1. Field of the Invention
This invention relates to true phase matched parametric generation of light in monolithically integrated intersubband (ISB) semiconductor optical devices in general and, more particularly, to true phase matched second harmonic (SH) generation of light in monolithically integrated quantum cascade (QC) semiconductor optical devices.
2. Discussion of the Related Art
By the phrase parametric generation we mean any optical process by which light of one center frequency is made incident on or is generated inside a nonlinear optical region (e.g., a suitably designed quantum well structure; a bulk Group III-V compound semiconductor; LiNbO3) to generate light of a different center frequency and often at harmonics thereof. As such, parametric generation includes two-wave interactions such as second harmonic (SH) generation and three-wave interactions such as sum and difference frequency generation, as well as interactions involving more than three waves (e.g., four-wave mixing).
In the prior art realm of non-integrated nonlinear optical devices, it is known to utilize discrete optical pump sources and discrete nonlinear bodies to generate light at a frequency different from that of the pump source. Thus, for example, in IEEE Photonics Technology Lett., Vol. 12, No. 5, pp. 486–488 (2000) Chowdhury et al. analyze continuously phase matched M-waveguides in LiNbO3 for second-order nonlinear up-conversion. Implicit in the analysis is that the pump source is external to and not integrated with the LiNbO3 waveguide. Similarly, in Appl. Phys. Lett., Vol. 83, No. 4, pp. 620–622 (2003) Moutzouris et al. describe SH generation through optimized modal phase matching in GaAs/AlGaAs waveguides. Explicit in the experimental work is the use of a 1.55 μm optical pump source external to and not integrated with the semiconductor waveguide. Inherently, however, these types of discrete parametric devices suffer from relatively low nonlinear conversion efficiency as well as optical coupling losses between the source and nonlinear body.
In contrast, bodies of nonlinear optical semiconductor materials show promise for achieving high-efficiency, nonlinear conversion of light when monolithically integrated within compact, injection-pumped semiconductor light emitters. These integrated optical devices would extend the spectral range accessible to semiconductor optical sources and could find applications in fields ranging from high-resolution chemical sensing to quantum cryptography.
Although resonant ISB transitions have been studied extensively as a source of nonlinear (or parametric) light generation, so far practical applications have been limited by two obstacles: the lack of both a powerful integrated pump source and a tunable phase matching scheme. [See, for example, Gurnick et al., IEEE J. Quantum Electron., Vol. QE-19, No. 5, pp. 791–794 (1983); Rosencher et al., Science, Vol. 271, pp. 168–273 (1996); Capasso et al., IEEE J. Quantum Electron., Vol. 30, No. 5, pp. 1313–1326 (1994); and Vurgaftman et al., IEEE J. Quantum Electron., Vol. 32, No. 8, pp. 1334–1346 (1996), all of which are incorporated herein by reference.] Only recently has the first obstacle been overcome by monolithically integrating the nonlinear ISB transitions within the structure of a QC laser. [See, for example, Owschimikow et al., Phys. Rev. Lett., Vol. 90, No. 4, pp. 043902-(1–4) (2003) and Gmachl et al., IEEE J. Quantum Electron., Vol. 39, No. 11, pp. 1345–1355 (2003), both of which are incorporated herein by reference.] On the other hand, the challenge of phase-matching the modes of the pump (laser) light and the modes of the parametric (e.g., SH) light continues to be an obstacle to achieving high efficiency parametric conversion.
Common techniques for phase matching, such as birefringence phase-matching or quasi phase matching, are not practical for ISB lasers. [Regarding the former, see, for example, Fiore et al., Nature, Vol. 391, pp. 463–466 (1998), which is incorporated herein by reference.] Moreover, the various schemes proposed for parametric (e.g., SH) phase matching in asymmetric double quantum well structures cannot be easily applied to ISB lasers because of the intrinsic waveguide dispersion, weak voltage tunability, and strict current transport requirements. [These schemes are described by Vurgaftman et al., supra, Meyer et al., Appl. Phys. Lett., Vol. 67, No. 5, pp. 608–610 (1995) and Vodopyanov et al., Appl. Phys. Lett., Vol. 72, No. 21, pp. 2654–2656 (1998), all of which are incorporated herein by reference.]
Thus, a need remains in the art for a monolithically integrated semiconductor optical source that exhibits true phase matching and efficient parametric light generation.
By the phrase true phase matching we mean that the nonlinear body is characterized in that the effective refractive indices of the mode of the pump light and that of the parametrically generated light are essentially equal to one another. True phase matching is to be distinguished from prior art techniques such as quasi phase matching, which relies on periodically resetting the phase of the parametric light at the layer interfaces of a multilayer nonlinear body, and birefringence phase matching, which utilizes the different indices of refraction of ordinary and extraordinary waves to make the effective refractive index of the pump light equal to that of the parametric light.