Periodic Nanostructures with Interfering Femtosecond Lasers

Encyclopedia of Nanoscience and Nanotechnology

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Periodic Nanostructures with Interfering Femtosecond Lasers Masahiro Hirano, Ken-ichi Kawamura, Hayato Kamioka, Taisuke Miura Japan Science and Technology Agency, Kawasaki, Japan

Hideo Hosono Tokyo Institute of Technology, Nagatsuda, Japan

CONTENTS 1. Introduction 2. Progress in Encoding Technology 3. Features of Encoding 4. Encoding Mechanism 5. Encoding System 6. Encoded Hologram Glossary References

1. INTRODUCTION Introduction of femtosecond (fs) laser pulse, which is characterized as an ultrashort time domain with good coherence over the whole pulse duration, significantly influences trends in various laser application fields. Specifically, when the fs pulse is applied to material processing, it provides additional advantages to conventional laser processing, including the capability of processing in transparent materials, nanosized patterning, and clean processing nearly free from thermal effects. Those features originate from the inherent nature of the pulse. First, fs laser pulses have extremely short time durations, leading to very high peak powers due mostly to the temporal compression of the laser energy. Such high peak powers can process almost all kinds of materials, typically by laser ablation [1–3]. When the energy is a little smaller than the ablation threshold, it can still induce structural changes, which are inevitably accompanied by refractive index modulation [4–8]. As the laser energy can be absorbed efficiently through a multiphoton ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

process in such a high-power pulse, materials to be processed are not necessarily opaque against the laser wavelength. In other words, the fs laser processing is particularly suited to transparent materials, which are difficult to process by conventional lasers such as CO2 and YAG:Nd3+ lasers. Second, with the aid of nonlinear effects such as multiphoton absorption and selffocusing, we can concentrate the laser energy onto a very small spot. In addition, materials suffered ablation or structural changes only in a limited area where the accumulated energy exceeded a certain threshold value. Nanoscale structures may be formed in this way. Finally, the pulse terminates before the completion of energy transfer from photoexcited electrons or electron-hole plasma to the lattice, which minimizes thermal effects during the machining process and makes the process clean [1, 2, 6–9], specifically when only a single pulse is involved. By using these features, a large number of excellent works have been performed recently in fabricating various kinds of fine-scale structures on the surface and inside of transparent dielectrics, semiconductors, polymers, and metals. Further, if we combine another distinct feature of the fs pulse, which is good coherence [10], with the above mentioned features, we can encode holograms, having a very fine internal structure in various materials, by splitting a single pulse into multiple beams and allowing interference. In this chapter, we will focus on an emerging technology of the formation of periodic microstructures with interfering fs laser pulses.

2. PROGRESS IN ENCODING TECHNOLOGY Phillips et al. [11] at Rice University, performed an initial work, where they used a fs KrF eximer laser to record holographic gratings in polyimide with the intention to realize Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (457–468)

458 fine patterning resulting from the thermal diffusion-free process. Also, an observation of a periodical fringe in diamond has been reported, which was accidentally encoded, presumably due to interference of the incident fs laser beam with a reflected beam [12]. In spite of these reports, material processing with the interfering fs pulses had not attracted much attention before Hosono’s group succeeded in encoding holographic gratings in versatile materials, including transparent dielectrics, semiconductors, polymers, and metals with an interfering Ti:Sapphire laser pulse [13–18]. Since then, several groups have reported the formation of the periodic structures in polymers and glasses with the interfering fs pulse. Misawa’s group, at Tokushima University, recorded three-dimensional structures in photoresist material with the interference of five beams split from a single fs pulse with the intention to form a photonic crystal [19]. Hirao’s group at Kyoto University [20], and Ito’s group at Osaka University [21], have succeeded in encoding embedded gratings in polymers and glass materials. The number of research groups involved in this technique is increasing, with an expectation that the interfering fs pulses will open a new frontier in laser processing.

3. FEATURES OF ENCODING Features of the encoding technique that uses interference fs laser pulses are summarized. As is fs laser processing, almost all kinds of materials can be processed with the interfering fs pulses, and the structures are formed in a designated position in materials. Especially, embedded structures can be formed in transparent materials. The process can be completed in a single shot, thus, throughput of the process is potentially very high. Overall size of gratings is as small as ∼100
m, where the fringe spacing is in the range of 0.1– 5
m, depending on the colliding angle of the beams and laser wavelength. As the spacing is proportional to the laser wavelength for a fixed crossing angle of the beams, use of fs pulse with a shorter wavelength is effective in narrowing the fringe spacing. On the other hand, as is common to the fs laser processing, minimum structure sizes for line width or dot diameter in the periodic structures can be reduced to the nanometer scale, which seems to be almost insensitive to the irradiated laser wavelength [22]. An additional novel feature of the technique is that two- and three-dimensional periodic structures can be fabricated by a double exposure and multiple-beam interfering methods.

4. ENCODING MECHANISM As fs laser pulses exhibit good coherency over the entire pulse duration, interference takes place when multiple fs pulse beams split from a single pulse and overlap with each other both spatially and temporally. The interference pattern resulting from this overlapping can immediately lead to the modulation of the electron-hole plasma density in the irradiated materials through multiphoton absorption or avalanche ionization processes, where the plasma is excited more densely in an enhanced region of the interference than in a depressed region. The density modulation of the plasma causes the refractive index modulation. That is, a transient

Periodic Nanostructures with Interfering Femtosecond Lasers

grating is immediately formed in the material just after the irradiation of the interfering fs pulse. The transient grating, which likely lasts for several picoseconds (ps), may induce reconstructed beams that could interfere with the incident beams to form additional gratings. The plasma energy in the material is likely relaxed to the lattice within ∼1 ps to accumulate locally in the lattice. With the relaxation proceeding, the transient grating is converted into a permanent grating when the local energy in the lattice exceeds a threshold for laser ablation or structural changes of the material. That is, the permanent gratings can be formed when the laser energy in the enhanced region of the interference is over a threshold value.

5. ENCODING SYSTEM 5.1. General Feature A hologram encoding system by fs laser pulses does not differ significantly from conventional laser hologram exposure systems. A major difference between the two systems is that an fs laser pulse is used instead of a continuous wave light, typically from a gas laser having a long coherent length. A fs KrF excimer laser (248 nm, ∼400 fs) [11], Ti:sapphire laser (800 nm, ∼100 fs) [13–18, 20–25], second harmonics of Ti:sapphire (380 nm, ∼80 fs) [19], and third harmonics of Ti:sapphire (290 nm, ∼100 fs) [26,27] have been used so far as a fs pulse source. Because of a very short coherent length of the fs pulse, which is restricted by the spatial extent of the pulse, an optical delay line and a mechanism for detecting the time coincidence of the two beams, with a spatial accuracy of ∼
m, is, in general, required to ensure a spatial and temporal overlap. It also is desirable for the timecoincidence measurement to be independent of the angle between interference beams. By varying this angle, we can control over-fringe intervals in gratings and over shapes of unit elements in two-dimensional periodic structures. The sum of the frequency generation of the pulses due to optical nonlinear crystals (colliding angle dependent) [20] or third harmonic generation in air (colliding angle independent) [23] is effectively used for most of the systems, but alternative time-coincidence detecting system, without using the up-frequency conversion process, is necessary when a shorter wavelength fs pulse is used. A distributed feedback dye laser technique was used for the fs KrF laser [11], and for the pump and prove techniques, based on the optical Kerr effect and transient absorption, have been developed for the THG of the Ti:sapphire laser [26, 27]. On the other hand, use of a diffractive beam splitter in a multiple beam interfering system allows for the temporal coincidence without installing any adjusting mechanism [19]. When comparing the encoding systems developed so far, optical configurations are basically the same, and differences exist in the types of fs lasers and the detecting mechanisms for the temporal coincidence of the split beams, which are shown in Table 1.

5.2. Experimental Setup Figure 1 shows, as a representative example, a schematic diagram of an experimental setup for grating encoding that uses near infrared (IR) fs laser pulses developed by

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Periodic Nanostructures with Interfering Femtosecond Lasers

Table 1. Type of lasers and detection mechanisms for temporal coincidence used in encoding systems developed so far, together with encoded structures and materials in each encoding system. Type of laser

Detection for temporal coincidence

Encoded structure

Materials

Ref.

I

KrF excimer 274 nm ∼400 fs

Distributed feedback dye laser

Surface grating

Polyimide

[11]

II-1

Ti: sapphire 800 nm ∼100 fs

THG in air

Surface grating Embedded grating Two-dimensional periodic structure

SiO2 glass, SiO2 thin film LiNbO3 , ZnO, ZrO2 , MgO, TiO2 , diamond, carbon, SiC, ZnSe, Si, WC, Pt, Au

[13, 14, 15, 16, 17, 18, 22, 23]

II-2

Ti: sapphire 800 nm 100 fs∼5 ps (chirped)

THG in air before chirping

Embedded grating Embedded two-dimensional periodic structure

SiO2 glass

[24, 25]

III

Ti: sapphire 380 nm (SHG) ∼80 fs

none

Multidimensional periodic structures

Negative photoresist (SU-8)

[19]

IV

Ti: sapphire 290 nm (SHG) ∼100 fs

Transient optical absorption and optical Kerr gate

Surface grating

SiO2 glass ZnO

[26, 27]

V

Ti: sapphire 800 nm ∼150 fs

SFC in nonlinear optical crystal

Embedded grating

Azodye-doped PMMA

[20]

VI

Ti: sapphire 800 nm ∼130 fs

Transilluminated optical microscope

Embedded grating

Soda-lime glass

[21]

Kawamura et al. [13]. A Ti:sapphire laser system, consisting of a Ti:sapphire oscillator, a stretcher, a regenerative amplifier pumped by the second harmonic of an Nd:YAG laser, and a compressor, is used as a light source. It generated 800-nm light pulses with a repetition rate of 10 Hz, a pulse duration of ∼100 fs (an effective coherence length of ∼30
m), and a maximum energy of 3 mJ/pulse. A fs laser pulse from the laser system was divided into two beams by a half-silvered mirror. The separated beams propagated along different optical paths, with one having a variable path length (optical delay line). The two beams were focused onto a single spot of 50–100
m in diameter by lenses with a focal length of 5–10 cm. The angle of intersection () between these two beams was varied from 10 to 160 . The spatial

length of each optical path was equalized with a precise movement of mirrors in the optical delay line, while monitoring the time coincidence signal. Third harmonic generation (THG) from air was used for the signal [23], as will be described. Once the time coincidence of the two beams was realized, samples were placed at the beam-focus position. Encoding of the gratings was confirmed in-situ by detecting diffracted light from an incident He–Ne laser. Figure 2 (a)

(b)

sample optical delay

ccd (c)

θ lens beam1

stage f: 100 mm

beam2

half mirror

Pulse compressor prism grating 100fs 0.1~5 ps

from regenerative amplifier Figure 1. Experimental setup for hologram encoding system using IR fs laser pulses. Reprinted with permission from [24], M. Hirano et al., Proc. SPIE 506, 89 (2003). © 2003, SPIE.

Figure 2. Images showing that spatial and temporal coincidence of two fs laser beams are required for encoding gratings. (a) No gratings are recorded when two fs beams are not in spatial coincidence. (b) No gratings are recoded when two fs beams are in coincidence spatially, but not temporally. (c) Gratings are encoded when two fs beams are in coincidence both spatially and temporally. Reprinted with permission from [18], M. Hirano et al., Appl. Surf. Sci. 197–198, 688 (2002). © 2002, Elsevier Science.

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Periodic Nanostructures with Interfering Femtosecond Lasers

shows that both spatial and temporal coincidences were simultaneously required for encoding gratings. As will be described, chirping of the fs pulse, which results in the stretching of the pulse width from ∼100 fs to ∼10 ps, keeping the total pulse energy unchanged, is very effective in encoding embedded gratings due mostly to the reduction of the surface damage [24, 25]. In using the stretched pulse for the encoding, the fs pulse is chirped by controlling the degree of the compression in the compressor, while keeping other optical configurations unchanged.

5.3. Detecting Mechanism Representative examples of the detecting mechanism for the temporal coincidence of the beams are introduced in this session.

5.3.1. Third Harmonic Generation in Air It has been reported that THG is induced when an intense fs laser pulse is focused in various gases such as air [28] or argon [29]; the mechanism has been discussed theoretically [30]. This phenomenon has been applied to monitoring the time coincidence of the two fs pulses [23]. When pulse energy of the first fs laser beam exceeds 1.5 mJ/pulse, a blue spot is observed at the beam center of the far field pattern, which is confirmed as the third harmonic of the laser fundamental (800 nm) at 266 nm. The THG was not observed when the pulse was focused in a vacuum, indicating that air was responsible for the THG. Then, a second focused beam, having an energy of 50
J/pulse, which was too small to induce the THG light by itself, was directed at the focal point so as to overlap spatially with the first beam. The far field pattern of the second beam on the luminescence screen yielded only weak white continuum light without the blue spot, likely due to the self-phase modulation in air when the relative time delay of the two pulses was large. While tuning the optical path length, the blue THG spot suddenly appeared at the center of the white continuum spot. Figure 3 shows the THG intensity in the

second beam as a function of the delay time calculated from the change in the optical path length. The intensity increases abruptly as the delay time approaches zero from below. The rise time for the THG enhancement estimated from the inset figure was ∼200 fs, which is in reasonable agreement with a pulse width of ∼100 fs. With a further increase in the delay time, the intensity, at first, decreases sharply. The decrease then becomes gradual and is still observed at delay times above 50 ps. Despite the gradual decay on the long delay time side, the peak in Figure 3 is sharp enough to determine the exact time coincidence of the two fs pulses. The trend in Figure 3 was independent of the angle of intersection of the two beams, as expected from the inherent nature of THG in the gas phase. The use of THG in air makes it possible to arrange the time coincidence of two fs pulses over a wide range of angles of intersection.

5.3.2. Pump and Prove Technique The techniques using up-frequency conversion processes are not applicable for monitoring the time coincidence of ultraviolet fs pulses, because the resultant sum frequency light in the process has a too short wavelength to propagate in nonlinear crystals or even in air. To overcome this problem, alternative methods using a distributed feedback dye laser technique and two kinds of pump-probe techniques based on optical Kerr and transient absorption effects have been developed. In the pump and prove techniques, the two beams to be interfered are used, respectively, as pump and probe pulses under the low intensity condition below the threshold for the ablation [26, 27]. In the optical Kerr gating method, the pump pulse is polarized perpendicular to the probe pulse by using a /2 plate, which is inserted in the pump beam path before a quartz plate (Fig. 4). On the other hand, the probe pulse goes into another /2 plate after passing through the quartz plate and then into polarized beam splitter (PBS) placed at the front of a photodiode detector (PD) equipped with a phase-sensitive detection system.

–10

Chopper

THG Intensity

THG Intensity (arb. unit)

From laser source

Rail

α –0.4

–0.2

0.0

0.2

0.4

0.6

0.8

1.0

Delay Time (ps)

0

10

20

30

40

BS

PBS

Sample λ /2

λ /2

PD

50

Delay Time (ps) Figure 3. Third harmonic generation light intensity as a function of delay time between two fs laser pulses. Inset: expanded curve around zero delay time. Reprinted with permission from [18], M. Hirano et al. Appl. Surf. Sci. 197–198, 688 (2002). © 2002, Elsevier Science.

Delay Figure 4. Experimental setup for hologram encoding system by using interfering UV fs pulse beams. Reprinted with permission from [26], H. Kamioka et al., Proc. SPIE 4760, 994 (2002). © 2002, SPIE.

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Periodic Nanostructures with Interfering Femtosecond Lasers

a–SiO2

a–siO2

30 deg. 60 90

Intensity (arbitrary units)

Silica has third-order susceptibility, with a very fast time response, and thus the pump pulse induces a birefringence through the electronic Kerr effect, also known as the nonlinear refractive index change. When the /2 plates are rotated by 45 degree with each other, the probe pulse can go through PBS only when the silica plate is irradiated simultaneously with the pump pulse (Fig. 5). That is, this geometry yields an autocorrelation measurement of the pulse. Although the observed pulse width stretches from that of the fundamental wavelength (870 nm) and the exact reason for that has not been clarified, the time zero is determined at the center position of the band. In the transient absorption method, the probe pulse is directly collected at the PD without both the /2 plates and the PBS device. The absorbance change of the probe pulse perturbed by the pump-pulse irradiation is detected by the same procedure as the Kerr measurement. Because the laser energy (4.3 eV) is below the bandgap energy of silica, the absorption occurs through two photon processes. The electron hole plasma is momentary created and decays to a self-trapped exciton or transient defect centers with a fast relaxation time. Figure 6 shows the experimental result of the transient absorption measurement. The pulse width (FWHM) is obtained as 300 fs, which is a little larger compared to that obtained by the optical Kerr measurement. However, it is accurate enough to determine the time zero between the two pulses from the absorption band profile. Spectra for different colliding angles of two pulses also were demonstrated in Figure 6, indicating that the peak width has hardly changed with the angle, although dip structures become prominent in a longer delay time region with increasing the angle. These results indicate that the transient absorption measurement is a useful method to obtain the time zero, not restricted by the colliding angle between two pulses.

–1.0

–0.5

0.0

0.5

1.0

Delay time (ps) Figure 6. Optical transient transmission intensity as a function of delay time between pump and prove beams for several crossing angles. Solid line shows Gaussian fitting curve. Reprinted with permission from [26], H. Kamioka et al., Proc. SPIE 4760, 994 (2002). © 2002, SPIE.

5.3.3. Distributed Feedback Dye Laser Technique When the beams overlap with each other spatially and temporally in a dye cell placed at the focal point of colliding fs KrF laser beams, a grating is induced in the cell, which in turn acts as a Bragg mirror, thereby forming a distributed feedback dye laser [11]. In this way, monitoring the output of the dye cell while tuning an optical delay line makes it possible to achieve the temporal coincidence of the two KrF laser beams.

6. ENCODED HOLOGRAM Kerr signal (arb. units)

Several types of holographic structures have been encoded by the above mentioned exposure techniques in various materials, including dielectrics (SiO2 glass, SiO2 thin films, LiNbO3 , ZnO, ZrO2 , MgO, TiO2 , diamond, carbon), semiconductors (SiC, ZnSe, Si), metals (WC, Pt, Au, Al) and polymers (polyimide, PMMA, negative photoresist SU-8). Table 1 also summarizes types of periodic structures and encoded materials. Representative examples among them are shown in detail in this section.

6.1. Surface Relief Hologram –1.0

–0.5

0.0

0.5

1.0

Delay time (ps) Figure 5. Optical Kerr gate signal in silica plate as a function of delay time between pump and prove pulses. The crossing angle between the two beams is 30 . Reprinted with permission from [26], H. Kamioka et al., Proc. SPIE 4760, 994 (2002). © 2002, SPIE.

6.1.1. Silica Glass by IR fs Laser When the intersecting beams were focused onto the surface, surface-reliefs type gratings were encoded because of material ablation. Figure 7 shows surface-relief gratings encoded in silica glass for several  values, with a total fluence of 0.3 mJ/pulse. Each grating forms a circle with a diameter of ∼100
m and

462

Periodic Nanostructures with Interfering Femtosecond Lasers θ=20°

θ=90°

θ=160°

1 µm 15nm 2.6 µm

0.58 µm

0.43 µm

Figure 7. Optical microscope images of gratings recorded in silica glasses for various beam intersecting angles (). Fringe interval d varies with  according to the equation d = /2 sin/2 . Reprinted with permission from [18], M. Hirano et al., Appl. Surf. Sci. 197–198, 688 (2002). © 2002, Elsevier Science.

2 6

Depth (nm)

is composed of parallel fringes with a constant spacing equal to /[2 sin/2], where  is the laser wavelength (800 nm). This observation constitutes clear evidence that the gratings were encoded as the result of interference between the two fs pulses of the fundamental wavelength (800 nm). The formation of a periodic valley structure at the surface is revealed by (AFM) images. The deposition of debris or molten materials onto the surface indicates that the grating structure results from laser ablation at this fluence level. Infrared spectra suggested O–Si–O bond angles in the laser-irradiated area suffered photo-induced changes, leading to the densification of the silica glass. Such structural changes have been observed in silica glasses during highenergy ion implantation or neutron bombardment; densification typically saturates at ∼3% [31]. As this compaction is accompanied by a refraction index increase of ∼0 7%, a refraction index modulation type grating can be simultaneously formed beneath the surface relief type grating. High-resolution images of the grating acquired with a field emission scanning electron microscope (SEM) (Fig. 8) show periodic valleys in the internal structure of the grating. These valleys are very narrow, down to 15 nm. The compaction of silica glass in very localized areas due to intense laser irradiation generates tensile forces directed parallel to the surface, which likely play an important role in the formation of such narrow valleys.

4

[µm]

0

[µm]

600 200 0 0

4

2

6

Distance (µm) Figure 8. SEM image (upper) and AFM profile (lower) of a grating encoded in silica glass. Reprinted with permission from [18], M. Hirano et al., Appl. Surf. Sci. 197–198, 688 (2002). © 2002, Elsevier Science.

smaller spacing between grooves is expected when increasing the colliding angle of the two pulses. The SEM picture of the grating is shown in Figure 10 for the colliding angle of 60 . The spacing agrees with the predicted value of 290 nm, which will be further narrowed down to ∼150 nm with increasing of the angle.

560 nm

6.1.2. Silica Glass by Ultraviolet fs Laser When the interference UV fs pulse (290 nm), with a total energy of 20
J, was irradiated at bulk silica glass, the micrograting structure was encoded at the surface through an ablation, as shown in Figure 9. A colliding angle was set to be 30 . The fringe spacing is in good agreement with the calculated value of 560 nm for  = 290 nm. The net fluence of the two laser pulses at the sample surface was 0.25 J/cm2 , which is similar to that of the 800 nm fs laser that encoded the surface grating on silicon surface. The slight dependence of the threshold power on the laser wavelength implies that the multiphoton absorption is not a major process, but the avalanche ionization is responsible for the excitation of electron-hole pairs in silica glass. The

2

0

10 µm

Figure 9. Optical microscope image of a grating on silica surface encoded by UV fs pulse (290 nm). The colliding angle is 30 . Reprinted with permission from [26], H. Kamioka et al., Proc. SPIE 4760, 994 (2002). © 2002, SPIE.

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Periodic Nanostructures with Interfering Femtosecond Lasers

lies 0.45–1.05
m beneath the surface. Raman spectra for the grating encoded area suggest that the refractive index modulation was caused by a structural change from diamond to diamond-like carbon or amorphous carbon [33]. Similar embedded gratings in a multicomponent glass plate [21] and

[nm]

1.50 µm

0

200 0

Surface-relief-type gratings with a spacing of 167 nm were produced at polyimide surface by a single shot of a 400 fs KrF excimer laser with an average energy of 83 mJ/cm2 . The colliding angle was 45 degree.

5

[µm]

HEIGHT ( nm )

6.1.3. Polyimide by fs KrF Laser

5

0

Figure 10. SEM image of a grating on silica surface encoded by UV fs pulse (290 nm). The colliding angle is 60 . Reprinted with permission from [26], H. Kamioka et al., Proc. SPIE 4760, 994 (2002). © 2002, SPIE. 200

0 0

5

Surface-relief-type gratings with a valley depth as shallow as 3.5 nm were recorded in thin-film silica on a silicon wafer by reducing the total fluence to 0.015 mJ. The surface profile of the fabricated grating was smooth and neither small deposits nor macroscopic laser damage or cracking was observed at this fluence, suggesting that the formed grating resulted from the densification of the films, not from laser ablation. The valley depth of the grating (3.5 nm) relative to the film thickness (114 nm) supported this suggestion, as it coincides with the saturation level (∼3%) of radiation-induced densification in amorphous SiO2 . A difference in the properties of undensified and densified amorphous SiO2 is manifested in the etching rates during stress-enhanced corrosion [32]. An AFM image and a cross section of grating structures formed by etching with 1% HF solution (Fig. 11) show that etching increases the depth of the valley by a factor of 5–6 relative to that before etching. In other words, chemical etching converted the refractive index modulation-type grating into a surface-relief-type grating.

When the focal point of the interfering fs laser pulses is positioned inside the target materials, an embedded grating is recorded through a refractive index modulation, either due to structural alternations such as densification in silica glass, crystallographic phase changes from crystalline to amorphous states, or the formation of micropores due to the evaporation of a small amount of the materials. Such an embedded grating was recorded inside a diamond crystal [14]. Confocal microscopic images reveal the grating

1%-HF solution

[nm]

0

200 0

0

5 5

10

[µm]

[µm]

10

200 HEIGHT (nm)

6.2.1. Full Compressed Pulse

10

DISTANCE (µm)

6.1.4. Silica Thin Film

6.2. Embedded Hologram

10

10

[µm]

0 0

5 DISTANCE (µm)

10

Figure 11. AFM images of surface-relief-type gratings encoded in silica thin films before and after etching by 1% HF solution. The valley depth was deepened due to preferential etching in the densified areas. Reprinted with permission from [18], M. Hirano et al., Appl. Surf. Sci. 197–198, 688 (2002). © 2002, Elsevier Science.

464

Periodic Nanostructures with Interfering Femtosecond Lasers

polymer [20] have been reported by using fully compressed fs laser pulse.

(a)

(b)

6.2.2. Chirped Pulse In spite of the successes in encoding the embedded gratings by fully compressed fs pulse (∼100 fs), it is still difficult to encode embedded gratings deep inside versatile materials. The major reason for such a shallow processing depth is likely attributed to the fact that absorption coefficient is larger at the surface than the inside of materials, thereby, most of the pulse energy being lost around surface and the penetrating pulse is not intense enough for the encoding. The deterioration of the coherency of the pulse due to the interaction between the intense pulse and materials during the propagation may be an additional cause for the difficulty in encoding the gating inside of materials. One possible approach to overcome the difficulty is to use focal lenses with a large numerical aperture, which makes it possible to reduce the laser-power density at the surface much lower than that at the focal area inside the material, thereby keeping accumulated energy at the surface below the threshold, while enhancing the energy at the focal point above the threshold. This approach is successfully used to encode embedded gratings for specific materials such as diamond [14], multicomponent glass [21], and polymers [20], as mentioned before. But, this approach alone unlikely provides enough controllability and flexibility to the encoding process of the embedded gratings. An essential solution to overcome the difficulty is to reduce the peak energy of the pulse keeping the total energy of the pulse unchanged by expanding the pulse width. A chirped pulse, which is a partially compressed fs pulse, may meet the requirements. Before encoding embedded gratings with the chirped pulses, surface-relief-type gratings were encoded to confirm their effectiveness [24, 25]. The diameters of the surface-relief-type gratings at the surface slightly decrease with an increase in the pulse width from 100 to 5000 fs, each pulse having a constant total energy. As the energy density at the periphery of the recorded area corresponds to the threshold for the encoding, this observation implies that the threshold is more dominantly governed by the total energy than by the peak energy of the pulse in this pulse width range. Figure 12 shows (FE)–SEM images of a cross section of the surface-relief-type gratings recorded by 100 fs and 400 fs pulses with the total energy of 50
J/pulse. The 100 fs pulse encodes the grating with very shallow grooves of ∼1
m located at the surface. On the other hand, the groove depth becomes deeper for the 400 fs pulse irradiation, indicating the pulse energy penetrates deeper with an increase in the pulse width or a decrease in the peak power. The shallow processing depth by 100 fs pulse may be explained as follows. When the 100 fs pulse with an energy of ∼5 TW/cm3 is irradiated at the silica glass, it generates the electron-hole pairs with a density of ∼1021 /cm3 around the surface within the pulse duration via multiphoton absorption and avalanche ionization processes as predicated by Stuart et al. [34]. The resultant dense plasma strongly absorbs IR light through one photon process, thereby dramatically enhancing the plasma density and thus preventing the penetration of the pulse into

1.50 µm

3.00 µm

Figure 12. FE-SEM images of cross section of the surface-relief-type gratings recorded by 100 fs (a) and 400 fs (a) pulses in silica glass. Reprinted with permission from [24], M. Hirano et al., Proc. SPIE 506, 89 (2003). © 2003, SPIE.

the silica more than ∼1
m. On the other hand, the front part of the stretched pulse may penetrate into materials before the electron-hole pair density at the surface becomes greater than ∼1021 /cm3 . These observations encourage the use of a stretched pulse for the formation of embedded gratings. Then, the cross area of the two stretched pulse beams was shifted to the inside of the material to encode the embedded grating, as demonstrated in Figure 13, where the beam colliding angle of 45 , cross area of 30
m below the surface, laser pulse width of 500 fs, and total pulse energy of 50
J/pulse were used. No gratings are observed in the crossing area in the as-cut sample, but faint gratings are seen in the beam entrance areas on the surface. After the chemical etching, an embedded grating appears at the crossing area due to the stress-enhanced etching, as described previously. An expanded SEM image for the cross area shown in Figure 14, clearly demonstrates that the grating is encoded inside the silica glass at a depth of ∼30
m. Periodic line grooves with a constant spacing (d) of ∼1
m is visible in the grating. The spacing agrees with that given by the equation of d = /[2 sin/2], where  is the wavelength of the laser (800 nm), and  is the colliding angle between two incident pulses (45 ). Those gratings are observed directly without the chemical etching by a confocal microscope as shown in Figure 15b. The faint gratings in the beam entrance

E RFAC

CE SU

AN ENTR

30 µm

30 µm Figure 13. FE-SEM image of the propagation of the two stretched pulses. Reprinted with permission from [24], M. Hirano et al., Proc. SPIE 506, 89 (2003). © 2003, SPIE.

465

Periodic Nanostructures with Interfering Femtosecond Lasers 1st

2nd

3rd

4th layer

Top surface

200 µm

Top surface Figure 16. Cross-sectional SEM images for multiple gratings located vertically inside silica glass. Reprinted with permission from [25], K. Kawamura et al., Appl. Phys. Lett. 81, 1137 (2002). © 2002, American Institute of Physics.

30.0 µm

Figure 14. Expanded FE-SEM image of the crossed area of the two stretched pulse beams. Reprinted with permission from [25], K. Kawamura et al., Appl. Phys. Lett. 81, 1137 (2002). © 2002, American Institute of Physics.

areas on the surface, whose line spacing is ∼1
m, equaling to that of the embedded grating, are most likely created as results of the interference between an incident beam and a conjugated beam reconstructed from the transient grating, formed by the periodic density distribution of the laser-induced excited state or by electron-hole plasma. Figure 16 shows a cross-sectional FE–SEM image of multiple gratings vertically located inside the silica glass. Each grating was recorded by a single shot. These gratings, which were encoded in four layers separated by 300
m, show long tails downward to the beam propagation direction. When an array of gratings is encoded in each layer, it can be visualized by illuminating white light, as shown in Figure 15a.

0 20

5 mm

40

(µm) 0

(a)

40

20

60

(b)

Figure 15. Embedded gratings in silica glass. (a) Array of embedded gratings in two layers are visualized by illuminating white light. (b) Confocal microscope image of embedded grating. Two surface gratings at the beam entrances also are observed. Reprinted with permission from [25], K. Kawamura et al., Appl. Phys. Lett. 81, 1137 (2002). © 2002, American Institute of Physics.

No noticeable difference is observed between the gratings encoded by down- and up-chirped pulses, suggesting the nonlinear optical interaction, which may degrade the coherence of the pulse, does not play a significant role, at least in the stretched pulse. These results clearly indicate that use of chirped pulses instead of a fully compressed fs pulse, which results in stretching of the pulse width from ∼100 to ∼5000 fs, provides controllability and flexibility to the encoding system, particularly giving rise to the capability in encoding embedded gratings in versatile materials. The optimized pulse width, which may be of material dependent, is ∼500 fs for silica glass.

6.3. Multidimensional Periodic Structure 6.3.1. Double Exposure Technique Two-dimensional periodic structures can be formed by a double-exposure technique [22]. A series of optical microscope images of the gratings in Figure 17 demonstrate the double-exposure procedure. Photos (a) and (b) in this figure show gratings with diameters of ∼15
m and ∼50
m encoded by laser pulse energies of 40
J and 100
J, respectively. After the first grating was encoding, the samples were rotated by 90 and a second grating was recorded superposed on the first grating. The resultant structure is not always a simple superposition of two kinds of gratings, but a complicated structure results from interactions between the two encoding processes. Figure 18 is an SEM image of a crossed grating encoded when energy in the second pulse (80 mJ) was larger than in the first one (40 mJ) with a 45 angle between the beams for both exposures. The periodic vertical lines represent fringes of the grating encoded by the first exposure, since the periodicity of 1.0
m equals /2 sin/2 . It is noted that an array of round dots with a diameter of ∼140 nm is observed in the center part of the crossed grating. The dots become ellipsoidal as one moves toward the edge of the grating and connect with each other to form periodic horizontal lines. Since the first grating was not encoded in the outer area and the spacing is consistent with the second exposure, the horizontal lines here are fringes of the second grating. The formation of the dot array in the central part results from the interaction between the first grating and the incident beams in the second exposure.

466

Periodic Nanostructures with Interfering Femtosecond Lasers

When the first pulse energy (80
J/pulse) is larger than that of the second pulse (40
J/pulse), a resultant structure is composed of periodic narrow valleys recorded by the first exposure and a two-dimensional dot array with exfoliations and cracks around the dot (Fig. 19). Diameters of the dots are ∼100 nm and each dot seems to have a smaller dot (a)

40 µJ

6 µm

Figure 18. Scanning electron microscope image of a crossed grating encoded at a total fluence of 40
J/pulse for the first exposure and 80
J/pulse for the second exposure. The beams intersected at an angle of 45 for both exposures. Reprinted with permission from [18], M. Hirano et al., Appl. Surf. Sci. 197–198, 688 (2002). © 2002, Elsevier Science.

(b)

100 µJ

1.5 µm

(c) 40 µJ+100 µJ

Figure 19. Scanning electron microscope image of a crossed grating encoded at a total fluence of 80
J/pulse for the first exposure and 40
J/pulse for the second exposure. The beams intersected at an angle of 45 for both exposures. Reprinted with permission from [18], M. Hirano et al., Appl. Surf. Sci. 197–198, 688 (2002). © 2002, Elsevier Science.

25 µm

Figure 17. Procedure for double exposure. (a) Optical microscopic image of a grating encoded at a total laser fluence of 40
J/pulse (first grating). (b) Optical microscopic image of a grating encoded at a total laser fluence of 80
J/pulse after the sample was rotated by 90 (second grating). (c) Optical microscopic image of a crossed grating, encoded by superposing the second grating onto the first grating. Reprinted with permission from [18], M. Hirano et al., Appl. Surf. Sci. 197–198, 688 (2002). © 2002, Elsevier Science.

750 nm Figure 20. Scanning electron microscope image of a crossed grating encoded at a total fluence of 100
J/pulse and the beams intersected at right angle for each exposure. Reprinted with permission from [18], M. Hirano et al., Appl. Surf. Sci. 197–198, 688 (2002). © 2002, Elsevier Science.

467

Periodic Nanostructures with Interfering Femtosecond Lasers

These results indicate that the double-exposure technique of using fs laser pulses offers a new tool for the formation of two-dimensional nanostructures in various kinds of materials.

UNDER

6.3.2. Multibeam Interference Technique

0. DEV

250 ms

Screen

Laser beam

6.4. Application to Optical Device

UNDER

0. DEV

Results of the double-exposure process suggest that multibeam interference provides a novel technique in encoding multidimensional periodic structures. Especially, when we use the interference among five beams, we can fabricate three-dimensional periodic structures in transparent materials. Such three-dimensional structures have been encoded in polymers by using a Q-switched YAG:Nd3+ laser [35]. Three-dimensional periodic structures were encoded in negative photoresist SU-8 with interfering five beams split from a SHG of Ti:sapphire laser pulse [19], where the pulse was split into five beams by a diffractive beam splitter, making it possible to achieve the temporal coincidence without the detecting mechanism.

250 ms

Because sizes of the recorded gratings (∼50
m) are slightly smaller than that of optical devices, gratings could be used as building blocks of the optical devices. For example, two gratings capable of coupling and decoupling of light to waveguide were encoded on a waveguide fabricated on a LiNbO3 substrate. Figure 21 demonstrates that He–Ne laser light is coupled and decoupled to the waveguide through these gratings.

GLOSSARY Screen

Laser beam Figure 21. Photographs of two gratings encoded on an optical waveguide fabricated in LiNbO3 . Light coupling and decoupling functions of the gratings are demonstrated. Reprinted with permission from [18], M. Hirano et al., Appl. Surf. Sci. 197–198, 688 (2002). © 2002, Elsevier Science.

inside, whose diameter becomes smaller with a decrease in the pulse energy. The dot array results from the interaction between the periodic valleys and the incident beams in the second exposure, most likely due to the interference among two incident beams and reconstructed beams from the first grating. If this were the case, encoded structures would be equivalent with those by the interference among four beams divided from a pulse. By increasing  from 45 to 90 , an array of trapezoid structures could be produced; this array looks like a simple superposition of two orthogonal gratings (Fig. 20).

Chirped femto second laser pulse Not fully compressed pulse with a pulse width of 100∼3,000 fs from regenerated amplified fs pulse laser system. Double exposure technique An interference fs pulse are irradiated exactly on as encoded grating which was rotated by 90 degree. Embedded grating Refractive index type grating encoded inside transparent materials. Interference fs laser pulse When two fs laser pulses split from a single pulse collided with spatial and temporal coincidences, interference of fs laser pulse occurs due to good coherency of the pulse. Multi-dimensional periodic structure Two or three dimensional periodic structures are encoded by the interference among four of five fs laser pulses. Two dimensional structures are also fabricated by a double exposure technique. Structural alternation Structural changes induced by the irradiation of fs laser pulse including densification of silica glass, crystallographic phase change from crystal to amorphous states in diamond, which accompany with the refractive index modulation. Surface grating Surface relief type grating encoded by material ablation.

468

Periodic Nanostructures with Interfering Femtosecond Lasers

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