Transcript poster

Towards Circularly-Polarized Light Emission from
Vertical-Cavity Surface-Emitting Lasers
Fan Zhang, Jian Xu and Akhlesh Lakhtakia*
Department of Engineering Science and Mechanics, Penn State University, State College, PA, 16802
*[email protected], Tel: (814)863-4319, Fax: (814)865-9974
Polarization control in
external cavity diode laser
CP emission from QDs LEDs
Introduction
Device structure
System setup
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.
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
STF deposition
Quartz
crystal
monitor
Vacuum
chamber
Schematic of the device
CP ratio=112
Chiral mirrors: structurally left-handed STFs made of TiO2 with the circular Bragg
regime centered at 610 nm
Substrate
Device characterization
CTF
cv
CP ratio=32
Vapor
Source
Light output energy as a function
of driving current (Inset: spectrum
of the LCP laser emission)
chiral STF
Schematics of depositions
of CTFs and chiral STFs
Reflectance (%)
80
60
RCP
RCP
RCP
LCP
40
RCP
RCP
RCP
LCP
20
400
450
500
550
Right-handed
chiral STF mirror
Conventional
mirror
500
240
Free-space LCP Emission
Free-space RCP Emission
Microcavity LCP Emission
Microcavity RCP Emission
400
300
200
100
200
160
120
p-contact layer
1.0
Microcavity emission
Lambertian emission
0
0.8
0.6
5
0.4
10
0
10
20
30
40
50
Pumping Power (mW)
Active layer
(MQWs)
o
o
n-contact layer
o
0.2
0
Bottom DBR
mirror
10 20 30 40 50 60 70 80 90
Azimuthal Angle  (deg)
15
o
80
20
40
560
580
o
600
620
Wavelength (nm)
Large discriminable difference between CP handedness is persistent under
different pumping light power.
Spatially: narrower emission angle (strongly directed normal to the surface).
Wavelength (nm)
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 mirrors
Chiral STF mirror: CP states preserved by reflection.
 Conventional mirror: CP states NOT preserved, due to p shift.
 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.
o
45
0
ml/2 cavity
LCP
RCP
Conventional
mirror
Top chiral STF
Acknowledgement
The authors thank Sean M. Pursel and Dr. Mark W. Horn for providing help on initial
STF depositions.
References
A. 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).
Device characterization
100
80
Material: TiO2
lcenter=840nm
6
5
60
40
LCP
RCP
20
0
600
4
3
2
2.5
1.5
1.0
0
800
900
Wavelength (nm)
Reflectance spectra of the CTF and
RH chiral STF bilayers (Inset: cross-section
SEM image of the CTF and STF bilayers)
lpeak=841.8nm
FWHM=2.06nm
0.5mJ pump energy
Temperature=300K
0.5
0.0
830
835
840
845
Wavelength (nm)
1
700
RCP
LCP
2.0
Intensity (a.u.)
Right-handed
chiral STF mirror
Device design
Reflection (%)
Chiral-mirror microcavity
CP emission from VCSELs
CTF (QWP)
PL Intensity (normalized)
A tilted and rotating/fixed substrate corresponds to chiral STF/CTF deposition.
Atomic self-shadowing (Low energy adatom diffusion).
LCP and RCP emission spectra
Measured reflectance spectrum
of the NQDs confined in the
of the microcavity device
chiral-STF-based microcavity
for incident LCP light
Spectrally: narrower FWHM; higher peak intensity; large discriminable
difference between CP handedness.
LCP emission peak in good agreement with the position of spectral hole.
PL Intensity (a.u.)
Oblique angle deposition
Photoluminescence Intesity (a.u.)
Schematic of the basic system
for PVD of STFs
100
Ith = 46 mA
LCP lasing output
Side-mode suppression ratio is 26 dB
Intensity (a.u.)
Vacuum
pump
0
Polar plot of the normalized analyzer
transmission vs the angle between the
optical axes of the analyzer and the
Fresnel-rhomb retarder
0.44
0.46
0.48
0.50
Pump Source Energy (mJ)
Light output as a function of the pumping
light energy (Inset: spectrum of the RCP
lasing emission)