The future goal of achieving optical computation, manifesting all optical processes, seems brighter for photons, although there is still a long way ahead. The midpoint between solely electrical ICs and optical ICs is a hybrid containing both types of circuit on a single chip. This type of mixed chip is used already in many applications to modulate optical signals, either in phase or amplitude, by means of a high density of free electrons and holes that change the refractive index of a semiconductor, especially in a p-i-n junction. This electro optical (EO) effect is at the core of this work.
An important concept in this article is that of optical wavelength shifting. Nonlinear materials are considered by many as the only approach to shift the wavelength of light, but nonlinear processes require high intensities and complicated structures to produce phase-matching. These two requirements make the integration of nonlinear materials into optical ICs extremely difficult. In a series of papers, the authors have developed an optical wavelength shifting technique that they named “adiabatic shifting.” This name implies that this process is not a finite frequency sum or substraction, as in nonlinear optics. Instead, as intuitively explained in this article, the process of adiabatic shifting is analogous to what happens when a guitar string is plucked and immediately afterwards its tension is changed, resulting in a change of pitch. The crucial point is that the detuning of the cavity needs to take place within the wave lifetime inside it. For the case at hand, a p-i-n junction was integrated with a PC microcavity. The current of holes and electrons through the p-i-n junction tuned the resonance of the cavity and shifted adiabatically the wavelength of the trapped light. The adiabatically shifted light was used to couple light out of the cavity efficiently to a waveguide (also an integrated PC waveguide).
Through the use of adiabatic wavelength shifting, the major result of this work was achieved: a dynamic EO Q-switching. Q-switching is the creation of delayed and high intensity pulses by closing and opening a cavity or by simply modulating its QF. When the cavity is closed, it stores and builds up light intensity inside it. At chosen intervals the cavity is opened so that it releases high-intensity pulses. Adiabatic wavelength shifting thus enhances the fast discharges of energy inside the cavity by high coupling of shifted wavelengths out.
This is the first demonstration of integrated EO Q-switching of a microcavity. The authors' next goal is to achieve higher modulation speeds and higher intensity pulses, which are feasible with fabrication optimization.
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