Femtosecond optical vortex-induced flower-shaped surface relief structures in an azo-polymer film

Azo-polymers exhibit a unique photoinduced mass transport mechanism facilitated by a trans–cis photoisomerization reaction, which can be used for the creation of reversible surface relief structures (relief structures can be erased simply by the uniform illumination of visible light).^1,2 Such structures have significant potential as rewritable optical integrated circuits and rewritable optical data storage devices.^3–5 The formation of surface relief structures in azo-polymers using laser beams occurs in the following stages: Azo-polymers are first transformed from their stable trans-form into their unstable cis-form at room temperature under illumination by visible light.^6–8 This trans–cis/cis–trans photoisomerization cycle (cycle time is typically 1–2 ps) induces a spring-like motion of the azo polymers, thus resulting in spatially anisotropic photo-fluidity. Further illumination gradually increases the population of cis-phase azo-polymers and results in softening of the azo-polymer film. The azo-polymers in the bright region (region illuminated by light) then move along the polarization direction toward the dark region (region non-illuminated by light) of the irradiating light via photon radiation force, referred to herein as photoinduced mass transport, following the polarization direction of the irradiating light via photon radiation force (photoinduced mass transport); this results in the formation of the surface relief structure.^9,10 The resulting structure, therefore, typically reflects the spatial intensity distribution and polarization of the irradiating beam.^11,12

Laguerre–Gaussian (LG) modes are a common example of optical vortices, which carry a ring-shaped spatial intensity profile (an annular spatial form with a dark core) and orbital angular momentum (OAM), which can be characterized by the topological charge, ℓ, which is associated with its helical wavefront and on-axis phase singularity.¹³ Circularly polarized light also carries spin angular momentum (SAM), s, which is originated from its helical electric field. Consequently, circularly polarized optical vortices possess a total angular momentum (J = ℓ + s), which is given by the sum of its orbital and spin angular momentum. These unique features enable optical vortices to be applied to a variety of applications, such as optical trapping and manipulation,¹⁴ ultrahigh density telecommunication,¹⁵ high-dimensional teleportation,¹⁶ super-resolution fluorescence microscopy,^17,18 and nano-/microfabrication of chiral materials.^19,20

To date, a number of researchers have reported the formation of chiral structures via the irradiation of materials (including azo-polymers) with optical vortex laser beams, wherein OAM twists the irradiated materials.^21–23 Such chiral structures formed in azo-polymers may be potentially applied to the development of ultrahigh density optical data storage technologies. We should note that prior studies on surface relief formation in azo-polymers have been based on single-photon-absorption (SPA) processes.^24,25 Optical vortex-induced surface relief formation in azo-polymers via two-photon-absorption (TPA), in contrast, is the subject of this body of work. TPA process, in which the trans–cis photoisomerization occurs only at the focal point of the laser beam, enables three-dimensional microfabrication with a high spatial resolution,^26,27 and it is thus an important stepping-stone in the development of advanced high-density rewritable data storage with a high spatial resolution.

In this work, we report the formation of exotic TPA-induced flower-shaped surface relief structures, which exhibit petals along the azimuthal direction in azo-polymers, via irradiation by a near-infrared femtosecond optical vortex laser beam with a rather shorter pulse duration than the trans–cis/cis–trans photoisomerization cycle time. The work presented here also highlights new nonlinear phenomena, including mode instability and SAM–OAM coupling.

The azo-polymer used in our experiments, Poly-Orange Tom-1 (POT) (monomer weight: 484, molecular weight: ∼190 000 g/mol) [Fig. 1(a)], has strong absorption in the wavelength range 300–550 nm [Fig. 1(b)], and it is thus a candidate for two-photon isomerization by irradiation using near-infrared ultrafast laser pulses. It was spin coated onto a glass slide to form a film with a nominal thickness of 1 μm.

A near-infrared femtosecond laser with a wavelength of 800 nm, a pulse width of 51 fs, and a pulse repetition frequency of 80 MHz was used as the laser source in our experiments. Its output was converted into a circularly polarized optical vortex beam with ℓ = 1 or 2 and s = ±1 by using a spiral phase plate and a quarter wave plate. The generated optical vortex was focused onto the surface of the azo-polymer film using a high numerical aperture objective lens (NA = 0.9) [Fig. 1(c)]; the focused annular spot had a diameter of ∼4 µm. The maximum average power, peak power, and estimated intensity of the focused optical vortex beam were ∼40 mW, ∼9.8 kW, and ∼78 GW/cm², respectively, and an exposure time of 200 s was used throughout the experiments. The high density azo-polymer film will exhibit rather high TPA absorption, thus enabling the shortening of the exposure time for flower-shaped relief formation.

Images of the surface relief structures fabricated in azo-polymer films were observed by using an atomic force microscope (SPM-9700, Shimadzu). Images of the surface relief structures formed when using a circularly polarized optical vortex with J = 2 (ℓ = 1, s = 1) with a range of irradiation intensities are shown in Fig. 2. Under low intensity illumination (∼14 GW/cm²), the optical vortex twisted and directed azo-polymers from the bright illumination region to the dark core of the vortex beam, forming a slightly twisted surface relief structure with a single-arm, which is reflective of the annular spatial form and OAM of the irradiating vortex beam. Interestingly, when the intensity of the irradiating vortex beam exceeded 20 GW/cm², the surface relief structures transformed into flower-shaped structures with well-defined petals. For example, surface relief structures with four petals along the azimuthal direction were generated in the azo-polymer when irradiating with an intensity of 37 GW/cm². Such flower-shaped surface reliefs typically had a diameter of ∼3.6 µm and a height of ∼420 nm. In addition, we observed that azimuthally or radially polarized beams with an annular spatial form and without OAM only produced simple ring or dip-shaped surface reliefs without any petals, even under high illumination intensities [Figs. 2(b) and 2(c)]. Furthermore, illumination of the azo-polymer with higher-order optical vortices and higher intensities further reinforced the spatial mode instability, thus resulting in the formation of flower-shaped surface reliefs with 5–8 petals, as shown in Fig. 3.

These flower-shaped surface relief structures have not been reported in prior publications when using SPA. In fact, the illumination of high power continuous-wave or pulsed green laser will induce easily the melting, bleaching, and ablating of the azo-polymers with strong absorption for visible light, owing to the extremely high temperature rise beyond glass temperature.²⁸ 

Intriguingly, it was discovered that negative SAM also influenced the formation of flower-shaped surface reliefs. When illuminating using an optical vortex with J = 0 (ℓ = 1, s = −1), the azo-polymer was observed to aggregate at the central dark core of the illuminating beam, producing a dip-shaped surface relief structure with a height of 1.05 µm, and was devoid of any petals. Such dip-shaped relief structures were also generated when illuminating with an optical vortex with J = 1 (ℓ = 2, s = −1) [Fig. 4(a)]. These observations demonstrate the existence of longitudinal gradient forces, which are induced via destructive SAM–OAM coupling under tight focusing conditions.²⁹ 

As mentioned in a previous publication,²⁶ the photoinduced mass transport of azo-polymers always occurs along the polarization direction of the irradiation light by trans–cis–trans photoisomerization via both single and two photon absorption processes. In fact, linearly polarized optical vortex beams were found to generate inclined, rod-shaped surface relief structures, owing to azimuthal mass transport of the azo-polymers around the dark core of the beam, and along the polarization direction [Fig. 4(b)]; this is consistent with our prior publications.

As mentioned above, surface reliefs were fabricated by vortex and vector beams with an annular spatial intensity form and with different polarization states, such as linear, circular, azimuthal, and radial polarizations. Only circularly polarized vortex beam with non-zero total angular momentum enabled the production of flower-shaped surface reliefs. These results manifest that the flower-shaped surface relief formation will be understood by considering the spatial mode instability of optical vortices with OAM (that is collapse of optical vortices) associated with third-order nonlinear effects in azo-polymers.^30,31

When μ is an imaginary number, the azimuthal modulation term determined by M (which depends upon the topological charge and the intensity I of the incident optical vortex) is amplified, thereby resulting in the formation of flower-shaped surface relief structures with 2M petals via the collapse of the optical vortex.

It is therefore concluded that the formation of the flower-shaped surface relief structures is due to the instability of the vortex beams associated with third-order nonlinear effects in the azo-polymer. In addition, the asymmetric optical radiation force of linearly polarized optical vortices break the symmetry and uniformity of the azo-polymer surface, thereby preventing the collapse of optical vortices associated with third-order nonlinear effects.

We have observed the formation of exotic flower-shaped surface relief structures in an azo-polymer film via two photon absorption, induced using femtosecond pulsed, optical vortex laser beams. The shape of the surface relief structures is a function of the azimuthal mode instability of the illuminating vortex beam, and this instability is associated with third-order nonlinear effects in the azo-polymer film. We also observe that destructive SAM–OAM coupling effects manifest under strong focusing conditions and can negatively impact the formation of these flower-shaped surface relief structures.

This phenomenon, in which the flower-shaped surface relief structures are formed, offers new insights into light–matter interactions. We anticipate that this fabrication technique and the generated surface relief structures may benefit the development of advanced optical data storage technologies, wherein the topological charge of the illuminating vortex beam may offer an additional degree of freedom and increased data density.

Takashige Omatsu would like to acknowledge the support of this research by the JSPS KAKENHI Grants-in-Aid (Grant Nos. JP22H05131, JP22H05138, JP22K18981, and JP23H00270) and by the JST Core Research for Evolutional Science and Technology program (Grant No. JPMJCR1903). We thank Prof. Hiromi Okamoto, Institute for Molecular Science, Japan, for fruitful discussions.

The authors have no conflicts to disclose.

Kana Ishihara: Data curation (lead); Formal analysis (lead); Visualization (lead); Writing – original draft (lead). Takashige Omatsu: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Project administration (equal); Resources (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.