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NLO Webinar 01-25-16 R. A. Norwood 3

Exploring Recent Advances in

NLO Materials

Robert A. Norwood

College of Optical Sciences

University of Arizona



in enchanting Tucson, Arizona


Outline

• Introduction to nonlinear optics and motivation

• Laser developments: Key driver

• Materials: Research becomes commercial

• Devices: Microresonators, nanotechnology and

fiber innovation

• Conclusion


What is nonlinear optics?

1. A nonlinear relationship between the

induced polarization and the electric

field.

2. A phenomenon that enables EO

modulators and switches

3. An efficient way to generate new

frequencies of light in certain media.

4. Physical phenomena that both impair

optical fiber networks and can be used to

make optical communications devices

5. The basis for Nico Bloembergen’s Nobel

),(),(),(),(),(:),(),( )3(

0

)2(

0

)1(

0 trEtrEtrEtrEtrEtrEtrP


The nonlinear polarization

P(

r ,t) = e0c(1) × E(

r ,t) + e0c

(2) : E(

r ,t)E(

r ,t) + e0c(3) E(

r ,t)E(

r ,t)E(

r ,t)

Vanishes in centrosymmetric media like ordinary glass

These expressions assume a completely instantaneous

response; i.e. no dispersion

Rigorous quantum theory has been established for

predicting the values of the susceptibilities for atoms,

molecules and, increasingly, condensed matter ensembles

Present in all media


Second order nonlinear optical

effects

• For situations in which a single high power optical beam

at frequency w is present

Second harmonic generation (SHG) (w w 2w)

Optical rectification (OR) (w w 0)

• With multiple frequencies are present (w1 and w2 ) new

phenomena emerge such as

Sum frequency generation (SFG) (w1 w2 w3)

Difference frequency generation (DFG) (w1 w2 w3)

Electro-optic effect (EO) (w1 0 w1)


Second harmonic generation

(2)w

w

2w

w

w

2w

Primary applications

• Convert infrared lasers to

visible – laser pointers!

• Spectroscopy

• Microscopy (biology)

P(2w) =e0c (2)Ew

2


Sum frequency generation

(2)

w1

w1 w2


• Convert infrared lasers to visible

• Photon counting spectroscopy

(optical gating)

• Dermatologic lasers

• Microscopy (biology)

w2

P(w1 +w2 ) =e0c (2)Ew1Ew2

w1

w2

w1 w2


Difference frequency generation

(2)

w1

w3 w1 - w2

w2

w3

w1


• Optical parametric oscillation (OPO)

• Mid infrared frequency generation

• Terahertz generation

w2

P(w1 -w2 ) =e0c (2)Ew1Ew22

*


The Pockels Effect

A change of refractive index induced by a DC or low frequency (sub-THz) electric field and linearly dependent on the applied field. This is really just sum and difference frequency mixing with a low frequency field.

n(E) = n - 12rn3E

r is the linear EO coefficient or the Pockels coefficient

Today available values for r range from 1 to 250pm/V


• Electro-optic modulators – phase

and amplitude

• Electro-optic switches

• Pockels cells


Third order nonlinear optical effects

• For situations in which a single high power optical beam

at frequency w is present

Self phase modulation (SPM) (w w - w w)

Third harmonic generation (THG) (w w w 3w)

• With multiple frequencies are present (including w 0)

new phenomena emerge such as

D.C. Kerr effect (0 + 0 + w = w)

Cross phase modulation (XPM) (w1 w2 - w1 w2)

D.C. SHG (0 + w w 2w)

Four wave mixing (w1± w2± w3 w4)


Third harmonic generation

(3)w

w

3w

w

w 3w


• Very few - - it is very difficult to

create a phase matched

interaction – frequency tripled

systems always use two (2)

• Useful for unambiguously

measuring the ultrafast part of

(3)

P(3w) =e0c (3)Ew

3

w


Intensity dependent refractive index

(3)

w w


• Laser modelocking

• All-optical switching

• Optical limiting

wn(I) = n 0+n2I

w

w

w

w

w

n2

Self-focusing


Sum frequency generation

(3)

w1

w4 w1 w2 w3

w3

w1

w4


• Optical parametric frequency generation

• Supercontinuum generation

• Optical parametric oscillation

w2

P(w1 +w2 +w3) = 6e0c (3)Ew1Ew2

Ew3

w3

w2


Difference frequency generation

(3)

w1

w4 w1 w2 - w3

w1

w3Primary applications

• Optical parametric amplification

• Optical parametric oscillation

• Supercontinuum generation

w2

P(w1 +w2 -w3) = 6e0c (3)Ew1Ew2

Ew3

*w3

w2

w4


Two-photon absorption

Im (3)

w1 w1

w1

The process pictured is nonresonant – the probability for TPA is low

since two photons must be in the same place at the same time

w1

w1

TPA cross section s(2) leads to an intensity dependent absorption coefficient

s =s (2)IApplications in nonlinear microscopy,lithography, optical switching, limiting


Stimulated Raman scattering

Raman medium

ws w wv

w

ws Primary applications

• Raman amplification

• Spectroscopy

• Optical switching

w

wv


Lasers beat Moore’s law

• Most solid-state laser technologies have been

advancing faster than Moore’s law for more than a

decade

• Most recently there has been tremendous progress in

modelocked fiber laser technology, enable by low cost

high quality semiconductor laser diodes

• These lasers are turn-key, compact systems that

completely change how ultrafast photonics gets done

(you may never turn on your Ti:sapphire laser again)


Example: Commercial

supercontinuum light sources

A pulsed seed laser (in this case a semiconductor laser pumped Q-switched YAG laser)

Pumping a highly non-linear photonic crystal fiber, which in turn generates a super-continuum


Spectral performance

Optical Spectral Specification

-60,0

-50,0

-40,0

-30,0

-20,0

-10,0

0,0

400 600 800 1000 1200 1400 1600 1800

Wavelength (nm)

Op

tic

al

Po

we

r D

en

sit

y (

dB

m/n

m)

Incandescent lamp

SuperK™ RED

SLEDsO

-ba

nd

E-b

an

d

S-b

an

d

C-b

an

dL

-ba

nd

Flat and stable

output

spectrum in

visible and all

telecom bands

Incandescent lamp

Super K Compact

SLED


• Take advantage of compatibility with established optical

fiber technology and components

• Very long interaction lengths possible

• Compact and generally quite efficient

Mode-locked erbium fiber laser


Carbon nanotubes

(source: http://en.wikipedia.org/wiki/Carbon_nanotube)

• ~1nm diameter

• Possess ultrafast carrier recovery time (<1ps)

• Robust

• Low cost

Great material for making a saturable absorber!


Single walled carbon nanotubes

SEM picture of SWCNTs Absorption spectrum of SWCNTs


Carbon nanotubes with fiber taper

Polymer

with SWCNTs

Fiber taper coated with SWCNTs in PDMS

Solutions of SWCNTs


980/1550nm

WDM fiber coupler

980 nm pump

50/50 Output

couplerCNTs SA

Polarization

Controller

Er3+-doped fiber

Laser output

Isolator7m SMF28 fiber

for dispersion control

Fiber laser with CNT SA

K. Kieu and M. Mansuripur, Opt. Lett. 32, 2242 (2007)


Mode-locked laser with SWCNTs

incorporated in fiber taper

980/1550nmWDM fiber coupler

outputfiber coupler

Erbiumdoped fiber Isolator

Fiber taper with SWCNTs

980 nm pump

splice

Laser output

splice

All fiber modelocked laser ~ 150fsec pulses at

50-80MHz repetition rate


Hand held gain block


Multiphoton imaging (MPI)system

K. Kieu et al., Biomedical Optics Express 4, 2187 (2013)


Achievements

Multiphoton imaging of diatoms

THG imaging of diatom frustule

(~100m in diameter)THG (green) and 3PEF (red) imaging

of living diatoms

100m


SHG imaging to assess poling

efficiency

Nonlinear image of a poled EO modulator

using SEO100, SHG (red), THG (green)

Nonlinear image of a unpoled EOmodulator

using SEO100, SHG (red), THG (green)

Au + SEO100

SEO100

R. Himmelhuber et al., Appl. Phys. Lett. 104, 161109 (2014)


Radially poled ring resonator

Y polarized fundamental X polarized fundamental


MPI materials characterization

Graphene flakes

Red: fluorescence

Green: THG

Standard optical (left) and SHG image of

MoS2 flakes

MPI image of gallium

selenide flakes

L. Karvonen et al.,

Scientific Reports 5,

10334 (2015).

A. Saynatjoki et al., ACS

Nano 7, 8441 (2014).


Photoinduced material degradation

with MPI

THG

SHG

FL

Absorption

Degradation

ProbesSources

S. Shahin et al., Optics Express 22, 30955 (2014)


Graphene (single layer)

60s

FTIR spectra indicate that

graphene oxide is being formed as

the main photodegradation product

mW

mW

mW

mW


DR1 EO polymer


• Very small, power independent decay in lithium niobate suggests

slow system drift rather than photodegradation

Lithium niobate


Gold standard materials

• Second order frequency generation effects at moderate

optical powers: periodically poled lithium niobate (PPLN)

• Second order frequency generation at high powers: beta-

barium borate (BBO), potassium dihydrogen phosphate

(KDP)

• Pockels effect EO modulators: lithium niobate

• Third order nonlinear optics: standard single mode optical

fiber (SMF28), dispersion shifted fiber (for frequency

generation in the C-band)


Emerging commercial materials

• Electro-optic polymers for high speed electro-optic

modulators, integrated silicon EO modulators, and terahertz

generation

• Mid- and long-wave infrared second order frequency

generation materials - AgGaS2 crystal

• High nonlinear refractive index optical fibers such as

bismuth doped optical fiber

• Very low dispersion, low effective area silica based fibers for

third order frequency conversion


Host polymer E/O chromophore

High temperature,

high voltage

Poling of EO polymers creates a non-centrosymetric material. Typical state-of-the-art

electro-optic (Pockels) coeficients, r33, are between 100 and 200 pm/V.

Dn =n3r33E

2

Idealized view

Electro-optic polymers


Material comparison

Material n r33max (10-12m/V) FOM (V-1), n3r33

Lithium niobate 2.2 30 320

First generation

EOP

1.7 70 344

New polymers

(from Soluxra)

1.8 250 1460


Material comparison with loss

Material n r33max (10-12m/V) a (dB/cm) FOM (V-1)

Lithium niobate 2.2 30 0.5 0.96

First generation

EOP

1.7 70 2.0 0.26

SEO 100

(Soluxra)

1.7 120 2.0 0.44

SEO250

(Soluxra)

1.8 250 1.5 1.46

This simple FOM ignores electrode loss - - at 40GHz, device length is limited to 2cm

FOM (V-1)

(electrode

loss limited)

0.24

0.26

0.44

1.46


EO polymer progress: r33, thermal

stability, power handling & material loss

Remarkable progress has been made in developing high r33 (>200

pm/V) EO polymers with excellent thermal stability (85 C), low optical

loss (~ 1 dB/cm), and high power photo-stability (~100 mW).

Slide courteously provided by J. D. Luo at Soluxra, LLC


• Using advanced EO polymers

• Novel sol-gel based hybrid design

• Unique device design allows > 100% poling

efficiency

• Record high EO coefficient r33 of 170pm/V

demonstrated

• Concept now being extended to silicon photonics

Hybrid EO polymer/sol-gel devices

2.5V, 5.5dB loss modulator

R. Hiimmelhuber, et. al., IEEE J. Lightwave Tech 31, 4067 (2013)

R. Himmelhuber, et. al., Opt. Mat. Express 1, 252 (2011)

C. Greenlee, et. al., Opt. Express 19, 12750 (2011)

I. E. Araci, et. al., Opt. Express 18, 21038 (2010)

C. T. DeRose, et. al., Opt. Express 17, 3316 (2009)

Y. Enami, et. al., Appl. Phys. Lett. 94 213513 (2009)

C. T. DeRose, et. al., IEEE Phot. Tech. Lett. 20, 1051 (2008)

Y. Enami, et. al., Appl. Phys. Lett. 92, 193508 (2008)

Y. Enami, et. al., Appl. Phys. Lett. 91, 093505 (2007)

Y. Enami, et. al., Nature Photonics 1, 180 (2007)

C. T. DeRose, et. al., Appl. Phys. Lett. 89 131102 (2006)


Hybrid sol-gel/EO polymer MZ with

5.7 dB IL, Vp of 2.8V: AJLS 102

r33 = 70pm/V

Vp = 2.8V

Insertion Loss: 5.7dB

C. DeRose, et al, Optics Express 17, 3316 (2009)

Low loss internal taper with

high confinement factor


RF-over-fiber using EO polymer

modulator

EO polymer waveguide together with antenna shaped drive electrodes is

used to put a free-space wireless signal onto an optical carrier

O. Herrera et al., J. Lightwave Tech. 32, 3861 (2014)


RF over fiber demonstration


Ultrahigh activity EO polymers


Device platforms• Silicon photonics is becoming a standard platform for

integrated photonics and benefits from hybrid device

approaches using highly nonlinear materials

• Microresonators have increasingly been perfected to provide

ultrahigh Q and thus very large intensity for modest in put

powers

• Both photonic crystal fiber and capillary fiber provide new

platforms for highly nonlinear liquids such as carbon

disulfide

• Plasmonics provides the potential for nanoresonators,

concentrating optical fields by many orders of magnitude ins

nanometer scale volumes


EO polymer/Si hybrid modulator

Confinement factor multiplied by neff

3vs. waveguide width is plotted for different waveguide

heights. The dots are for guided light at 1310nm and the solid lines are for 1550nm light. The

inset shows the electrical field of a TE mode in a 260nm wide and 300nm high silicon

waveguide with EO cladding on SiO2

at 1550nm.


EO polymer/Si phase modulator

Proof-of-concept phase modulator demonstrated – estimated in-device r33

was 132pm/V

R. Himmelhuber, et al., J. Lightwave Tech. 31, 4067 (2013)


Microresonators for nonlinear

photonics• Circulating power inside the resonator is proportional to the

Q-factor

• A few mW input power can provide hundreds of watts of

circulating power with high Q-factors

• Microresonators have small mode volumes

• High power circulating in small volumes creates a localized

high intensity field and gives us access to the nonlinear

photonics regime with low input power

Raman lasing, third harmonic generation, and frequency comb

generation have been demonstrated in microdisk, sphere, and toroid

resonators


Whispering gallery mode

resonators

WGMRs are a class of optical

resonators in which light

propagates in a circular path

around the resonator.

WGMs have high Q-factors, or

finesse, and low mode volumes

and come in a wide variety of

shapes and sizes


Achievements

Microresonator characterization

Transmission vs.

microresonator/ fiber taper gap

Transmission vs. wavelength

showing many resonant modes


Lithographically fabricated

microtoroids

• Combination of lithographic/etching processes and CO2 laser thermal

reflow of silica

• Optimum dimensions are D ~ 30 microns, d ~ 6 microns

T. J. Kippenberg, et. al., APL 85, 613 (2004)


WGM Raman laser

• 70 micron diameter sphere

• 2 mW pump power

• Q ~ 108

Raman oscillations

separated by FSR

Four wave

mixing

Pump

S. M. Spillane, et. al.

Nature 415, 621 (2002)

Tapered fiber

~ 1.5 micron

diameter


SHG in Si3N4 microrings

J. S. Levy et al., Optics Express 19, 11415 (2011)

100 microwatts of SHG for

315 mW of pump


THG in Si3N4 microrings

J. S. Levy et al., Optics Express 19, 11415 (2011)


Achievements

Microbubble resonators

• Thin walled (2 micron) microbubbles made by torch and fusion splicing

processes

• Solutions of biochromophores and organic chromophores can be introduced

through the capillary

• Cavity walls are thin enough to provide good penetration of WGM mode into

luminescing solutions


• Rhodamine6G @ 5x10-3 M

concentration

• Pump wavelength @

980nm

• Fluorescence imaged using

relay of a .1NA microscope

objective and Ocean Optics

fiber lens through 2 filters

centered around

580nm.(400-715nm)

• Minimum measureable

fluorescence was with 700

microwatts pump.

R6G two-photon fluorescence

in a microbubble resonator

G. Cohoon et al., Optics Letters 39, 3098 (2014)


Liquid photonics platform

• Very long pathlengths (1m) are possible

• For liquids of suitable refractive index,

waveguiding is achieved and thus high

intensity maintained over the fiber length

• A new process has been developed where

capillary fiber (core diameter 1 – 10 m) can

be fusion spliced to SMF-28 fiber, allowing

for convenient filling, low coupling loss and

repeated use for characterization

• Fiber optics technology and theory can be

readily used

Liquid core

optical fiber

Special splice

50cm – 3 m

Platform for ultra-efficient nonlinear photonics


LCOF platform requirements

• LCOF solvent – requirements

refractive index > 1.46 at 1550nm

basic vibrational absorption loss guidelines

70dB/m at 1550nm (mainly aliphatic)

40dB/m at 1550nm (mainly aromatic)

25dB/m at 1310nm

drives choice of halogenated or partially halogenated solvents

with few or no aliphatic carbon-hydrogen bonds

the classic nonlinear optical liquid carbon disulphide works well

since it only has low energy vibrations and a refractive index ~

1.6 at 1550nm


CS2 - filled 50cm fiber with core diameter of

2 micron and input pulse length of ~300fs

n2 ~ 150*10-16 cm2/W

good fit with experiment and the

literature at these pulse lengths

SPM in CS2 LCOF

Experiment Theory

n2 value in 10-16 cm2/W


SRS in CS2 LCOF

pump Stokes lines

No SRS observed in un-filled

fiber at these powers

600ps pump pulse

1.5kHz rep rate

30W average power

K. Kieu, et. al., Optics Express 20, 8148 (2012)


Fiber laser source for inverse

Raman scattering switch

SRS IRS


Inverse Raman scattering in LCOF

Liquid CS2

filled fiber

Loss of 18 dB observed

2 mW needed

Extracted large Raman gain

coefficient

Main resonance of CS2

K. Kieu, et. al., Optics Letters 37, 942 (2012)

~17.5 dB


Mode-locked

laser Circulator

Circulator

EDFATunable filter

2x2 43/57

coupler

Sagnac loop with i-LCOF

Port 1Port 2

Single beam all-optical switching

Single beam switching data

• 0.5m of CS2 filled fiber (2m core diameter)

• Switching peak power < 2W

• > 50% switching efficiency achieved


LCOF system for slow light

0 1 2

5

15

25

35

45

P0 (W)

De

lay (

ps)

(a)

0 1 2

5

15

25

P0 (W)

Ga

in (

dB

)

(b)


SPM spectral broadening reduction

with negative n2 dyes

• Pulse spectrum narrows after adding a negative n2 dye to the solution

S. Shahin et al., Optical Fiber Communications Conference (OFC) 2013


Surface plasmon enhanced TP

fluorescence in GFP - fabrication

Grating etched in silicon using optical lithography and reactive ion etching

• Green fluorescent protein (GFP),

a highly luminescent protein, was

used as the probe molecule

• GFP was spun directly onto

the lithographically fabricated

grating with ~ 1m periodicity

- RIE used to pattern the Si

- Ag thickness is 100nm


Surface plasmon enhanced TP

fluorescence in GFP using MPM

Enhancement of the signal in the patterned region ~ 30x

• MPI specs

- l: 1040nm

- Fluorescence filter: 560-750nm

- Power: 26mW

- Objective: 20x

• Measurements were performed on the

silver plain and grating edge to enable

determination of the fluorescence

enhancement due to the grating

B. Cocilovo et al., Journal of Lightwave Technology, DOI10.1109/JLT.2015.2412126 (2015)


Conclusion• The development of compact, turn-key modelocked solid-state laser

systems enables practical nonlinear optics in instruments and devices

• EO polymers have now been developed that have exceptional properties

for use in EO modulators and the combination of optimize sol-gel materials

and EO polymers has resulted in several generations of high performance

EO modulators with applications in standard optical communications, and

RF photonics; EO polymers have also been show to be compatible with the

silicon photonics platform

• Liquid core optical fiber provides a flexible platform for nonlinear photonics

which has been used to explore a variety of all-optical effects and devices

including efficient SRS, IRS and IRS switching, Brillouin lasing, slow light

generation and compensation for self-phase modulation

• Microbubble resonators provide a promising vehicle for exploring the

interface between resonator enhanced nonlinear photonics and

microfluidics


Acknowledgements


• B. Cocilovo

• G. Cohoon

• V. Demir

• O. Herrera

• R. Himmelhuber

• K. Kieu

• K. Kim

• O. Kropachev

• S. Mehravar

• S. Namnabat

• N. Peyghambarian

• S. Shahin

• R. Voorakaranam

Third-order Materials• J. Hales, S. Marder, and J. Perry,

Georgia Tech

Funding• AFOSR

• AFOSR COMAS MURI

• AFOSR BioPAINTS MURI

• DARPA ZOE

• NSF – CIAN ERC

• NSF – CMDITR

• ONR NECom MURI

• SRC/Intel

EO Materials• A. Jen and J. Luo at UW and Soluxra

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