Observation of Nondegenerate Two-Photon Gain in GaAs

Abstract

Two-photon lasers require materials with large two-photon gain (2PG) coefficients and low linear and nonlinear losses. Our previous demonstration of large enhancement of two-photon absorption in semiconductors for very different photon energies translates directly into enhancement of 2PG. We experimentally demonstrate nondegenerate 2PG in optically excited bulk GaAs via femtosecond pump-probe measurements. 2PG is isolated from other pump induced effects through the difference between measurements performed with parallel and perpendicular polarizations of pump and probe. An enhancement in the 2PG coefficient of nearly two orders-of-magnitude is reported. The results point a possible way toward two-photon semiconductor lasers.Multi-photon processes, predicted by Dirac [1], have been understood theoretically since the foundational work of Göppert-Mayer [2]. The simultaneous absorption or emission of two quanta by a single electronic transition have come to be known as two-photon absorption (2PA) and two-photon emission (2PE), respectively. 2PE may be spontaneous or stimulated by either one or two photons, referred to as singlyand doubly-stimulated [3]. Utilizing doubly-stimulated 2PE, or two-photon gain (2PG), to realize a two-photon laser (2PL) has been a goal of nonlinear optics since shortly after the invention of the original laser [4,5]. 2PL's are predicted to have many desirable properties due to the inherent nonlinearity of the gain, including pulse compression [6], self-mode-locking [7], and unique photon statistics [8,9]. While observation of 2PG has been reported in a variety of materials [3,[10][11][12][13], the development of 2PL's remains largely elusive. This is because 2PG is a weak process, and gain media suffer from various completing processes, e.g., excited state absorption, one-photon lasing, and parametric wave mixing [3]. Thus far, one group has demonstrated twophoton lasers using atomic barium [14] and potassium [15]. These, however, involved transitions from dressed states rather than eigenstates of unperturbed atoms [3]. All experimental work on 2PG thus far has focused only on the degenerate case where both emitted photons have the same frequency. However, there is no restriction on the energies of the individual photons, so long as their sum matches the energy difference between the initial and final states. Thus, 2PG may occur for both degenerate (D) and nondegenerate (ND) photon pairs.Semiconductors have been proposed as potential 2PG media [16], and recent investigations have shown D-2PG in bulk AlGaAs waveguides [13]. It has been shown theoretically [17] and experimentally [18] that 2PA in bulk semiconductors may be increased by over two orders of magnitude when using photons of very different energies, so called extremely nondegenerate (END) photon pairs, e.g., ħωa ≈ 10ħωb. This enhancement has been utilized for applications such as mid-IR detection [19] and 3D imaging [20]. Enhancement of 2PA translates into enhancement of 2PG, as the relation between the absorption and stimulated e...

Nonlinear Optical Signatures of the Transition from Semiconductor to Semimetal in PtSe2

“…The insets in Figure a,b show the values of ΔT/T 0 at time zero as a function of pump intensity. For 4 and 7 L films, the linear dependence confirmed 2PA was the dominant NLO response . While for the 17 and 55 L ones, the differential transmission signals saturate upon increasing pump intensity and show a large modulation depth.…”

Nondegenerate two- and three-photon nonlinearities in semiconductors

“…Thus two-photon gain is enhanced whenever 2PA is enhanced, as shown in Figure 2. ND-2PG has recently been experimentally demonstrated in optically excited GaAs, using pump-probe techniques, demonstrating such enhancement [21]. END-2PG holds potential for the creation of a two-photon semiconductor laser.…”

Nondegenerate two-photon absorption in GaAs/AlGaAs multiple quantum well waveguides

“…Applications of ND-2PA include detection [1], imaging [2], and all-optical switching [3]. Inverting carrier populations can also transform ND-2PA into nondegenerate two-photon gain [4][5][6], which is critical for realizing a two-photon semiconductor laser [7][8][9]. Waveguides are especially interesting for nonlinear optical applications because they enable strong effects through long interaction lengths.…”