Vertical integration of ultrafast semiconductor lasers

Lasers generating short pulses - referred to as ultrafast lasers - enable many applications in science and technology. Numerous laboratory experiments have confirmed that ultrafast lasers can significantly increase telecommunication data rates [1], improve computer interconnects, and optically clock microprocessors [2, 3]. New applications in metrology [4], supercontinuum generation [5], and life sciences with two-photon microscopy [6] only work with ultrashort pulses but have relied on bulky and complex ultrafast solid-state lasers. Semiconductor lasers are ideally suited for mass production and widespread applications, because they are based on a wafer-scale technology with a high level of integration. Not surprisingly, the first lasers entering virtually every household were semiconductor lasers in compact disk players. Here we introduce a new concept and make the first feasibility demonstration of a new class of ultrafast semiconductor lasers which are power scalable, support both optical and electrical pumping and allow for wafer-scale fabrication. The laser beam propagates vertically (perpendicularly) through the epitaxial layer structure which has both gain and absorber layers integrated. In contrast to edge-emitters, these lasers have semiconductor layers that can be optimized separately by using different growth parameters and with no regrowth. This is especially important to integrate the gain and absorber layers, which require different quantum confinement. A saturable absorber is required for pulse generation and we optimized its parameters with a single self-assembled InAs quantum dot layer at low growth temperatures. We refer to this class of devices as modelocked integrated external-cavity surface emitting lasers (MIXSEL). Vertical integration supports a diffraction-limited circular output beam, transform-limited pulses, lower timing jitter, and synchronization to an external electronic clock. The pulse repetition rate scales from 1-GHz to 100-GHz by simply changing the laser cavity length. This result holds promise for semiconductor-based high-volume wafer-scale fabrication of compact, ultrafast laser

Towards wafer-scale integration of high repetition rate passively mode-locked surface-emitting semiconductor lasers

One of the most application-relevant milestones that remain to be achieved in the field of passively mode-locked surface-emitting semiconductor lasers is the integration of the semiconductor absorber into the gain structure, enabling the realization of ultra-compact high-repetition-rate laser devices suitable for wafer-scale integration. We have recently succeeded in fabricating the key element in this concept, a quantum-dot-based saturable absorber with a very low saturation fluence, which for the first time allows stable mode locking of surface-emitting semiconductor lasers with the same mode areas on gain and absorber. Experimental results at high repetition rates of up to 30GHz are show

Guided self-assembly of lateral InAs/GaAs quantum-dot molecules for single molecule spectroscopy

We report on the growth and characterization of lateral InAs/GaAs (001) quantum-dot molecules (QDMs) suitable for single QDM optical spectroscopy. The QDMs, forming by depositing InAs on GaAs surfaces with self-assembled nanoholes, are aligned along the [] direction. The relative number of isolated single quantum dots (QDs) is substantially reduced by performing the growth on GaAs surfaces containing stepped mounds. Surface morphology and X-ray measurements suggest that the strain produced by InGaAs-filled nanoholes superimposed to the strain relaxation at the step edges are responsible for the improved QDM properties. QDMs are Ga-richer compared to single QDs, consistent with strain- enhanced intermixing. The high optical quality of single QDMs is probed by micro-photoluminescence spectroscopy in samples with QDM densities lower than 108 cm−2

Single-mode, single-polarization VCSELs via elliptical surface etching: experiments and theory

Polarization control in vertical-cavity surface-emitting lasers (VCSELs) has attracted a lot of interest and different techniques have been proposed to achieve it. Among them, one of the most attractive relies on slight modifications of existing and well-behaving devices, i.e., by introducing a noncircular transverse section layer somewhere in the device. Even though experimental verifications of this principle have already been carried out, a theoretical framework to better understand and possibly optimize such devices is still missing. This mainly originates from the need of a fully vectorial and three-dimensional approach. In this paper, we will undertake a joint experimental and theoretical effort: first, we give experimental evidence of polarization control by applying elliptical surface etching. Then, after having validated the vectorial electromagnetic model by comparing numerical and experimental results, we are able for the first time to explain and compare the polarization selection mechanisms and, consequently, to provide guidelines for optimized structures

Towards wafer-scale integration of high repetition rate passively mode-locked surface-emitting semiconductor lasers

ISSN:0946-2171ISSN:1432-0649ISSN:0721-7269ISSN:0340-379