Title: Design and Characterization Tools for Building Optical Functionalities at the Nanometer Scales
1Design and Characterization Tools for Building
Optical Functionalities at the Nanometer Scales
- Jung Y. Huang
- IEO, NCTU
- Nov. 19, 2003
2Current Status of Optical Chip Technology
? State of the art of photonic chip circuit WDM
channel selector ?Currently only a few optical
functions are available ?Bends make up most of
the surface of optical chip
3Hybrid Solution for the Near Future
- Possible Solution use photonic crystal devices
to enhance optical functions and chip complexity
but only when it is absolute needed - Efficient and compact input/output coupling and
spot-size converters required.
4 PSoC --- Overall Objectives
- Ultra dense all-optical integration engineering
of PLC-PBG chip technology for ultrafast
(terabit/sec) and parallel optical system
applications
5PSoC --- Problems need to be solved
- How to mold photon flow at sub micrometer
scales?--- unlike to electrons within material,
which are firmly confined by work function,
photons can readily radiate into free space. - How to find an optimal PLC-PBG structure for a
predefined optical functionality? - How to evaluate interface and finite size effects
of a realistic PLC-PBG and discover new geometry
of photon scattering centers? - What is the most cost effective PLC-PBG
fabrication technology? - How to evaluate optical properties at the
nanometer scales?
6PSoC --- the Strategy
- Design/fabrication/characterization of toolbox
devices for future photonic chip technology
(especially with ultra small device area) - Employ optical via concept for chip scale and
interchip connection - Develop and integrate various simulation/optimizat
ion tools for PLC/PBG optical circuit level
design - Cost effective photonic chip fabrication
procedures.
7Brief Tutorial of PBG
81-D PBG --- Bragg Mirrors
The first thin layer has a higher index than the
second and the third is the same as the first.
Therefore, there is a phase jump at the first
interface (I) if the film is quarter-wavelength
thick, the reflections from the first interface
and that from the second are 2pphase difference
and therefore add constructively. The same
happens between the second and third interfaces
there is no phase change at (II) if the film is
also quarter-wavelength thick, the reflections
from (II) and (III) have 2p phase delay and again
add constructively.
91-D PBG --- Bragg Mirrors
- What happens if you change
- the number of layers NL ?
- the index contrast ?n ?
- the incident angle ? ?
10Small ?n
Large ?n
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121-D Periodic System
- 1. Introduce periodicity via the specific 1-D
Bragg mirror with quarter-wavelength thick layers - 2. The general case can be the one with periodic
scatterers spaced at half-wavelength al/2 ?
kp/a - 3. For electrons, these periodic scatterers
correspond to the atomic potentials.
13The Formation of Bandgaps
3. At the band edges, standing waves form, with
the energy being either in the high or the low
index regions
1. Dispersion curve for free space
2. In a periodic system, when half the
wavelength corresponds to the periodicity the
Bragg effect prohibits photon propagation.
4. Standing waves transport no energy with zero
group velocity
14The Dispersion Relationship
Plot the dispersion curves for both the positive
and the negative sides, and then shift the
curve segments with kgtp/a upward or downward
one reciprocal lattice vectors.
This reduced range of wave vectors is called the
Brillouin zone
152-D Photonic Crystals
- In 2-D PBG, different layer spacing, a, can be
met along different - direction. Strong interaction occurs when l/2
a.
2. PBG (Photonic band gap) stop bands overlap
in all directions
16Band Diagram
17Four Possible Functionalities of PBG
- 1. Stop Band use PBG as high reflectivity
omni-directional mirror (PBG waveguides)
18Dispersion Curve of Slab Waveguide
Because guiding modes redistribute themselves
with frequency, for small ?, the dispersion curve
of guiding modes approaches the cladding
line For large ?, it approaches the core line.
19Dispersion of a PBG Line Defect Waveguide
Dispersion diagram of W1 line-defect photonic
crystal waveguide Waveguide modes exist within
the bandgap. Photons are prohibited in the 2D
PBG, which lead to lossless confinement of
photons in the line defect area.
20Four Possible Functionalities of PBG
- 2. Dielectric Band Uses the strong dispersion
available in a photonic crystal (dispersion
engineering with form birefringence)
21Dispersion relation to wave vector diagram
1. In a homogeneous material in absence of
material dispersion n(w)constant n, the
dispersion diagram is simply a straight line
wkc/n.
2. In 2D systems, one can think of this line as
a cone. For a given frequency w, this cone
becomes a circle.
22Wavevector diagram for an interface between two
isotropic media
Real space
The wave vector diagram tells us the direction
and magnitude of the refracted and reflected
beams. Their direction is normal to the
iso-frequency curve and corresponds to Snells
law.
23Four Possible Functionalities of PBG
- (3) Air Band Couples to radiative modes for
light extraction from high-efficiency LEDs and
fibre coupling.
24Four Possible Functionalities of PBG
- (4) Defect Band Couples to waveguide/cavity
modes for spectral control such as PBG point
defect laser or PBG line defect filter, etc.
25What simulator features needed for the design and
optimization of nano photonic devices
261. Engineer a Suitable Wavevector diagram
(dispersion engineering) for an interface between
free space and a PBG medium
The artificially distorted dispersion surface is
reminiscent of a birefringent crystal with an
origin from geometric form. The key outcome is
that a small change in output angle (superprism).
272. Building an Optical Beam Shaping/Imaging
Functionality with Iso-frequency Curve of PBG ---
effective medium concept for superprism and
meta-materials with negative index of refraction.
The artificially engineered PBG structure with a
proper dispersion curve can also be employed for
a beam collimator or even a microscopic objective
lens.
A slab of material with refractive index n -1
mimics the action of a conventional lens by
refracting beams of light emitted by an object to
refocus them in the image plane.
283. Defect Engineering for Wavelength-Sensitive
PBG Splitter
(a) and (b) are vacancy line defects obtained by
removing one column and one row of cylinders,
respectively. (c) and (d) are obtained by sliding
two parts of the crystal with respect to each
other a distance asqrt(3)/2. In (c) a channel
waveguide with dispersion similar to (a) is
obtained, whereas in (d) a coupled-cavity
waveguide (2-moded in the whole k range) results.
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304. Defect Engineering for enhancing spontaneous
emission with PBG structure
Problem Escape cone is 2 of 4p steradian
Active Region
Random Texturing 100 extraction possible
31Electric Field and Energy Distribution in a PBG
cavity show reduction in EM mode Volume and
therefore increase Purcells Enhancement factor.
32 Design Platform for Nano Photonic Devices
33 Web-based Framework
34Application examples (1) an extremely compact
optical mode transformer
- (1) A mode transformer (from 5 ?m to 0.5 ?m)
with a device length of 8.5 ?m and transmission
efficiency (0-0) 94 for wavelength 1.5 ?m.
35Application examples (2) output coupler
- (2) Optimized output coupler with aperiodic
grating structure, which increases coupling
efficiency (to radiation modes) from 40 to
99.6.
36Application examples (3) Broadband Polarization
Beam Splitter
37Device Characteriztion (1) complete-field
characterization at nanometer scales
38Device Characteriztion (2) control and
complete-field characterization at femtosecond
scales
measured
theory
39(2) Coherent control at femtosecond scales
Right Schematic of coherent optical microscopy
with selective excitation by invoking femtosecond
pulse shaping for real-time controlling the
photon and electron interaction. Left SHG
intensity from BBO SHG is improved adaptively by
phase compensation with an adaptive pulse shaper.
40(2) Coherent control at femtosecond scales
SHG intensity generated from BBO excited by phase
distorted fs pulse reflected from self-assembled
InAs layer is improved adaptively by an
femtosecond pulse shaper.
41(3) complete-field characterization at
femtosecond scales Optical parametric
amplification frequency-resolved optical gating
(OPA-FROG)
Single-shot design of OPA-FROG for extremely weak
optical signal from ultrafast and non repeatable
event
42(3) complete-field characterization with gain at
femtosecond scales OPA-FROG
Measured and retrieved OPA-FROG traces at a
wavelength near 840 nm
Measured OPA-FROG trace at a wavelength of 840 nm
from a highly excited super continuum white light
generator
43Device Characteriztion (4) optical anisotropic
characterization at nanometer scales
- Optical anisotropy mapping of a photonic crystal
device at nanometer scales by using a SNOM and
polarization modulation scheme. To reveal its
functionality, a periodic poled LiNbO3 sample was
used and the result is presented on the left
picture.
44Device Characteriztion (4) optical anisotropic
characterization for two-dimensional distribution
of the thickness and optical axis of an uniaxial
crystal film
45Conclusions
- PBG structure offers the possibility to mold
photon flow at nanometer scales. - Photonic bandgap materials (prepared from
conventional materials) with novel functionality
could play a role of silicon in photonics. - New research tools with femtosecond control and
nanometer resolution shall be developed for
probing nano photonic materials and devices.