Theory Of Two-photon Absorption Research Articles

Theory of two-photon absorption with broadband squeezed vacuum

We present an analytical quantum theoretic model for nonresonant molecular two-photon absorption (TPA) of broadband, spectrally multimode squeezed vacuum, including low-gain (isolated entangled photon pairs) and high-gain (bright squeezed vacuum or BSV) regimes. The results are relevant to the potential use of entangled-light TPA as a spectroscopic and imaging method. We treat the scenario that the exciting light is spatially single mode and is nonresonant with all intermediate molecular states. In the case of high gain, we find that in the case that the linewidth of the final molecular state is much narrower than the bandwidth of the exciting light, bright squeezed vacuum is found to be equally (but no more) effective in driving TPA as is a quasimonochromatic coherent-state (classical) pulse of the same temporal shape, duration, and mean photon number. Therefore, in this case the sought-for advantage of observing TPA at extremely low optical flux is not provided by broadband bright squeezed vacuum. In the opposite case that the final-state linewidth is much broader than the bandwidth of the BSV exciting light, we show that the TPA rate is proportional to the second-order intensity autocorrelation function at zero time delay ${g}^{(2)}(0)$, as expected. We derive and evaluate formulas describing the transition between these two limiting cases, that is, including the regime where the molecular linewidth and optical bandwidth are comparable, as is often the case in experimental studies. We also show that for ${g}^{(2)}(0)$ to reach the idealized form ${g}^{(2)}(0)=3+1/\overline{n}$, with $\overline{n}$ being the mean number of photons per temporal mode, it is required to compensate the dispersion inherent in the nonlinear-optical crystal used to generate the BSV.

A [111]-Cut Si Hemisphere Two-Photon Response Photodetector

Properties of two-photon response in a [111]-cut nearly-intrinsic Si hemisphere photodetector are studied. The measured photocurrent of the photodetector responding to the 1.32μm continuous wave laser shows a quadratic dependence on the coupled optical power and is saturated with the bias voltage. Also, the photocurrent is independent of polarization. Such properties are in good agreement with the theory of two-photon absorption. The isotropic photocurrent generated from the [111]-cut Si hemisphere is compared to the anisotropic one induced in the [110]-cut Si sample and the ratio of χxxxx/χxxyy for silicon performing at 1.32 μm is calculated to be 2.4 via the fitted function of the anisotropic photocurrent from the [110]-cut sample.

Theory of two-photon absorption in poly(diphenyl) polyacetylenes

In this paper, we present a theoretical study of the nonlinear optical response of the newly discovered conjugated polymer poly(diphenyl)polyacetylene (PDPA). In particular, we compute the third-order nonlinear susceptibility corresponding to two-photon absorption process in PDPA using: (a) independent-particle Hückel model, and (b) using the correlated-electron Pariser-Parr-Pople (P-P-P) model coupled with various configuration-interaction methodologies such as the singles-configuration-interaction (SCI), the multi-reference-singles-doubles CI (MRSDCI), and the quadruples-CI (QCI) method. At all levels of theory, the polymer is found to exhibit highly anisotropic nonlinear optical response, distributed over two distinct energy scales. The low-energy response is predominantly polarized in the conjugation direction, and can be explained in terms of chain-based orbitals. The high-energy response of the polymer is found to be polarized perpendicular to the conjugation direction, and can be explained in terms of orbitals based on the side phenylene rings. Moreover, the intensity of the nonlinear optical response is also enhanced as compared to the corresponding polyenes, and can be understood in terms of reduced optical gap.

Spectral profiles of two-photon absorption: Coherent versus two-step two-photon absorption

ABSTRACTWe present a theory of two-photon absorption in solutions which addresses the formation of spectral shapes taking account of the vibrational degrees of freedom. The theory is used to rationalize observed differences between spectral shapes of one- and two-photon absorption. We elaborate on two underlying causes, one trivial and one non-trivial, behind these differences. The first refers simply to the fact that the set of excited electronic states constituting the spectra will have different relative cross sections for one-and two- photon absorption. The second reason is that the two-step and coherent two-photon absorption processes are competing, making the one-and two-photon spectral bands different even considering a single final state. The theory is applied to the N-101 molecule [di-phenyl-amino-nitro-stilbene] which was recently studied experimentally in the paper [ T.-C. Lin, G.S. He, P.N. Prasad, and L.-S. Tan, J. Mater. Chem., 14, 982, 2004.]

Two-photon absorption and two-photon induced absorption of liquid toluene at 347.15 nm

Pump pulse transmission and time-delayed probe pulse transmission measurements through liquid toluene were performed with linear and circular polarized second harmonic pulses of a mode-locked ruby laser system. Two-photon absorption and two-photon induced absorption are observed. The induced absorption anisotropy is investigated. A theory of two-photon absorption in isotropic media is presented. The two-photon absorption cross-section components, the effective excited-state absorption cross-section, the absorption anisotropy relaxation time and the excited singlet-state relaxation time are determined.

Two-photon absorption in GaAs/GaAlAs superlattices

A theory of two-photon absorption in GaAs/GaAlAs superlattices is given in a zero and a quantizing magnetic field. Radiation is assumed to be polarized in the plane of the layer. Numerical results for the absorption coefficient are given for two superlattice periods. The threshold energy for two-photon absorption is found to be smaller for superlattices of smaller period, both in zero and non-zero magnetic fields. The selection rules for transition between Landau levels for left and right circular polarization are Delta n=+1 and -1, respectively, in contrast to the selection rule Delta n=0 for one-photon absorption.

A special up-conversion phenomenon of Sm 3+ ion in fluoride glass — single-photon absorption followed by two-photon absorption

The first observation of a special up-conversion fluorescence from the 4G 5/2 level of Sm 3+ ions induced by 1.06 μm pulsed laser excitation is reported. Based on the one-photon and two-photon absorption theory, the transition probability and rate are estimated, the fluorescence process is modeled with rate equations. The dominating population of the 4G 5/2 state is attributed to a single photon excitation from ground state 6H 5/2 followed by two-photon absorption from excited state 6F 1/2. Due to their much smaller transition probabilities, the other possible up-conversion luminescence processes are unobserved.

Two-photon transitions from 3H 6 to 1I 6 of Tm 3+ in LaF 3 single crystal

Direct two-photon transitions between the crystal field components of the 3H 6 ground state and the 1I 6 excited state within the 4f 12 manifold of Tm 3+: LaF 3 have been observed at 77 K. The dependence of the induced fluorescence intensity on the pumping laser intensity agrees with the prediction by two-photon absorption theory. Several new crystal field components of 1I 6 were observed from the two-photon transitions of the absorption spectra.

Rydberg wave packets in many-electron atoms excited by short laser pulses.

An atomic electron excited to a coherent superposition of Rydberg states by a short laser pulse corresponds to a wave packet moving on a radial Kepler orbit. The dynamics of the motion of the wave packet can be observed in a two-photon process where a first laser pulse excites the wave packet, which at a later time is probed by a second pulse. In a many-electron atom a single valence electron excited to the Rydberg wave packet can exchange energy with the atomic ion core (electron correlation), whenever the Rydberg wave packet passes through the atomic core region. We can view this orbiting of the wave packet as a succession of below-threshold inelastic scattering events from the atomic ion core. A theory of two-photon absorption with time-delayed short laser pulses is developed which is based on a ``smooth'' multichannel quantum-defect Green function.

Resonantly enhanced 2+2 multiphoton ionization spectra of N2 via the ≈a 1Π g state: a line intensity study

The rotational state-selective detection of nitrogen is demonstrated employing 2 + 2 multiphoton ionization through the intermediate ã 1Π( v' = 3) state, and the method is seen to be very sensitive. A comparison between experiment and theory is presented using intensity data from the resonantly enhanced multiphoton spectrum reported here and the CO-sensitized fluorescence spectrum previously recorded by another group. The relative line intensities are calculated using transition strength factors derived from the theory of two-photon absorption. Agreement is good and opens the possibility of performing state-selective experiments on a quantitative basis using this relatively simple technique. Reliable sample temperatures are obtained using graphical or deconvolution methods from spectral intensity data.

The exponential index of multiphoton processes in two-photon absorption

A verified theory of two-photon absorption is computationally investigated to study the dependence of the exponential index of multiphoton processes, K, on incoherent relaxation bandwidths as a function of detuning.

The use of CNDO/S in spectroscopy. XVI. Multiple photon ICR photodissociation spectra

The theory of two-photon absorption has been implemented within the CNDO/S-CI formalism to calculate two-photon absorptivity of doublet states. The method was utilized to investigate the origins of below threshold optical absorption in ion cyclotron resonance photodissociation spectra of benzene, t-butylbenzene, styrene, and 1-methylnaphthalene. The computations suggest that these below threshold processes may be due to simultaneous two-photon absorption rather than sequential absorption as postulated by Freiser and Beauchamp. Some photodissociation pathways are found to be both mono- and biphotonic in nature. Some wavelength dependent processes found in t-butylbenzene may well be biphotonic and could be competitive with monophotonic absorption below 3.08 eV.

Semiclassical theory of two-photon absorption: Absorption and level shifts in a model system

Numerical calculations of level shifts and power-broadening linewidths for double-photon absorption in truncated rigid-rotor models are presented. This work demonstrates the feasibility of a previously suggested calculational method. It is shown that level shifts in double-photon absorption deduced from three-state models are misleading since other states dipole-coupled to states directly involved in the transition make important contributions to the shifts.

Theory of two-photon absorption and emission second-order saturation effect

A simplified theory of small-signal two-photon absorption is developed, based on the solution of rate equations involving an infinitely short-living intimediate state. It is shown that in a first approximation this theory leads to results identical to those obtained using second-order perturbation techniques. Moreover, the present approach leads to a built-in saturation effect at high intensities depending on the parameters of the laser and of the absorbing medium. An analysis expression is given for the intensity dependent saturation.

Polarization dependence of the two-photon absorption coefficient in crystals

The theory of two-photon absorption with provision for exciton effects is built on the basis of general principles without using any model of electronic states of the crystal. Electron-phonon interaction is neglected. The splitting of exciton levels into levels of transverse and longitudinal excitons is taken into account. The polarization dependence of two-photon absorption coefficient of allowed two-photon dipole transitions from the ground state of crystal for all 32 crystallographic classes is calculated. The results obtained are compared with the experimental data available.

Theory of Two-Photon Absorption by Polaritons

We treat theoretically the simultaneous two-photon absorption and sum-frequency generation associated with exciton polaritons in noncentrosymmetric crystals. Expressions are derived for the two-photon absorption coefficient and for the intensity of the polariton beam. We describe the symmetry properties of two-photon absorption by longitudinal excitons and by transverse polaritons.

Theory of Two-Photon Absorption Allowed and Forbidden Transitions

はじめにMayerによって行なわれた2光子吸収の理論をわかりやすく書きかえ,つぎにそれが振動・電子相互作用を含むように拡張した。これらの取り扱いで電子と輻射場との相互作用および電子と分子振動との相互作用は量子化されている。1光子吸収の場合と同様に,ここでも許容遷移と禁制遷移の間には簡単な関係が存在する

Two-Photon Laser Excitation of Polycyclic Aromatic Molecules

The theory of two-photon absorption is extended to include vibronic mixing between electronic states. Theoretical expressions are derived which relate the possible transition pathways with measured quantities. The experimentally determined quantities are: the relative absorption cross sections of linearly versus circularly polarized exciting light, and the polarization of the fluorescence resulting from two-photon excitation by linearly polarized light. The theory is applied to measurements on two molecules of D2h symmetry, anthracene and 9,10 dichloroanthracene. Utilizing the equations developed, the relative contributions of the three pathways, (1) both photons absorbed along the short axis, (2) both photons absorbed along the long axis, and (3) one photon absorbed along the long axis and the other photon absorbed along the short axis, are determined. From the deduced values of the transition pathways the relative contributions of different final vibronic states to the absorption of two laser photons (ℏω=14 400 cm−1) are determined.