Chunfang Cao

High performance external cavity InAs/InP quantum dot lasers

We report on high performance InAs/InP quantum dot tunable external cavity lasers (ECLs) operating in continuous-wave mode at room temperature. A tuning range of 70 nm has been achieved, covering the wavelengths from 1563 to 1633 nm. The threshold current densities are lower than 1625 A/cm2 in the tuning range. More than 23 mW output power was obtained at lasing wavelength of 1594 nm with an external differential quantum efficiency of 10.3%. An even wider tuning range of 98 nm has been obtained from the ECL based on the QD laser lasing in a longer wavelength.

Detailed Study of the Influence of InGaAs Matrix on the Strain Reduction in the InAs Dot-In-Well Structure.

InAs/InGaAs dot-in-well (DWELL) structures have been investigated with the systematically varied InGaAs thickness. Both the strained buffer layer (SBL) below the dot layer and the strain-reducing layer (SRL) above the dot layer were found to be responsible for the redshift in photoluminescence (PL) emission of the InAs/InGaAs DWELL structure. A linear followed by a saturation behavior of the emission redshift was observed as a function of the SBL and SRL thickness, respectively. The PL intensity is greatly enhanced by applying both of the SRL and SBL. Finite element analysis simulation and transmission electron microscopy (TEM) measurement were carried out to analyze the strain distribution in the InAs QD and the InGaAs SBL. The results clearly indicate the strain reduction in the QD induced by the SBL, which are likely the main cause for the emission redshift.

Relative intensity noise of InAs quantum dot lasers epitaxially grown on Ge

We report the relative intensity noise (RIN) characteristics of an InAs quantum dot (Qdot) laser epitaxially grown on the Ge substrate. It is found that the minimum RIN of the Ge-based Qdot laser is around −120 dB/Hz, which is 15 dB higher than that of a native GaAs-based Qdot laser with the same layer structure. The higher RIN in the Ge-based laser can be attributed to the high-density epitaxial defects of threading dislocations and antiphase domain boundaries.

A novel semiconductor compatible path for nano-graphene synthesis using CBr4 precursor and Ga catalyst

We propose a novel semiconductor compatible path for nano-graphene synthesis using precursors containing C-Br bonding and liquid catalyst. The unique combination of CBr4 as precursor and Ga as catalyst leads to efficient C precipitation at a synthesis temperature of 200°C or lower. The non-wetting nature of liquid Ga on tested substrates limits nano-scale graphene to form on Ga droplets and substrate surfaces at low synthesis temperatures of T ≤ 450°C and at droplet/substrate interfaces by C diffusion via droplet edges when T ≥ 400°C. Good quality interface nano-graphene is demonstrated and the quality can be further improved by optimization of synthesis conditions and proper selection of substrate type and orientation. The proposed method provides a scalable and transfer-free route to synthesize graphene/semiconductor heterostructures, graphene quantum dots as well as patterned graphene nano-structures at a medium temperature range of 400–700°C suitable for most important elementary and compound semiconductors.

Influence of GaAsBi Matrix on Optical and Structural Properties of InAs Quantum Dots

InAs/GaAsBi dot-in-well structures were fabricated using gas-source molecular beam epitaxy and investigated for its optical and structural properties. GaAsBi-strained buffer layer and strain reduction layer are both effective to extend the photoluminescence (PL) emission wavelength of InAs quantum dot (QD). In addition, a remarkable PL intensity enhancement is also obtained compared with low-temperature-grown GaAs-capped InAs QD sample. The GaAsBi matrix also preserves the shape of InAs QDs and leads to increase the activation energy for nonradiative recombination process at low temperature. Lower density and larger size of InAs QDs are obtained on the GaAsBi surface compared with the QDs grown on GaAs surface.

Heteroepitaxy growth of GaAsBi on Ge(100) substrate by gas source molecular beam epitaxy

We have investigated the growth of GaAsBi single-crystal film on Ge substrate by gas source molecular beam epitaxy. A high-quality GaAsBi epilayer has been obtained. It has been found that the surfactant effect of Bi suppresses the interdiffusion of Ge at the GaAsBi/Ge interface and reduces the misfit dislocation density. The Bi atoms occupy the As sites, as indicated by the appearance of GaBi-like TO(G) and LO(G) phonon modes in Raman spectra. In addition, the redshift of the GaAs-like LO(G) phonon frequency has been observed in the Raman spectra, owing to the Bi-induced biaxial strain and the alloying effect as well.

Analysis of mode-hop free tuning of folded cavity grating feedback lasers.

We analytically study an external cavity laser structure including a folded cavity. A steering mirror is utilized in the folded cavity to deflect the intracavity laser beam. A mode-hop free tuning range of ∼400  GHz can be achieved by control of the steering mirror, and a fast tuning rate is expected because of the small mass of the steering mirror. This technique has potential for applications in spectroscopy for turbulent media, especially in the mid-infrared region.

Near-infrared GaAsBi quantum well laser diodes (Conference Presentation)

Dilute bismide is a novel class of III-V semiconductor compound possessing a number of attractive physical properties such as a large band-gap bowing effect, a large spin-orbit split band and a less temperature sensitive band-gap etc. In this talk, I will present electrically pumped near infrared GaAsBi quantum well (QW) laser diodes (LDs) grown by molecular beam epitaxy with room-temperature lasing up to 1.14 m. Epitaxial growth is carefully optimized to ensure high bismuth incorporation and high optical quality at the same time. The LDs reveal an output power over 120 mW under pulsed excitation at 300 K and can operate under CW excitation up to 273 K. They also show high performance with an internal quantum efficiency of 86% and an internal optical loss of 10 cm-1. The characteristic temperature is 79 K in the temperature range of 225-350 K and the temperature coefficient of the lasing wavelength is 0.26 nm/K at 77-350 K, much smaller than 0.35-0.40 nm/K for InGaAs and InGaAsP QW LDs. These results suggest that GaAsBi LDs are attractive candidates for uncooled near infrared lasers on GaAs.

Electrically pumped GaAsBi laser diodes

In this paper, we present electrically pumped GaAsBi quantum well (QW) laser diodes (LDs) grown by molecular beam epitaxy. The LDs reveal a record long lasing wavelength of 1.14 µm at 300 K and can be operated under CW excitation up to 273 K. They also show high performance with an internal quantum efficiency of 86% and an internal optical loss of 10 cm−1. The characteristic temperature is 79 K in the temperature range of 225–350 K and the temperature coefficient of the lasing wavelength is 0.26 nm/K at 77 – 350 K, much smaller than 0.35 – 0.40 nm/K for InGaAs and InGaAsP QW LDs. These results suggest that GaAsBi LDs are attractive candidates for uncooled near infrared lasers on GaAs.

External cavity lasers using stripe mirrors with different mirror width

Abstract We present the study of an external cavity laser using a stripe mirror and a grating for the optical feedback. As the most important parameter in this system, the width of the stripe mirror has been systematically studied, which strongly affects the device performances. Furthermore, the size of the focused laser spots and the distance between the spots of adjacent modes have been obtained. The measured results agree with the theoretical calculation very well. These parameters are very important for the design and construction of this kind of external cavity lasers.