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Spotlight on Optics


  • October 2013

Optics InfoBase > Spotlight on Optics > Anisotropic electro-optic effect on InGaAs quantum dot chain modulators

Anisotropic electro-optic effect on InGaAs quantum dot chain modulators

Published in Optics Letters, Vol. 38 Issue 20, pp.4262-4264 (2013)
by Wei Liu, Baolai Liang, Diana Huffaker, and Harold Fetterman

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Spotlight summary: Advances in material science have made it possible to grow and fabricate optoelectronic devices with characteristic dimensions of a few to tens of nanometers. The ability to tailor geometry and material composition at these length scales has ushered in an era of designer quantum engineering utilizing quantum-confined states to control how a device interacts with light. In general, semiconductor heterostructures can exhibit quantum confinement along one, two or all three spatial directions. The prototypical quantum confined semiconductor is the quantum well. Spatial confinement along one direction realizes the textbook particle-in-a-well scenario and imparts a degree of control to the spectral region of the well’s optical response. A second control knob for engineering the energetic location of a quantum well’s optical response is through application of an external electric field. The application of this field results in a Stark shift of the well’s energy levels – the Quantum Confined Stark Effect (QCSE). Quantum wells utilizing the QCSE are routinely engineered to serve as the active region in devices ranging from lasers to optical modulators.

More recently, devices based on the full three-dimensional confinement of a semiconductor’s electrons and holes are being investigated. The confinement results in optical transitions that occur ideally at discrete energies. Further, application of external electric fields results in a large QCSE that can be exploited for efficient optoelectronic device applications. Although quantum dots have many desirable attributes – large oscillator strengths, size dependent optical transition energies, large QCSE – the standard approach to growth, the Stranski-Krastanow method, relies on a random self-assembly process. The result is an inhomogeneous distribution in quantum dot sizes and little control over precise quantum dot location. Devices fabricated with dots reflect this inhomogeneity by exhibiting a broad optical response. Many research groups are currently trying to devise strategies that enable growth control over both quantum dot size and location. Ultimately, it is this lack of growth control that has slightly muted the full impact of quantum dot based optoelectronic devices.

The advances described by Liu et al in “The anisotropic electro-optic effect on InGaAs quantum dot chain modulators,” leverage recent improvements in quantum dot growth that enable ordering of the quantum dot heterostructures along well defined crystal directions. By stacking multiple dot layers the authors are able to ensure the quantum dot active material – embedded in a ridge optical waveguide (modulation regions with lengths from 0.5 to 2 mm and ridge widths of 4 μm) – efficiently interacts with the waveguide mode. The result is a waveguide based electro-optic modulator exploiting the QCSE that exhibits low drive voltages and a large operating bandwidth.

Specifically, the paper demonstrates device operation at the telecommunication wavelengths of
1.55 μm and 1.32 μm. Depending on the orientation of the waveguide, with respect to the crystal
axes, the linear electro-optic coefficient is found to change revealing an anisotropy in the device’s electro-optic response. This anisotropy is not surprising in that the underlying quantum- dot quantum states are sensitive to the local strain. The authors report linear electro-optic coefficients in the range of 24 – 40 pm/V depending on the exact device orientation and operating wavelength. When compared with bulk gallium arsenide the quantum dot based devices exhibit more than an order of magnitude larger linear electro-optic response resulting in half-wave voltages in the range of 3-6 volts. The authors also report operating bandwidths as large as 12.3 GHz. The results reported in the article by Liu et al make clear that advances in controlled quantum dot growth can have a dramatic impact on quantum dot optoelectronic device applications.

--Nick Vamivakas

Technical Division: Optoelectronics
ToC Category: Optical Devices
OCIS Codes: (230.2090) Optical devices : Electro-optical devices
(160.4236) Materials : Nanomaterials
(250.4110) Optoelectronics : Modulators

Posted on October 15, 2013

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