We demonstrate, experimentally and theoretically, excitable nanosecond optical pulses in optoelectronic integrated circuits operating at telecommunication wavelengths (1550 nm) comprising a nanoscale double barrier quantum well resonant tunneling diode (RTD) photo-detector driving a laser diode (LD). When perturbed either electrically or optically by an input signal above a certain threshold, the optoelectronic circuit generates short electrical and optical excitable pulses mimicking the spiking behavior of biological neurons. Interestingly, the asymmetric nonlinear characteristic of the RTD-LD allows for two different regimes where one obtain either single pulses or a burst of multiple pulses. The high-speed excitable response capabilities are promising for neurally inspired information applications in photonics.

We report measurements of the spectral dispersion and the magnitude of three-photon absorption in Ale,sGa,,s,As for photon energies between one half and one third the band gap and show that a two-parabolic-band model is valid in this material. These results indicate that there is a limited spectral range below half the band gap in AlGaAs (and presumably all semiconductors) in which the bound electronic optical nonlinearity can be used without significant multiphoton absorption.One of the hardest aspects of nonlinear optics is finding a suitable nonlinear material for various applications. For example, we have been interested in all-optical waveguide switching devices which require solid-state materials that exhibit a large, ultrafast third-order nonlinear response with little or no absorption at the communication wavelengths of 1.3 and 1.55 ,UIL There are many materials with a large third-order nonlinear response but the major problem has been excessive absorption. The total absorption can have a number of contributions so that (Y= czr + c@= (Y$ + . . . . where a2 and cr, are the two-(2PA) and three-(3PA) photon absorption coefficients, respectively, and I is the intensity. For example, 2PA in semiconductors between the band gap and half the band gap is known to be detrimental to alloptical switching.' Below half band gap the 2PA coefficient decreases rapidly with increasing wavelength so that the two-photon figure of merit T-Cl, as required.lY2 AtGaAs, when used with photons of energy just less than half the band-gap energy, was found to be an ideal material for alloptical switching, leading recently to subpicosecond waveguide switching with 65-pJ energies.3'4 However, when an AlGaAs device is operated at wavelengths too far below half the band gap, 3PA appears to become a limiting factor for all-optical switching.'There are a number of detailed calculations which have been made on multiphoton absorption in different materials based on models of varying sophistication.6*7 There is now a wealth of 2PA data available for comparison with theory.* In particular, a simple analysis using a two-parabolic-band model by Wherrett has provided a reasonable prediction of 2PA coefficients for semiconductor materials, including AlGaAs."@ For 3PA, experimental data is very limited. No spectral dispersion has been reported and agreement with existing theories is unclear. For AlGaAs in particular, a single value for CL-+ has been published near 1.67 pm, which agrees within a factor of two with theory3 In this letter, we report measurements of the spectral dispersion and magnitude of LU, below half the band gap, show that it agrees well with Wherrett's two-parabolic-band model, and evaluate the spectral range over which AlGaAs can be used for efficient nonlinear optics.The 3PA coefficient can be calculated from Eq. Ref. 6:(1) in where P is the Kane parameter, E, the band-gap energy, hw the energy of the photon, n is the refractive index of the material, and e'/& the fine-structure constant. For these calculations the wavelength d...

Very few nonlinear optical materials are actually useful for high throughput all-optical devices. However, AlGaAs does satisfy all of the nonlinear optical figures of merit when used with photons of energy less than one half the semiconductor bandgap. Here we review our measurements of the pertinent nonlinear coefficients in waveguides and various device applications to all-optical switching in the communications band around 1550 nm.

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We report on photo-detectors consisting of an optical waveguide that incorporates a resonant tunneling diode (RTD). Operating at wavelengths around 1.55 μm in the optical communications C band we achieve maximum sensitivities of around 0.29 A/W which is dependent on the bias voltage. This is due to the nature of RTD nonlinear current-voltage characteristic that has a negative differential resistance (NDR) region. The resonant tunneling diode photo-detector (RTD-PD) can be operated in either non-oscillating or oscillating regimes depending on the bias voltage quiescent point. The oscillating regime is apparent when the RTD-PD is biased in the NDR region giving rise to electrical gain and microwave self-sustained oscillations Taking advantage of the RTD's NDR distinctive characteristics, we demonstrate efficient detection of gigahertz (GHz) modulated optical carriers and optical control of a RTD GHz oscillator. RTD-PD based devices can have applications in generation and optical control of GHz low-phase noise oscillators, clock recovery systems, and fiber optic enabled radio frequency communication systems.