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Abstract
While single wavelength all-optical information encoding through optically induced orientation of azobenzene dyes is being extensively pursued, we propose multi-wavelength optical data processing and recording based on disperse red 1 (DR1) and 4-(4-hydroxybutyloxy) azobenzene doped organic-inorganic hybrid films to increase the density of recording data. By investigating the change of absorbance spectrum of the doped film under different irradiations, results indicate a laser pulses around 470 nm would be suitable as the probe beam. In the measurement of optical data processing and recording, two cw lasers pulse at 532 nm and 355 nm induce trans-cis isomerization of the azo-dyes in the film, while the output of the probe beam record the processed data as {(−1), (0), (1)} according to different inputs of the pump beams. Since the light induced isomerization has a sensitive response in the as-prepared solid organic-inorganic matrix system, the films is promising as recording and monitoring element in all-optical devices over a wide range of repetition rates.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
All-optical network is the future of worldwide communication. Trends in optical data processing and storage are higher data rate, higher recording density, and higher storage capacity [1–5]. Current commercial data recording techniques are based on heat-mode recording systems, which limits the size of the recorded pit, and each kind of the heat-mode storage system has a temperature threshold in the recording process, which limits the writing speed [6,7]. To solve those problems, photon-mode recording system based on photochromic materials is becoming a promising candidate. To establish this, materials containing photo-responsive dye molecules are becoming attractive [8–12]. Determination of an appropriate chromophores-matrix combination is not a simple choice to make. Among various classes of organic molecules for nonlinear optical applications, organic azo-dyes have been shown to be very promising. Optically bistable azo-dye molecule with two phenyl ring separated by a switchable -N = N- bond are attractive for the second and the third order nonlinear optical effects and polarized-light induced anisotropy [13,14]. This effect can be simply descripted as that under certain wavelength of irradiation, due to the selectivity excitation, azobenzene molecules change from rod-like trans isomer to a strongly kinked-shape cis isomer, and eventually align perpendicularly to the light polarization and become stable [15]. The response time of such light induced isomerization is subpicosecond in solid state materials, which can be suppressed or enhanced by specific interaction between the azobenzene molecules and the matrices which are used for doping [16]. That is, the high performance is related to both high nonlinear chromophore landing, and maintenance of the nonlinear chromophore alignment. In this sense, the guest-host systems in which the azo molecules are not covalently bonded to the matrix are clearly underperforming. The candidates for matrix should offer both a stable network for the azo molecules to survive, and also enough space for the -N = N- bond to switch [17].
While the polymeric materials are widely due to their easy fabrication and excellent surface characteristics [18,19], the inorganic photorefractive crystals are superior in long-term stability and memory capacity [20], organically modified silanes (Ormosils) based organic-inorganic hybrid system have both rigid inorganic networks and organic free volume for doping dye groups [21], for which the azo molecules can be covalently bonded to a hybrid organic-inorganic network to maintain its high performance. Another key feature of Ormosil materials is their plasticity, not at a macroscopic scale but all the way down to the molecular level. For instance, it is possible to change the refractive index of the materials by simply adjusting the ratio between organic and inorganic components. Besides, employing a hybrid material also has an advantage of depositing thicker films than inorganic sol-gel films which have a critical thickness of around 1~0.5μm. In several applications, a thick film without multilayers is an important requirement to avoid unnecessary propagation loss. By incorporating azo-dyes into an Ormosil network, it is possible to obtain high quality nonlinear optical medium with excellent optical properties.
Multi-wavelength data processing and storage is a promising approach to increase the recording density and data capacity. For instance, in single-wavelength system, the data are coded by {(0), (1)}; but in two wavelength recording, the data could be coded as {(−1), (0), (1)}, thus the recording density is increased. Photochromic material with different absorption bands can be used in the multi-wavelength recording system. Laser beam corresponding to each of the absorption band of the material would record by light induced isomerization independently. To achieve this goal, multi-kinds of azo-dyes could be employed as dopant to fabricate photochromic material with different absorption bands. As members of classic azobenzene derivative chromophores, disperse red 1 (DR1) and 4-(4-hydroxybutyloxy) azobenzene have extraordinary behavior, which is related to the light induced trans-cis isomerization, and succeeding molecular alignment for the optical data recording and optical information processing in visible and ultraviolet region, respectively. In this work, those optical quality azo-dyes coexistence TiO2/Ormosil composite films have been prepared by a sol-gel technique at low temperature. The performance of multi-wavelength optical data processing and recording based on the as-prepared films was investigated by using a cw Nd:YVO4 laser at 355nm and a SHG cw Nd:YVO4 laser at 532 nm as two inputs, respectively.
2. Preparation and experimental setup
The hybrid films containing DR1 and 4-(4-hydroxybutyloxy) azobenzene molecules used for multi-wavelength optical data processing and recording were prepared by spin-coating the hybrid solution onto glass slides. Here, in the preparation of TiO2/Ormosil sol, γ-glycidoxypropyltrimethoxysilane (GLYMO, CH2OCHCH2O(CH2)3Si(OCH3)3) was used as the Ormosil source, tetrabutyl titanate (Ti (OC4H9)4) as the TiO2 source, hydrochloric acid (HCl, 37wt% in water) as catalyst, ethanol as solvent, de-ionized water for hydrolysis, and acetylacetone was used to weaken the hydrolysis rate of titanium isopropoxide. Two groups of solutions were prepared in our experiment, respectively. Especially, the TiO2/Ormosil organic-inorganic hybrid sols were prepared at room temperature by a sol-gel process. In the preparation, de-ionized water and hydrochloric acid were added drop by drop and the molar ratio of HCl/GLYMO was 0.01. The details of the preparation processing of the TiO2/Ormosil matrix were described in our previous work [22]. In the present study, the molar ratio of GLYMO to TiO2 was 8:2. To prepare the azo-dyes doped TiO2/Ormosil sol, the commercial DR1 and 4-(4-hydroxybutyloxy) azobenzene compound in the weight of 1% of the sol-gel hybrid solution was added into the matrix, respectively. Following the common practice for spin coating, one layer of the sol-gel film was spun onto the glass slides at 3000 rpm for 30 seconds. Then, the film-coated samples were heated at 50°C for about 10 minutes.
The photo-responsive measurements of the hybrid films were carried out by a Jasco V-570 UV-visible (UV-vis) spectrophotometer after irradiation by UV light for various time intervals. The UV irradiation light was produced by a Driel Instrument 66901 500W mercury lamp through filters centered at 532 nm and 355 nm, respectively. Absorbance spectra change of the as-prepared hybrid films were measured by UV-vis spectrometer in the wavelength range of 200~800 nm. The experimental arrangement as shown in Fig. 1 was employed to form and investigate the performance of multi-wavelength optical data processing and recording of the azo-dyes doped hybrid films, here, a cw Nd:YVO4 laser at 355 nm and a SHG cw Nd:YVO4 laser at 532 nm were used as two pump beams with the power varied at about 39.4 mW/cm2 and 41.8 mW/cm2, respectively; while a tunable cw laser pulses at a wavelength range from 320~1750 nm was used as probe beam with the power around 20.6 mW/cm2. Firstly, the two S polarized Nd:YVO4 laser is split into two pump beams by beam splitters, and then reflected by mirrors to meet each other and obtain a spot on the surface of the hybrid film. In order to obtain the performance of the multi-wavelength optical data processing and recording, the two pump beams were chopped manually, or by optical choppers. Once the azo-dyes doped hybrid film was placed at the spot, the light induced isomerization occurred. To monitor the change of the absorption properties due to the isomerization, the output probe beam was received by a photomultiplier tube PMT and the signal was displayed by a high-speed digital phosphorous oscilloscope.
Fig. 1 Schematic diagram of the multi-wavelength optical data processing and recording effect measurement experimental set-up. P1, P2: polarizer; VA1, VA2: variable attenuator; M1, M2: mirrors; BS1, BS2: beam splitters; D1, D2, D3: detectors.
3. Results and discussion
Azo-dyes molecules most likely exist in the trans state at the lowest energy, in which exhibits a strong anisotropic dipolar transition moment tensor. When azo-dyes molecules are exposed to the appropriate irradiation near the frequency of its main absorption peak, they change from trans to cis configuration. During this process, the distance between the acceptor and the donor groups reduces from about 0.9 nm to 0.55 nm, hence, leading to changes of refractive index, optical anisotropy, and absorption coefficient. In our previous work [23], it has been reported that in 4-(4-hydroxybutyloxy) azobenzene molecules doped organic-inorganic films, in the range of 300~600 nm that there is a major absorption peak at 352 nm and a weak one at 437 nm, which are related to the π→π* electronic transition of the trans isomers and the n→π* electronic transition of the cis isomers, respectively. When the hybrid film is irradiated by UV light of 370 nm wavelength, azobenzene chromophores undergoes a trans-cis photoisomerization process. The intensity of the absorption peak at 352 nm gradually decreases and the weak peak at 437 nm becomes more pronounced. While in DR1 doped hybrid films, there is an intense absorption peak around 410 nm corresponding to the n→π* electronic transition of the cis and a weak absorption band around 514 nm which originates from the π→π* electronic transition of the trans. Under the irradiation around 514 nm, there is a decrease in intensity of the trans isomer's peak, while an increase of the cis isomer's peak, demonstrating a photoisomerization process inside the hybrid film [24].
Figure 2 shows the absorbance spectrum change of the DR1 and 4-(4-hydroxybutyloxy) azobenzene molecules coexistence organic-inorganic hybrid films after different irradiations of the action light from a mercury lamp through filters. The light exposure induced a slow temporal change of the absorbance spectrum. It can be observed that a few equi-absorbing points could be found after each exposure. To be specific, in Fig. 2(a), it shows the UV-visible absorption spectra change of the as-prepared hybrid film due to the photoisomerization under an irradiation at 532 nm. Absorption bands of the material could be observed around 330 nm, 410 nm, and 505 nm, respectively, while almost no absorption after the wavelength of 600 nm. According our previous works [23, 24], the first band is mainly related to the π→π* electronic transition of the trans isomers of 4-(4-hydroxybutyloxy) azobenzene molecules; while the second one is related to both the n→π* electronic transition of the cis isomers of 4-(4-hydroxybutyloxy) azobenzene and DR1 molecules; and the third one is corresponding to the π→π* electronic transition of the trans isomers of DR1 molecules. When the film is irradiated only by the light of 532 nm, both of the absorption peaks around 330 nm and 505 nm gradually decrease with an increase of the irradiation time, while the absorption valley around 410nm appears slightly increasing. Equi-absorbing points can be found located around 380 nm and 430 nm after exposure. In Fig. 2(b), it shows UV-visible absorption spectra change of the as-prepared hybrid film due to the photoisomerization under an irradiation at 355 nm, which appears different from that in Fig. 2(a). It can be observed that the two absorption peaks around 330 nm and 505nm gradually decrease by the irradiation time, while the absorption valley around 410 nm appears slightly increasing. And the equi-absorbing point only can be found around 400 nm. In Fig. 2(c), the change of absorbance spectra appears more complicated. When the film is irradiated by both of the light at 532 nm and 355 nm simultaneously, the absorption peak around 330 nm decreases, and the absorption valley around 410 nm increases by the irradiation time, while the peak around 505 nm appears an obviously blue shift, which could be related to the combined effects of n→π* electronic transition of 4-(4-hydroxybutyloxy) azobenzene molecules and π→π* electronic transition of DR1 molecules. Equi-absorbing points also can be found around 400 nm and 470 nm. Between those two points, the absorption intensity around the former one under different irradiations almost remains the same. While, the latter one appears to decrease, increase and remain the same, respectively. Therefore, if we want to obtain different performance under different irradiations, using a probe beam which pulses at a wavelength around 470nm would be suitable.
Fig. 2 UV-Vis absorbance change of the DR1 and 4-(4-hydroxybutyloxy) azobenzene molecules doped organic-inorganic hybrid films after different irradiations. (a) 532nm; (b) 355nm; (c) both 532 nm and 355 nm simultaneously.
Figure 3 shows the recording curve of the multi-wavelength optical data processing and recording effect of the film by manually operation. For the observation of the recording process, the two pump lasers were chopped manually to be turned off for 3s, 5s, and 1 min successively, while the probe beam kept on all the time. It can be observed from Fig. 3 that when the two pump beams were turned on simultaneously, the output signal remained as digital “0”; when the 355nm laser was turned off, while the 532nm laser was still on, the output signal appears as digital “-1”; when the 355nm laser was on, while the 532nm laser was turned off, the output signal appears as digital “1”. The curve of output data has good contrast to be identified. The noise should be mainly attributed to the scattering of the as-prepared film. Two reasons may be involved for the scattering: the first is the non-uniform hydrolysis and condensation of the silicon alkoxides and titanium alkoxides; and the second is the possibility of undissolved azo-dyes molecule groups. Considering the scattering as the source of the noise, the S/N ratio of the output could be estimated as 8.94 dB. It should be noticed that a tunable cw laser pulses at wavelength range from 320~1750 nm was used as probe beam. The specific wavelength of probe beam was determined under the condition that the both lasers were turned on. As discussed above, there exist an equi-point around 470 nm which behaves differently under different irradiations. Particularly, when the two pump beams are turned on simultaneously, the absorption intensity of the as-prepared film at this very point of wavelength would remain the same. Therefore, considering this characteristic, we found the equi-point specifically located at 472.3 nm in our current system, which could always keep the output signal changeless when the two pump beams are turned on. Therefore, the wavelength of the probe beam was determined as 472.3nm. It also can be noticed from Fig. 3 that during the manually operation, the intensity change of the valleys and the peaks are different in output curve. The longer time for the 355 nm laser turned off, the deeper the valley would be. Similarly, the longer time for the 532 nm laser turned off, the higher the peak would be. Consequently, deeper valley and higher peak lead to longer recover time to go back to digital “0”. Results in Fig. 3 demonstrate that the as-prepared films have potential in all-optical neural networks. The current two pump beams could be seen as two different optical neural stimulations, while the output of the probe beam could be seen as the calculated result of this optical neural network. For instance, the whole system could be used for monitoring and processing the two optical stimulations. When the stimulations remain the same, the system would record the calculated data as digital “0”, which means nothing change; once one of the stimulations is varied, the data would be recorded as digital “-1” or “1” according to the specific variations. Such property could help the recording easier to be read, and simpler to be decoded.
Fig. 3 Recording curve of the multi-wavelength optical data processing and recording effect of the film by manually operation.
The as-prepared films not only could be used in the situation below under manually controlled switching, or in the processing of low frequency events, the good sensitivity makes it promising in higher operating repetition. Figure 4 shows the effect of multi-wavelength optical data processing when the pump beams were chopped by an optical chopper with a frequency of 1k Hz, while the probe beam kept on all the time. Specifically, Fig. 4(a) shows the process that the two inputs of 355nm and 532 nm changed from {(0), (0)} to 1k Hz square wave, and the Fig. 4(b) shows the process that the inputs changed from 1k Hz square wave to {(1), (1)}. To be clear, the 355 nm pump laser beam is identified as the first input, while the 532 nm pump laser beam is the second one. It can be observed that when the two inputs were {(0), (0)}, the output would be digital “0”; when the two inputs were {(1), (0)}, the output would end up as digital “1”; when the two inputs were {(0), (1)}, the output would end up as digital “-1”; when the two inputs were {(1), (1)}, the output would be digital “0”. That is, if the inputs are the same, the output would be digital “0”, while if the inputs are different from each other, the output would be digital “1” or “-1”. It can be observed from Fig. 4 that the curves of the output signal is clear enough to be identified with an estimated S/N ratio of 5.09 dB. Since the frequency of the inputs is 1k Hz, which does not give the azo-dyes in the as-prepared film enough time to proceed a relative fully isomerization, the shape of the output curves appear to like triangular wave, consequently showing a worse S/N ratio than that in Fig. 3.Such properties could be used in high speed all-optical signal processing and recording for multi-wavelength inputs. Unlike single-wavelength system, which the data are coded by {(0), (1)}, in two wavelength storage, the data could be coded as {(−1), (0), (1)}, thus the recording density is increased.
Fig. 4 Data processing curves of multi-wavelength optical data processing and recording effect of the film by an optical chopper with a frequency of 1k Hz.
The mechanism of the below multi-wavelength optical data processing and recording effects can be explained by a simplified level diagram as shown in Fig. 5. Based on the discussion below, it can be found that the major absorption peak around 330 nm is corresponding to the trans isomers of 4-(4-hydroxybutyloxy) azobenzene molecules, while the broad absorption band located around 505 nm is corresponding to the trans isomers of DR1 molecules, and the weak absorption valley around 410 nm is corresponding to the cis isomers of both the two dyes. It should be noted that the absorption valley of the cis isomer in both two azobenzene dyes are overlapped with each other around 410 nm. In optical applications, the two optical inputs might have four different combinations, which makes it has four different cases: {(0), (0)}, {(0), (1)}, {(1), (0)}, and {(1), (1)}. Also, the 355 nm pump laser beam is identified as the first input, while the 532 nm pump laser beam is the second one.
Fig. 5 Simplified level diagram to explain the trans-cis isomerization in two azo-dyes coexistence hybrid system.
Case 1: the inputs are {(0), (0)}
In the absence of two pump beams, the two kinds of azo-dyes are both in the trans configuration in the lowest singlet state St10 and St20. The absorption intensity of the film at the wavelength of the probe beam, which is 472.3 nm, remains the same. In this case, we identify the output intensity of the probe beam as digital “0”.
Case 2: the inputs are {(1), (0)}
Upon the irradiation of 355 nm, which near the resonant irradiation of the 4-(4-hydroxybutyloxy) azobenzene dye, the molecules absorb light and undergo a transition to the excited state of the trans isomer, St1′. From St1′ the molecules could decay to St10 or Sc10. The trans-cis light-induced isomerization of 4-(4-hydroxybutyloxy) azobenzene molecules results in the latter case. From Sc10 the molecules could be excited to Sc1′ and then decay back. Therefore, according to Fig. 2(b), the absorption intensity of the film at 472.3 nm tends to increase. As compared to Case 1, such an increased absorption intensity of the output is identified as digital “1”.
Case 3: the inputs are {(0), (1)}
Similar as Case 2, upon the irradiation of 532 nm, which near the resonant irradiation of the DR1 dye, the molecules absorb light and undergo a transition to the excited state of the trans isomer, St2′. From St2′, the molecules decay to Sc20, and then the trans-cis light-induced isomerization of DR1 molecules is achieved. Therefore, according to Fig. 2(a), the absorption intensity of the film at 472.3 nm tends to decrease. As compared to Case 1, such a decreased absorption intensity of the output is identified as digital “-1”.
Case 4: the inputs are {(1), (1)}
Upon both resonant irradiation of the two dyes, based on the discussion below, both 4-(4-hydroxybutyloxy) azobenzene and DR1 molecules occur trans-cis isomerization simultaneously. From Fig. 2(c), it can be found that the absorption intensity of the film at 472.3 nm remains the same. We also identify the output intensity of the probe beam as digital “0”.
4. Conclusions
In the present study, we have designed and prepared two different kinds of azo-dyes coexistence Ormosils based organic-inorganic hybrid films. Based on the property of trans-cis isomerization of the photosensitive dyes, the combination of DR1 and 4-(4-hydroxybutyloxy) azobenzene molecules make the hybrid system show more complicated optical-induced responses. By employing a mercury lamp through different filters, the photo-responsive measurements were carried. The change of absorbance spectrum of the as-prepared film shows several equi-absorbing points, among which the wavelength around 470 nm is perfect for the wavelength selection of the probe beam for its different behavior under different irradiations. By employing a measurement arrangement with two pump beams, two-wavelength all-optical processing and recording performance was investigated. In those process, the data could be coded as {(−1), (0), (1)}, which could help increase the density of the data. Specifically, when the two input signals were the same, as they were both “0” or “1”, the output signal would be recorded as digital “0”; otherwise, the output signal would be digital “1” or “-1” according to different inputs. The good optical sensitivity makes it promising over a wide range of frequency up to 1k Hz. All the results indicate that the as-prepared azo-dyes doped Ormosils based organic-inorganic films is potential for multi-wavelength data processing and recording. And also, the easy preparation makes the films compatible for traditional devices manufacture.
Funding
National Natural Science Foundation of China (NSFC) (61704009); Natural Science Foundation of Shaanxi Province (2017JQ6078, 2017JQ6042); Fundamental Research Funds for the Central Universities (310832171009); Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2015KF07).
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