modification of glass by fs laser for … of glass by fs laser for optical/memory applications...
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Modification of Glass by FS Laser for Optical/Memory Applications
Kazuyuki Hirao and Kiyotaka Miura
Department of Material ChemistryKyoto University
International Workshop on Scientific Challenges of New Functionalities in Glass
Review of the interaction effect between glass and laser
Characteristics of the femtosecond laser in glass processing Mechanism of the refractive index change
Application of photo-induced refractive index changes
Other structural changesPrecipitation of gold nano-particlesValence state change of samarium ionsWriting of nano-gratingGrowth of non-linear optical crystal
Precipitation and growth of silicon in the glass
Femtosecond laser can be used to modify transparent materials like glass microscopically and three-dimensionally.
Resonance wavelength(e.g., UV laser or light)
The surface
Nonresonance wavelength(femtosecond laser)
The interior
high peak power
Nonresonance wavelength(e.g., CW, nanosecond laser
or visible light)
Non-reaction(one-photon process)
Glass
Reaction area Reaction areaNon-reaction
When a transparent material like glass is irradiated by a tightly focused femtosecond laser, photo-induced reactions should occur only near the focused part of the laser beam.
１００μｍ１０μｍ
0.5 μJ, f 300, 1 kHz 、 3 μJ, x 50, 200 kHz1 μJ, x 50, single pulse
１００μｍ
(A) (B) (C)
Energy density
(A) Coloring line due to defect formation(B) Refractive index changes due to local densification by a single pulse(C) Melting due to heat accumulation
alkali silicate glass
Low High
・The structural changes induced by a femtosecond laser take various forms depending on the condition of laser irradiation.
We found that a shock wave forms at the focal point inside the glass of the beam created by a femtosecond laser and propagates to the surrounding area.
The change in the spatial pattern of the probe beam was monitored to obtain the refractive-index distribution.
Objective
Probe beam(388 nm)
Pump Beam(775 nm; 500 fs)
20X
Harmonic Separator f=30 mmIris
Prism
Blue filter
CCD Camera
Optical setup used to observe the shock waveLaser of the second harmonic from part of the pump laser
Al: Silicate glass
Transient lens method
The density at the center decreases and the high-density area due to pressure-wave generation rapidly expands into the surrounding area.
8
6
4
2
0
Δφ(r
) pha
se c
hang
e
14121086420r/μm
5000 ps2000 ps1600 ps
1000 ps
1200 ps
1400 ps
800 ps
1 ps
100 ps
200 ps
400 ps
600 ps
300 ps
The positive peak due to the pressure wave propagates outward with a constant velocity.
~6.2 μm/nsThe temperature and pressure of this region
rises very rapidly and dramatically. >3000 K, ~1 GPsFig. The phase distribution obtained by a phase
retrieval calculation of the spatial pattern.
The center of the focusing laser beam
Den
sity
fsfs laserlaser
Shock waveShock wave
The irradiation of a single pulse
Temperature increase and thermo-elastic stress should be induced in a very limited
volume.↓
The relaxation of the thermo-elastic stress produces the force driving the shock wave.
↓The shock wave propagates outward.
↓The structure of the glass is extended outside.
After pulse irradiation
Compressive stress moves back toward the center.
↓A graduated high-density region is formed in the laser-focusing area.
↓After the thermal diffusion from the
irradiated region, the structural change becomes a permanent
refractive-index change.
One pulse
If laser pulses are repeatedly irradiated, the high-temperature area rapidly expands, and melting regions can be formed at arbitrary sites within the glass.
Fig. Photo-written waveguides were written by translating the sample (a) parallel or (b) perpendicular to the axis of the laser beam
(a) (b)
Waveguide
Laser Beam
Microscope Objective
Refractive index change
Waveguide
Laser Beam
Microscope Objective
Refractive index change
(a)
20μｍ
(b)
20μｍ
(a)
20μｍ20μｍ
(b)
20μｍ20μｍ
Array of refractive index changes inside silica glass
By using a femtosecond laser with a high repetition rate, refractive index changes are continuously induced along a path traversed by the focal point.
↓
We can write waveguides in glass.
Similar waveguides can be written inside various types of glass, such as silica, borosilicate, fluoride and chalcogenide glass.
250 kHz
Common Writing ConditionWavelength : 800 nmRepetition rate : 250 kHzPulse width : 270 fsScanning rate : 50 μm/s
Objective power(NA) (mW)
Ⅰ 0.25 425Ⅱ 0.13 425Ⅲ 0.13 350
LP01
LP01, LP11
LP01, LP11
Calculated guide [email protected]μm
LP01
LP01, LP11
LP01, LP02, LP11
Calculated guide [email protected]μm
13.5
10.3
6.7
12.0110.27III
9.0180.53II
6.0201.32I
Change diameter[μm]
Refractive index
reference[%]
Table Guide mode and mode field diameter calculated from the refractive-index profile
Refr
active
Inde
x
Radial Distance (μm)
-10 -5 0 5 101.455
1.460
1.465
1.470
1.475
1.480
1.485
Refr
active
Inde
x
Radial Distance (μm)
-10 -5 0 5 101.455
1.460
1.465
1.470
1.475
1.480
1.485
Fig. The refractive index profiles of the core of waveguides written in silica glass at various numerical apertures and average powers.
The characteristics of a writing waveguide can be controlled by adjusting the writing conditions
Characterization• This device has matrix-arrayed multichannel optical
I/Os.• The optical path are redirected by 90 degrees.• This structure and size enable optical interconnects
to be integrated in a board-to-board or board-to-backplane communication structure more densely.
LD or PD
Optical circui PCB Optical-path redirected waveguide
LSI
2.5 x 3.75 x 1 mm
It's not easy to fabricate an optical-path redirected waveguide using the conventional method, so this is an example where the laser writing technique has a clear advantage.
Grating & Binary lens (microscopic view)
Δ=46.5 μm
1024 μm
f=9 mm
10 mm
CCD
λ=633 nm
5 mm
5 mm Beam diameter = 12-13 μm
Beam profile at the focal plane
254 μm
Split beam power ~ 27 % of input beam
Particle size: Small
TEM
100 nm
300 400 500 600 700 800
Abs
orba
nce
[a.u
.]
Wavelength [nm]
(a)
(b)
(c)
(a)
(b)
(c)
Laser intensity: (a) < (b) < (c)
Blue shift
550 ℃ for 1 hr
1014 1015 1016400
450
500
550
600
Peak
pos
ition
[nm
]
Laser intensity [W/cm2]
Applications may also include developments in techniques such assurface plasmon resonance and near-field scanning optical microscopy.
LASER BEAMWavelength: 800 nmPulse width: 50 fsRepetition rate: 250 kHzAverage power: 100 mWOvjective: 50x, NA=0.6
Photo-reduction Sm3+ →Sm2+
GLASSSmF3 : AlF3-YF3-BaF2-SrF2-CaF2-MgF2
500 550 600 650 700 750 800
Emis
sion
inte
nsity
(a.u
.)Wavelength (nm)
5 D0→7 F0
5 D0→7 F2
Sm2+
Sm3+
5 D0→7 F1
: 4G5/2 -6HJ: 4G5/2 -6HJ
J=5/27/2
9/2
Fig. Photoluminescence spectra for glass excited at 488 nm : red line shows spectrum for laser-irradiated areas; yellow line shows spectrum for non-irradiated area.
Laser-irradiated areas (photo-reduced areas) recorded inside glass can be detected only by emissions at 680, 700 or 725 nm.
Irradiated areaIrradiated area
200 nm
Alphabetical character comprised 300 to 500 photo-reduction bits. The bits had a diameter of 200 nm.The spacing between each characters was 1 μm.
Recording : Sm3+→Sm2+
Femtosecond laser100 nJ/pulse at 800nm
Readout :Ar+ laser 1 mW at 488 nm
Erasure : Sm2+→Sm3+
Ar+ laser 10 mW at 488 nm
Photoluminescence images of alphabetic characters recorded by using the photo-reduction of samarium ion on different layers.
Image after a femtosecond laser irradiation to areas I and II.
(a)
(b)Ⅰ Ⅱ
Ⅰ Ⅱ
(c)
Photoluminescence image before the erasure.
Image after an Ar+ laser irradiation to photoreduction bit I and II
Three-dimensional optical memory with rewrite capability is achievable.
Sm2+→Sm3+
200 kHz, 150 fs, 600 mW
SiO2 glass
BEI
E Polish
PolarizationLaser
Hot plasma
longitudinal plasma wave
Oxygen defectSiO2-x (x ~ 0.4)
ΔΔnn ~ ~ --0.10.1The refractive index of nano lines is lower than that of the circumference, and periodic nano lines function as a grating.
Elemental distributions of Ba and Al at around the focal point of the laser.
30 μm
SEM1 sec. 30 min.
BaO-Al2O3-B2O3 glass
Laser Beam
Melting region
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
） Ba
Al
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
） Ba
Al
1 psIn
tens
ity (a
.u.)
Scanning distance (μm)
Ba
Al
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
） Ba
Al
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
） Ba
Al
1 psIn
tens
ity (a
.u.)
Scanning distance (μm)
Ba
Al
1 sec.
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
） Ba
Al
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
） Ba
Al
1 psIn
tens
ity (a
.u.)
Scanning distance (μm)
Ba
Al
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
） Ba
Al
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
） Ba
Al
1 psIn
tens
ity (a
.u.)
Scanning distance (μm)
Ba
Al
1 sec.
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
）
Ba
Al
40 min
Inte
nsity
(a.u
.)
Scanning distance (μm)
g μ
Ba
Al
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
）
Ba
Al
40 min
Inte
nsity
(a.u
.)
Scanning distance (μm)
g μ
Ba
Al
30 min.
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
）
Ba
Al
40 min
Inte
nsity
(a.u
.)
Scanning distance (μm)
g μ
Ba
Al
0 20 40 60 80 100
走査距離(μm）
Ｘ線
強度
(任意
）
Ba
Al
40 min
Inte
nsity
(a.u
.)
Scanning distance (μm)
g μ
Ba
Al
30 min.
The concentration of Al increased at the center of the irradiated area, and Ba increased outside the center with the passage of time.
Measurement of the x-ray diffraction patterns confirmed that only beta-BBO crystals grew in the focused area.
LASERWavelength: 80 0nmPulse width: 50 fsRepetition rate: 250 kHzAverage power: 400 mWObjective: 100x, NA=0.9
• Metallic clusters or particles can be precipitated.• Specific ion can be reduced or oxidized.• Oxygen-deficiency centers (≡Si-Si ≡, etc. ) can be formed. • Specific ions can be diffused at the micrometer order.
Silicon cluster or particle can be extracted from silica or silicate glass.
General oxide glasses can not trap superfluous oxygen ions generated by cutting the Si-O bond and growth of the silicon in a closed space like the glass inside it.
We tried to deposit silicon from silicate glass prepared using metallic aluminum as the starting material.
～10cm~ 10 cm
～1mm~ 1 mm
～10cm~ 1 cm
Past Present Future
Silicon as a photonic medium has unique advantages for photonic integrated circuits
Refractive indices of core and cladding layers are approximately 1.50 and 1.45, respectively.
Refractive indices of silicon and conventional oxide glass are approximately 3.5 and 1.5, respectively.
Three-dimensional integration of different optical components in the glass
In order to achieve compact and integrated devices, high refractive index difference is required.The high refractive index contrast between the silicon waveguide core and a surrounding glass cladding enables the fabrication of small optical structures.
If silicon can be deposited in the glass at three dimensions, this leads to further compactness and higher integration of optical devices.
500 1000 15000
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Abs
orba
nce
Si - Na – Al - OSiSi -- CaCa –– Al Al -- OOSi - Ca - Na – Al - O
Si - Ca - B – Al - O
Metallic aluminum : ~20 mol% in air
Melting Temp. : 1400 -1500 ℃Crucible : Al2O3
SiOSiO2 2 -- CaCOCaCO3 3 -- AlAlSiO2 - CaCO3 - Al2O3
200 300 400 500 600 700 800Wavelength(nm)
Emis
sion
inte
nsity
(arb
.uni
ts)
PLE（em:310 nm）
PL（ex:256 nm）
PLE（em:480 nm）
200 300 400 500 600 700 800Wavelength(nm)
Emis
sion
inte
nsity
(arb
.uni
ts)
PLE（em:310 nm）
PL（ex:256 nm）
PLE（em:480 nm）
SiO2 –CaCO3 (CaO) –Al ( 20 mol%)
Oxygen deficiency defect : O3≡Si-Si ≡O3
LASERPulse width: 130 fsPulse energy: 3 μJRepetition rate: 200kHzObjective: 50x, NA=0.85Irradiation Time: 5 sec. Irradiation of fs laser
Sample glass
Polish
Irradiation of fs laser
Sample glass
Polish
Microscope
Backscattered electron
20 μm
10 μm
SiSi -- CaCa –– Al Al –– O glassO glass
20 μm
Al Si
Ca O
Al Si
Ca O
Al Si
Ca O
Al Si
Ca O
EPMA
Si
Al Si
Ca O
Al Si
Ca O
Al Si
Ca O
Al Si
Ca O
SiOAl
Al: Silicate glass
fs laser
Silicon rich structure≡Si-Si ≡
+≡Si-Si-Si ≡
: moreSilicon cluster
Oxygen deficiency centers≡Si-Si ≡ etc.
+Aluminum rich structures
Heat treatment
Si particleThermite reaction
Silicon rich structure grows into a particle because of the thermite reaction promoted by the heat treatment
SiOAl
Al: Silicate glass
fs laser
Silicon rich structure≡Si-Si ≡
+≡Si-Si-Si ≡
: moreSilicon cluster
Oxygen deficiency centers≡Si-Si ≡ etc.
+Aluminum rich structures
Heat treatment
Si particleThermite reaction
Silicon rich structure grows into a particle because of the thermite reaction promoted by the heat treatment
・Glass prepared using metallic aluminum as the source material has oxygen deficiency centers・Si-O bonds are continuously broken and re-formed with numerous unbound atoms ・Si and Al-rich structures are formed at the center of the irradiated area.・Si rich structure grows into a particle because of the thermite-like reaction.