The potential advantages of using femtosecondpulses of laser radiation for ablation of structuralmaterials of power plants of high energy density areassociated with the existence of modes of stimulation,which are characterized by the absence of plasmascreening of incident coherent radiation, and with thesmall depth of heating of ablating matter during thetime of laser stimulation [1]. This results in the insig�nificance or total absence of the liquid phase, in thepossibility of direct transfer of radiant energy into thethin layer of evaporating condensed matter, and in thehigh�accuracy spatial localization of laser stimulationlimited by the diffraction effects alone [2]. Further�more, the pulsed femtosecond laser ablation is a pow�erful scientific tool and is of general physical interestbecause it opens up new opportunities in the field ofnonequilibrium thermodynamics, generation ofnanostructures, acceleration equipment, laboratorysimulation of processes in stellar atmospheres, super�nova outbursts, and so on [3].
Results are known of investigations of spectral�energy thresholds of femtosecond laser ablation ofmetals [4] and of crystal and amorphous dielectrics[5]; as to the number of papers devoted to the investi�gation of thresholds of ablation of polymer materialsby ultrashort laser pulses, it is very limited, and thedata given in these papers are difficult to analyzebecause of significant scatter of the values of parame�ters of stimulation and the absence of unified proce�dure of determination of the spectral�energy thresholdof laser ablation.
The effect made by ultrashort (femtosecond) laserpulses on polymer media is characterized by a numberof special features of dynamics of solid�liquid�gas�plasma phase transitions, which are primarily associ�ated with the variation of the mechanism of absorptionof coherent radiation and of the macrostructure anddynamics of surface plasma formation. An importantoptical�and�thermophysical characteristic of matterand one of the most informative parameters definingthe effectiveness of interaction between laser radiationpulses and condensed media is the spectral�energythreshold of laser ablation, i.e., the density of energy oflaser radiation at which begins the removal of matterfrom the surface being irradiated. In addition, theexperimental data on spectral�energy thresholds andrates of laser ablation are of fundamental importancefrom the standpoint of both determining the charac�teristics of physicochemical processes of laser stimula�tion of matter and constructing many�parameter andmany�factor mathematical models of optical�and�thermophysical and radiation�gasdynamic unsteady�state processes and critical phenomena of interaction.
The present paper describes the experimental�diagnostic module with femtosecond laser complex(τ0.5 ~ 45–70 fs, λ1 = 266 nm, λ2 = 400 nm, and λ3 =800 nm) for combined ultrahigh�speed interferometryof surface plasma formation (Mach–Zehnder scheme)and interference microscopy of the surface (Michel�son scheme), which were used for recording the pro�cesses of interaction between ultrashort laser pulsesand condensed media in vacuum and 3D geometry ofcrater. Results are given for the first time of the exper�
An Investigation of the Optical�and�Thermophysical and Gasdynamic Characteristics of Femtosecond Laser Ablation
of Structural Materials of the Polymer SeriesE. Yu. Loktionova, A. V. Ovchinnikovb, Yu. Yu. Protasova, and D. S. Sitnikovb
a Bauman Moscow State Technical University, Moscow, 105005 Russiab Joint Institute for High Temperatures, Russian Academy of Sciences (IVT RAN), Moscow, 125412 Russia
Abstract—A description is made of the experimental�diagnostic module with femtosecond terawatt lasercomplex (τ0.5 ~ 45–70 fs, λ = 266, 400, and 800 nm) and of the pioneering procedure of combined ultrahigh�speed interferometry and interference microscopy (Michelson and Mach–Zehnder schemes) of the pro�cesses of interaction between ultrashort laser pulses and condensed media in vacuum. Results are given forthe first time of the investigation of spectral�energy thresholds and rates of laser ablation of a number of solid�state media using elements of the polymer series (C2F4)n, (CH2O)n in the UV–NIR wavelength range of laserradiation in atmospheric conditions and in vacuum p ~ 10–2 Pa.
DOI: 10.1134/S0018151X10050159
HEAT AND MASS TRANSFERAND PHYSICAL GASDYNAMICS
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imental determination of spectral�energy thresholdsand rates of femtosecond laser ablation of condensedmedia using elements of the polymer series (such asformaldehyde (СН2О)n and fluoroplastic (С2F4)n) inthe UV–NIR spectral range in vacuum.
DESCRIPTION OF THE EXPERIMENTAL SETUP
The experimental�diagnostic module (Fig. 1) con�sists of four basic units, namely, 1) titanium�sapphirefemtosecond terawatt laser complex (Coherent),2) a module for transport and conversion of laser radi�ation, 3) a unit for recording and processing of exper�imental data, and 4) a unit for providing the requiredgas�vacuum conditions.
The experiments involved the radiation of a femto�second terawatt laser system (Fig. 2) which consistedof a master oscillator 1, a stretcher and regenerativeamplifier 2, a multipass amplifier 3, and a light com�pressor 4. The radiation of a diode�pumped continu�ous�wave solid�state laser 5 (Coherent Verdi V5) wasused for pumping the master oscillator (CoherentMira). The master oscillator shaped pulses of femto�second duration on wavelength λ ~ 790 nm with spec�tral width Δλ ~ 40 nm on the level of 0.5 of the maxi�mal value of intensity. The average power of opticalradiation was 650 mW, with the energy of individualpulse reaching 7 nJ. The method of amplification ofchirp pulses was used for further amplification of fem�tosecond laser radiation. The first amplification stagewas provided by a regenerative amplifier (CoherentLegend) with a linear resonator circuit, whichincreases the pulse energy to 1.2 mJ. The pulse repeti�tion rate of regenerative amplifier was 1 kHz and wasdefined by a pumping laser 6 (Coherent Evolution 15)which shaped laser pulses of duration τ0.5 ~ 500 ns onwavelength λ = 532 nm. A Faraday shutter was used asthe optical isolation between the master oscillator andregenerative amplifier. The contrast in intensitybetween the main pulse and pre�pulses coming outfrom the regenerative amplifier was ~104. A contrastimproving circuit consisting of two crossed polarizersand a Pockels cell located between them was used afterthe regenerative amplifier for effective amplification ofsingle pulse in the multipass amplifier and for provid�ing a higher contrast required for high�power femto�second laser systems, as well as for reducing the laserpulse repetition rate to 10 Hz. This circuit provided fora three orders of magnitude higher contrast in inten�sity between the main pulse and pre�pulses in thenanosecond time range. The outlet multipass ampli�fier (four passes) increased the pulse energy to 350 mJusing two pumping lasers 7 (Positive Light, Contin�uum) with the pulse energy of 750 mJ each. For attain�ing a high quality of radiation at the amplifier output,use was made of the circuit of transfer of spatial distri�bution of pumping laser beam from the end of secondharmonic crystal to the end of active element of the
amplifier. Immediately before the amplifier, the laserbeam divergence was corrected by a telescope for thecompensation of the thermal lens arising in the activeelement, as well as for increasing the beam diameterfrom pass to pass. As a result, the beam radius variedfrom 3 mm in the first pass to 8 mm after the fourthpass in the multipass amplifier. A single pulse isolationcircuit consisting of two polarizers and an electroopticmodulator was provided for protecting the optical ele�ments of amplifying stages against the radiationreflected from the target after the multipass amplifier.The shutter opening time was 15 ns, which made itpossible, on the one hand, to pass an amplified pulseinto the optical compressor and, on the other hand,not to pass the light pulse reflected from the target orfrom generated plasma, because the time after whichthe reflected radiation comes (with optical arm lengthof ~6 m) could be as long as 40 ns.
The compression of coherent radiation pulse afteramplification occurs in a vacuum optical compressorassembled in a circuit with two diffraction gratings;before the compressor, the beam size increases to30 mm to provide for the energy density of100 mJ/cm2 (below the damage threshold of diffrac�tion grating). The pulse duration at the output of thelaser system is τ0.5 = 35 ± 5 fs with energy up to 250 mJ.The module for transport and conversion of laser radi�ation provides for the distribution of radiation betweenthe circuits of laser stimulation, interference micros�copy, and interferometry of the flow; for the diagnos�tics of radiation, for generation of the first, second,and third harmonics in nonlinear crystals of BаB2O4,and for wave front interference in the planes of sensi�tive elements of recording equipment.
The unit for recording and processing of experi�mental data comprises CCD cameras (NPK Vid�eoskan), a compact spectrometer (S�150, Solar LS),equipment for monitoring the parameters of laserradiation (calorimeter, photodiode, or photomulti�plier), and a personal computer with software packagefor the processing of interference patterns (Phasemeasurement, VNIIOFI—All�Russia Research Insti�tute of Opticophysical Measurements) and spectro�grams. The unit for generating and maintaining thegas�vacuum conditions in the zone of stimulationincludes a specially made vacuum chamber (400 mmin diameter, 300 mm high) evacuated by an oilless vac�uum unit (TSH 071, Pfeiffer vacuum) to a pressure p ~5 × 10–3 Pa and systems for monitoring, control ofresidual pressure, and gas bleeding.
It is known that the use of interference microscopyis informatively important for characterization of mul�tifactor optical�and�thermophysical and gasdynamicprocesses of laser stimulation of the surface of laser�irradiated condensed target, and analysis of these pro�cesses requires data on the optical characteristics ofsurface plasma formation through which the objectbeam passes twice (Fig. 1). The effect of surface
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14 8
16
11
10
12315 4
F = 25 mm
F = 133 mm
7
Dielectric mirror
Metal non�transmitting mirror
Beam�splitting plate
Optical fiber
Diaphragm
Interference filter
Glan prism
713
7
9 8
10
1
2
5
6with selective coating
Fig. 1. The optical scheme of femtosecond experimental diagnostic complex: (1) ablating target, (2) titanium�sapphire femtosec�ond laser, (3) Michelson interferometer, (4) Mach–Zehnder interferometer, (5) optical delay line, (6) vacuum chamber, (7) crys�tal of second harmonic generation, (8) micro�objective, (9) lens objective, (10) CCD camera, (11) crystal of third harmonic gen�eration, (12) photomultiplier, (13) half�wave plate, (14) optical fiber lens, (15) compact spectrometer with CCD array,(16) mechanized shutter.
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plasma formation on the interference pattern of targetsurface being recorded may be analyzed theoretically[6] or experimentally. The necessary experimentaldata may be obtained using an additional interferome�ter which probes the surface plasma formation in par�allel to the surface being irradiated. At the same time,the shadow and interference methods employed forcharacterization of gas�plasma flows cannot be fullyutilized for qualitative assessment of the space�timedistribution of parameters if the mass flow rate andtemperature characteristics of flow in the zone of sur�face plasma formation are unknown. Therefore, thesimultaneous application of several noncontact exper�imental procedures for the investigation of the pro�cesses of interaction between laser radiation and mat�ter enables one both to obtain the desired characteris�tics and the space�time distribution of parameters andto increase the informative value and reliability ofexperimental data obtained using each one of theseprocedures separately.
For realizing the scheme of interference micros�copy of ablating target with high time resolution, aMichelson interferometer was assembled in the exper�imental�diagnostic module (Fig. 1) with the image ofthe surface of sample under investigation being trans�ferred to the plane of CCD matrix. In this circuit, thefemtosecond laser pulse is divided by a beam�splittingplate into heating and probing pulses. A P�polarizedlaser pulse incident on sample 1 under investigation atan angle of 45° is used for exciting the target material.An attenuator unit consisting of a polarizer and a λ/2half�wave plate 13 is used in the optical scheme forvarying the energy density in the zone of stimulation.The energy of laser radiation in each pulse is moni�tored using a calibrated photomultiplier 12 (H6780�04, Hamamatsu) which registers the radiation
reflected by the quartz plate. The photodetector is cal�ibrated against the readings of pyroelectric energymeter (J�10MT�10 kHz, Coherent) within the rangeof employed energy levels. The use of photomultiplierplaced into volume 6 being evacuated makes for themost accurate recording of the laser radiation energyincident on the target, which is especially important inthe case of laser stimulation in the UV spectral region.The probing laser pulse with variable delay relative tothe heating pulse is intended for lighting the targetregion being investigated. An autocorrelator was usedfor determining the pulse duration. The time delay isvaried using a multipass circuit of delay line 5 (withlarge step of variation of ~6 ns) in combination withmotorized circuit (8MT160�300, Standa, range ofvariation of 0–1.8 ns); this enables one to vary thedelay in the range of values of Δt = 0–75 ns with theaccuracy of Δτ < 100 fs defined by the duration of laserpulse of lighting. The possibility of simultaneous prob�ing by radiation of both a single wavelength and differ�ent wavelengths was realized instrumentally (in Mich�elson 3 and Mach–Zehnder 4 interferometers). Thesecond arm of Michelson interferometer is formed bymicro�objective 8 and a reference mirror. A system ofneutral light filters located between the reference mir�ror and micro�objective was used for equalizing theintensity in the arms of interferometer. The probing(“object”) beam reflected from the sample surfaceinterferes with the “reference” beam in the plane ofCCD matrix 10. The thermal radiation of plasma is cutoff by an interference light filter with transmission onthe respective wavelength, which is located before theCCD matrix 10. The use of a 12�bit CCD cameramakes it possible to record interference patterns with ashorter step of intensity quantization than that in thecase of 8�bit video cameras and to increase the sensi�
AN INVESTIGATION OF THE OPTICAL�AND�THERMOPHYSICAL ... CHARACTERISTICS 733
tivity of the experimental optical scheme. The focalplane was determined by the image of target surfacewith the reference arm closed. For precluding thelighting of frame by scattered radiation, the selectedwavelength of probing pulse was different from that ofthe wavelength of heating pulse.
The transfer of image in the Mach–Zehnderscheme from the plane of laser stimulation to the planeof CCD matrix was accomplished using two objectives:the lens objective 8 constructed an intermediate imagewhich was transferred by means of micro�objective 9with magnification to the plane of CCD matrix 10.The focal plane of laser radiation in the flow, whichcorresponded to the plane of location of stimulationspot, was determined using a wire located vertically atthe target surface so that its end fell on the zone ofstimulation (this was monitored by the image from thecamera of interference microscope), after which thealignment of the optical scheme of Mach–Zehnderinterferometer 4 was performed for obtaining a sharpimage of the wire. The use of a mechanized shutter 16made it possible to obtain both interference andabsorption (shadow) photographs of gas�plasma flowwithout changes in the optical scheme and loss of sealof the evacuated target chamber 6. For reducing lossesduring the transfer of UV laser radiation, the crystal 11of generation of third harmonic is located as close aspossible to the irradiated surface directly in the vac�uum chamber.
EXPERIMENTAL PROCEDURE
In performing the interference microscopy, it isnecessary that the surface of ablating target would havea high coefficient of specular reflection on the wave�length of probing radiation. This condition is hard tomeet; therefore, the targets were provided by cuts ofpolymer materials 5–7 μm thick made using a rotation
microtome (Cut 4055, Slee Mainz) and placed on adielectric mirror, the thickness of these cuts beingmonitored by means of a profilometer (170622, ZAOKhK Instrumental’nye Zavody). Delrin® 500 NC�010films of fluoroplastic (C2F4)n and polyformaldehyde(CH2O)n and massive samples of these materials wereused as polymer targets. The spectral reflectance andabsorptance [7] and the surface roughness (for(CH2O)n—Ra 0.13, Rz 0.3; for (C2F4)n—Ra 0.05, Rz0.21) were determined for the employed samples. Aseries of measurements of the parameters of crater andsurface plasma formations for different time delays ofthe probing pulse of radiation relative to the heatingone made it possible to obtain the required data on thedynamics of formation of crater on the target surfaceand on the evolution of surface plasma formation. Forthis purpose, three interference patterns are recordedfor each time delay, namely, (a) the interference pat�tern of unexcited surface or surface region (initial),(b) the interference pattern under the effect of heatingpulse of laser radiation with a delay of the probingpulse relative to the heating one (time), and (c) theinterference pattern of target surface taken several sec�onds after the effect of the heating pulse (final, notrecorded for the flow) (Fig. 3). The intensity of laserradiation was controlled by means of polarizationattenuator and half�wave plate and recorded using aphotomultiplier with subsequent recalculation inaccordance with the calibration curve. The processingof interference patterns was automated using special�purpose software package for obtaining phase andamplitude patterns and subsequent processing of thesedata with a view to determining the crater parametersin the zone of stimulation (depth, diameter, volume),mass flow rate from the target surface, spectral�energythresholds of laser ablation, and the density of elec�trons and neutral particles in the gas�plasma flow. Theprocessing of experimental data was partly automated
(а) (b)
Fig. 3. (a) The interference pattern of surface and (b) the reconstructed profile of the target crater after irradiation a (C2F4)n film
by two pulses of laser radiation (λ = 266 nm) with energy density W = 0.62 J/cm2 (the scale rule corresponds to 20 μm).
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for obtaining the rates of scattering of particles and ofpropagation of shock�wave fronts. Several parametersof ablation crater were used for improving the accu�racy of determining the threshold values of energydensity of laser radiation, namely, the diameters ontwo axes at half�height and the maximal depth of cra�ter. The data on crater diameter were used for obtain�ing the values of spectral�energy thresholds of laserablation by interpolation of experimental dataassuming that we have the following for the Gaussianbeam [8]:
, (1)
where rx is the crater radius on the major axis, r0 is theequivalent radius of focusing spot, E is the energy oflaser pulse, Wa is the threshold value of energy density,and θ is the angle of incidence of laser radiation on thetarget. The values of spectral�energy thresholds oflaser ablation, obtained using the crater diameters onthe major and minor axes, almost coincide; however,they markedly exceed the values determined in thecase of interpolation of data on the maximal depth ofcrater in accordance with the Bouguer–Lambert–Beer law [9, 10],
, (2)
where h is the maximal depth of crater, and leff is theeffective absorption depth. In so doing, it must betaken to account that, in the case of reflection of radi�ation directly from the target surface, the depth of cra�ter is determined by the interference pattern as
, (3)
where Δϕ is the phase shift, and λ is the probing radi�ation wavelength. In studying the crater in a transpar�ent film, one must take into account the double trans�mission of probing radiation through the film and thedifference between the refraction indexes of targetmaterial nt and of ambient medium na on the probingradiation wavelength,
. (4)
This procedure is based on the assumption that thespatial shape of distribution of laser radiation intensityover the beam profile corresponds to the Gaussianone. Therefore, the deviation of the shape of laserpulse from Gaussian leads to increasing instrumentalerror in determining the threshold value of energydensity of laser radiation. At the same time, the depthof crater on irradiated polymer film, determined byinterferometry, may differ from the actual depthbecause of the variation of optical properties (first ofall, refraction index) of the material located betweenthe crater bottom and substrate mirror. We used thedata on the sizes of craters formed after stimulation by
⎛ ⎞= θ⎜ ⎟
π⎝ ⎠
2 20 2
0
ln cosxa
Er rr W
⎛ ⎞= ⎜ ⎟
π⎝ ⎠eff 2
0
lna
Eh lr W
Δϕλ=
π2h
( )
Δϕλ=
π −2 2 t a
hn n
single laser pulses for determining the values of spec�tral�energy thresholds of laser ablation; repeated mea�surements were performed for reducing the effectmade by the probabilistic behavior of optical break�down on the value of ablation threshold being deter�mined. Because of the small depth of etching as aresult of single irradiation of surface, experimentaldata are employed which were obtained underrepeated stimulation, including those obtained withthe pulse repetition rate up to 1 kHz [11]. Such datamay be incorrect (distortion occurs due to accumula�tion effects which manifest themselves both in thevariation of optical�and�thermophysical properties ofrepeatedly irradiated surface as a result of heating andphase and photochemical transformations and in theemergence in the surface zone of the target underhigh�frequency stimulation of regions in which thelaser pulse parameters vary). At the same time, theresults of analysis of the depth and shape of cratersformed as a result of stimulation by a laser pulse packet(up to 105 and more) enable one to record the minimalrates of ablation down to elementary (atomic ormolecular) layers [12]. In such cases, it is more correctto speak of laser desorption rather than of laser abla�tion [13].
Because the profile of laser radiation intensity maybe other than Gaussian in the case of increasing dis�tance from the center of focusing spot, the error ofdetermination of dependences of amplitude and phaseshifts for reflected wave front of probing radiationincreases in the case of small flows; the statistical scat�ter of deviations of intensity profile from Gaussian was~±3%. The absolute error of employed algorithm ofdetermination of phase shift of wave front is Δψ ~±(π/100) [14]; therefore, the error of determination ofcrater depth is Δh ~ ±(λ/200), where λ is the probingradiation wavelength. The scale of transfer of image tothe plane of CCD camera of interference microscopeis ~ ~±0.8 μm/pixel, i.e., the absolute error of determi�nation of crater radius is Δr ~ ±1.2 μm (at r0 ~ 20 μm,the relative error is Δr/r0 ~ ±6%); in view of the error ofenergy calibration of the photomultiplier of control ofheating radiation power ΔE/E ~ 5%, this gives theerror of determination of spectral�energy thresholds oflaser ablation ΔWa/Wa ~ ±17%. Proceeding from thesize of light�erosion crater on the surface being irradi�ated, the absolute error of determination of mass flowrate amounts to Δm ~ ±10–14 kg.
EXPERIMENTAL RESULTS AND DISCUSSION
The developed optical procedures for the investiga�tion of interaction between ultrashort laser pulsesmade it possible to analyze the dynamics of a numberof optical�and�thermophysical and gasdynamicparameters of the processes which accompany theinteraction between laser radiation and condensed
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matter. The values of threshold of laser ablation for(CH2O)n targets were determined using the data oncrater radius (Fig. 4). No ablation is registered for thepower density I ~ 1.2 × 1013 W/cm2 which is limitingfor the given laser setup on wavelength λ = 266 nm; thevalue of coefficient of specular reflection of surface atnormal incidence, which was measured on this wave�length for the investigated (CH2O)n sample, amountsto ~24%, i.e., with due regard for diffuse reflection, a(CH2O)n target absorbs approximately a half of theincident radiation. Figure 5 gives the dependence ofthe rate of laser ablation on wavelength of stimulatinglaser radiation for a (C2F4)n plane polymer target. Thevalues of spectral�energy thresholds of laser ablationobtained using the data on crater depth are 2.5 timeslower than those obtained using the data based on theanalysis of diametric dimensions of crater (Fig. 4 andtable). This difference is especially significant in thecase of stimulation by UV laser radiation (λ =266 nm); this is apparently associated with the varia�tion of optical properties (refraction index) of (C2F4)n
target under the effect of high�energy quanta of coher�ent radiation.
Figure 6 illustrates the dynamics of the shape ofcrater in the mid�section under repeated effect ofultrashort laser pulses on (C2F4)n target (N = 10, f ~0.1 Hz). The asymmetry of the crater profile is associ�ated with the fact that the crater wall facing the inci�dent radiation is steeper (the angle of incidence oflaser radiation on the target surface is ϕ ~ 45°).
Figure 7 gives values of spectral�energy thresholdsof laser ablation of (С2F4)n fluoroplastic, both the val�ues obtained by us and those borrowed for comparisonfrom [11, 15–18]. The differences in the resultant dataare associated with the use of different approaches todetermination of the spectral�energy threshold ofablation, with the scatter of values of parameters ofstimulation, and with the special features of micro�structure of employed targets. For example, Hashidaet al. [16] used a sample of ePTFE as target, i.e., a filmof thickness δ ~ 50 μm with porosity π ~ 60% drawnfrom pressed powder of (С2F4)n and made up by inter�woven fibers 10–100 nm thick; these features of thestructure of employed material may lead to a signifi�cant lowering of the threshold of ablation compared toa massive sample of sintered powder. The value of thethreshold of ablation was obtained by the results ofmeasurement of crater diameter using a scanning elec�tron microscope after 100 pulses of laser radiation witha frequency of 10 Hz. A decrease in the threshold value
8
7
6
5
4
3
2
1
00.1 1 10
W, J/cm2
r2 × 10−3, μm2
266 nm400 nm800 nm
Fig. 4. The square of crater radius as a function of radiation energy density for three wavelengths of probing radiation of 266, 400,and 800 nm on (C2F4)n; lines indicate interpolation by formula (1); open symbols and dotted lines indicate measurementsin vacuum.
The values of spectral�energy thresholds Wa (J/cm2) of fem�tosecond laser ablation of polymer materials
λ, nm(CH2O)n (C2F4)n
atmosphere vacuum atmosphere vacuum
266 >0.8 >0.8 0.25 0.23
400 2.04 1.6 1.35 1.15
800 7.05 6.7 3.34 2.1
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of energy density proportionally to is registeredin the range of laser pulse durations of 130–700 fs [16].Womack et al. [15] investigated the possibility of pre�paring thin films by way of deposition of Teflon® evap�orated by laser radiation (Good Fellow), with theinvestigation performed in an argon atmosphere (p =0.5 mbar); in so doing, the criterion of threshold oflaser ablation is not defined. In [11, 15], use was madeof fluoroplastic targets made using different technolo�gies and different procedures for determination of
τ390.
0.5spectral�energy thresholds; therefore, the results ofcomparative analysis of respective data cannot beregarded as fully reliable. The threshold value ofenergy density of laser radiation Wa ~ 2.1 J/cm2
obtained by us in vacuum is in good agreement withthe data of Womack et al. [15]. The appreciable differ�ence of the value of spectral�energy threshold of abla�tion found by Wang et al. [11] from our data and fromthe data of Womack et al. [15] is apparently associatedwith the conditions of laser stimulation rather than
0.1 1
I0, W/cm2
0
0.4
0.8
1.2
1.6
2.0
Wa
= 0
.1 ±
0.0
2 J/
cm2
12
1 10
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m/p
ulse
0.5
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0
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1013 1014
Wa
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.2 J
/cm
2
h', µ
m/p
ulse
W, J/cm2 W, J/cm2
1012 1013
I0, W/cm2
(b)(а)
Fig. 5. The rate (μm/pulse) of laser ablation of (C2F4)n target in the atmosphere (solid symbols) and in vacuum (hollow symbols)under the effect of femtosecond pulses of laser radiation of wavelength (a) λ1 = 266 nm, (b) λ2 = 400 nm; solid lines indicate inter�polation by formula (2); dashed lines bound the confidence interval of 0.95.
x, μm
−3.2−3.0−2.8
−2.4−2.2−2.0−1.8−1.6−1.4−1.2−1.0−0.8−0.6−0.4−0.2
0
−2.6
1
2
3
4
5
6
7
8
9
10
10 20 30 40 50 60 70
h, μm
−3.4
Fig. 6. The dynamics of crater in (C2F4)n film under repeated effect of UV laser radiation (λ = 266 nm) with energy density W =
0.62 J/cm2 (the numerals by the curves correspond to the ordinal number of laser pulse).
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with procedural features of experiments and processdifferences in the materials of polymer targets. In thecase of stimulation of a (С2F4)n target under atmo�spheric conditions by single pulses of laser radiation,the value of threshold of ablation is higher, and in thecase of stimulation by laser pulses with a frequency of1 kHz, lower [11] than that in the case of stimulationin vacuum [15] where the value of threshold dependsonly slightly on the pulse repetition rate.
Analysis of the effect made by buffer gas (whencomparing experimental data with calculation data forvacuum) performed by Wang et al. [11] reveals that, inthe vicinity of the threshold of laser ablation, thebuffer gas makes no appreciable effect on the dynam�ics and macrostructure of the zone of laser stimula�tion; in so doing, the values of thresholds are taken tobe equal both for atmospheric conditions and for vac�uum, disregarding the pulse repetition rate in the the�oretical model of Kumagai et al. [19]. Wang et al. [11]give the profilogram of crater obtained after the stim�ulation by 100 laser pulses: a narrow deep channel isobserved at the energy density of 5 J/cm2, the emer�gence of which is not registered by the profilometerbut is only observed with the aid of scanning electronmicroscope. The emergence of this channel and thesignificant decrease in the threshold of laser ablationare possibly associated with the emergence and main�tenance of surface plasma (at 1 kHz) until the nextlaser pulse in the air at the target surface, which maylead to self�focusing of radiation while increasing theactual energy density in the zone of stimulation com�pared to the predicted energy density and to additionalevaporation of the target material as a result of absorp�tion of broadband short�wave radiation of plasma bythis material.
The effective coefficient of absorption αeff = 1/leff isdetermined by experimental data using formula (2).Its dependence on the quantum energy of laser radia�tion and on long pulses of laser radiation, which isgiven in Fig. 8, demonstrates that, in the case of stim�ulation (10 ns–10 μs) by short pulses in the NIR�UVrange, the value of αeff is significantly (by 2–2.5 ordersof magnitude) lower than the values obtained by spec�trophotometric methods using radiation of low inten�sity, while in the case of ultrashort laser pulses (τ <1 ps) the value of αeff is five to ten times these values; inso doing, the deviations in both cases are inverselyproportional to the photon energy. The decrease inαeff in the case of duration of laser radiation pulses τ >10⎯9 s is associated with the fact that the plasma for�mation arising in the surface zone of the target beingirradiated absorbs a part of radiation, because thethreshold of plasma formation under nanosecondstimulation is higher than the threshold of evaporation(there is not enough time for the surface plasma toform under stimulation with energy density in thevicinity of evaporation threshold); however, the valuesof αeff obtained by the experimental results in vacuum
turn out to be much closer to those measured spectro�photometrically. Therefore, the results of experimen�tal determination of spectral�energy thresholds oflaser ablation in vacuum demonstrate that the mecha�nism of absorption of radiation under the effect ofnanosecond laser pulses insignificantly varies in thecase of low�intensity radiation. In the presence ofbuffer gas (air), the decrease in the coefficient ofabsorption αeff is caused by the variation of chemicalcomposition and macrostructure of the absorbingmedium rather than by the variation of the mechanismof absorption. The values of αeff obtained under theeffect of ultrashort pulses in atmospheric conditionsand in vacuum exhibit no significant differences fromone another because, with the laser pulse durationbeing shorter than the time of electron relaxation, thebuffer gas does not have enough time for influencingthe mode of delivery of radiation to the target surface.Also indicative of this is the insignificant differencebetween the values of αeff obtained under laser stimu�lation in the single and pulse�periodic modes with afrequency of ~1 kHz in the atmosphere. In the case offemtosecond laser ablation of polymer materials in theinfrared and visible spectral regions, the multiphotonabsorption is most probable; this leads to a significantincrease in the values of αeff compared to the spectro�photometric data; in the UV spectral region, this dif�ference decreases (or disappears), because the quan�tum energy of laser radiation exceeds the energy ofinteratomic and intermolecular bonds of investigatedpolymers, which brings about their photodecomposi�tion (i.e., one and the same mechanism of absorptionprevails in the UV and VUV spectral regions for both
200 250 300 350 400 750 800 850
4.00
2.00
1.00
0.50
0.25
123456
Wа, J/cm2
λ, nm
Fig. 7. The values of thresholds of ablation of (C2F4)n tar�get under the effect of ultrashort laser pulses: (1) thepresent study, (2) τ0.5 = 110 [11], (3) 120 [15], (4) 130–700 [16], (5) 300 [17], (6) 300 fs [18]; the hollow symbolscorrespond to the vacuum conditions.
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LOKTIONOV et al.
ultrashort and longer pulses of laser radiation). Underthe effect of ultrashort laser pulses, the values of effec�tive linear coefficient of absorption of polymers inatmospheric conditions significantly exceed the valuesobtained under the effect of nanosecond laser pulses.This difference decreases proportionally to the wave�length of stimulating laser radiation in vacuum as well;the values of spectral�energy thresholds of femtosec�ond laser ablation of polymers decrease proportionallyto the wavelength and significantly decrease in the UV
spectral region; in the case of stimulation under vac�uum conditions, the values of thresholds are lowerthan those under normal pressure, with the effect ofbuffer gas pressure being more significant in the IRregion than in the UV region.
The distribution of electron concentration in a sur�face plasma formation under laser stimulation of(CH2O)n target is given in Fig. 9; according to thesedata, the velocity of motion of vapor front 75 ns after
105
104
103
102
101
100
αeff, cm–1
1
3
45
2
0.125 0.25 0.5 1 2 4 8hν, eV
Fig. 8. Linear coefficients of absorption of (C2F4)n target: (1, 5) femtosecond ablation, (5) [11, 18]; (2) micro�nanosecond abla�tion [20–27]; (3) spectrophotometrically determined [7] spectral dependence of the coefficient of absorption; (4) IR spectropho�tometry [28]; hollow symbols correspond to measurements in vacuum.
600450
700 800 900 1000 1100 1200 350
500
550
600
650
500
600
700
800
900
(a) (b)
μm
μm μm
0
400 450 500 550
15.7516.0016.2516.5016.7517.0017.2517.50
logn [сm–3]
0
logn [сm–3]
16.616.817.017.217.417.617.818.0
μm
Fig. 9. The distribution of electron concentration in a surface plasma formation 75 ns after stimulation by laser radiation (λ =400 nm) with energy density W ~ 13 J/cm2 of (CH2O)n target in (a) atmospheric and (b) vacuum conditions.
HIGH TEMPERATURE Vol. 48 No. 5 2010
AN INVESTIGATION OF THE OPTICAL�AND�THERMOPHYSICAL ... CHARACTERISTICS 739
stimulation is v ~ 2 km/s in vacuum and v ~ 1.35 km/sin atmospheric conditions; in so doing, the axial com�ponent of the velocity of propagation of shock wave isvz ~ 3.1 km/s, and the radial component—vr ~2.3 km/s.
The effectiveness of laser ablation may be estimatedproceeding from the dependence of mass flow rate onthe energy of stimulating pulse. Figure 10 gives thisdependence for a (C2F4)n target: the maximal value ofspecific mass flow rate in this case is attained in theenergy density range W ~ 0.6–0.9 J/cm2, which corre�sponds to a three� or fourfold excess over the spectral�energy threshold of ablation, and amounts to m' ~120 μg/J.
CONCLUSIONS
The realization of combined (in two planes) preci�sion laser pulse microinterferometry of the surface ofcondensed target and of surface plasma formationwith high space (~10–6 m) and time (~10–13 s) resolu�tion enables one to determine the dynamics of theoptical�and�thermophysical and gasdynamic charac�teristics of laser ablation of condensed media. Deter�mined for the first time for a number of polymer mate�rials ((C2F4)n and (CH2O)n) are the values of spectral�energy thresholds, rates and specific mass flow rate ofablation, and of the effective coefficient of absorptionunder single stimulation by femtosecond pulses ofUV–NIR radiation in both atmospheric and vacuumconditions. A significant increase is demonstrated inthe effective linear coefficient of absorption of radia�tion by the target material under the effect of
ultrashort pulses compared to longer pulses of laserradiation.
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2. Ovsianikov, A., Passinger, S., Houbertz, R., et al.,Three Dimensional Material Processing with Femto�second Lasers, in Laser Ablation and Its Applications,Berlin: Springer, 2007, p. 121.
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10. Srinivasan, R. and Braren, B., Chem. Rev., 1989,vol. 89, no. 6, p. 1303.
u°
12
140
120
100
80
60
40
20
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4W, J/cm2
m', μg/J
Fig. 10. The specific mass flow rate as a function of energy density of laser radiation (λ = 266 nm) under conditions of stimulationof (C2F4)n target: (1) in atmospheric conditions, (2) in vacuum.
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11. Wang, Z.B., Hong, M.H., Lu, Y.F., et al., J. Appl. Phys.,2003, vol. 93, no. 10, p. 6375.
12. Hashida, M., Fujita, M., Tsukamoto, M., et al., Fem�tosecond Laser Ablation of Metals: Precise Measure�ment and Analytical Model for Crater Profiles, in 3rdInt. Symp. on Laser Precision Microfabrication, Osaka:SPIE, 2003, p. 452.
13. Laser Ablation and Desorption. Experimental Methods inthe Physical Sciences, Haglund, R.F. and Miller, J.C.,Eds., London: Academic, 1998.
