Bo Zhang， Liang Zhong， Ting He， and Jing-Ling Shen

Photo-Doped Active Electrically Controlled Terahertz Modulator

Bo Zhang， Liang Zhong， Ting He， and Jing-Ling Shen

—We demonstrate an electric-controlled terahertz （THz） modulator which can be used to realize amplitude modulation of terahertz waves with slight photo-doping. The THz pulse transmission was efficiently modulated by electrically controlling the monolayer silicon-based device. The modulation depth reached 100% almost when the applied voltage was 7 V at an external laser intensity of 0.6 W/cm2. The saturation voltage reduced with the increase of the photo-excited intensity. In a THz continuous wave （CW）system， a significant fall in both THz transmission and reflection was also observed with the increase of applied voltage. This reduction in the THz transmission and reflection was induced by the absorption for electron injection. The results show that a high-efficiency and high modulation depth broadband electric-controlled terahertz modulator in a pure Si structure has been realized.

Index Terms—Electrically controlled， electrode structure， terahertz modulator.

1. Introduction

A terahertz （THz） modulator is one of the key components which can actively control the spatial transmission/reflection of an incident THz wave for telecommunication， spectroscopy， and imaging［1］，［2］. Various approaches have been developed to achieve the amplitude and frequency modulation including optical，electromagnetic， temperature， and electrical control［3］-［8］. In particular， the active electrically tunable terahertz modulator is desired for high switch speed and easy control. The field effect transistor can be used to tune the carrier concentration for modulating the transmission/reflection of the THz waves based on graphene， GaAs， and other optoelectronic materials［9］-［15］. For instance， an intensity modulation depth of 22% and a modulation speed of 170 kHz have been successfully achieved by a graphene field effect transistor［16］. Another area of significant research is the use of two-dimensional electron gases （2DEGs） in high electron mobility transistors and modulation depths of up to 33% at 0.46 THz with all electrical control［17］. However，these terahertz modulators are constrained by the complex processing and low modulation depth. Therefore， an electrically tunable modulator with easy processing and high modulation depth is highly required in the terahertz frequency region.

In this work， we fabricated a THz modulator， where a double electrode is deposited on a monolayer silicon wafer with easy processing. The THz pulse transmission was efficiently modulated by electrically controlling silicon-based device. The modulation depth reached 100% almost when the applied voltage was 7 V at an external laser intensity of 0.6 W/cm2. The saturation voltage reduced with the increase of the photo-excited intensity. In the THz continuous waves （CW） system， a significant fall in both THz transmission and reflection was also observed with the increase of applied voltage. This reduction in THz transmission and reflection was induced by the absorption for electron injection.

2. Experimental Details

A 2 mm thick silicon （Si） wafer that has a high resistance （＞10000 ?/sq.） was selected as a substrate. The 100 nm thick electrodes were deposited on the top of the Si wafer by using thermal evaporation. The gap of the electrodes is 1 mm. The experimental setup and the sample are shown in Fig. 1. A typical terahertz time domain spectroscopy （THz-TDS） with a reliable bandwidth of 0.2 THz to 2.6 THz was used to obtain the THz optical parameters of the Si wafer. In the THz-amplitudemodulation experiment， a 450 nm semiconductor CW laser was used to irradiate the Si wafer. The wavelength of laser performs a key role in producing photo-induced carriers because the wavelength is strongly absorbed by the Si. The output power of the excitation laser was controlled by an externally digital-tunable and DC-stabilized voltage supplywith a power range from 0 mW to 420 mW. An optical power meter （S302C， Thorlabs） was used to measure the output laser power in the experiment. The THz beam is incident normally to the Si wafer， whereas the 450 nm laser beam is incident at an oblique angle of 45°.

Fig. 1. Schematic of the electrode deposited on the silicon with excitation light and bias voltage Vgapplied.

3. Results and Discussions

The THz transmission spectra through the electrode/Si structure under various laser irradiance levels are shown in Fig. 2 （a）. The THz transmission decreased gradually with increasing the laser intensity， dropping almost to 22% of the original value under the laser intensity of 1.45 W/cm2. THz transmission intensities through an electrode/Si structure under various bias voltages with different photo-doping are shown in Fig. 2 （b）. Obviously， the modulation depth reached 100% almost when the applied voltage was 7 V at an external laser intensity of 0.6 W/cm2. The saturation voltage reduced with the increase of the photo-excited intensity. Fig. 2 （c） shows the normalized power for THz transmission through the electrode/Si structure under various levels of bias voltage with slight photo-doping of 0.6 W/cm2. The THz transmission distributed over a frequency window ranging from 0.2 THz to 2.6 THz and decreased gradually with increasing the bias voltage，dropping to less than 5% of the original value at the bias voltage of only 6 V. As a comparison with the graphene field effect transistor， the saturation voltage is 4 V， which only can provide the modulation depth of 22%. Generally，the electrode/Si structure is shown to be an advantageous device that can be used as a highly depth electric-controlled terahertz modulator for THz waves in a wide frequency range from 0.2 THz to 2.6 THz with slight photo-doping.

Knowledge of the THz transmission and reflection intensities of the electrode/Si hybrid structure is essential to reveal the modulation mechanism. The THz continuous waves system （THz-CWS） experimental setups used for the transmission and reflection measurement were described in detail in our previous work［13］，［14］. Fig. 3 （a） shows the THz transmission intensity distribution through the electrode/Si structure without optical doping. The THz transmission intensity drops to 10% at the bias voltage of 8 V， as shown in Fig. 3 （b）. In addition， the THz reflection intensity distribution of the electrode/Si structure without bias voltage in the reflection experiment is shown in Fig. 3 （c）. The THz reflection intensity drops to 80% of the original value under the bias voltage of 8 V， as shown in Fig. 3 （d）. The dependence of the bias voltage on both the THz transmission and THz reflection shows that the THz transmission and reflection intensities decrease while the THz absorption increases nonlinearly with increasing the bias voltage， as shown in Fig. 3 （e）. This reduction in THz transmission and reflection was induced by absorption for electron injection.

Fig. 2. THz intensities under different conditions for the electrode/Si structure in THz-TDS: （a） THz transmission spectra through an electrode/Si structure under different laser light irradiances， （b） THz transmission intensities through an electrode/Si structure under various bias voltages with different photo-doping， and （c） THz transmission distributed over a frequency window ranging from 0.2 THz to 2.6 THz decreased gradually with an increasing bias voltage.

Fig. 3. THz intensities under different conditions for the electrode/Si structure in THz-CWS: THz transmission intensity distributions through the electrode/Si structure under bias voltage:（a） 0 V and （b） 8 V； THz reflection intensity distributions on the electrode/Si structure under bias voltage: （c） 0 V and （d） 8 V； （e）bias voltage of the THz intensities for transmission and reflection，and absorption from the electrode/Si structure.

To evaluate the modulation performance of the samples，the modulation factor MF， which is defined as the change in the integrated transmitted THz power caused by the photo-excited intensity， is introduced as follows［13］，［14］:

Fig. 4. Carrier density ratio （N/N0） and THz transmission modulation factor （MF） through an electrode/Si structure under various levels of bias voltage.

4. Conclusions

In summary， we demonstrated an electric-controlled terahertz （THz） modulator which can be used to realize amplitude modulation of terahertz waves with slight photo-doping. The THz pulse transmission was efficiently modulated by electrically controlling the monolayer silicon-based device. The modulation depth reached 100% almost when the applied voltage was 7 V at an external laser intensity of 0.6 W/cm2. The saturation voltage reduced with the increase of the photo-excited intensity. In the THz-CW system， a significant fall in both THz transmission and reflection was also observed with the increase of the applied voltage. This reduction in THz transmission and reflection was induced by the absorption for electron injection. The results showed that a high-efficiency and high modulation depth broadband electric-controlled terahertz modulator in a pure Si structure has been realized.

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Bo Zhang was born in Beijing， China in 1984. He received the B.S. degree from the Capital Normal University （CNU），Beijing in 2007 and the Ph.D. degree from the Beijing Jiaotong University， Beijing in 2012， respectively. He is currently a lecturer with Capital Normal University. His research interests include terahertz spectroscopy， THz modulator， and organic lasers.

Liang Zhong was born in Beijing， China in 1991. He received the B.S. degree from CNU， Beijing in 2014. He is currently pursuing the M.S. degree with CNU. His research interests include terahertz spectroscopy and THz modulator.

Ting He was born in Beijing， China in 1984. He received the B.S. degree from CNU，Beijing in 2007 and the M.S. degree from CNU in 2010 in optics， respectively. He is currently pursuing the Ph.D. degree. His research interests include terahertz spectroscopy， THz modulator， and THz communications.

Jing-Ling Shen was born in Beijing， China in 1957. She received the B.S. degree from the Beijing University of Technology （BUT），Beijing in 1982 and the Ph.D. degree from the Institute of Physics， Chinese Academy of Science， Beijing in 1998， respectively. Her research interests include terahertz spectroscopy， THz modulator， and THz communications.

Manuscript received March 6， 2015； revised May 16， 2015. This work was supported by the Natural Science Foundation of Beijing under Grant No. 4144069 and the Science and Technology Project of Beijing Municipal Education Commission under Grant No. KM201410028004.

B. Zhang is with the Department of Physics， Capital Normal University，Beijing 100048， China （Corresponding author e-mail: bzhang@cnu.edu.cn）.

L. Zhong， T. He， and J.-L. Shen are with the Department of Physics，Capital Normal University， Beijing 100048， China （e-mail: 2140602045@cnu.edu.cn； heting54@163.com； sjl-phy@cnu.edu.cn）.

Color versions of one or more of the figures in this paper are available online at http://www.journal.uestc.edu.cn.

Digital Object Identifier: 10.3969/j.issn.1674-862X.2015.02.005