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Abstract
An optically rewritable and electrically erasable terahertz (THz) wavefront modulator based on indium oxide (In2O3) and DMSO-doped PEDOT:PSS is proposed. The modulator has a three-layer structure of In2O3/PEDOT:PSS/quartz, which can weaken the THz transmission under the action of light excitation. Optically written THz Fresnel plates, which can focus the input Gaussian beam into a point, were realized. After optical excitation, the function of the device reduces slowly if it is stored in the room environment. However, the function can be stored for a long time if it is encapsulated in the nitrogen environment. If a bias voltage of 22 V is applied on the device, the function of the device can be erased in 10 seconds. The new function can be written into the device after wiping. Experiments on THz rewritable holographic devices are carried out to show the validity of this approach. This method can provide new devices for THz wavefront modulation and develop tunable optical imaging elements.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The special position of terahertz (THz) radiation in the electromagnetic spectrum makes it has a series of special properties, including fingerprint spectrum, low photon energy, high penetration, water molecule absorption, as well as wide bandwidth. The THz radiation has been applied in the fields of molecular identification, substance composition detection, high-resolution imaging, and other fields [1–3]. Comparing with the quick development of THz sources and detectors, THz functional devices have yet to be developed. Many efforts have been devoted to develop functional THz devices, such as modulators [4–7], filters [8], absorbers [9–14], and polarizers [15]. THz active devices have also been studied because they can provide more functions. Different approaches, such as mechanical [16,17], electronic [18,19], magnetic [20], and thermal [21,22] methods, are adopted to regulate THz radiation. The optical approach, which has the advantages of high speed, stability, and flexibility, is becoming the main direction [23–25]. Deng et al. reported a subwavelength grating structure based on InSb that can respond to THz plasma [26]. Busch et al. studied a semiconductor that can use transient carrier absorption of THz radiation during light modulation to implement virtual gratings and filters [27,28]. Rizza et al. theoretically investigated an IR driven tunable dielectric metamaterial for THz radiation [29]. However, the active device mentioned above is partially transient, which means that the tuning function of the device is lost once the external trigger is turned off. In many applications, the function of the device is expected to be maintained for a long time without external trigger. Recently, the THz nonvolatile memory device have been proposed. Using indium oxide (In2O3) and imethyl sulfoxide (DMSO) doped poly (3,4-ethylenedioxythiophene):poly (4-styrenesulfonate) (PEDOT:PSS), the state of THz transmission can be stored for a relative long time and can be erased with applied voltage [30]. It was found that this kind of devices can keep their states for more than 15 days in a nitrogen environment [31]. However, all these proposed devices are employed to modulate the transmission of THz wave, no wavefront modulation is considered.
In 2022, Zhang et al. proposed a THz nonvolatile reconfigurable metasurface device based on phase change materials [32], which inspires us to expand previous works into wavefront modulation. In this work, we propose an optically rewritable and electrically erasable THz wavefront modulator based on In2O3 and DMSO-doped PEDOT:PSS. The In2O3 is used as state keeping material and the PEDOT:PSS plays a role in promoting photogenerated carrier migration [30]. Special patterns such as computer-generated holograms can be optically written into the device according to requirement. The pattern written in the device will gradually weaken over time in the natural environment, and the pattern will disappear after about 2-3 hours. In the absence of oxygen, the pattern written in the sample can be maintained for a month. Furthermore, the pattern in device can be quickly erased by applying a bias voltage on the sample and a new pattern can be written into the sample again. It is expected that this kind of rewriteable wavefront modulator can provide a series of reusable devices for THz technology.
2. Experiment
2.1 Sample fabrication
The DMSO and PEDOT:PSS (Xi'an Bath Solar Energy Technology Co.) are used to prepare PEDOT:PSS/DMSO solution. The volume ratio of PEDOT:PSS solution and DMSO solution is 9:1. Then the DMSO-doped PEDOT:PSS solution was spin coated on a clean quartz substrate with a thickness of 1 mm, the resulting film was dried at 100°C for 15 minutes. Two parallel silver wire electrodes with separation of 1 cm were deposited on the DMSO-doped PEDOT:PSS layer with thermal evaporation. Then, the In2O3 nanoparticles (size:10 nm, ALADDIN) were dissolved in ethanol and spin coated on the DMSO-doped PEDOT:PSS film. The resulting sample was dried at 100°C for 15 minutes again [30,31,33–35]. A scanning electron microscope image of a fabricated sample is shown in Fig. 1(a), it can be found that the surface of the sample is relative uniform. A cross image of the sample is shown in the insert of Fig. 1(a), which indicates the thicknesses of the In2O3 and DMSO doped PEDOT: PSS films are ∼250 µm and 300 nm, respectively.
Fig. 1. Properties of the fabricated sample. (a) Scanning electron microscope image of the sample, (b) signals in the THz frequency domain, and (c) THz image of the sample.
The THz time-domain signals of the prepared sample are measured by a THz time-domain spectroscopy, the corresponding frequency spectra after Fourier transform are shown in Fig. 1(b). The black line is the transmission signal of the prepared sample. After 10 minutes of excitation with external 450 nm light, the transmission of the sample is reduced by about 80%, as shown by the red line, which means that the modulation depth (MD) can be up to 20%. The MD is defined as [36]:
In order to show the wavefront modulation ability of the fabricated sample, the THz Fresnel zone plates (FZPs) are demonstrated firstly. The equation of FZP can be expressed as:
where n represents the n-th FZP, rn is the radius of concentric circles, λ and f are the central wavelength and focal length of FZP, respectively. In the experiment, the THz FZPs with different center wavelengths and focal lengths can be achieved if the value of rn is changed. Three commercial metal THz FZPs with different focal lengths are selected as templates to fabricate active THz FZPs. The templates are manufactured with the conventional focused ion beam etching technology. The patterns are deposited in a 1 µm thick chromium film on a silica substrate with a thickness of 2 mm. Due to the limited size of the THz beam which is about 1 cm, only seven clear rings were selected to ensure uniform intensity of the FZP pattern.2.2 Experimental setup
As shown in Fig. 2(a), a semiconductor laser with a wavelength of 450 nm is used to project the pattern of templates on to the sample directly. The flux is 360 mW/cm2. An external voltage is connected to the sample for pattern erasing. The performances of the fabricated devices were measured with a home-built THz imaging system [37]. The dynamic subtraction technique is used to effectively remove the background of measured THz signals [38]. The schematic of the sample testing setup is shown in Fig. 2(b). The THz radiation is generated from a 1 mm thick ZnTe crystal. The distance between the sample and detection crystal ZnTe is d. All measurements were made at room temperature and humidity is less than 5%.
Fig. 2. Schematics of the setup for sample printing (a) and testing (b). FZP: Fresnel zone plates template, L: lens, HWP, half wave plate, BS: beam splitter, QWP: quarter wave plate, PBS: polarization beam splitter.
3. Results and discussion
Three THz FZPs with focal length f of 6.0, 6.5, and 7.0 mm were designed for three frequencies of 0.70, 0.75, and 0.80 THz, respectively. The samples are hided directly behind the templates and illuminated by the 450 nm CW laser light for 10 minutes, then they are insert into the THz imaging system for performances characterization. The distances between the samples and ZnTe detector crystal were carefully adjusted to d = 6.0, 6.5, and 7.0 mm, respectively. The intensity distributions on the preset focal planes are shown in Fig. 3. The THz FZPs designed for 0.70 THz has a focal length of 6.0 mm. The intensity distributions for 0.70, 0.75, and 0.80 THz on this focal plane are shown in Figs. 3(a1)-(a3), respectively. A strong focal point is appeared in Fig. 3(a1) and intensity distributions in Figs. 3(a2) and (a3) exhibit obvious defocus characteristics, which indicates the fabricated FZP can achieve the preset function well. For the FZP designed for 0.75 THz, the detection crystal is moved to 6.5 mm. The intensity distribution for 0.70, 0.75, and 0.80 THz on this plane are shown in Figs. 3(b1)-(b3), respectively, a strong focal point is shown in Fig. 3(b2). For the FZP designed for 0.80 THz, the corresponding intensity distributions are shown in Figs. 3(c1)-(c3). It can be concluded that when the detection distance and wavelength are matched, the corresponding THz intensity distributions will show obvious focus characteristics. All experimental results demonstrated that the optically printed FZPs can achieve the preset function well.
Fig. 3. Intensity distributions on the preset plane for THz FZPs. Intensity distributions on the focal plane of the THz FZP with a focal length of 6.0 mm (a), 6.5 mm (b) and 7.0 mm for 0.7 THz (1), 0.75 THz (2) and 0.8 THz (3), respectively.
The function of printed device can be stored for a long time. Figure 4(a) presents the intensity distribution on the focal plane of the FZP designed for 0.75 THz after being stored in a nitrogen environment for 7 days. The result is nearly the same as the initial state shown in Fig. 4(b), the intensity was only reduced 10%. The expectation of storage time when the modulation depth is reduced to 0.5 is 29 days. The pattern printed in the sample can be quickly erased by applying a voltage of 22 V on it for 10 seconds. After erasing, the sample was imaged again. The intensity distribution on the plane which is 6.5 mm away from the sample is shown in Fig. 4(b). It exhibits a homogeneous wavefront which indicates that the THz wave has not been modulated. Then the pattern of the FZP was printed into the sample, the corresponding intensity distribution is shown in Fig. 4(c). The focal point appears again, which proved that the sample could be reused.
Fig. 4. Storage, erasing, and rewriteable quality of the sample. (a) Intensity distribution on the focal plane for the FZP designed for 0.75 THz after being stored in a nitrogen environment for 7 days, (b) for the sample after electrical erasing and (c) rewriting.
The proposed sample can also be used to generate more complex THz field. Three pure intensity holograms are designed and laser printed into the sample. The holograms are designed for 0.75 THz and can generate the intensity distribution of letters “C”, “N”, and “U”, respectively. The holograms have 100*100 pixels. Each pixel is 100 *100 µm2 in size, thus the total size of the hologram is 1.0*1.0 cm2. The holograms are designed with an optimization algorithm and fabricated by projecting templates onto the samples with the light of 450 nm wavelength. The intensity distributions on the preset plane generated by the corresponding holograms are shown in Figs. 5(a1)-(a3). The letters have been displayed well. After being stored in an anaerobic environment for 7 days, the samples were measured again and the intensity distributions were shown in Figs. 5(b1)-(b3). Apart from the intensity has been reduced a little bit, the letters are nearly the same as they are shown in Figs. 5(a1)-(a3). After electronical erasing, the holograms are rewritten into the samples, the corresponding intensity distributions are shown in Figs. 5(c1)-(c3). The displayed letters are similar to the initial state which demonstrates that the samples are reusable.
Fig. 5. Intensity distributions of rewritten holograms. (a1)-(a3) Constructed intensity distributions for optical written holograms, (b1)-(b3) constructed intensity distributions for the holograms after stored in an oxygen-free environment for 7 days, and (c1)-(c3) Intensity distribution for the rewritten samples.
The working mechanism of the device can be understood as follows: In the sample, oxygen molecules are adsorbed on the surface and combined with free electrons to form an N-type oxide semiconductor. After photoexcitation, the electron hole pairs are produced. The positive holes adsorbed oxygen and release electrons back into the conduction band of the sample, thus the conductivity of sample is increased and the transmission of THz is decreased [36]. If the sample is exposed into the air, the oxygen will enter the sample and reduce the carrier concentration, thus the transmission of THz will be recovered. However, if the sample is placed in nitrogen environment, the oxygen adhesion will be prevented and the information can be storage for a long time. By applying a voltage, the carriers will accelerate the attachment of oxygen, thus, the transmission of THz wave will be recovered and the information in the sample will be erased. In a nitrogen environment, the store time can arrive 7 days without significant changes. However, the storage time is less than 3 hours if the sample is kept in the atmospheric environment. If the sample is put into an oxygen rich environment, the function of device can be erased more quickly. Applying voltage is a faster approach to erase the function of device. In the practical applications, the function of templates can be implemented by a spatial light modulator or digital mirror device which can generate writing pattern easily.
4. Conclusions
In a conclusion, a THz rewriteable wavefront modulator based on indium oxide nanoparticles is investigated. The In2O3/DMSO-doped PEDOT:PSS/quartz sample attenuates the THz transmission under photoexcitation. When the photoexcitation is terminated, the modulated THz transmission can slowly recover to its original value in the air environment within 3 hours, however, the function of device can be stored for a long time (about 29 days) in a nitrogen environment. The modulated THz transmission can be recovered quickly by applying a bias voltage on it and the pattern can be rewritten into the device after erasing. It is expected that this kind of in- situ electrically erasable rewritable wavefront modulation devices can nudge the development of THz communication, THz sensing, and THz imaging.
Funding
The National Natural Science Foundation of China (12174270, 12174271, 62175168, 62275175); National Key Research and Development Program of China (Grant No. 2019YFC1711905); Sino-German Mobility Program of the Sino-German Center for Science Funding (M-0225); Capacity Building for Science & Technology Innovation-Fundamental Scientific Research Funds (Grant No. 00820531120017).
Disclosures
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Data availability
The data that support the findings of this study are available from the corresponding authors upon reasonable request.
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