Abstract

Semiconductor-based nonlinear optical and electro-optic materials would be useful for switching and modulation devices in photonic integrated circuits. Due to the modification in the density of states, quantum dots and quantum wells are expected to exhibit enhanced optical nonlinearities and enhanced electro-optic effects [1]. In this regard, we characterized the electro-optic coefficient and loss of multilayer InAs quantum dot laser structures at 1309 and 1515nm. Quantum dot waveguides were grown by molecular beam epitaxy, where the active region is formed by three or five layers of self-assembled InAs QDs, covered by a 5-nm In_0.15 Ga_0.85As QW and separated from each other by a 40-nm GaAs spacer layer. The areal dot density of the lens-shaped QDs is 3×l0⁻¹⁰cm⁻². The laser cavity is clad by 1.5 μm of Al_0.7Ga_0.3 As, which is n-doped on the substrate side and p-doped on the top side. Devices are ridge-waveguide lasers with 8 and 15 pm ridge widths and lasing is peaked at around 1285nm [2], Measurement of the electro-optic coefficients away from EL emission peak was carried out by coupling 1.5 pm light from a tunable laser (Santee Tunable LD Light Source TSL-520) onto one end of the waveguide with a lensed fiber. A controlled DC voltage source was used to apply 0 to 6 Volts reverse bias to the samples. Transmission through the device was recorded as a function of wavelength. The index of refraction was inferred from the Fabry-Perot cavity response and the linear electro-optic coefficients (r₄₁) are obtained by fitting the measured data to the equation of transmission through a Fabry Perot modulator and calculating the refractive index difference between two specific voltages. The average values for electro-optic coefficients (r₄₁ / Γ) of 3QDs and 5QDs samples are given as 7.7×l0⁻¹³ m/V and 17×l0⁻¹³ m/V. Loss characterization were carried out by using a 1.3 pm light from a thermally tunable laser. Transmission through the device was recorded as a function of wavelength. Loss coefficient is found to be wavelength and bias voltage dependent. At 1.3 pm, typical loss differences are ~3dB/cm for a bias voltage difference of 5V for the 3QDs sample and ~ 4dB/cm for a bias voltage difference of 4V for the 5QDs sample.

© 2007 IEEE