Effect of pulse duration on plasmonic enhanced ultrafast laser-induced

Appl Phys A
DOI 10.1007/s00339-012-7210-1
<strong>Effect</strong> <strong>of</strong> <strong>pulse</strong> <strong>duration</strong> on plasmonic enhanced ultrafast
laser-induced bubble generation in water
R. Lachaine · E. Boulais · E. Bourbeau · M. Meunier
Received: 21 January 2011
© Springer-Verlag 2012
Abstract Bubbles generated in water by focusing femtosecond
and picosecond laser <strong>pulse</strong>s in the presence <strong>of</strong>
100 nm gold nanoparticles have been investigated in the fluence
range usually used for efficient cell transfection (100–
200 mJ/cm 2 ). Since resulting bubbles are at the nanoscale,
direct observation using optical microscopy is not possible.
An optical in-situ method has been developed to monitor the
time-resolved variation in the extinction cross-section <strong>of</strong> an
irradiated nanoparticle solution sample. This method is used
to measure the bubbles lifetime and deduce their average diameter.
We show that bubbles generated with femtosecond
<strong>pulse</strong>s (40–500 fs) last two times longer and are larger in
average than those generated with picosecond <strong>pulse</strong>s (0.5–
5 ps). Controlling those bubble properties is necessary for
optimizing <strong>of</strong>f-resonance plasmonic enhanced ultrafast laser
cell transfection.
1 Introduction
Cell transfection is a technique widely used by biologists
to modify cells behavior [1–8]. While many methods, including
viral vectors [1], electroporation [2–4], and lip<strong>of</strong>ection
[5–8] are currently available, efficient transfection
still faces major challenges [9]. A promising way to achieve
cell transfection is the ultrafast laser-induced cell perforation
[10–15]. In this method, a femtosecond laser <strong>pulse</strong> is
R. Lachaine (�) · E. Boulais · E. Bourbeau · M. Meunier
Laser Processing and Plasmonics Laboratory, Ecole
Polytechnique <strong>of</strong> Montreal, Montreal, Quebec H3C 3A7, Canada
e-mail: remi.lachaine@polymtl.ca
M. Meunier
e-mail: michel.meunier@polymtl.ca
tightly focused onto the cell membrane to induce membrane
permeabilization. However, tight focusing limits the
throughput and makes the method inappropriate for most biological
and medical application where large population <strong>of</strong>
cells must be transfected. To recover a high throughput, it
is possible to amplify locally the laser electromagnetic field
using plasmonic nanostructures deposited on the cells, such
as gold spherical nanoparticles (NPs) [16–19]. Nanoparticles,
through the plasmonic effect, amplify and absorb
the laser near-field and generate cavitation bubbles that are
thought to permeabilize the cell membrane, enabling cell
perforation and transfection with high efficiency [20, 21].
Measurement <strong>of</strong> the bubble average dimension and lifetime
as a function <strong>of</strong> laser parameters is important to get
a better understanding <strong>of</strong> the basic mechanisms underlying
nanocavitation and to optimize the plasmonic enhanced
laser transfection process. Major difficulty comes from the
reduced dimensions <strong>of</strong> the bubbles that are generated, preventing
the use <strong>of</strong> optical microscopy to properly measure
their diameter. In this paper, we present an in-situ optical
method to measure the variation <strong>of</strong> the extinction crosssection
<strong>of</strong> an irradiated colloidal nanosphere suspension
sample and then determine both the lifetime and the average
dimension <strong>of</strong> the bubbles. This method is then used to
study the effect <strong>of</strong> laser <strong>pulse</strong> <strong>duration</strong> on the bubble characteristics.
2 In-situ nanobubble optical detection setup
Figure 1 presents the in-situ optical setup used to characterize
the generated bubbles. A first laser, called pump
(Ti:Sapphire, 6 mJ/<strong>pulse</strong>, τ<strong>pulse</strong> =[40 fs–5 ps], 800 nm), is
focused on the sample (gold NPs, 100 nm, 1.4×10 9 part/ml)
using a lens (focal = 10 cm) and generates the bubbles.

Fig. 1 In-situ nanobubble detection setup. Fs pump laser <strong>pulse</strong> <strong>duration</strong>
can be changed from 40 fs up to 5 ps. The sample is composed
<strong>of</strong> 100 nm gold NPs in water solution. Bubbles are detected using the
variation <strong>of</strong> the extinction cross-section
A second laser, called probe (He:Ne, CW, 2 mW), is
collinear with the pump laser and focused in the sample.
A dichroic mirror separates the two laser beams after the
sample. The probe beam is then spatially filtered using a
10 μm pinhole. This spatial filter blocks light that is scattered
by cavitation bubbles. When this occurs, a drop in the
intensity <strong>of</strong> the probe signal is detected by the photodiode
(Si 1-GHz photoreceiver, trise = 0.6 ns) located behind the
pinhole. Note that this intensity loss is related to the total extinction,
since it comes from both absorption and scattering,
which is blocked by the spatial filter.
3 Results and discussion
Upon irradiation, plasmonic nanostructures enhance the
near-field and create a plasma in the medium. This plasma
eventually relaxes, leading to a local energy deposition
into the water surrounding the NPs [21]. This generates a
fast, quasi-isochoric, temperature rise along with a strong
stress confinement. A pressure wave is then released in the
medium and, if the energy is sufficient, a cavitation bubble
is created.
Using the experimental setup described earlier, it is possible
to detect those bubbles. A typical signal obtained at
200 mJ/cm 2 and <strong>pulse</strong> <strong>duration</strong> 1 ps is shown in Fig. 2.
If the fluence is increased at values over the transfection
range (which is between 100–200 mJ/cm 2 ), it is possible
to observe the characteristic oscillatory behavior coming
from successive growths and collapses <strong>of</strong> cavitation bubbles
[22],asshownintheinset<strong>of</strong>Fig.2 (the fluence used
was 450 mJ/cm 2 ). This observation strongly suggests that
R. Lachaine et al.
Fig. 2 Typical signal acquired with the experimental setup. Fluence
used was 200 mJ/cm 2 and <strong>pulse</strong> <strong>duration</strong> was 1 ps. The inset shows the
oscillatory behavior observed at higher fluences (450 mJ/cm 2 )
the measured signal comes from cavitation bubbles and not
from any other phenomenon. At fluences usable for the
transfection experiment, this oscillatory behavior is however
not observed, either because the bubbles are too small to initiate
oscillation or because the secondary bubble is beyond
the resolution <strong>of</strong> the optical method.
3.1 Bubble lifetime
The bubble lifetime measurement is quite direct. As shown
in Fig. 2, the bubble lifetime is defined as the time between
the beginning <strong>of</strong> the intensity drop and the moment where
the intensity comes back to zero for the first time. Note that
the asymmetry between rise and collapse times is due to both
the presence <strong>of</strong> bubbles <strong>of</strong> different lifetimes within the focal
volume and the <strong>of</strong>f-resonance irradiation cavitation mechanism
itself. Details are explained elsewhere [21].
Bubble lifetime was measured as a function <strong>of</strong> average
fluences for different <strong>pulse</strong> <strong>duration</strong>s (Fig 3a). For fluences
below 300 mJ/cm 2 , which include the transfection fluence
range (100–200 mJ/cm 2 ), results show that bubble lifetimes
are longer for femtosecond (fs) <strong>pulse</strong>s (below 500 fs) than
for picosecond (ps) <strong>pulse</strong>s (above 500 fs). For example, at
200 mJ/cm 2 , bubble lifetime is 600 ns when irradiating with
a40fs<strong>pulse</strong>comparedto350nswitha1ps<strong>pulse</strong>.
For all <strong>pulse</strong> <strong>duration</strong>s, results show that bubble lifetimes
range from a few hundreds <strong>of</strong> nanoseconds to about 1 μs.
However, permeabilization <strong>of</strong> the membrane remains for a
few microseconds, as shown by patch-clamp experiments
[12], allowing the foreign DNA to enter the cell. In that
regard, bubble lifetime is probably less important than the
bubble dimension in the perforation process.

<strong>Effect</strong> <strong>of</strong> <strong>pulse</strong> <strong>duration</strong> on plasmonic enhanced ultrafast laser-induced bubble generation in water
Fig. 3 (a) Bubble lifetime and
(b) average diameter for
different average fluences at
different <strong>pulse</strong> <strong>duration</strong>
Fig. 4 Cth ext comparison with and without a NP in the center <strong>of</strong> the
bubble
3.2 Bubble dimension
It is possible to deduce an approximate average bubble diameter
from the signal amplitude A defined as the maximal
loss in transmission (see Fig. 2). First, a Mie code was used
to calculate the theoretical extinction coefficient Cth ext <strong>of</strong> a
vapor bubble <strong>of</strong> different diameters at the probe wavelength
(633 nm). Of course, for small bubbles, Cth ext is affected by
the presence <strong>of</strong> the NP in the center <strong>of</strong> the bubble. A Mie
coated code [23] has thus been used to calculate the extinction
coefficient <strong>of</strong> a NP in water coated with a vapor layer <strong>of</strong>
different thickness in order to evaluate the importance <strong>of</strong> the
NP. Figure 4 shows that the presence <strong>of</strong> the NP is not negligible
for bubble diameters under 140 nm (20 nm vapor layer
thickness). As it will be shown later, the smallest bubbles
that have been detected have a diameter <strong>of</strong> about 240 nm.
Therefore, the presence <strong>of</strong> the NP at the center <strong>of</strong> the bubble
has been neglected for simplicity reason.
Two main approximations were made to evaluate the bubbles
diameter: (i) there is one and only one bubble created
around each NP and those bubbles remain disjoint; (ii) all
bubbles have the same diameter within the focal volume,
thus an average bubble diameter could be found. Knowing
the normalized signal amplitude A, the NPs concentration C
(1.4 × 109 part/ml) as well as the focal depth L (755 μm),
Beer–Lambert law was applied to deduce the experimental
extinction coefficient C exp
ext . The ratio between the pump
and probe laser focal spots was also considered because the
pump spot was smaller than the probe spot.
C exp
ext =
ln (1 − A) r
CL
2 probe
r2 pump
The average bubble diameter is then deduced by finding
the bubble diameter matching Cth ext and Cexp ext .Usingthis
method, we were able to measure average bubble diameter
down to 240 nm.
Fluences useful for cell transfection usually range from
100 to 200 mJ/cm2 [20]. Results presented in Fig. 3b show
that fs <strong>pulse</strong>s produce larger average bubbles than ps <strong>pulse</strong>s
in that range . For example, at 200 mJ/cm2 , fs <strong>pulse</strong>s (40 fs)
create 750 nm average diameter bubbles while ps <strong>pulse</strong>s
(1 ps) generate a smaller 380 nm average diameter bubble.
This is probably because longer <strong>pulse</strong>s are less efficient
to photoionize the water around the particle. Indeed, simulations
were performed to calculate the amount <strong>of</strong> energy
deposited into the water for a 40 fs <strong>pulse</strong> in comparison
with a 1 ps <strong>pulse</strong> when irradiating a 100 nm gold NP and
results show that the deposited energy is approximately 3
times larger for fs <strong>pulse</strong>s than for ps <strong>pulse</strong>s. Those results
are shown in Fig. 5. Classical rate equations in 0D, including
electron generation with Keldysh theory [24, 25], collision
term [26, 27], and recombination term [24], have been
solved to determine the plasma density and the temperature.
Further details are given elsewhere [21].
Fs <strong>pulse</strong>s seem thus more effective to generate larger average
bubbles than ps <strong>pulse</strong>s below 200 mJ/cm2 . However,
the fact that the average bubbles are measured to be larger
does not necessarily imply that the maximum bubbles are
also larger. Indeed, NPs located near the center <strong>of</strong> the focal
spot are irradiated with a fluence higher than the average
(1)

Fig. 5 Simulation results for the deposited energy into water when
irradiating a 100 nm gold NP with a 800 nm, 40 fs, and 1 ps laser
<strong>pulse</strong>. The deposited energy is about 3 times larger for fs <strong>pulse</strong>s than
for ps <strong>pulse</strong>s
fluence, thus creating maximum bubbles larger than the average
bubbles. Further investigations would be required to
make a statement concerning the maximum bubble dimensions.
4 Conclusion
In conclusion, the effect <strong>of</strong> <strong>pulse</strong> <strong>duration</strong> on laser induced
bubble generation in the presence <strong>of</strong> gold nanoparticles was
investigated. Using the bubbles scattering signal and a relatively
simple model, we were able to measure the bubble
lifetime and average diameter. Those characteristics were
measured for different pump <strong>pulse</strong> <strong>duration</strong>s, from 40 fs
up to 5 ps. It has been shown that fs <strong>pulse</strong>s produce larger
and longer average bubbles than ps <strong>pulse</strong>s for laser fluences
used for cell transfection. Works are underway to relate the
bubble characteristics and dynamics to the efficiency <strong>of</strong> <strong>of</strong>fresonance
plasmonic enhanced ultrafast laser cell transfection.
Acknowledgements The authors would like to thank the Natural
Science and Engineering Research Council (NSERC) and Le Fonds
Quebecois de la Recherche sur la Nature et les Technologies (FQRNT)
for financial support. The technical assistance by Yves Drolet is also
acknowledged.
References
R. Lachaine et al.
1. W. Walther, U. Stein, Drugs 60, 2 (2000)
2. T.K. Wong, E. Neumann, Biochem. Biophys. Res. Commun. 107,
2 (1982)
3. K. Shigekawa, W.J. Dower, BioTechniques 6, 8 (1988)
4. J. Weaver, Y. Chizmadzhev, Bioelectrochem. Bioenerg. 41, 135
(1996)
5. F.L.Graham,J.Smiley,W.C.Russell,R.Nairn,J.Gen.Virol.36,
1 (1977)
6. T. Wilson, D. Papahadjopoulos, R. Taber, Cell 17, 1 (1979)
7. R. Fraley, S. Subramani, P. Berg, D. Papahadjopoulos, J. Biol.
Chem. 255, 21 (1980)
8. P.L. Felgner, Y.J. Tsai, L. Sukhu, C.J. Wheeler, M. Manthorpe,
J. Marshall, S.H. Cheng, Ann. N.Y. Acad. Sci. 772, 126 (1995)
9. J.L.Stilwell,D.M.McCarty,A.Negishi,R.Superfine,R.J.Samulski,
J. Virol. 77, 23 (2003)
10. U.K. Tirlapur, K. Konig, Nature 418, 6895 (2002)
11. D. Stevenson, B. Agate, X. Tsampoula, P. Fischer, C.T.A. Brown,
W. Sibbett, A. Riches, F. Gunn-Moore, K. Dholakia, Opt. Express
14, 16 (2006)
12. J. Baumgart, W. Bintig, A. Ngezahayo, S. Willenbrock, H.M. Escobar,
W. Ertmer, H. Lubatschowski, A. Heisterkamp, Opt. Express
16, 5 (2008)
13. V. Kohli, J.P. Acker, A.Y. Elezzabi, Biotechnol. Bioeng. 92, 7
(2005)
14. E. Zeira, A. Manevitch, Z. Manevitch, E. Kedar, M. Gropp,
N. Daudi, R. Barsuk, M. Harati, H. Yotvat, P.J. Troilo, T.G. Griffiths,
S.J. Pacchione, D.F. Roden, Z. Niu, O. Nussbaum, G. Zamir,
O. Papo, I. Hemo, A. Lewis, E. Galun, FASEB J. 21, 13 (2007)
15. A. Vogel, N. Linz, S. Freidank, G. Paltauf, Phys. Rev. Lett. 100,
038102 (2008)
16. A. Uchugonova, K. Konig, R. Bueckle, A. Isemann, G. Tempea,
Opt. Express 16, 13 (2008)
17. C. Yao, R. Rahmanzadeh, E. Endl, Z. Zhang, J. Gerdes,
G. Huttmann, J. Biomed. Opt. 10, 6 (2005)
18. D. Lapotko, Nanomedicine 4, 7 (2009)
19. V.K. Pustalkov, A.S. Smetannikov, V.P. Zharov, Laser Phys. Lett.
11, 775 (2008)
20. J. Baumgart, L. Humbert, E. Boulais, R. Lachaine, J.J. Lebrun,
M. Meunier, Biomaterials 33, 7 (2012)
21. E. Boulais, R. Lachaine, M. Meunier, Nano Lett. (2012).
doi:10.1021/nl302200w
22. E.M. Glinsky, S.D. Bailey, A.R. London, A.P. Amendt,
M.A. Rubenchik, M. Strauss, Phys. Fluids 13, 20 (2001)
23. E.C. Le Ru, P.G. Etchegoin, Principles <strong>of</strong> Surface-Enhanced Raman
Spectroscopy (Elsevier, Amsterdam, 2009)
24. A. Vogel, G. Noack, G. Huttmann, G. Paltauf, Appl. Phys. B,
Lasers Opt. 81, 8 (2005)
25. L. Keldysh, Zh. Èksp. Teor. Fiz. 47, 5 (1965)
26. L. Hallo, A. Bourgeade, V. Tikhonchuk, C. Mezel, J. Breil, Phys.
Rev. B, Condens. Matter Mater. Phys. 76, 2 (2007)
27. N. Andreev, M. Veisman, V. Efremov, V. Fortov, High Temp. 41,
5 (2003)