Letter Vol. 46, No. 5 / 1 March 2021 / Optics Letters 1037

High-speed programmable lithium niobate thinfilm spatial light modulatorXuanchao Ye,1,† Fengchao Ni,1,† Honggen Li,1 Haigang Liu,1,5 Yuanlin Zheng,1,2,6 ANDXianfeng Chen1,2,3,4,7

1State Key Laboratory of AdvancedOptical Communication Systems andNetworks, School of Physics and Astronomy, Shanghai Jiao TongUniversity, Shanghai 200240, China2Shanghai Research Center for QuantumSciences, Shanghai 201315, China3Jinan Institute of Quantum Technology, Jinan 250101, China4Collaborative Innovation Center of LightManipulations and Applications, ShandongNormal University, Jinan 250358, China5e-mail: liuhaigang@sjtu.edu.cn6e-mail: ylzheng@sjtu.edu.cn7e-mail: xfchen@sjtu.edu.cn

Received 12 January 2021; accepted 25 January 2021; posted 28 January 2021 (Doc. ID 419623); published 19 February 2021

High-speed spatial modulation of light is the key technologyin various applications, such as optical communications,imaging through scattering media, video projection, pulseshaping, and beam steering, in which spatial light modu-lators (SLMs) are the underpinning devices. ConventionalSLMs, such as liquid crystal (LC), digital micromirrordevice (DMD), and micro-electro-mechanical system(MEMS) ones, operate at a typical speed on the order ofseveral kilohertz as limited by the slow response of the pixels.Achieving high-speed spatial modulation is still challengingand highly desired. Here, we demonstrate a one-dimensional(1D) high-speed programmable spatial light modulatorbased on the electro-optic effect in lithium niobate thin film,which achieves a low driving voltage of 10 V and an overallhigh-speed modulation speed of 5 MHz. Furthermore,we transfer an image by using parallel data transmissionbased on the proposed lithium niobate SLM as a proof-of-principle demonstration. Our device exhibits improvedperformance over traditional SLMs and opens new avenuesfor future high-speed and real-time applications, such aslight detection and ranging (LiDAR), pulse shaping, andbeam steering. © 2021 Optical Society of America

https://doi.org/10.1364/OL.419623

Spatial light modulators (SLMs) are a class of devices that modu-late the amplitude, phase, or polarization of a free-space lightbeam. Recently, SLMs have been widely used in various fields,such as optical communication [1], wavefront shaping [2–4],three-dimensional video projection [5,6], femtosecond pulseshaping [7–9], and beam steering [10–12]. Conventional SLMsmainly include two types: (1) liquid crystal SLM (LCSLM)based on liquid crystal substrate [13–16]; and (2) digitalmicromirror device (DMD) based on micromirror deflectiontechnology [17–20]. One limitation of LCSLMs is that theresponse time of liquid crystal molecules is relatively long,typically on the scale of microseconds. The modulation speedof LCSLMs is mostly kilohertz and is difficult to increase. The

DMD is composed of a large number of tiltable micromirrors,of which the tilt direction of the micromirror array is drivenby electrical signals to realize the modulation of light. Themodulation speed of DMD can reach 10 kHz, besides its binarymodulation characteristic also restricts its applications. Therelative low modulation speed of current SLM extremely limitsthe further applications in high-speed circumstances.

Recently, there have been several reports on high-speed SLMson different material platforms. An electro-optic SLM basedon graphene integrated on a grating resonator [21] utilizes theelectro-optic characteristics of graphene and can theoreticallyachieve a modulation speed up to 0.78 GHz, but only about 9%modulation depth was achieved due to graphene damage dur-ing the transfer process. A novel tuned subwavelength gratingresonator based on an interferometric microelectromechanicalsystem (MEMS) [22] has demonstrated a modulation speed ashigh as 0.5 MHz, but the MEMS structures are usually compli-cated and sophisticated. In addition, a programmable plasmonicphase modulator (PPPM) [23], which is extremely sensitive tochange of refractive index at the interface of metal-dielectric,has demonstrated a phase modulation speed at 1 GHz basedon surface plasmon resonance of Kretschmann configuration.However, this PPPM has a relatively high insertion loss, and it isextremely sensitive to the incidence angle.

Lithium niobate (LN) is widely considered as one of themost appealing material platforms for ultrafast modulators.The strong electro-optic effect [24] of LN makes its refractiveindex to change linearly with the applied electrical field withinfemtosecond timescales. In addition, LN also has excellentproperties such as ultrawide transparent window and ultralowoptical loss. Therefore, most electro-optic beam deflectors[25,26], optical couplers [27], and optical modulators [28–31],whose modulation rate can reach up to Gbit/s, are proposedbased on the LN platform. However, these modulation unitshave only one single channel, which can only manipulate oneoptical signal at once but not multiple parallel channels to formspatial light modulation. In addition, previous proposed SLM

0146-9592/21/051037-04 Journal © 2021 Optical Society of America

1038 Vol. 46, No. 5 / 1March 2021 /Optics Letters Letter

based on the LN platform has a relatively complex structure, andthe modulation speed can only reach 30 Hz [32].

Here, we demonstrate a one-dimensional SLM based onelectro-optic effect using LN thin film (LN-SLM) and achievea high-speed programmable spatial modulation of free-spacelight. In our LN-SLM device, we achieve a low driving voltageof 10 V and high-speed modulation at 5 MHz. In addition,the LN-SLM proposed here can perform at three modulationmodes, which are phase, polarization, and amplitude mode.To further demonstrate its validity of programmable property,we then realize a parallel communication system to transmit animage by encoded optical field pattern.

Figure 1 shows the schematic of the LN-SLM structure.Figures 1(a) and 1(b) are the top and bottom of the LN-SLMstructure, respectively. Figure 1(c) is the mount to hold theelectrode-LN crystal. There are 16 electrodes on each side of thesupport device, which are connected with the electrodes on topof the electrode-LN crystal. The schematic of the electrode-LNcrystal is illustrated in Fig. 1(d), which consists of the top elec-trodes layer, the middle crystal layer, and the bottom electrodelayer. The top layer has 16 parallel electrodes, each of which isabout 1.7 cm long, 425 µm wide, and 70 nm thick, as shownin Fig. 1(e). The gap between adjacent electrodes is 213 µm.On account of the weak adhesion strength between gold andsilicon dioxide, a 10 nm thick chrome adhesive layer betweenthe gold electrodes and the crystal layer is introduced. As shownin Fig. 1(f ), the middle crystal layer consisted of a thin cuboidz-cut LN crystal with a dimension of 1 cm× 1.7 cm× 100 µmand a thin silicon dioxide (SiO2) layer 2µm thick on the top andbottom surfaces of LN. The SiO2 layer reduces ohmic loss dueto the metallic layer. The bottom electrode layer is connectedwith a grounded electrode of the support device, and it is com-posed of a gold electrode 70 nm thick and a 10 nm thick chromeadhesive layer, as shown in Fig. 1(g). Furthermore, the laserinput and output surfaces of the SLM, as shown in Fig. 1(d), arepolished in order to reduce scattering.

As demonstrated in the methods above, 16 plate capaci-tors are formed between the top and bottom electrodes layer.Voltages can be applied to the 16 top electrodes, which resultin the 16 uniform electric fields in the LN crystal layer (for the

Fig. 1. (a) and (b) show the top and bottom of the LN-SLM struc-ture, respectively. (c) The support device, which is used to hold theelectrode-LN crystal. The electrodes on the electrode-LN crystal andthe support device are connected by metal wires. (d) The view of theLN thin film with electrodes, which consists of three parts: the topelectrodes layer; the middle crystal layer; and the bottom ground elec-trode layer. The detailed structures of these three parts are, respectively,shown in (e), (f ), and (g).

Fig. 2. Three modulation modes using the LN-SLM. (a) The phasemodulation mode. The wavefront information is modulated when ver-tical linear polarization light passes through the SLM. (b) The polari-zation modulation mode. The spatial variation polarization state of theLN-SLM is realized when 45◦ linear polarization light passes throughthe device. (c) The amplitude modulation mode. A polarizer is used totransfer the polarization mode to the intensity modulation mode.

electric field distribution, see Supplement 1 section 1). The elec-tric field can induce a refractive index variation (1n) in differentchannels of the LN crystal on account of the electro-optic effect.

The incident light can be divided into ordinary light (o-light)and extraordinary light (e-light) due to the birefringence of LN.The equation relating the phase shift and the applied voltage forthe e-light is1ϕe = πn3

e γ33LV /(λd), where1ϕe is the phaseshift of e-light, V is the applied external voltage, ne is the refrac-tive index for the e-light, γ33 = 30.9 pm/V is the electro-opticcoefficient, λ is the wavelength of the laser beam, L = 1.7 cmis the length of the SLM, and d = 100 µm is the thickness ofthe LN layer. Thus, the theoretical half-wave voltage is 12.5 Vaccording to the experimental parameters. Similarly, for theo-light, the induced phase shift is 1ϕo = πno

3γ13LV /(λd),no is the refractive index for the o-light, and γ13 = 9.6 pm/V isthe electro-optic coefficient of the LN crystal. The calculatedhalf-wave voltage is 36.1 V.

The LN-SLM has three modulation modes, as illustrated inFig. 2, which are the phase, polarization, and intensity modu-lation mode. Obviously, for the phase modulation mode,the e-light incident light is more effective because γ33 > γ13.Therefore, the vertical linear polarization light can achieve ahigh phase modulation depth. When different electrodes areapplied with different voltages, the wavefront of the laser beamcan be changed when it passes through the LN-SLM, as shownin Fig. 2(a). The crystal birefringence effect is used when theLN-SLM operates at the polarization modulation mode. Inthis situation, the input light is tuned to 45◦ linear polarization.The phase difference between o-light and e-light varies with thedriving voltage, which causes the polarization change of the lightat the output, as shown in Fig. 2(b). In addition, the amplitudemodulation mode can be easily achieved when a polarizer isadded after the LN-SLM on the basis of the polarization modu-lation mode, which transfers the polarization information to theintensity information of the LN-SLM, as shown in Fig. 2(c). Fora proof-of-principle demonstration, we choose the amplitudemode to investigate the modulation effect of our SLM withoutloss of generality hereafter.

The experimental setup for high-speed modulation is illus-trated in Fig. 3. The laser at the wavelength of 671 nm withhorizontal linear polarization is used as the light source. Thelens L1 and L2 are used to expand the laser beam. The half-waveplate is used to change the polarization of incident laser beam to45◦ linear polarization incidence. The cylindrical lens is used tovertically focus the laser beam spatially to fit the SLM aperture.All the channels of the SLM are fed with a sinusoidal waveformfrom a signal generator with a frequency of 5 MHz. A slit afterthe SLM is used to remove residual light which is not coupled

Letter Vol. 46, No. 5 / 1March 2021 /Optics Letters 1039

Laser

L1

L2

HWP

Mirror

High-speed SLM CLSlitGTPPhotoelectric detector

Oscilloscope Signal Generator

Fig. 3. Schematic of the experimental setup for high-speed modu-lation. Lenses L1 and L2, f1 = 50 mm, f2 = 300 mm, respectively;HWP, half-wave plate; CL, the cylindrical lens, fcl = 100 mm; GTP,the Glan-Taylor polarizer.

into the SLM. After a Glan-Taylor polarizer, a photoelectricdetector is used to detect the output light intensity, monitoredby the oscilloscope.

Mathematically, there are two ingredients in the output laserof the LN-SLM. One part passes the dead spaces (gaps betweenelectrodes), and the phase difference between the o-light ande-light is δ1 = 2π(no − ne )L/λ. And the other part passesthrough the electric field area in the LN crystal, which corre-sponds to each metal electrode of the top electrodes layer. Thephase difference between the o-light and the e-light in this part

is δ2 =2π(no−ne )L

λ+

πne3γ33V Lλd −

πno3γ13V Lλd . Therefore, we

can obtain the intensity of the output light after the GTP in theintensity modulation mode (see Supplement 1 section 2):

Iout =1

3

1

1+ tan2 12δ1

Iin +2

3

1

1+ tan2 12δ2

Iin, (1)

where Iin is the input light intensity. The validity of the model-ing is proved in the following experiment.

Figure 4(a) presents the 20 V peak-to-peak sinusoidal drivingsignal at 5 MHz from the signal generator. The red curve inFig. 4(b) presents the measured intensity of the output light asa function of time with a 5 MHz sinusoidal signal, which is ingood agreement with the theoretical curve. Besides, the nor-malized intensity of output light versus applied voltages is alsogiven in Fig. 4(c). The experimental result is well in agreementwith the theoretical curve calculated using Eq. (1). We pointout that the modulation depth decreases with higher frequencyof the driving signal. The limitation of the modulation speed ismainly caused by the electrode in our LN-SLM rather than theresponse time of LN crystal. Therefore, the modulation speedcan be further improved by carefully designing the electrode ofthe LN-SLM.

Fig. 4. (a) Temporal characteristics of the driving voltage fromthe signal generator. (b) Normalized intensity of the modulationlight after Glan-Taylor polarizer response to 5 MHz driving voltage.(c) Normalized intensity of the modulation light after Glan-Taylorpolarizer as a function of applied voltage.

In addition to the demonstration of the high-speed modu-lation of the LN-SLM, we also experimentally implement aLN-SLM-based parallel transfer system to further demonstratethe availability of the programmable space-variant modulation.We separately modulate different channels of the SLM to con-vert the multibit sequence into an encoded optical intensitypattern. The experimental setup is similar to that in Fig. 3,except that the photoelectric detector is replaced by a charge-coupled device (CCD), and the signal generator is replaced by acomputer with an analog output card (ART PCI8304), whichhas 32 simultaneous analog signal outputs.

We use eight channels to investigate the digital coding infor-mation transferred through the encoding field pattern by usingthe proposed LN-SLM. Figure 5(a) shows three examples ofencoding field patterns corresponding to three different eight-bit sequences, where “0” corresponds to a low light intensity,and “1” corresponds to a high light intensity. Figures 5(a-1),5(a-2), and 5(a-3), respectively, show the field pattern withdata “10010010” (value: 146), “00100101” (value: 37), and“11011010” (value: 218). A good contrast of the field patternis accomplished. Then, we transfer an image using the paralleldata transmission system constructed above to further verify theperformance of the proposed LN-SLM. A 256-color bitmapof a parrot with a resolution of 60 × 112 pixels is chosen inour experiment. Each pixel value of the image is an integerbetween 0 and 255, which corresponds to an eight-bit sequence.By loading the information on each pixel of the LN-SLM, wetransfer the image pixel by pixel, as shown in Fig. 5(b). The 6720sequential codes have high accuracy after transmitting throughthe LN-SLM with 0.067% bit error ratio (BER).

In the above experiments, we have demonstrated the high-speed and multichannel programmable modulation capabilityof our LN-SLM. In our experiment, only 16 channels are usedin our LN-SLM, which is extensible when it is used in differentcircumstances. Besides, there is still much room to improvethe speed of the modulation and the data transmission. For thesecond experiment, the low frame rate of the CCD is the mainlimitation for the rate of data transmission, which is only 60frames/s. We can further improve this problem by using a higherframe rate device or a photoelectric array.

Compared with the state-of-the-art SLMs, including ferro-electric LCSLMs [13], DMDs [33], and MEMS SLMs [2,34],the modulation speed of our SLMs based on LN thin film can

Fig. 5. (a) Encoding field pattern with eight-bits coding sequences:(a-1) 10010010; (a-2) 00100101; (a-3) 11011010. (b) The process oftransmitting a parrot illustration utilizing the LN-SLM to build a par-allel data transmission system.

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realize an over tens to hundreds times faster modulation speed.In addition to the advantages in modulation speed, the excellentcharacteristics of LN also provide more advantages. First, LNhas a high optical damage threshold and is suitable for modula-tion of high-power pulsed lasers. Second, LN has an ultrawidetransparent window and ultralow optical loss. Therefore, SLMbased on an LN platform has an ultrawide modulation band-width. Third, the proposed SLMs are solid-state devices, andoffer excellent mechanical stability compared to DMDs andother mechanical devices. Conventional DMDs [17] can onlywork in amplitude mode, or need to build a complex opticalsystem to achieve phase and polarization modulation. However,the LN-SLM proposed here shows its versatile property and canswitch among phase, amplitude, and polarization modulationmodes by simply changing the polarization state of the incidentlight and adding a polarizer.

In conclusion, we have proposed and experimentallydemonstrated a high-speed programmable SLM based onthe electro-optic effect of an LN platform that can achieveoverall high-speed modulation up to 5 MHz, which improvesorders of magnitude compared to the conventional LCSLMsand DMDs. Moreover, we utilize the multichannel modulationcharacteristic of the LN-SLM to develop a data parallel trans-mission system and evaluate it using an eight-bit encode fieldpattern. The high modulation speed shows a great potential ofsuch SLMs for real-time operation, which is strongly desired inapplications of high-speed circumstances such as holographicdisplay, spectrum shaping, and optical communication.

Funding. National Key Research and Development Program ofChina (2017YFA0303701, 2018YFA0306301); National NaturalScience Foundation of China (11734011, 12004245, 12074252,62022058); Shanghai Municipal Science and Technology Major Project(2019SHZDZX01-ZX06); Shanghai Rising-Star Program (20QA1405400);Shandong Quancheng Scholarship (00242019024) .

Disclosures. The authors declare no conflicts of interest.

Supplemental document. See Supplement 1 for supporting content.

†These authors contributed equally to this Letter.

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