Optical Nonlinearities of GaAs‐Based Epitaxial Structures for All‐Optical Switching

Abstract

An experimental study is made of GaAs/AlGaAs monolithic bistable etalons fabricated by metalorganic vapor-phase epitaxy. Optical bistability is observed in the reflective mode, with a minimum threshold power of less than 3 mW and a switching contrast as high as 30: 1, with a cavity resonator designed to achieve simultaneously a high finesse and a high reflectivity contrast. The dispersive optical nonlinearity of the multiple quantum well active medium is measured as a function of the intracavity intensity, and a strong saturation behavior is observed. The origin of this saturation and its implication on the design of future devices is discussed briefly.On decrit les rksultats d'une etude experimentale du regime bistable de cavites monolithiques en GaAs/AlGaAs fabriquees par epitaxie organo-metallique. La bistabilite optique a t t t observee en reflection, avec un seuil en puissance inferieur a 3 mW et un contraste de commutation de 30 : 1, avec des paramttres de cavite spkcialement etudies pour obtenir simultanement une grande finesse et un contraste eleve en reflection. La non-linearite optique dispersive des puits quantiques multiples constituant la couche active a ete mesuree en fonction de l'intensite intracavite, mettant en evidence une saturation marquee a haute intensite. L'origine de cette saturation est brievement discutee, ainsi que son incidence sur la conception de futurs dispositifs.

Saturation of Band‐Tail Absorption in Multiple Quantum Wells and Its Link with the Nonlinear Index Saturation

“…However where the carrier density dependence of the absorption a and the carrier lifetime z are written explicitly, and h u~ is the photon energy. The only possibility for the solution of (4) to display an asymptotic behavior like the right-hand side of (3) is that either s ( N ) or a ( N ) vanishes for a finite value of N . Although T is known to decrease with N due to bimolecular or Auger recombination, it does not vanish at a finite value of N .…”

“…These attractive properties are obtained using the large excitonic-resonant nonlinear refractive index Sn,, of GaAs MQWs at room temperature [3], which involves the photoexcitation of electron-hole pairs, initially created in the low-energy tail of the excitonic and band-to-band optical transitions (Urbach's tail), and subsequently thermalized in higherenergy states through interaction with the lattice vibrations. In the course of our studies of such bistable microcavities, it was found that, due to the short nonlinear medium length (typically 100 QWs with a 20 nm periodicity), optical bistability could only be observed in the highest finesse cavities [4], which suggested that the available nonlinear phase shift was limited by some saturation mechanism.In the nonsaturating regime, Sn,, is simply proportional to the intensity I , and it is well known that n, defined as &,,(I) = n,l is proportional to the linear absorption coefficient a0 [5]. It is thus possible to define a figure of merit nJao which gives information about the nonlinear efficiency of the material.…”