Ultra-flat broadband microwave frequency comb generation based on optical frequency comb with a multiple-quantum-well electro-absorption modulator in critical state

Abstract

In this paper, we proposed a novel ultra-flat broadband microwave frequency comb (MFC) generation based on optical frequency comb (OFC) with a multiple-quantum-well electro-absorption modulator (MQW-EAM) in critical state. The scheme is simple and easy to adjust. The performance of the MFC generation scheme is investigated using software Optisystem. The results show that the comb spacing of MFC can be adjusted from 5 to 20 GHz by changing RF signal’s frequency and the MFC is almost independent on the linewidth of the tunable laser diode. The performance of the MFC can be improved by reasonably increasing the voltage of the RF, the small-signal gain of the Erbium-doped fiber amplifier (EDFA) and the responsivity of the photodetector. The MFC generated by this scheme has 300 GHz effective bandwidth with 15 comb lines, whose power variation is 0.02 dB, when the components’ parameters in the Optisystem are set as follows: the power of tunable laser diode (TLD) is 0 dBm, the wavelength is 1552.52 nm, and linewidth is 1 MHz; RF signal’s frequency is 20 GHz and the voltage is 10 V; the reverse bias voltage of MQW-EAM is 6.92 V; the small-signal gain of the EDFA is 40 dB; the responsivity of the photodetector (PD) is 1 A/W.

Keywords

microwave frequency comb (MFC) / multiple-quantum-well electro-absorption modulator (MQW-EAM) / flatness

1 Introduction

One-dimensional (1D) semiconductors with well-controlled size, morphology, and chemical composition have been the focus of scientific research due to their unique chemical-physical properties and potential applications in electrons, photonics, chemical sensing, and biological imaging [1-6]. To date, a wide range of materials have been prepared in the form of 1D nanomaterial by a variety of methods [7-9]. Among them, various functional nanowires are a hot topic and have been successfully synthesized and applied in different fields, such as nanolasers [2], nanocatalytic [7], and nanosensors [10-13].

Silver based ionic conductors, such as Ag₂S and AgI, have served as model compounds in studies on ion and electron transport/transfer in solid-state physics [14,15]. AgVO₃, Ag_xV₂O₅, and Ag₂V₄O₁₁ are the most commonly occurring phases in the solid state, and the detailed structural and phase analysis on them had been widely investigated [16]. Silver molybdate/chromate are an important family of inorganic materials, which have wide potential applications in various fields. Yu et al. have successfully prepared Ag₂MoO₄, Ag₆Mo₁₀O₃₃, and Ag₂Mo₂O₇ nanoribbons [17]; Li et al. have synthesized Ag₂CrO₄ nanorods. These works are all labeled as important and valuable [18]. Our group also synthesized 1D Ag₂MO₄ (M= Cr, Mo, and W) nanomaterials through the modified hydrothermal method, and the photoresponse of the nanomaterials were also investigated [19]. Attention was also devoted to inserting other anions into these compounds to obtain new materials with high conductivities.

Today, phosphate salts have attracted great attention due to the variety of their applications in the electrical, optical, prosthetics, structural fields, etc. [20]. Also, transition metal phosphates are important as they are used as inorganic and biomaterials finding applications as catalysts, ion exchangers and in low thermal expansion ceramic materials [21]. Cutroni et al. have conducted studies on ionic conduction and dielectric behavior of Ag₃PO₄ glasses [22,23]. George studied the characterization of nano-Ag₃PO₄, and found that the nano-Ag₃PO₄ is a directric relaxor material of high dielectric constant [24]. However, the Ag₃PO₄ nanowires have not been reported.

In this paper, for the first time, Ag₃PO₄ nanowires with the width of 100 nm and the length up to several micrometers were obtained via a simple hydrothermal route without any surfactant or template. The current-voltage (I-V) characteristics of Ag₃PO₄ nanowires and the photoswitching response were also investigated, and we believe it will be widely used in nanodevices.

2 Experiment

2.1 Preparation of Ag₃PO₄ nanowires

All of the chemical regents used in our experiments were of analytical grade and without further purification. Aqueous solutions were prepared using distilled water.

In a typical procedure, 0.5 mM AgNO₃ and 0.25 mM Na₃PO₄ were dissolved in 25 mL distilled water, respectively. Then the AgNO₃ solution was added into Na₃PO₄ solution slowly under magnetic stirring to form a mixture at room temperature. The resulting mixture was transferred into a Teflon-lined autoclave with a capacity of 60 mL and heated to 160°C for 12 h, then cooled to room temperature naturally. The resultant was collected and washed several times with absolute ethanol and distilled water, and dried under vacuum at 60°C for 12 h.

2.2 Characterization

The phase and crystallography of the products were characterized by a Shimadzu XRD-6000 X-ray diffractometer equipped with Cu Kα radiation (λ = 0.15406 nm). A scanning rate of 0.05°·s^-1 was applied to record the pattern in the 2θ range of 10°–80°.

The morphologies of the products were analyzed with a FEI Quanta 200F scanning electron microscope equipped with an energy dispersive X-ray (EDX) spectroscope. Transmission electron microscopy (TEM), selected area electron diffraction (SAED) pattern and high-resolution transmission electron microscope (HRTEM) were captured with a JEOL-2010 transmission electron microscope, using an accelerating voltage of 200 kV. The photo image was taken from an Olympus optical microscope.

2.3 Measurements of photoconductive properties

The photoconductivity measurements were tracked with a CHI 620B (CHI Instruments, Chenhua Corporation, Shanghai, China) electrochemical workstation. A bundle of Ag₃PO₄ nanowires were dispersed and bridged over the ITO electrodes. Light was supplied with an incandescence lamp (12 V, 10 W).

3 Results and discussion

3.1 Crystallography and morphology

Figure 1 displays the XRD pattern of the Ag₃PO₄ obtained at the pH= 2. The diffraction peaks may be indexed as the cubic phase of Ag₃PO₄ with the calculated lattice constant a = 0.6046±0.0011 nm, which is consistent with the reported value, a = 0.6048 nm (JCPDS card No. 84-0512). No impurity is detected, indicating that the products have a high purity.

Fig.1 XRD pattern of as-synthesized products (all diffractions peaks can be indexed as cubic Ag3PO4)

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Field emission scanning electron microscopy (FESEM) is employed to describe the morphologies of the products. Figure 2(a) gives the low magnification image of the as-synthesized Ag₃PO₄ nanowires, with the length up to several micrometers. The high magnification shows that the diameter of the wire is about 100 nm (Fig. 2(b)). The chemical composition of these nanowires was checked by EDX in Fig. 2(b) (upper right). Besides the elements Si from the substrate, the peaks of the elements Ag, P, and O are detected in the EDX pattern.

Fig.2 SEM images revealing wires several micrometers in length and ~100 nm in diameter. (a) Low magnification; (b) high magnification and EDX spectrum (inset indicates nanowires are composed of Ag, P, and O elements)

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Figure 3 shows a typical TEM image of the prepared Ag₃PO₄ nanowires, revealing the wire-like morphology features. Each wire has a uniform width along its entire length, and the average diameter is about 100 nm, which complies with the scanning electron microscopy (SEM) result. The HRTEM image (Fig. 3, inset) illustrates that the wire has lattice planes with spacing of 0.305 nm, corresponding to the d spacing of the (200) plane of the cubic phase of Ag₃PO₄ respectively. The SAED (Fig. 3, inset) taken from this area displays the crystalline structure of Ag₃PO₄, whose bright diffraction spots may be indexed as (200) and (011) respectively.

Fig.3 TEM image of Ag3PO4 nanowires with diameter of 100 nm (HRTEM image of Ag3PO4 (low inset) shows clear crystal lattice, which can be indexed as (111) crystal planes; SAED pattern (upper inset) suggests single crystallinity of Ag3PO4 nanowire)

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To understand the growth process of the Ag₃PO₄ nanowires, a series of parallel experiments were performed, which were checked with FESEM. When the Ag⁺ came across $\mathrm{P}{\mathrm{O}}_4^{3-}$, it must be synthesized Ag₃PO₄ nanopaticles (Fig. 4(a)). With the hydrothermal time increased to 5 h, the wires began to grow from the center of the particle, as shown in Fig. 4(b). There are several other factors that influence the crystallinity and the morphology of Ag₃PO₄ nanowires, such as temperature and concentration. Learnt from a series of experiments, the optical reaction conditions for the synthesis of high quality Ag₃PO₄ nanowires are at 160°C for 12 h. Although the precise mechanism for the formation of Ag₃PO₄ nanowires in still under discussion, we can infer from the above analysis that the growth of the Ag₃PO₄ nanowires is through the solid-solution-solid transformation mechanism.

Fig.4 SEM images of Ag3PO4 samples prepared after hydrothermal reaction showing morphological evolution of Ag3PO4 nanowires. (a) 2 h; (b) 5 h

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3.2 Photoswitch of Ag₃PO₄ nanowires

In order to measure the current signals through the Ag₃PO₄ nanowires, indium tin oxide (ITO) coated glass with the electrode gap of 50 μm being employed as the substrate. A bundle of Ag₃PO₄ nanowires was dispersed and bridged over the electrodes with an effective length of 60 μm, as shown in Fig. 5(a) (inset). Gold gap electrodes were fabricated on the substrate through thermal evaporation with a micrometer-sized Au wire as the mask; with a slight movement of the Au-wire mask, Au-Au gap electrodes were deposited. The distance between the wires and light source was 10 cm.

Figure 5(a) shows the I-V curves measured in dark (curve I) and under illumination (curve II) for comparison. Both of them exhibit a good linear behavior, which proves a fine ohmic contact between the Ag₃PO₄ nanowires and Au electrodes. The conductivity of the wires rapidly increased under illumination with an incandescence lamp. In these cases, the energy from the light excites the electrons in the ion conductor Ag₃PO₄ nanowires from the valence band into the conduction band, increasing the charge carrier concentration via direct electron-hole pair creation and thus enhancing the conductivity of the wires.

Figure 5(b) shows the reversible photoconductive characteristics of the Ag₃PO₄ nanowires. A voltage of 0.1 V was applied across the two electrodes and the current recorded during the light was alternatively on and off at 10 s intervals. Obviously, the current through the Ag₃PO₄ nanowires got promptly increased and decreased with the illumination on and off, which proves that the device is highly sensitive and are capable of repeatable activities. The measurements were performed for ten periods and proved that the photoconduction behavior was reproducible.

Fig.5 Photoswitch of Ag3PO4 nanowires. (a) I-V curves of a bundle of Ag3PO4 nanowires measured in dark (curve I) and under illumination by using an incandescence lamp (12 V, 10 W) (curve II), and image of a bundle of Ag3PO4 nanowires bridging over ITO electrodes from optical microscope (inset); (b) photoconductive characteristics of device during light switching on/off (voltage of 0.1 V was applied across Au-Au electrodes and current was recorded during light alternatively on and off at 10 s intervals)

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4 Conclusions

In summary, this work describes a facile hydrothermal method for the synthesis of large-scale, high-purity, and uniform AgPO₄ nanowires without employing surfactants or templates for the first time. Based on the ionic conductors of the AgPO₄, the as-prepared Ag₃PO₄ nanowires exhibited linear current-voltage (I-V) characteristics and excellent photoresponse. As the light source was switched on and off, the currents could be reversibly switched between high and low value at the voltage of 0.1 V, which will find wide applications in photo-electro nanodevices. Further work is needed in a full investigation of Ag₃PO₄ nanowires.

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Acknowledgements

Related studies were supported by the National Natural Science Foundation of China (Grant No. 61275067).

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