Two-photon Polarization Spectroscopy Research Articles

Cavity effect on a biexciton in a CuCl microcavity

We studied the population and coherence dynamics of a biexciton in a planar CuCl microcavity. We developed a time-resolved two-photon polarization spectroscopy technique, by which we extracted the population dynamics of the biexciton even when the biexciton was spectrally overlapped with the lower cavity polariton. The observed lifetime of the cavity biexciton in the weak-coupling regime was much shorter than that of a bare CuCl thin film without a cavity, which directly demonstrates cavity enhancement of the radiative decay rate of the biexciton. The cavity enhancement originates from the hybridization of the unbound two-cavity-polariton state to the biexciton wave function by the biexciton-cavity coupling. By comparing the population and coherence decay curves of the biexciton, we found that the coherence of the cavity biexciton was limited by the radiative population decay.

Two-color polarization spectroscopy of rubidium

We report on two-color, two-photon polarization spectroscopy in a room-temperature rubidium vapor. We use two separate lasers, a strong pump at 780nm to induce an anisotropy in the atomic polarization and a weak probe at 776nm to interrogate this anisotropy. The lasers are resonant with the 52S1/2→52P3/2 and 52P3/2→52D5/2 transitions in rubidium, respectively. Finally, we have used our polarization spectroscopy signal as an error signal to lock the 776nm laser. This modulation-free locking scheme allows us to detune the lock point of the second laser by adjusting the detuning of the laser used for the first transition.

Measurements of two-photon absorption cross-section of Xe for laser spectroscopic techniques calibration

Two-photon transition cross section for one xenon transition (5p6 1S0-5p56p[1/2]0) will be measured with very low uncertainty by two-photon polarization spectroscopy. The method, which is explained in this paper, is based on a comparison of direct measurements of the two-photon absorption in xenon gas at two closed wavelengths (resonant and no resonant).

Electric field measurements in a hollow cathode discharge by two-photon polarization spectroscopy of atomic deuterium

The local electric field strength (E-field) is an important parameter to be known in low pressure plasmas such as glow discharges, RF and microwave discharges, plasma boundaries in tokamaks etc. In this paper, we demonstrate, for the first time, the potential of two-photon polarization spectroscopy measuring the E-field in the cathode fall region of a hollow cathode discharge, via Doppler-free spectra of the Stark splitting of the 2S level of atomic deuterium. Electric field strength is determined in the range from 2 to 5 kV cm−1. Compared with LIF, this method has several advantages: it is not affected by background radiation, it can be applied without limitation at elevated pressure and it allows simultaneous measurement of absolute local atomic ground state densities of hydrogen isotopes.

Two-color, two-photon laser-induced polarization spectroscopy (LIPS) measurements of atomic hydrogen in near-adiabatic, atmospheric pressure hydrogen/air flames

Two-color, two-photon laser-induced polarization spectroscopy (LIPS) measurements of atomic hydrogen in near-adiabatic, atmospheric pressure hydrogen/air flames

Two-color, two-photon laser-induced polarization spectroscopy (LIPS) measurements of atomic hydrogen in near-adiabatic, atmospheric pressure hydrogen/air flames

Two-color, two-photon laser-induced polarization spectroscopy (LIPS) of atomic hydrogen has been demonstrated and applied in atmospheric pressure hydrogen/air flames. Fundamental and frequency-doubled beams from a single 486-nm dye laser were used in the experiments. The 243-nm pump beam in the measurements was tuned to the two-photon n=1→ n=2 resonance of the hydrogen atom. The 486-nm probe beam was tuned to the single-photon n=2→ n=4 resonance of the hydrogen atom. Measurements were performed in an atmospheric pressure H 2/air flame stabilized on a near-adiabatic, flat-flame calibration burner (the Hencken burner). For the range of pump beam intensities used, the LIPS signal was found to be nearly proportional to the square of the pump beam intensity over a wide range of flame equivalence ratios. Spectral lineshapes were recorded at flame equivalence ratios ranging from 0.85 to 2.10. Vertical H-atom number density distribution profiles were measured in the Hencken burner. The vertical H-atom number density profiles measured along the burner centerline for various flame equivalence ratios were compared with the results of a numerical flame calculation using the UNICORN (Unsteady Ignition and Combustion with Reactions) code. Good agreement between theory and experiment was obtained for stoichiometric and rich flame conditions. For flames with equivalence ratios greater than 1.5, the H-atom concentration was substantially above the adiabatic equilibrium value, even at 50 mm above the burner surface. The slow approach to the adiabatic equilibrium H-atom concentration value can be explained by assuming partial equilibrium in the postflame gases; the H-atom concentration is proportional to the O 2 concentration which requires significant residence time to decrease to its very low equilibrium concentration. These results suggest that the use of the Hencken burner as a radical measurement technique calibration source may be of questionable value for equivalence ratios greater than 1.5 and less than 0.8.

Absolute atomic hydrogen density distribution in a hollow cathode discharge by two-photon polarization spectroscopy

We report on quantitative measurements of ground-state atomic hydrogen densities in a stationary plasma far off thermodynamic equilibrium, generated in a hollow cathode discharge, by two-photon polarization spectroscopy via the 1S–2S transition. Absolute densities are obtained using a well established calibration method based on the non-resonant two-photon polarization signal of xenon gas at room temperature, which serves as the reference at the wavelength of the hydrogen transition. This study is dedicated to demonstrating the capability of two-photon polarization spectroscopy close to the detection limit. Therefore, it requires single-longitudinal mode UV-laser radiation provided by an advanced UV-laser spectrometer.

Two-photon polarization spectroscopy applied for quantitative measurements of atomic hydrogen in atmospheric pressure flames

Two-photon – polarization spectroscopy has been applied for the first time to determine absolute number densities and kinetic temperatures of atomic hydrogen via the 1S–2Stransition in flames at atmospheric pressure. This sensitive laser spectroscopic technique, so far mainly developed and applied for quantitative plasma diagnostics, is also well suited for real combustion processes, because the signal is directly related to the two-photon absorption itself and therefore not limited by quenching or photo-ionization. It can be applied with high spatial and temporal resolution in a wide range of pressure and temperature. Furthermore, a well-established calibration procedure allows for a precise determination of the absolute ground-state density of atomic hydrogen. The measurements also demonstrate that a certain potential of this method comes especially into its own in conjunction with laser radiation of highest spectral quality, i.e. pulsed single-longitudinal mode UV radiation.

Optical Ramsey fringes induced by Zeeman coherence

We utilize a two-photon polarization spectroscopy technique to observe Ramsey fringes that appear due to light scattering by coherently prepared rubidium atoms. We examine the effect in a vapor cell for spatially and temporally separated laser fields. We present a theoretical interpretation of the experimental results.

Two-photon polarization spectroscopy of F3+ and F2 color centers in LiF crystals

We identified energy levels, and their corresponding symmetries, of F3+ and F2 color centers in LiF crystals, using the Feofilov method of polarized luminescence as applied to two-photon excitation. For this purpose, we measured the azimuthal dependencies of the polarization degree, P(ϕ), and the intensity, I(ϕ), of the F3+ and F2 color center luminescence in LiF crystals under two-photon excitation. To increase the reliability of the results we measured the polarization dependencies of three LiF crystal plates cut parallel to the three main crystallographic planes (100), (110) and (111), respectively. The measured polarization dependencies were seen to be in good agreement with the theoretical polarization dependencies derived for P(ϕ) and I(ϕ). We established that the new energy level of the F2 center has the same parity as the ground state. Consequently, the one-photon electric-dipole transitions are forbidden between the states. The symmetry of the new electronic state of the F3+center was found to be 1A1.

Two-photon polarization spectroscopy of LiF crystals with F 2 laser color centers

The paper reports on a measurement and calculation of the dependence of LiF: F2 luminescence polarization and intensity on the polarization of high-power laser radiation (λex=1064 nm) producing two-photon excitation of F2 centers. It is shown that the two-photon transition results in the excitation of a previously unknown electronic level of F2 centers of the A1g symmetry.

Intracavity two-photon polarization spectroscopy on the basis of light-induced peaks of radiation power

A calculation of the selective losses of a spectrometer based on a broad-band laser with an anisotropic three-mirror ring resonator containing a two-photon-absorbing atomic gas medium with a light-induced resonance anisotropy has been performed. The conditions under which laser radiation power resonances appear in the output radiation spectrum and the contribution of different factors to their formation have been revealed. Possible ways of obtaining information on the polarization effects induced by a powerful light wave in a medium at the frequencies of two-quantum transitions were investigated.

Spectroscopy of F 3 + -color centers in lithium fluoride crystals (review)

Results of investigations of radiative F 3 + -color centers in lithium fluoride obtained by the methods of one- and two-photon absorption, polarization spectroscopy, and luminescence in singlet, triplet and triplet-singlet channels are reported. The scheme of energy levels of the F 3 + -center is revised. The probabilities of singlet-triplet conversion and the rates of depletion of the lower triplet state in the range of 80–350 K are presented. Recommendations are given on the technology of radiative LiF coloration that provide an increase in the F 3 + -center concentration and a decrease in the concentration of some other centers that prevent lasing on the LiF:F 3 + active medium from being obtained. The phototransformations of color centers in LiF are analyzed. The parameters of lasers with LiF:F 3 + active media that provide generation of radiation tunable in the green region of the spectrum are considered. Based on spectroscopic data the maximum attainable time characteristics of such lasers are discussed. The prospects for further investigations of LiF:F 3 + lasers are outlined in brief.

Two-photon polarization spectroscopy and (2 + 3)-photon laser induced fluorescence of N 2

Two-photon induced polarization spectroscopy (TIPS) for the detection of molecular nitrogen is demonstrated. Laser induced fluorescence (LIF) from N 2 +, which was created via a (2 + 3)-photon resonance from N 2, was monitored simultaneously to allow a quantitative comparison between the two techniques in different operating regimes. Power dependence checks for both techniques were performed and the effects of collisional partners were investigated in different gas flow mixtures. These preliminary results suggest that TIPS is not as severely affected by molecular quenching collisions as LIF. The high signal levels obtained prove that the technique is sensitive and may have potential as diagnostic tool for combustive and non combustive flows.

Absolute determination of local ground-state densities of atomic hydrogen in nonlocal-thermodynamic-equilibrium environments by two-photon polarization spectroscopy.

Two-photon polarization spectroscopy was applied in order to establish an absolute method for the determination of atomic ground-state densities in gases and plasmas. The method has high spatial and temporal resolution, can be used in a wide range of densities, and does not require any knowledge of the thermodynamic state of the system, because the measured two-photon absorption is strictly proportional to the atomic ground-state density. For atomic hydrogen, which is of basic physical importance as well as of great interest in technology and basic plasma research, the method has carefully been worked out. The measurement range is ${\mathit{N}}_{\mathit{H}}$\ensuremath{\ge}${10}^{19}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$ with an uncertainty of less than 10%. To achieve this accuracy, an arc plasma was shown to provide a standard of atomic hydrogen density with 5% uncertainty, and nonresonant two-photon absorption in xenon gas was established to serve as a transfer standard for easy application. In addition to the density measurements, the developed method allows for precise measurements of two-photon transition probabilities of other atomic species.

Two-photon induced polarization spectroscopy applied to the detection of NH 3 and CO molecules in cold flows and flames

Two-photon induced polarization spectroscopy, TIPS, is demonstrated for the detection of NH 3 and CO molecules. Measurements were performed in flow as well as in flame environments. The signal dependence on the laser pulse intensity and on the pump beam polarization was investigated. Two-dimensional signal intensity maps were formed from one-dimensional signal distribution measurements at different heights in flames. With a proper choice of the laser wavelength two-photon polarization spectroscopy signals from NH 3 and one-photon polarization spectroscopy signals from OH could be measured simultaneously. The spectrum of the Q branch of the X 1Σ−B 1Σ +(0,0) band of the CO molecule at different temperatures and the dependence of the two-photon polarization signal on the number density of CO molecules in the presence of N 2 and He molecules were also measured. A comparison between the TIPS and two-photon DFWM techniques for flame studies is briefly discussed.

Doppler-free two-photon polarization-spectroscopic measurement of the Stark-broadened profile of the hydrogen L alpha line in a dense plasma.

It is demonstrated that Doppler-free two-photon polarization spectroscopy makes possible Stark profile measurements without Doppler-broadened background under high-density and high-temperature conditions, e.g., in dense arc plasmas, where other sub-Doppler laser-spectroscopic techniques introduced so far fail. This method, based on the polarization rotation connected with a two-photon transition, is not limited by collisions changing the velocity or the excitation of the atoms or by photoionization of the upper level.

Theory of two-photon polarization spectroscopy of plasma-broadened hydrogen L alpha line.

The Stark-broadened line profile observed with Doppler-free two-photon polarization spectroscopy of the hydrogen resonance line ${\mathrm{L}}_{\ensuremath{\alpha}}$ in a plasma is investigated theoretically. With use of computer simulations for the numerical evaluation of line profiles, strong ion dynamical effects in the line core are found at moderate plasma densities.

Two-Photon Polarization Spectroscopy in CuCl

Highly sensitive momentum-space spectroscopy, named two-photon polarization spectroscopy (TPPS), is presented in the Z 3 exciton resonant region of CuCl. This spectroscopy is based on the induced optical anisotropy associated with the formation of \( \varGamma_{1}\) excitonic molecules (EM's). By this method the following facts are confirmed: the upper branch polaritons (UBP's) and the lower branch polaritons (LBP's) coexist in the wide energy range above the longitudinal exciton energy; the conversion of UBP into LBP occurs in the reflection at the sample boundary; the EM relaxation depends on the wave number in the large wave number region. From the quantitative analysis of the spectra, it has been found that the damping of the Z 3 exciton depends strongly on its energy in the L-T gap region. The prospects of the ABC problem are also discussed.

Two-photon polarization spectroscopy in the spatially dispersive region of exciton polaritons in CuCl

Two-photon polarization spectroscopy (TPPS) is presented in the spatially dispersive region of Z 3 exciton polaritons in CuCl. The time evolution of the TPPS signal is discussed. The possibility of the application for the quantitative evaluation of k-dependent exciton damping is demonstrated experimentally.