Yi Xie

VCSEL With Photo-Aligned Liquid Crystal Overlay

A technological platform for a vertical cavity surface-emitting laser with a tunable polarization is presented. It is achieved by integrating an 850-nm vertical cavity surface-emitting laser chip in a liquid crystal cell that uses photo-alignment to orient the nematic liquid crystal. The vertical cavity surface-emitting laser and the liquid crystal layer can be electrically driven with separate electrodes. We demonstrate that the polarization state of the laser emission can be controlled by applying an appropriate voltage over the liquid crystal layer. The polarization of the emission depends on the emission angle, in particular, for voltages close to the liquid crystal threshold voltage, which is in agreement with expectations from the extended Jones calculus.

Vertical-cavity surface-emitting laser emitting circularly polarized light

We demonstrate both theoretically and experimentally that a vertical-cavity surface-emitting laser with an overlay of cholesteric liquid crystal (CLC-VCSEL) becomes a coupled system with different spectral, threshold and polarization characteristics from those of the stand-alone VCSEL. Due to the existence of a band gap for circularly polarized light in the liquid crystal, the output light becomes almost circularly polarized with the opposite handedness of the CLC. We also show that it is even possible to replace the VCSEL top distributed Bragg reflector by a CLC overlay.

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

A tuneable external cavity consisting of a thin layer of nematic liquid crystal (LC) and a dielectric reflector is placed on the top of a vertical-cavity surface-emitting laser (VCSEL). By changing the voltage across the LC layer, the optical path length of the external cavity can be tuned. As a result, the emitting properties of the LC-VCSEL, including polarization state and emission wavelength, can be controlled by the voltage applied over the LC layer. Stable polarization switching with high contrast is obtained by voltage driving. This device can be integrated in applications which require electrically tuneable VCSEL emission.

Vertical-Cavity Surface-Emitting Laser With a Chiral Nematic Liquid Crystal Overlay

We demonstrate measurement results of the light emission of vertical-cavity surface-emitting lasers (VCSELs) with chiral liquid crystal (CLC) overlay (CLC-VCSEL). Two different CLC materials with different chiral-phase temperature range are presented and compared. The CLC layer acts as an external cavity for the VCSEL for one circular polarization state that results in complex polarization behavior inside the VCSEL cavity. We find that the emission of the device is purely circularly polarized with a degree of circular polarization higher than the degree of linear polarization of the stand-alone VCSEL. The threshold current of the CLC-VCSEL is decreased, and the emission wavelength of the CLC-VCSEL can be temperature tuned over a larger range compared with that of the stand-alone VCSEL. Numerical results reveal that the small reflection at the top CLC has a much larger influence on the laser dynamics than the stand-alone configuration.

Vertical-Cavity Surface-Emitting Laser With Cholesteric Liquid Crystal Overlay

We study theoretically the spectral and polarization threshold characteristics of vertical-cavity surface-emitting lasers with an overlay of cholesteric liquid crystal (CLC-VCSELs). Due to the existence of CLC band gap for circularly polarized (CP) light the polarization state of the resonant modes in the compound system evolves from linear (LP) inside the VCSEL to elliptical and circular (CP) inside the CLC. As a result, the output mirror reflectivity is increased and CLC-VCSEL threshold gain is decreased. We elucidate the transition from LP to CP state of the generated light by studying the role of the CLC thickness and the top distributed Bragg reflector reflectivity. This transition takes place for 5-6 μm thick CLC and is not gradual but happens in an oscillatory fashion. When the CLC thickness is fixed and the VCSEL top mirror reflectivity is decreased a profound gradual red shift of the resonant mode wavelength and increase of threshold gain are observed.

Hybrid VCSEL: liquid crystal systems

We study theoretically and experimentally spectral and polarization characteristics of hybrid systems of VCSELs integrated within liquid crystal (LC) cells. Three cases are considered: Nematic or cholesteric LC on top of VCSEL, coupled-cavity system with the second cavity next to the VCSEL’s one filled in with nematic LC and a system with a nematic LC inside the VCSEL cavity. For the case of nematic liquid crystal - VCSEL coupled cavity system we demonstrate selection between two orthogonal directions of linear polarization of the fundamental mode by changing the LC length or by electro-optical tuning of the LC director. For the case of cholesteric liquid crystal-VCSEL system we demonstrate lasing on circularly polarized (CP) modes due to the LC band gap for CP light. The transition from nematic to isotropic phase of the LC when increasing temperature leads to a drastic change of the polarization of the generated light from left-handed circular to linear polarization. Finally, we investigate the possibility of efficient wavelength tuning by utilizing electrooptical effect in nematic LC layer integrated next to the active region in a VCSEL cavity.

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

We have developed 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. The 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 (SiO2/Ta2O5). 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.

VCSELs with nematic and cholesteric liquid crystal overlays

We study theoretically the spectral and polarization threshold characteristics of Vertical-Cavity Surface-Emitting Lasers with nematic and cholesteric liquid crystal overlay: LC-VCSELs. In the first case, we demonstrate the possibility of selecting between two orthogonal directions of linear polarization (LP) of the fundamental mode (x or y LP) by choosing appropriate NLC length and to achieve strong polarization discrimination: threshold gain difference as large as several times the threshold gain. We also demonstrate an active control of light polarization by electro-optically tuning the LC director and show that either polarization switching between x and y LP modes or continuous change of the LP direction is possible. For cholesteric LC-VCSEL we show that it becomes a coupled system with different spectral, threshold and polarization characteristics than the ones of the stand-alone VCSEL. Due to the existence of a band gap for circularly polarized light in the liquid crystal, lasing occurs in almost circularly polarized modes at the LC side.

Vertical cavity surface emitting laser with nematic and chiral liquid crystals overlay

A technological platform for a vertical cavity surface emitting laser (VCSEL) with tunable polarization is presented. It is realized by integrating an 850nm VCSEL chip in a liquid crystal (LC) cell that uses photo-alignment (PA) to orient the LC. Two kinds of LC are filled in and form a thin layer over the emitter of the VCSEL: nematic LC or chiral nematic LC (cLC). The VCSEL and the nematic LC layer can be electrically driven with separate electrodes. The polarization state of the laser emission can be controlled by applying an appropriate voltage over the nematic LC layer. The chiral nematic LC has a reflection band that contains the VCSEL emission wavelength, so that one circular polarized mode of the laser emission is reflected as a feedback into the VCSEL. We found that the emission from the VCSEL with cLC overlay is circularly polarized.

850nm VCSEL with a liquid crystal overlay

We developed an in- house technology to overlay liquid crystal (LC) on top of a 850nm Vertical Cavity Surface Emitting Laser (VCSEL) creating a so-called LC-VCSEL. Prior to this, the effect of the cell thickness on the planar alignment of the E7 LC is investigated. It is observed that the LC orientation is planar, uniformly aligned over the whole cell with an average pre-tilt of 22.50 in a thin a cell of 13μm thickness; such alignment uniformity is not observed in a thick cell of 125μm. Nevertheless, several domains of good uniformity are still present. Further, the polarization resolved LI characteristics of LC-VCSEL are investigated with and without the insertion of LC in a cell glued directly onto VCSEL package. Before filling in the LC, the VCSEL emits linearly polarized light and this linear polarization is lost after LC filling. The output intensity as a function of polarizer angle shows partial planar alignment of the E7 LC, which is very important for the further advancement of the LC-VCSEL integrated system.