Top DBR mirror replaced with CTF and chiral STF bilayers The CTF (QWP) introduces a pi/2 retardance to compensate the polarization mismatch between the two reflectors.

Towards Circularly-Polarized Light Emission from Towards Circularly-Polarized Light Emission from Vertical-Cavity Surface-Emitting LasersVertical-Cavity Surface-Emitting Lasers

Fan Zhang, Jian Xu and Akhlesh Lakhtakia*Fan Zhang, Jian Xu and Akhlesh Lakhtakia*

Department of Engineering Science and Mechanics, Penn State University, State College, PA, 16802Department of Engineering Science and Mechanics, Penn State University, State College, PA, 16802

*akhlesh@psu.edu, Tel: (814)863-4319, Fax: (814)865-9974*akhlesh@psu.edu, Tel: (814)863-4319, Fax: (814)865-9974

Compact circularly polarized (CP) light sources have recently attracted wide attention for direct chip-level integration due to potential applications in the fields of optical information processing and data storage, optical communication, quantum computing, and bio/chemical detection. So, it is highly desirable to have on-chip CP light emitters with precise controls over CP handedness and wavelength.

The authors report the development of a class of chiral-mirror-based vertical-cavity surface-emitting lasers (VCSELs). The advances in sculptured thin film (STF) technology will eventually lead to the development of a new family of CP photonic devices that are efficient, compact, and fully integrable into optical/optoelectronic chips for a wide range of applications of CP light.

Introduction

v

Vacuumpump

Source

Vacuumchamber

Vapor

Substrate

Quartz crystal monitor

CTF

chiral STF

Schematic of the basic system for PVD of STFs

Schematics of depositionsof CTFs and chiral STFs

Oblique angle depositionA tilted and rotating/fixed substrate corresponds to chiral STF/CTF deposition.Atomic self-shadowing (Low energy adatom diffusion).

STF deposition

400 450 500 5500

20

40

60

80

100

RCP

Right-handed chiral STF mirror

RCP

RCP RCP

Right-handed chiral STF mirror

RCP

Conventional mirror

LCP

RCP LCP

Conventionalmirror

An example of well-developed circular Bragg regime

Difference between chiral STF mirror and conventional mirror

Circular Bragg phenomenon (CBP)A well-developed CBP displays high selective reflection of CP light and is confined to a defined spectral regime.

Microcavity built with chiral mirrorsChiral STF mirror: CP states preserved by reflection. Conventional mirror: CP states NOT preserved, due to shift.

Chiral-mirror microcavity

Schematic of the device

Device structure

CP emission from QDs LEDs

Chiral mirrors: structurally left-handed STFs made of TiO2 with the circular Bragg regime centered at 610 nm

Device characterization

LCP and RCP emission spectra of the NQDs confined in the chiral-STF-based microcavity

Measured reflectance spectrum of the microcavity device

for incident LCP lightSpectrally: narrower FWHM; higher peak intensity; large discriminable difference between CP handedness.LCP emission peak in good agreement with the position of spectral hole.

560 580 600 620

40

80

120

160

200

240

0 10 20 30 40 50 60 70 80 90

0.2

0.4

0.6

0.8

1.0

PL

Inte

nsity

(no

rmal

ized

)

Azimuthal Angle (deg)

Microcavity emission Lambertian emission

45o

20o

5o

15o

10o

PL

Inte

nsity

(a.

u.)

Wavelength (nm)

0o

0 10 20 30 40 500

100

200

300

400

500

Ph

oto

lum

ine

sce

nce

In

tesi

ty (

a.u

.)

Pumping Power (mW)

Free-space LCP Emission Free-space RCP Emission Microcavity LCP Emission Microcavity RCP Emission

Large discriminable difference between CP handedness is persistent under different pumping light power.Spatially: narrower emission angle (strongly directed normal to the surface).

Polarization control inexternal cavity diode laser

System setup

1-Laser diode; 2-collimating lens; 3-Soleil Babinet Compensator; 4-Left-handed Chiral STF mirror

LD: one facet is coated for enhanced reflectivity; the other is antireflection-coated.The fast axis of the intra-cavity QWP was aligned at 45°with respect to the polarization of the TE mode in the LD.Chiral mirrors: left-handed STFs made of TiO2 with the circular Bragg regime

centered at 660nm.

System lasing behavior

Light output energy as a function of driving current (Inset: spectrum of the LCP laser emission)

Polar plot of the normalized analyzer transmission vs the angle between the optical axes of the analyzer and the Fresnel-rhomb retarder

CP ratio=112

CP ratio=32

Ith = 46 mALCP lasing outputSide-mode suppression ratio is 26 dB

Bottom DBR mirror

n-contact layer

Active layer (MQWs)

p-contact layer

Top chiral STF

m/

2 ca

vity

CTF (QWP)

0.44 0.46 0.48 0.500

1

2

3

4

5

6

830 835 840 8450.0

0.5

1.0

1.5

2.0

2.5

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

RCP LCP

Temperature=300K

FWHM=2.06nm0.5mJ pump energy

peak

=841.8nm

Inte

nsity

(a.

u.)

Pump Source Energy (mJ)

600 700 800 9000

20

40

60

80

100

center=840nmMaterial: TiO2

Re

flect

ion

(%

)

Wavelength (nm)

LCP RCP

CP emission from VCSELsDevice design

Device characterization

Reflectance spectra of the CTF and RH chiral STF bilayers (Inset: cross-section SEM image of the CTF and STF bilayers)

Light output as a function of the pumping light energy (Inset: spectrum of the RCP lasing emission)

ReferencesA. Lakhtakia and R. Messier. Sculptured Thin Films: Nanoengineered Morphology and Optics, SPIE Press (2005).F. Zhang, J. Xu, A. Lakhtakia, S. M. Pursel, M. W. Horn, and A. Wang, Appl. Phys. Lett. 91, 023102 (2007).F. Zhang, J. Xu, A. Lakhtakia, T. Zhu, S.M. Pursel, and M.W. Horn, Appl. Phys. Lett. 92, 111109 (2008).F. Zhang, Ph.D. Dissertation, Pennsylvania State University (2008).

AcknowledgementThe authors thank Sean M. Pursel and Dr. Mark W. Horn for providing help on initial STF depositions.