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Physical Review A

Intracavity self-induced transparency of a multilevel absorber

Michael Müller, Vladimir P. Kalosha, Joachım Herrmann

Published Aug 1, 1998• 13 citations
Publisher

Abstract

Intracavity self-induced transparency of a three-level absorber is studied in the scope of solid-state laser generation of an ultrabroadband electromagnetic pulse that drives the population of all absorber levels through complete Rabi flopping. We show that at sufficient pump rates a Ti:sapphire laser forces an intracavity GaAs single quantum-well absorber, which provides an inter-valence-band transition in the THz domain in addition to two direct optical interband transitions, into the self-induced transparency regime and acts as an all-solid-state ultrabroadband pulse emitter. In dependence on the resonator bandwidth, the intracavity pulse energy and the absorber dipole moments we obtain a multilevel self-induced transparency pulse spectrum which extends from the THz domain up to the ultraviolet. The steady-state sub-10-fs pulse consists of only a few optical cycles with the high-frequency components at its leading edge and a single to subcyclic THz component at its trailing edge.

DOI
10.1103/physreva.58.1372

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Coherent quantum-well absorber mode locking for excitation dependent dephasing

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Self-induced-transparency mode locking in a Ti:sapphire laser with an intracavity rubidium cell

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In self-induced-transparency (SIT) mode locking, 2\ensuremath{\pi} SIT solitons with a duration much smaller than the polarization relaxation time ${T}_{2}$ in the gain and absorber are formed in a laser cavity. This is in contrast to standard passive mode-locking schemes based on gain and absorption saturation. Despite the great promise, up to now SIT mode locking with 2\ensuremath{\pi} pulses was mainly studied theoretically. In this paper, a stable self-starting passive mode locking is demonstrated experimentally in a Ti:sapphire laser with a Rb vapor cell. We show that the mode locking indeed appears to be due to SIT in the Rb cell; that is, the pulse in the Rb cell is a 2\ensuremath{\pi} SIT soliton. We also confirm self-starting of the SIT mode locking. Self-starting takes place via a set of intermediate regimes, including the one containing zero-area pulses.

arXiv:1906.04587•Physical Review A•Jan 7, 2020•19 citations
Intracavity self-induced transparency of a three-level absorber is studied in the scope of solid-state laser generation of an ultrabroadband electromagnetic pulse that drives the population of all absorber levels through complete Rabi flopping. We show that at sufficient pump rates a Ti:sapphire laser forces an intracavity GaAs single quantum-well absorber, which provides an inter-valence-band transition in the THz domain in addition to two direct optical interband transitions, into the self-induced transparency regime and acts as an all-solid-state ultrabroadband pulse emitter. In dependence on the resonator bandwidth, the intracavity pulse energy and the absorber dipole moments we obtain a multilevel self-induced transparency pulse spectrum which extends from the THz domain up to the ultraviolet. The steady-state sub-10-fs pulse consists of only a few optical cycles with the high-frequency components at its leading edge and a single to subcyclic THz component at its trailing edge.
Recently the shortest pulses directly out of a Ti:sapphire laser as short as 6.5fs has been generated by use of single quantum well (SQW) GaAs absorber [1]. Since this laser produces pulses with durations shorter than or comparable to the polarization dephasing time and sufficient power for Rabi flopping of the excited carrier density, the noninstantaneous response of the absorber polarization has to be taken into account. We develope the theory of coherent absorber mode locking taking into account also inhomogeneous broadening of the semiconductor transition by freely movable carriers and Coulomb renormalization of the transition frequency in the scope of semiconductor Bloch equations. Photo-echo measurements in highly excited GaAs yield a decreasing of the effective dephasing with increasing of the laser excited carrier density as T2 ∝ N-1/3. In the SQW absorber the center wavelength of 750nm is well above the bandedge of the GaAs SQW at 870nm and the broad pulse spectrum excites mainly 3D carriers above the confinement depth. We have shown that Rabi-flopping is self-consistently produced due to sub10fs pulse formation at TW/cm2 intensities corresponding to an excited carrier density up to 1020cm-3.
The standard self-induced transparency (SIT) phenomenon represents the lossless propagation of a long multicycle stable soliton in an absorbing two-level resonant medium, if the soliton's envelope area equals $2\ensuremath{\pi}$ ($2\ensuremath{\pi}$ pulse). In this case, the leading half of the $2\ensuremath{\pi}$ pulse excites the medium, while the trailing half completely brings the medium back to the ground state. In this work, we theoretically demonstrate an analogous phenomenon for subcycle pulses propagating in an arbitrary multilevel resonant medium. The duration of the obtained solitary pulses must be smaller than the periods of resonant transitions in the medium. The obtained subcycle solitary solutions first turn an initially unexcited multi-level resonant medium to excited levels and then fully return it back to the ground state.
Bosonic cascade lasers are terahertz (THz) lasers based on stimulated radiative transitions between bosonic condensates of excitons or exciton-polaritons confined in a trap. We study the interaction of an incoming THz pulse resonant in frequency with the transitions between neighboring energy levels of the cascade. We show that at certain optical pump conditions the cascade becomes transparent to the incident pulse: it neither absorbs nor amplifies it in the mean-field approximation. The populations of intermediate levels of the bosonic cascade change as the THz pulse passes, nevertheless. In comparison, a fermionic cascade laser does not reveal any of these properties.
The regime of passive mode locking in a Ti:sapphire laser with a coherent absorber resulting in the emission of pulses with a duration considerably shorter than the absorber relaxation times is demonstrated experimentally. A cell filled with rubidium vapor placed in the laser cavity serves as a coherent absorber. Mode locking arises when the laser oscillation frequency is tuned to the resonance transitions in rubidium. In our opinion, 2π pulses of self-induced transparency appear in the cavity, and the reported results represent the first experimental demonstration of passive mode locking caused by the effect of self-induced transparency, which has previously been discussed only theoretically.
Quantum coherence and interference in atomic systems can lead to interesting optical phenomena such as nonabsorbing resonances, lasing without inversion, generating an ultrahigh index of refraction without absorption, propagation of light pulses without losses. In the last-named is given the title transparency. Up to now it is known some types of transparency in multi-level resonant media. One of them is electromagnetically induced transparency (EIT). The phenomenon of EIT in the propagation of laser radiation through an absorbing medium has been recently investigated and explained in terms of quantum coherence and interference for multi-level atomic system. For instance in the (Lambda) -system, if the atomic medium is prepared in a quantum superposition of states, matched pulses, i.e., a pair of optical pulses whose amplitude and phase have a well defined relation, propagate without absorption. The mechanism of coherent population trapping is responsible for EIT in a (Lambda) -system. There is another, well known mechanism for creating transparency in a two-state atom: self-induced transparency, or SIT. Here, a single pulse whose area (the integral of its Rabi frequency over time) is 2(pi) is applied to the medium. Such a pulse causes the atoms to cycle smoothly from the ground to the excited state and then back again. In principle, such a pulse can propagate over long distances. SIT can be generalized to multi-level systems and multiple quasi-monochromatic fields. Generalization of SIT in a (Lambda) -system was obtained in reference 2. In this report we present a new type of transparency (combined transparency). This type describes propagation of laser pulse through a (Lambda) -type medium where the initial atomic state is prepared in the form of spatially varying coherent superposition of low-energetic states (initial coherency gradient). The common envelope behaves as usual 2(pi) hyperbolic secant pulse, but the ratio between the components of frequencies transitions 1> yields 3> and 2> yields 3> changes due to coherent Raman scattering so (phi) equals atan(E1/E2) equals (pi) /2-Gz, where z is the coordinate, G is the constant determined by both the incident pulse parameters and the initial coherence between the atomic states. Our solution demonstrates a direct connection of EIT in adiabatic regime to the SIT effects. This type of a transparency is given the title screw-type transparency, because for the given form of incident pulse (pitch of screw) medium is transparent, if medium's the initial coherence gradient is correspondingly prepared (thread on a engine nut). Besides a transparency, this phenomenon contains a transformation of the incident pulse frequency during the process of the propagation. This property of the phenomena can be result in a complete conversion of the frequency.
In self-induced-transparency (SIT) mode locking, 2\ensuremath{\pi} SIT solitons with a duration much smaller than the polarization relaxation time T2{T}_{2}T2​ in the gain and absorber are formed in a laser cavity. This is in contrast to standard passive mode-locking schemes based on gain and absorption saturation. Despite the great promise, up to now SIT mode locking with 2\ensuremath{\pi} pulses was mainly studied theoretically. In this paper, a stable self-starting passive mode locking is demonstrated experimentally in a Ti:sapphire laser with a Rb vapor cell. We show that the mode locking indeed appears to be due to SIT in the Rb cell; that is, the pulse in the Rb cell is a 2\ensuremath{\pi} SIT soliton. We also confirm self-starting of the SIT mode locking. Self-starting takes place via a set of intermediate regimes, including the one containing zero-area pulses.