Efficient isolation of multiphoton processes and detection of collective resonances in dilute samples

Abstract

A novel technique to sensitively and selectively isolate multiple-quantum coherences in a femtosecond pump-probe setup is presented. Detecting incoherent observables and imparting lock-in amplification, even weak signals of highly dilute samples can be acquired. Applying this method, efficient isolation of one-and two-photon transitions in a rubidium-doped helium droplet beam experiment is demonstrated and collective resonances up to fourth order are observed in a potassium vapor for the first time. Our approach provides new perspectives for coherent experiments in the deep UV and novel multidimensional spectroscopy schemes, in particular when selective detection of particles in dilute gas-phase targets is possible.Multiphoton processes play an important role in many fields of science. Light conversion processes such as second harmonic generation or optical parametric amplification are routinely performed in many labs. Parametric downconversion[1] is the key technique to generate entangled photon pairs used in quantum cryptography applications or to study entanglement properties in various systems [2]. High harmonic generation has pioneered the development of state-of-the-art coherent light sources in the XUV spectral range having attosecond pulse duration which allow the real-time observation of electron dynamics [3,4]. Likewise, multiphoton absorption in a tight laser focus is used in nonlinear microscopy yielding 3D images of biological tissues with high spatial resolution and great penetration depth [5,6]. Furthermore, the energy conversion process in singlet fission incorporates a multiphoton process and has recently drawn great interest due to its potential application in solar light harvesting [7,8].The unique identification and efficient detection of multiphoton processes is however often challenging. Monitoring multiphoton absorptions for transitions with small cross sections as a function of intensity is cumbersome and identification by emission spectra is frequently compromised by multistep relaxation pathways or predominant dark relaxation channels. Coherent timeresolved spectroscopy offers a different approach to identify multiphoton processes. Here, the phase evolution of non-stationary states induced upon optical excitation is monitored. This allows not only unambiguous identification of one-and multiphoton processes but also provides high time resolution.The phase of superposition states induced by multiphoton transitions -commonly termed multiple-quantum coherences (MQCs) -evolves at a much higher frequency than for one-quantum coherences (1QCs) induced by onephoton transitions. In multidimensional spectroscopy, MQC signals have allowed to characterize the influence of electron correlations in molecular systems [9], probing the anharmonicity of molecular potentials [10] or unraveling the role of high-lying electronic states in ultrafast pho-toinduced processes [11]. Furthermore, many-body interactions in a weakly-interacting atomic gas [12] and in semiconductor nanstructures [13][14][15] have bee...

Probing weak dipole-dipole interaction using phase-modulated nonlinear spectroscopy

“…The first one is the case of organic molecules in solution or on a substrate, the second one are atoms in a gas, similar to the situation of the experiments of Bruder et al [10].…”

“…Lock-in detection can even be used to filter the fluorescence signal with respect to harmonics of the modulation frequency [10,18,19]. In this way individual many-body responses are separated into different signal channels.…”

Anomalous two-photon spectral features in warm rubidium vapor

“…It is also possible for atom-atom interactions, mediated via long-range dipole-dipole coupling, to produce observable effects in the optical absorption of atomic vapors [24][25][26]. Typically observed in ultracold atomic clouds [27][28][29], long-range dipole-dipole interactions have been recently observed in thermal rubidium and potassium vapors [24,26].…”

“…Typically observed in ultracold atomic clouds [27][28][29], long-range dipole-dipole interactions have been recently observed in thermal rubidium and potassium vapors [24,26]. Such interactions have also been proposed for applications as broad as quantum information processing schemes [30] to atomic clocks [30] as well as for creation of novel quantum states of matter [31] and Rydberg-state quantum gates and molecules [28].…”

Nature of relaxation processes revealed by the action signals of intensity-modulated light fields

“…Single frequency modulated light-fields have recently been used in phase synchronous detection of different coherent and incoherent nonlinear signals. 10,11 In our experiment, 10 two beams, whose phases are modulated at φ 1 and φ 2 , are used to excite PL from a fluorophore. Collinear combination of the two phase modulated beams leads to the modulation of the total intensity at the frequency φ = |φ 2 − φ 1 |.…”