BUYER'S GUIDE

SPATIAL LIGHT MODULATORS

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Aurélie JULLIEN Institut de Physique de Nice, Valbonne, France - [email protected]

Spatial Light Modulators (SLMs) are quasi- planar devices, allowing for the modulation of the amplitude, phase and polarization, or a combination of these parameters of an incident light beam according to the two spatial dimensions of the modulator. SLMs are employed in many different fields and are the subject of continuous technological development.

https://doi.org/10.1051/photon/202010159

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

patial light modulation is change of the optical path, the in- signal: electrical or optical. a well-established optical tensity, phase or polarization of an Although spatial light modulation technology with a wide incident light beam. They are usually has been made possible through range of applications. organized into categories according a plethora of technologies, among Spatial light modulators to (i) their use in reflection or trans- them mechanically or thermally de- S(SLMs) are two-dimensional objects, mission, (ii) the modulated optical formable mirrors, digital micro-mir- enabling to modulate, at any point parameter(s): amplitude, phase, pola- ror device (DMD), magneto-optic of the SLM surface, through a local rization, and (iii) the type of the driving devices or acoustic-optic Bragg cells,

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the denomination most often refers to Figure 1: Electro-optical properties of molecules that tend to line up in the non-mechanical components which nematic layers enable to direction where the strain energy exploit the electro-optical anisotropy of locally change the phase of the propagating is minimal. For LCs with a positive readout light. The application of the electric liquid crystals (LCs). Thereafter, we will field induces an average molecular rotation, dielectric anisotropy, the minimum focus onto the physical and technical which in return changes the refractive index, energy is reached when the mole- characteristics of the LC-based SLMs according to the input light polarization. cules are aligned in the direction of (LC-SLMs) [1]. Typical planar anchoring conditions can be the electric field. The strength of the vertical (VAN) or horizontal (PAN) or both, e.g. electric field modulates the average twisted (TN). Such a simple device allows for PRINCIPLE AND APPLICATIONS molecular orientation. Therefore, the modulation of the phase, amplitude or Principle. Liquid crystals are organic polarization of light according to the design the optical refractive index of the me- materials whose physico-chemical details and the presence or absence of dium is electrically-controlled and the properties are intermediate between additional polarizing elements. phase of a propagating light is modi- those of solids and liquids. The elon- fied accordingly. gated LC molecules therefore have Spatial control of the applied elec- both a structural order and anisotro- tric field, on one or two dimensions, → py specific to crystals such as optical, direction, defined by a vector n, the offers the ability to spatially modulate dielectric or even elastic anisotropy. so-called director axis. They fea- the phase of an incident optical wave. The optical anisotropy is generally ture properties of an anisotropic The latter is referred in the following higher than in crystals: an optical uniaxial medium with an optical axis as the “readout beam”, while the re- → ranging between 0.1 oriented along n. This direction can cording signal contains the informa- and 0.2 for example is common in be experimentally specified by de- tion to be “printed” on the phase of the LCs. The existence of a liquid order fining specific boundary conditions readout light. The readout beam has to also guarantees the fluid nature of the at the surface of the sample. Typical be polarized. In addition, its polariza- different LC mesophases and, to some anchoring conditions can be planar tion is a mean to control the parame- extent, the tunability of their proper- (PAN), homeotropic (VAN) or twisted ter modulated by the LC component, ties. These characteristics, combined (TN). Furthermore, the orientation whether it is the phase, amplitude or with a wide spectral transparency, of the director axis can be controlled polarization [2]. Indeed, projection of have made LCs materials widely used by applying external electric and / or an initially linearly polarized light at in . magnetic fields. As a matter of fact, 45° with respect to the LC extraordi- In the nematic mesophase, mole- the application of an electric field re- nary axis provides a phase-mismatch cules do not have a positional order sults in the creation of elastic forces between the two crossed-polarized but are oriented in a preferential leading to the reorientation of the components. This variable phase

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shift allows the modulation of the SLM MAJOR FAMILIES light amplitude if the component is AND PERFORMANCES Therefore, the nature placed between polarizer and ana- Most of commercial SLMs are electri- and concentration lyzer, and the modification of the cally-addressed, for instance through linear polarization state to elliptical standard digital video interface with of solutes in liquid water otherwise. Subsequently, phase-only, each grey level being related to a given is the main contrast factor amplitude-only, polarization, or the voltage. They can operate either in re- for biological systems. combination of phase-amplitude mo- flection or in transmission. dulation can be readily realized with a LC-SLM, as illustrated in Figure 1. Transmissive SLMs. Transmissive LCoS are microdisplays, composed SLMs include a nematic LC layer of a layer of nematic liquid crystals Applications. For decades, the large confined between two transparent enclosed between a transparent market of image projection and dis- conductive windows. At least one elec- electrode and a matrix of CMOS plays has fed the development of trode is segmented, in order to provide (complementary metal oxide semi- LC-SLMs that address otherwise un- individual electrical control over a cer- conductor) integrated circuitry on countable applications in a wide field tain amount of pixels. A particular type a silicon backplane. The latter ope- of scientific investigations. Nowadays, of transmissive SLM is the well-known rate in reflection through a reflective SLMs are used in fields as varied as LCD (Liquid Crystal Display), for am- treatment deposited on the CMOS imaging, digital , optical plitude light modulation. LCD relies matrix. Anchoring layers on one side switching, microstructure fabrica- on TN-type LC-SLM, placed between of the electrode and on the reflec- tion, optical vortex generation. In the parallel or crossed polarizers. tive layer allow the molecules to be context of adaptive optics, SLMs are oriented in a direction parallel to the employed to correct the wavefront of Reflective SLMs. Reflective SLMs are surface. An electric field maintained and optimize the point spread particularly sought as they enable to between the transparent electrode function for biomedical applications fold the associated optical system, while and the semiconductor controls the and microscopy. In addition, SLMs en- light propagates twice in the modula- local average molecular orientation able ultrashort optical pulse shaping ting layer, which, in turn, increases the of the liquid crystal and modulates through a process known as Fourier- dynamic range. In this family, the most its refractive index. domain pulse shaping. Recently, popular technology is LCoS (Fig. 3): such devices have also been used in Liquid Crystal on Silicium, mainly Performances criteria. The diffe- the field of telecommunications in used for phase-only or amplitude- rent components of the LC-SLM order to achieve modal multiplexing only light modulation. Most of current multi-layer structure can be indivi- in multimode optical fibers. Some il- commercial electrically-addressed LC- dually optimized according to the tar- lustrations are available in Figure 2. SLMs are based on this technology. geted application. Performances of

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LC-SLMs are then characterized as a Active area. LC-SLMs for scientific ap- to the number of electrode segments. priority by their active area, transmit- plications present an active area usual- In addition, at the junction between tance / reflectance, spectral accep- ly around 1–2 cm2, with some specific two adjacent segments, there are gaps tance, spatial resolution, response extension in the array configuration, where the SLM is inactive and / or time and modulation dynamics. up to 7 cm × 1 cm. has discontinuities in the modulated optical property. Inactive gaps are Modulation range and response Spatial resolution. The spatial reso- responsible for light scattering. This time. The modulation range is the lution is related to two parameters: feature is translated as the so-called maximum retardation that can be ap- the pixel density and the cross-talk filling factor, usually slightly above plied to a given wavelength while the between adjacent pixels. The pixel 90% for commercial systems. dynamic response time is defined as pitch depends on the category of the switching time from 10% to 90% SLMs. Higher pixel densities are Reflectivity (transmittance). The re- and from 90% to 10% (rise and fall achieved with LCoS, typically 1920 × flectivity (transmittance) is not 100% time). Theses two features are prima- 1080 pixels, but also up to 4160 × 2464. as some of the light may be diffracted rily determined by the LC layer mate- Transmissive SLMs are restricted to into higher orders due to the grating rial and thickness. Independently from a larger pixel size (a few tens of μm like structure of the pixel matrix while the technology, the thickness of the LC for transmissive SLMs, as opposed to some part of light is also scattered and layer is generally limited to 20 μm in a few μm for LCoS). When specified absorbed at the interpixel gaps. In ad- most SLMs, and results from a balance beyond the simple number of pixels, dition, the overall reflectivity (trans- between the desired modulation range the spatial resolution of an LC-SLM mittance) is limited by losses at the (e.g. maximum phase modulation), is around 40 lines per /mm, that is multiple interfaces of the multi-layer maximum control voltage, molecular between 20–30 μm. structure, the electrode transparency, disorder issues, and dynamic response the reflectivity of the metallic or die- time. For radiation in the visible spec- Filling factor. The electrically lectric coating in LCoS. Typical values tral range, the phase modulation evol- controlled LC-SLM makes it possible spread between 70% and 90%. ves between 0 and 2π or 0 and 4π. The to control the properties of the readout dynamic response time, meanwhile, light over a limited number of zones Spectral acceptance. Commercial SLMs ranges typically between 1–100 ms for predefined by the manufacturer, this make it possible to address different 10–90% rise and fall times. number being approximately equal spectral ranges, with bandwidth around 200 nm, centered in the visible, near- infrared or close to telecommunication bandwidths. A remaining drawback of the electrically-addressed LC-SLM tech- nology is the presence of a top electrode. Most of the time, Indium Tin Oxyde (ITO, typically transparent over 0.3–1 μm, with partial transmission up to 1.5 μm) is employed, which tends to reduce the spectral acceptance, by comparison with the LC mixture itself. Moreover, as the phase modulation range scales with the optical frequency, extending the spec- tral range towards the infrared tends to require thicker LC layers, which, in turn, constraints the electrical addres- sing scheme. These two features limit the practical use of LC-SLMs in the visible and near infrared spectral range.

Figure 2: Illustration of some applications of SLMs. Top: a spiral phase pattern leads to optical vortex beams [3]. Bottom: a SLM inserted in a zero- dispersion line enables the temporal shaping of a femtosecond pulse [4].

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Figure 3: Structure and cross-section of an LCoS SLM and photograph of an industrial product from Holoeye.

Flicker. The flickering phase cor- responds to the phase fluctuation due to electric polarization of the LC molecules and can be reduced to 0.01π by carefully designing control electronics.

Damage threshold. LC-SLM can tailor the properties of high power beams. Some damages might alter the SLM behavior, either due to laser ablation of one of the LC confi- LIMITATIONS challenging issues. Nevertheless, re- ning substrates, or to heating of the AND RECENT ADVANCES search and development of innovative LC layer. According to the available Although LC-SLMs are very performant LC-SLMs is still very active. In particu- data, the damage threshold is li- and popular optical systems, some li- lar, other solutions for controlling the mited by electrodes and/or metallic mitations can be deduced from the birefringence are being investigated. coating in LCoS and is around 5W/ performances detailed above. Among Replacing the electrical addressing cm2 for continuous light radiation, them, the pixelisation and limited by an optical addressing solves the and decreases to 0.1 J/cm2 for pulsed spectral acceptance in the mid-in- pixelisation issue and ensures arbitra- femtosecond lasers. frared spectral range are the most ry and continuous phase modulation.

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In so-called light valve modulators, or CONCLUSION LCoS technologies. Several performance OASLM, a biased photo-conductive Spatial light modulators, thanks to criteria, such as panel resolution, active substrate replaces the segmented their dynamic attractive optical capaci- optical area, refreshing rate and spectral electrode and the voltage across the ties and to their technological maturity, acceptance must be considered in order to LC layer is locally controlled by an an- are widespread in several scientific and define the most appropriate SLM for a tar- cillary absorbed control beam, often industrial domains. The most common geted application. It should be noted that referred as the "recording" beam, as components exploit the electro-opti- several developments towards innovating opposed to the "readout" beam. This cal anisotropy of liquid crystals and technologies could further boost the prac- electrode is, however, mandatory, are commercialized following LCD or tical applications in the coming years. as an oscillating electric field has to be maintained across the LC layer to control the average orientation of the molecular director. Moreover, conventional optical valves require REFERENCES an isolation layer to prevent cross- talk between the recording and [1] Spatial Light Modulator Technology: Materials, Devices, and Applications, readout beams. ed. Efron (1995) Finally, novel technological de- [2] C. Rosales-Guzmán and Andrew Forbes, How to Shape Light with Spatial Light velopments in this field are steadily Modulators, SPIE Spotlight (2017) proposed. Thermal or thermo- optical control of the LC layer or [3] A. A. Zinchik, Application of spatial light modulators for generation of laser beams with a spiral phase distribution, Sci. Tech. J. Inf. Technol., Mech. Opt. (2015) photo-polymerization of the ancho- ring layer might be promising me- [4] A. M. Weiner, Femtosecond pulse shaping using spatial light modulators, thods to provide continuous phase Rev. Sci. Instrum. 71, 1929-1960 (2000) modulation while eliminating the [5] N. Collings et al., The Applications and Technology of Phase-Only Liquid Crystal need for an electrode. Dielectric on Silicon Devices, J. Disp. Technol. 7, 112-119 (2011) metasurfaces might also be part of the next-generation of SLMs.

MAKES SUPPLIER PRODUCTS CONTACT

[email protected] Hamamatsu Hamamatsu Photonics France SARL LCoS +33 (0)1 69 53 71 00

[email protected] Jenoptik Jenoptik LCos, trans. SLMs +49 3641 65-4243

[email protected] Biophotonics Solution Biophotonics Headquarters LCoS +1-508-506-2941

[email protected] Meadowlark Optics Photon Lines LCoS, trans. SLMs +33 (0)1 30 08 99 00

[email protected] Thorlabs Thorlabs SAS France LCoS +33 (0) 970 444 844

[email protected] Holoeye Photonics AG Optoprim LCoS, trans. SLMs +33 (0) 141 90 61 80

www.santec.com/en/inquiry Santec Santec LCoS +1-201-488-5505

[email protected] Laser Components Laser Components S.A.S France LCoS, trans. SLMs +33 1 39595225

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