOOH, whose absorption coefficients are in the region 10-6-10-7 cm-1.

Optical multiple pulse sequences for multiphoton selective excitation and enhancement of forbidden transitions

In this paper we present novel and simple pulse sequences for enhancing the intensity of forbidden or highly nonresonant optical transitions. These sequences provide a straightforward approach to circumventing the most serious limitation of optical coherent transient spectroscopy: Available laser intensities are often insufficient to excite a significant fraction of the ground state population into desired excited states, either because of large inhomogeneous broadening or, in the case of multiphoton absorption, large anharmonicities. Optical phase modulation (which can be produced by an acousto-optic modulator) or amplitude modulation (which can be produced by an interferometer) with carefully chosen pulse flip angles and delays can effectively remove even very large energy mismatches, thus permitting essentially complete population inversions arbitrarily far from resonance. Coherent averaging theory and computer calculations are used to derive particularly valuable sequences. Pumping enhancement of 103–105 (depending on individual molecular parameters) for these modulated pulse sequences are predicted. Specific applications to multiphoton pumping of local vibrational modes are discussed.

Multiple sampling digital boxcar integrator, an efficient signal averager

A digital boxcar integrator of multiple sampling type which has a maximum sampling rate of 10 MByte/s has been developed. The efficiency, which is defined as the ratio of the number of sampling pulses applied during each cycle of a signal waveform to the total number of sampling points needed to cover a cycle of signal waveform, has been improved to 25% for a sampling time duration of 25 ns. The accumulation of a sampled signal at such a high rate has been accomplished by the parallel operation of 16 newly developed memory circuit blocks, each having arithmetic capability. As a demonstration, the present system was applied in optical coherent transient spectroscopy and shown to be far more efficient than a conventional boxcar integrator.

Millimeter/submillimeter nonlinear spectroscopy

The techniques of coherent transient spectroscopy have been successfully applied to the microwave and optical regions of the spectrum, but lack of suitable sources has prevented convenient extension to the millimeter/submillimeter region where several strong molecular transitions occur. Analysis of the experimental requirements and theory for such experiments considering several molecules including H2O and CH3Cl indicates how such experiments can be performed employing recently developed oscillators with increased power.

Coherent Transient Effects in Optical Spectroscopy

Coherent transient effects appear when a sample is illuminated by pulses of radiation and one observes the absorption or coherent emis­ sion by the sample on a time scale short compared to its relaxation time. Depending upon the type of pulses used, a wide variety of different transient effects can be produced, providing researchers with a set of powerful and sensitive tools that can be used to obtain many different kinds of spectroscopic data. These effects were first seen in nuclear magnetic resonance with the discovery of transient nutation in 1949 and spin echoes and free induction decay in 1950 (1, 2). Since that time, pulsed nuclear magnetic resonance has grown into a mature and important branch of NMR spectroscopy.� The history of coherent transient effects in optical spec­ troscopy is considerably shorter because the required coherent optical sources were not available until the advent of the laser in 1960. The first experimental breakthrough occurred in 1964 with the observation of photon echoes (the optical analog of spin echoes) in ruby by Kurnit, Abella & Hartmann (3, 4). Four years later coherent transient effects were also observed in gases (5, 6). These early experiments, which used pulsed laser sources, were quite difficult, however, and it was not until the 1970s with the development of new experimental techniques and better electronics that the field really began to grow and attract a substantial number of researchers. At present, this area is flourishing with many new results being produced in all areas of spectroscopy. For example, in recent years optical coherent transient spectroscopy has

Coherent optical transient spectroscopy in flames

The use of coherent optical transient spectroscopy to study collisional processes in flames has been considered. A short discussion of the density matrix equations is used to illustrate transient phenomena of possible interest. Characteristic times that arise for such phenomena are calculated for a variety of flame conditions, and it is concluded that both dephasing and energy decay rates for electronic and ir transitions in selected species may be measured. Available excitation sources and transient measurement techniques are examined and found suitable for making such measurements.

Determinations of relaxation rate constants on the 22 GHz rotational transition of H2O by coherent transient spectroscopy

The relaxation time constants, T2’s of the 616←523 rotational line of H2O, self-broadened by H2O and foreign-gas broadened by He, H2, N2, and CO, are determined by the frequency switched microwave transient method. The relaxation constants of the M=5 and M=4 components in the presence of Stark fields are also examined. A novel method to stabilize the modulated microwave frequency is successfully developed.