Vertical-cavity surface-emitting laser with liquid crystal external cavity

Abstract

ABSTRACT We have d eveloped a technology to integrate a thin layer of liquid crystal (LC) on top of a Vertical -Cavity Surface -Emitting Laser (VCSEL). Based on this technology, we demonstrate VCSELs with a chiral liquid crystal (CLC) layer , which acts as a tuneable mirror. Th e reflection properties of the CLC layer are controlled by temperature. Next we demonstrate VCSEL devices with tuneable external cavity using a nematic LC layer incorporated with an additional dielectric mirror (SiO 2 /Ta 2 O 5 ). The VCSEL and the LC layer can be electrically driven independently and the optical length in the external cavity can be tuned by the applied voltage on the LC layer. In both configurations we show that the emission properties of the VCSEL can be changed, in terms of emission wavelength , polarization state and/or lasing threshold. Keywords: Vertical Cavity Surface Emitting Laser , external cavity , ch iral liquid crystal 1. INTRODUCTION Because of its high emission efficiency and low operation power consumption, VCSELs ( Vertical -Cavity Sur face -Emitting Lasers) have been broadly used as light sources in optoelectronic devices in the past decade in optical interconnects and sensing applications [1-4]. The emission properties of VCSELs can be further improved to realize tunable polarization and wavelength [5-7]. In our previous publication, a technology to inte grate a VCSEL chip into a standard liquid crystal (LC) cell is developed [8]. Based on this technology, a new type of device is developed for a broader range of functionalities. The basic idea is to introduce reflection of light back into the VCSEL cavity . It is realized by using chiral liquid crystals (CLCs) or nematic LCs incorporated wit h an extra reflector. In the first case the extra reflection arises from the photonic band gap of the LC material itself. In this work, we fabricate and characterize electrically driven VCSEL devices , in which the emitting area of the VCSEL is covered with a thin LC layer. Two different device structures are demonstrated : VCSEL with CLC overlay (CLC -VCSEL) and VCSEL with nematic LC combined with a dielectric mirror (DE -LC -VCSEL) . For the first structure, the C LC layer has a photonic reflection band which co vers the VCSEL emission wavelength, so that one circular ly polarized mode of the laser emission which has the same handedness with the CLC helix is reflected and used as feedback into the VCSEL [9, 10 ]. For the second structure, the dielectric mirror provides strong refle ction and forms an external cavity, in which nematic LC can be filled. It is observed that the polarization state and the wavelength of the laser emission can be controlled by the applied voltage across the nematic LC overlay .