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Olaleye, T.M.; Raposo, M.; Ribeiro, P.A. Orbital Angular Momentum Light from Azopolymer Thin Films. Encyclopedia. Available online: https://encyclopedia.pub/entry/52369 (accessed on 18 May 2024).
Olaleye TM, Raposo M, Ribeiro PA. Orbital Angular Momentum Light from Azopolymer Thin Films. Encyclopedia. Available at: https://encyclopedia.pub/entry/52369. Accessed May 18, 2024.
Olaleye, Temitope M., Maria Raposo, Paulo A. Ribeiro. "Orbital Angular Momentum Light from Azopolymer Thin Films" Encyclopedia, https://encyclopedia.pub/entry/52369 (accessed May 18, 2024).
Olaleye, T.M., Raposo, M., & Ribeiro, P.A. (2023, December 05). Orbital Angular Momentum Light from Azopolymer Thin Films. In Encyclopedia. https://encyclopedia.pub/entry/52369
Olaleye, Temitope M., et al. "Orbital Angular Momentum Light from Azopolymer Thin Films." Encyclopedia. Web. 05 December, 2023.
Orbital Angular Momentum Light from Azopolymer Thin Films
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This entry is adapted from the peer-reviewed paper 10.3390/photonics10121319

Orbital angular momentum (OAM) encoding is a promising technique to boost data transmission capacity in optical communications. Azobenzene films have gained attention as a versatile tool for creating and altering OAM-carrying beams. Unique features of azobenzene films make it possible to control molecular alignment through light-induced isomerization about the azo bond. This feature enables the fabrication of diffractive optical devices such as spiral phase plates and holograms by accurately imprinting a phase profile on the incident light. By forming azobenzene sheets into diffractive optical elements, such as spiral phase plates, one can selectively create OAM-carrying beams. Due to the helical wavefront and phase variation shown by these beams, multiple distinct channels can be encoded within a single optical beam. This can significantly increase the data transmission capacity of optical communication systems with this OAM multiplexing technique. Additionally, holographic optical components made from azobenzene films can be used to build and reconstruct intricate wavefronts. It is possible to create OAM-based holograms by imprinting holographic designs on azobenzene films, which makes it simpler to control and shape optical beams for specific communication requirements. In addition, azobenzene-based materials can then be suitable for integration into optical communication devices because of their reconfigurability, compactness, and infrastructure compatibility, which are the main future perspectives for achieving OAM-based technologies for the next generation, among other factors.

orbital angular momentum azobenzene photoisomerization optical communication

1. Introduction

In the modern digital landscape, the effective functioning of the internet and the operation of data centers heavily rely on the high-speed transmission of data across long distances through optical fiber networks. This is a result of the revolutionary advancement that the field of optical communication has undergone over the course of several decades. Photonics, encompassing a vast spectrum of applications, has played a pivotal role in not only transforming optical communication but also reshaping various research domains. The core concept of photonics revolves around the generation, manipulation, and detection of light beams in numerous ways [1]. These beams have been enabling innovations in optical communications, imaging, sensing, and beyond. As the demand for higher data transmission capacity keeps increasing exponentially and as the capacity crunch draws nearer [2], the quest to explore more innovative technologies and materials has become more important for researchers in this field.
One of the researched technologies is in Orbital Angular Momentum (OAM) based communication systems has remained in the limelight since its groundbreaking discovery by Allen et al. in 1992 [3] following the initial theoretical research in 1991 [4]. Ongoing research has consistently demonstrated that harnessing the OAM of light presents a promising solution to significantly increasing the data transmission capacity of communication channels [5]. By exploiting unique phase profiles and orthogonal states inherent to OAM beams [6], these systems can not only enhance channel capacity but also effectively mitigate the channel capacity limitations of conventional systems reliant on the polarization of light [7][8]. Beams possess a spatial property characterized by a helical phase front. This has shown a lot of transformative capabilities in photonics, particularly because of its potential to enhance data transmission capacity by adding another degree of freedom for transmitting information in communication channels [9].
In addition to OAM systems, polarization-sensitive materials such as azobenzene [10] have emerged as a compelling candidate for research and investigation in the photonics field [11]. With the remarkable capacity of azobenzene for light-induced isomerization [12][13], the molecules of azobenzene films can dynamically change their orientation, resulting in the creation and modification of structured light such as OAM beams. Structured laser beams are laser beams that have been specifically manipulated to have predefined spatial intensity or phase distributions [14]. The photoisomerization capabilities of azobenzene, combined with the precision to imprint phase profiles onto incident light, enable azobenzene molecules to undergo reversible structural transformations as light passes through them [15]. With the use of optical devices such as spiral phase plates and holograms [16], azobenzene serves as a versatile tool, opening new methods for enhancing the fabrication of photonics devices [17][18][19], encoding information and information security [20] and transmitting data in optical communication systems.

2. Background, Generation, and Application of OAM Light

Electromagnetic waves are characterized by linear momentum and two distinct forms of angular momentum: spin angular momentum which is associated with circular polarization and the helicity of individual photons. It can take on values of ±1, corresponding to left- and right-handed circular polarization, and 0 for linear polarization.
On the other hand, OAM is associated with the spatial mode of a light beam with optical vortices [21][22], which are characterized by a twisted wavefront and a complex field amplitude [23]. The OAM of light is a new optical degree of freedom that arises from the spatial distribution of the wavefront and describes the rotation of the wave around its propagation axis with a measure of intensity distribution and phase information [24]. OAM beams are characterized by a helical wavefront, which imparts a rotational motion to the beam around its propagation axis. Each OAM beam is quantized, meaning the beam carries a specific value of OAM denoted by the topological charge (TC) ±ℓ. The ±ℓ represents the TC or the number of helical wavefronts the beam possesses in the clockwise (−) or anticlockwise (+) direction. Each photon in an OAM-carrying beam possesses an OAM of ℓħ, where ħ is the reduced Planck constant. The OAM value determines the number of helical wavefronts present in the beam. Unlike linear momentum or spin angular momentum, which are associated with polarization, OAM is a more intrinsic property of the optical field [25].
The study of OAM has primarily focused on Laguerre−Gaussian (LG) beams [26][27], which have well-defined values of OAM [28]. LG beams possess an azimuthal phase dependence of exp(iℓφ), where ℓ also known as the TC is the beam’s azimuthal mode number. It has a doughnut-shaped intensity profile determined by the beam’s radial mode number (p). Owing to their helical wavefront, LG beams carry a quantized orbital angular momentum (OAM) of ℓħ (ħ is the reduced Planck’s constant) per photon, where the amount of OAM is dependent on ℓ [29]. These beams exhibit vortex-like structures. They have been extensively researched for their fundamental properties, methods of production, measurement, and applications in this reference [30].
OAM has been recognized as potentially useful for a vast and diverse range of applications, such as microparticle manipulation [31][32], trapped particle rotation [33], encoding of information [34], transfer of OAM to atoms [35] [36]. Most especially, it has found utility in optical communications, where OAM multiplexing enables increased data transmission capacity [21].

3. Azobenzene Materials

Azobenzene is an organic compound with the chemical formula C6H5N=NC6H5, and it consists of two benzene rings bound by a nitrogen−nitrogen (-N=N-) double bond, which is known as the azo group [37]. Azobenzene groups present two distinct structural forms: the trans and the cis form, as shown in Figure 1. These forms are distinguished in the spatial arrangement of their atoms; specifically, the orientation of the nitrogen−nitrogen (N=N) double bond rings on either side of the molecule are aligned in a straight line, which gives the molecule a longer and extended shape. However, in the cis form, the N=N double bond is bent, bringing the two benzene rings closer together and giving the molecule a more compact, bent structure.
Figure 1. Schema of the trans and cis isomeric forms of azobenzene molecules or chemical groups.
A notable feature of azobenzene is its ability to undergo reversible isomerization, meaning it can switch between the trans and cis forms when exposed to light, particularly ultraviolet (UV) or visible light [38][39]. This is known as the photoisomerization process, during which the molecule can change between its trans and cis configurations upon exposure to specific light wavelengths [40]. While the trans form is thermodynamically preferred due to its stability, exposure to light or heat causes it to convert to the cis form, resulting in changes in its optical properties. The photochemical isomerization of azobenzene between its trans and cis forms was first discovered in 1937 [41]. The photoisomerization process of the azobenzene group leads to the change of the spatial geometric arrangement, through the conversion of the isomer from trans to cis (trans → cis), induced by light absorption, or from cis to trans (cis → trans), induced by the action of light or heat. This process is associated with a n-π* transition of low absorption intensity in the visible region together with a higher intensity transition in the ultraviolet region.
In 1984, Todorov and his collaborators [42] described for the first time the formation of photoinduced birefringence by linearly polarized light in polyvinyl alcohol (PVA) mixed with the polar orange methyl chromophore. Results demonstrated that the photoinduced photoisomerization process gives rise to alignment of dipolar chromophores in the direction perpendicular to the polarization of the light electric field which consequently creates photoinduced birefringence in the medium. This process is a statistical approximation, since a chromophore preferentially absorbs light polarized along the axis of its dipole. The major axis of the molecule, with the probability of absorbing photons, is proportional to cos2θ, where θ is the angle between the direction of the electric field of light and the molecular dipole moment [43], as demonstrated in Figure 2a. Thus, the chromophores oriented in the direction of polarization of light absorb the light with a greater probability, unlike those which are not oriented perpendicularly and are not able to absorb this light and experience isomerization (see Figure 2b).
Figure 2. (a) Schematic of the orientation of an azobenzene molecule relative to the electric field of light and its dipole moment 𝜇→; (b) schematization of the orientation of chromophores by the incidence of linearly polarized light: 𝐸→ represents the electric field vector, and 𝑘→ represents the wave vector. The region where the light falls tends to have chromophores oriented in the direction perpendicular to that of the light electric field.
If azobenzene is incorporated into a polymer chain, the photoisomerization reaction will occur in each azo group inserted in this chain [44]. This is a reversible reaction that does not involve the formation of secondary products [45], a so-called clean photoreaction. Photoisomerization was observed in solutions, in liquid crystals, in sol-gel systems, and in thin films of molecules with azobenzene groups or in mixtures of azobenzene with other molecules. Therefore, the azo group facilitates reversible photoisomerization, wherein the molecule can seamlessly transit between its trans and cis configurations upon exposure to specific light wavelengths. This inherent reversibility is fundamental for manipulating azobenzene materials in the context of structured light generation. The photoisomerization process in azobenzene involves the absorption of light energy, prompting the molecule to shift from its trans to cis configuration. This process exhibits high efficiency, with azobenzene displaying a notable quantum yield for photoisomerization. The cis configuration can then be converted back to the trans form, through either thermal or photochemical means, completing the reversible cycle. Azobenzene’s capacity for light-driven control makes it an ideal candidate for crafting structured light.
In parallel to the photoinduced birefringence phenomenon observed in materials that contain azobenzenes, Natansohn and Rochon [46] in collaboration with Tripathy [47] found that when linearly polarized light is incident on the medium in the form of an interference pattern, not only does photoisomerization take place, but also changes in the medium volume are observed, which are translated by the formation of a relief grating. The inscription of the grids occurs by impinging two light beams in a given area on the surface of the film, so that an interference pattern is formed. Modulation amplitudes can be on the order of 100 to 1000 nm, and the grating period can be adjusted depending on the incident interference [48]. The creation of optical relief gratings involves the net transport of mass, a mobility that is only possible thanks to the trans → cis → trans photoisomerization processes of azobenzene chromophores.
Azobenzene-based molecules enable precise control in optics, photonics, and nanotechnology by creating light-responsive materials and switches. This property allows the controlled manipulation of molecular structure and properties of azobenzene molecules upon response to light exposure. They vary widely in profuse chemical structures and properties with a wide range of applications in the textile as dyes, chemicals, and materials. These derivatives are used to create functional materials and molecular switches that respond to light, enabling precise control over various systems and properties. This inherent light-induced structural change through either thermal or photochemical means is the fundamental for manipulating azobenzene materials in the context of structured light generation.

3.1. Azobenzene Thin Films

Due to the importance of optical properties that materials containing azobenzene can offer, it is essential to develop and optimize molecular structures of this type. The basic idea is to incorporate azobenzene into a host matrix to create a structure that maintains its photoisomerization capabilities. Several techniques such as casting, spin coating, the Langmuir−Blodgett (LB) technique, and the layer-by-layer (LbL) technique have been used to produce ultrathin films of azobenzene molecules. Here, one should highlight the LbL technique, which was developed in the 1990s by Decher et al. [49][50] based on adsorption at a solid−liquid interface. In this process, a monolayer of a positively charged polymer is initially obtained by dipping a hydrophilized substrate in a positively charged polyelectrolyte solution. Subsequently, the monolayer is washed to remove polyelectrolyte molecules that are not completely adsorbed. The solid support with a positively charged monolayer is immersed in a negatively charged polyelectrolyte solution to be adsorbed on a layer of negative polyelectrolyte. Following this step, a new wash is performed to remove unadsorbed molecules. This iterative procedure results in the formation of a bilayer composed of two oppositely charged polyelectrolytes. Repetition of this procedure leads to the gradual buildup of multilayers, ultimately resulting in the formation of a self-assembled film [51].
Compared to other known techniques, such as the Langmuir−Blodgett technique [48][52] or spin coating [53][54][55], the LbL technique has demonstrated to be an effective method for obtaining thin films. The advantage of this LbL technique compared to those mentioned lies in the fact that it is a simple, economical method compatible with large-scale production. It should be noted that the LbL technique also allows thickness control, which depends on fundamental factors that condition adsorption at a solid/liquid interface, such as ionic strength, concentration, pH, and temperature. The LbL technique can also be used on any type of substrate, regardless of its size or shape, and allows the use of water as a solvent, thus having potential to cause no harm to the environment. Initially, it was used only to produce and study oppositely charged polyelectrolyte structures, but quickly extended to functional molecules such as azopolymers [56]. Oliveira and collaborators carried out a review of the work already published on LbL azobenzene films and concluded that these azobenzenes are more difficult to photoisomerize [57]. However, in more recent studies, it was demonstrated with the use of higher temperatures, the orientation can be well achieved [58].

4. Azobenzene for OAM Generation and Manipulation

Azobenzene materials possess unique properties that render them ideal for creating and controlling structured light, particularly photons with OAM. In several fields such as optical communications, quantum computing, and nanophotonics, OAM manipulation is essential. The reversible photoisomerization of azobenzene aids the manipulation of molecular structure and facilitates the encoding of information within light’s OAM. In addition, it allows quick modification of OAM states. This offers more flexibility in design options for photonic devices as well as increasing the response time for transmitting and processing high-speed information. The broad spectral range is another advantage of azobenzene isomerization, because it operates over a wide range of wavelengths such as in the UV, visible, and near-infrared ranges [59]. This adaptability aids the manipulation of OAM and makes it suitable to implement with many optical systems. High-quality azopolymer surface patterns are easily controllable and can be imprinted, erased and reconfigured as needed. These patterns remain stable for several years under normal storage conditions. Azopolymers have significant potential for creating various photonic elements, including diffraction gratings [60], photonic crystals [61], nanostructured polarizers [62], plasmonic nanostructures [63][64], data storage units [65][66], and optical metasurfaces [67][68].
Scalability has been one of the major drawbacks of commercializing the use of OAM beams. However, by incorporating azobenzene into patterned structures, the creation of large-scale OAM modulation devices may be possible for high-speed OAM multiplexing in optical communication systems and OAM-based quantum information processing. Another advantage that is notable about azobenzene for OAM manipulation is the compatibility with existing technologies and materials commonly used in optics and photonics, and it can be incorporated into various host material structures, such as polymers [69] or liquid crystals [70][71], to form thin films or bulk materials. It also enables the creation of OAM-generating structures such as spatial light modulator (SLM) to modulate light’s amplitude, phase, polarization, direction, and intensity [72][73]. To make an SLM with azobenzene polymers, a biphotonic holographic grating is used [74][75]. This grating is formed by the chromatic interference of light beams with different colors and polarizations [76], resulting in the creation of a physical grating structure/pattern with alternating regions of high and low intensity on the polymer surface. The photosensitive azobenzene polymer undergoes a change in its molecular structure in response to the light interference pattern where azobenzene molecules switch between different isomeric forms. This grating is then used as a spatial light modulator to diffract and control another light beam to change its phase and intensity [77]. The biphotonic holographic method in an azobenzene film and its reversible photoisomerization property is used for the storage of image/information [78]. In addition, liquid crystal systems that incorporates azobenzene enable the manipulation of refractive indices/birefringence and facilitate the development of optical elements that shape the OAM of light [79][80].
“Structured light” refers to light that can be controlled spatially in terms of its amplitude, phase, and polarization [81]. These parameters, together with the properties of an azobenzene film, must be manipulated to match certain criteria [14] for generation of structured light such as OAM beams:
i.
The azobenzene-enabled amplitude control plays a pivotal role in encoding information onto light by precisely modulating its intensity or brightness at different spatial points. This can be used to create patterns, enhance contrast, improve resolution and encode information. It is especially useful in microscopy and other optical applications. However, historically, lenses, prisms, apertures, and mirrors were the main static optical devices used in light manipulation, since accurate control over optical fields frequently required more complicated modifications [82]. One may accurately control the amplitude of a light beam as a first step toward better control, an idea that was crucial in the creation of holography. Amplitude masks were used in holography to simulate a “writing” laser beam which carries the information that is being encoded onto a holographic plate. Although clearly beneficial, this method is only able to use specified beam patterns [83].
ii.
Azobenzene materials also facilitate phase modulation by altering the timing or phases of different parts of a light wave. This process is integral to enabling beam shaping, producing structured wavefronts, and navigating light in desired directions in applications such as interference patterns, holography [84], and wavefront shaping within optical communication systems [85].
iii.
Polarization control alters the orientation of the electric field vector of light. It is used in applications such as LCDs, 3D cinema for 3D effects, and optical communications for transmission of information. Azobenzene plays a valuable role in enabling the manipulation of light’s polarization state for improving data-carrying capacity using polarization-based multiplexing and demultiplexing techniques in optical communication systems.
The formation of structures in azopolymer thin films strongly depends on the polarization state of the illuminating laser beam [86], even though unpolarized light could also separate chiral molecules [87]. Hence, understanding the polarization of light is crucial for manipulating and utilizing light waves. Polarization describes the orientation of the oscillation of light waves. In a process known as photoalignment [88], as polarized light oscillates, the molecules of azobenzene respond to the light exposure by aligning their orientation with it [1][89].

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Subjects: Instruments & Instrumentation
Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Temitope M. Olaleye
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Maria Raposo
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Paulo A. Ribeiro
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Revisions: 2 times (View History)
Update Date: 06 Dec 2023
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