Abstract

Graphene has received significant attention due to its many unique properties, such as its two-dimensionality, zero-mass and zero-gap band structure, and unsurpassed strength. Additionally, its terahertz (THz) properties are being studied for future ultrafast electronics for information and communication, sensing, and other applications. Although many theoretical studies exist, the basic properties of graphene in the presence of THz radiation are largely unexplored. In this study, we explore the THz dynamics of graphene on an indium phosphide (InP) substrate using THz time-domain spectroscopy (THz-TDS) and laser THz emission microscopy (LTEM). Using LTEM, we compared the THz radiation from InP to the radiation through graphene on InP to find that graphene decreases the amplitude of THz. Furthermore, we investigated the effect of continuous wave (cw) lasers of different wavelengths on THz radiation and discovered that a 365 nm cw laser greatly decreased THz transmittance through graphene on InP. In contrast, an 800 nm cw laser had no effect on transmittance, proving wavelength dependence of THz generation. We also studied the spatial variation of THz absorption of graphene on InP using LTEM, which allowed us to visualize the localized transmittance distribution of graphene. Both THz-TDS and LTEM results help us understand THz functionality in graphene on InP, and this understanding can potentially contribute to the development of future ultrafast electronics.

Introduction

Although microwave and visible light applications are prevalent in electronic and photonic devices, the terahertz (THz) regime (300 GHz to 30 THz) is largely unexplored and unutilized. This “terahertz gap” has captured much attention due to potential applications, such as sensing, communications, and imaging. Graphene has attracted much attention for its unique properties and many potential applications, such as ultrafast electronics, transistors, inert coatings, and biodevices. We are particularly interested in developing ultrafast electronics by utilizing graphene’s absorbance of THz radiation. There are many possible substrates for graphene, and this is the first study to analyze the THz dynamics of graphene on an indium phosphide (InP) substrate. Additionally, we do not know the effect of several phenomena, including the interface effects of graphene on THz absorption and the effect of continuous wave (cw) laser on THz emission and transmission. We aim to characterize graphene on InP, study the interface effects of graphene, and explore the effect of cw lasers on THz emission and transmission. Comparing the effects of different wavelength cw lasers on the THz emission can lead to interpretations that are in agreement with current theories of THz generation. Studying and understanding graphene on different substrates will contribute to the development of ultrafast electronic devices.

Background

Graphene and Indium Phosphide

Graphene is capturing the attention of researchers due to its astounding properties such as its zero bandgap, strength, high mobility, and low resistivity. There are several methods of fabricating graphene, and in this study, we use chemical vapor deposition (CVD) grown graphene.

Because graphene is a single layer of carbon atoms, we must find suitable substrates in order to handle graphene. We are interested in studying graphene on InP because it has a high electron mobility (5400 cm2/(Vs) at 300 K) and a direct bandgap, which may be useful for ultrafast electronics.

Background on Terahertz Radiation

Terahertz radiation is defined as electromagnetic radiation with a frequency in the range of 300 GHz to 30 THz and a wavelength of 1 millimeter to 10 micrometers. In this study, we use two techniques: THz time-domain spectroscopy (THz-TDS) and Laser THz Emission Microscopy (LTEM).

THz-TDS is a technique that probes the properties of a material with pulses of THz radiation. A Ti:Sapphire laser generates an output beam containing a train of femptosecond pulses. The beam is split into two: the pump and probe beams. The pump beam reaches the THz emitter, where the optical pulses are converted into THz electromagnetic pulses. The THz beam goes through the sample and then meets up with the probe beam at the detector, which measures the amplitude and phase of the THz electric field. Fourier analysis of the transmitted THz radiation, as compared with a reference spectrum without a sample, provides the transmission spectrum of the sample.

LTEM measures the near field absorption of the electric field. A pump laser is raster scanned across the sample surface, which produces THz waves. By monitoring the THz amplitude, THz emission or transmission images can be observed.

Methods

In this study, two different systems were used. Both systems can perform THz-TDS and LTEM. Both systems use a beam splitter to split the laser into pump and probe beams. Both use time delay stages, GaAs bowtie-shaped antenna detectors, and lock-in amplifiers. However, the differences are highlighted in Figures 1 and 2.

RESULTS

Data from system 1 led to three major results, but data from system 2 was inconclusive. The first finding from system 1 is that the THz emission decreases more than expected in the presence of graphene on InP. We would expect graphene to cause a 2.3% decrease in THz emission due to its theoretical interband absorbance of 2.3%. However, other factors, such as air, humidity, doping, and inhomogeneity of sample may have caused this decrease to be roughly 28%. Considering that the THz generation mechanism is the surge current effect (surface field effect), it is also possible that this decrease in THz emission is caused by a decrease in band bending when graphene is on the surface of InP, as explained in Figure 5.

The next result is that we can successfully image graphene and see inhomogeneity on the surface of graphene, while the substrate is uniform (Figure 6).

The final result is that a 365 nm cw laser decreases the THz emission, but an 800 nm cw laser has no effect on THz emission. Figure 6 shows this with THz images and Figure 7 shows this with THz-TDS responses. The reason for this wavelength dependence of THz emission is that the THz generation occurs at the surface of the sample, when carriers are excited. The penetration depth of an 800 nm cw laser is too deep and does not interfere with carriers on the surface, but a 365 nm cw laser has a short penetration depth, interfering with carrier interactions at the surface, decreasing THz emission.

DISCUSSION

This research is significant because it is the first THz study that uses InP as a substrate for graphene. We can conclude that graphene affects the surge current mechanism on InP, decreasing the THz emission. The local distribution of the surface of CVD graphene can be visualized using an LTEM system, which is useful for future THz imaging applications in a variety of fields, such as medicine and security. A 365 nm cw laser clearly affects the THz emission mechanism, while an 800 nm cw laser has no effect on THz emission. These results are in agreement with current models because the cw laser has no effect on THz emission when its wavelength is too long; the laser does not interact with carriers on the surface. This helps us understand the THz emission mechanism of InP.

There are several ways to expand upon and improve this research. Firstly, the results of system 1 would be more meaningful if performed in a vacuum and with all other environmental factors held constant. This would allow us to determine the decrease in THz emission caused by graphene alone, which is significant because it gives us insight on the interaction of graphene at the surface of InP (i.e., the decrease in band bending caused by graphene). We also need to compare the THz emission of graphene on InP to that of a mirror in order to know the effect of the reflection from graphene’s shiny surface.

A potentially useful application could be to generate more THz radiation by applying a gate voltage, effectively via a battery. This would be useful in situations where high-intensity THz generation is desired, and it may also be interesting to study graphene with a bandgap tuned by gate voltage.

FUTURE WORK

The data gathered from system 2 were inconclusive, so further study needs to be done in this area. When using system 1 and looking at the THz emission, the 365 nm cw laser causes a significantly greater decrease when graphene is present compared to InP substrate alone (Figure 8). However, when using system 2 and looking at the THz transmission, the 365 nm cw laser causes a significantly greater decrease when graphene is not present (Figure 9). This leads to confusion about how the cw laser is interacting with the surface of graphene and InP.

Although the 365 nm laser clearly affects the THz emission and transmittance, we cannot determine if this effect is primarily from graphene or InP because emission (system 1) and transmission (system 2) results are contradictory. Experiments with cw lasers on InP only and graphene on InP need to be continued in order to fully understand the cw laser effects and its effect on transmission versus emission.

Work by Yuki Sano at the Tonouchi Laboratory at Osaka University is currently doing related research to better characterize the THz dynamics of graphene on InP.

ACKNOWLEDGEMENTS

This research was conducted at Osaka University as part of the NanoJapan program. This material is based upon work supported by the National Science Foundation’s Partnerships for International Research & Education Program (OISE-0968405). Special thanks to the Tonouchi lab members for helping me with this research! Thank you to Sarah Phillips, Junichiro Kono, Cheryl Matherly, and Keiko Packard for organizing this program.

References

  1. Inoue, Ryotaro, Kazuhisa Takayama, and Masayoshi Tonouchi. "Angular Dependence of Terahertz Emission from Semiconductor Surfaces Photoexcited by Femtosecond Optical Pulses." Journal of the Optical Society of America B 26.9 (2009): A14. Print.
  2. Serita, Kazunori, S. Mizuno, H. Murakami, I. Kawayama, Y. Takahashi, M. Yoshimura, Y. Mori, J. Darma, and M. Tonouchi. "Scanning Laser Terahertz Near-field Imaging System." Optics Express 163639th ser. 20.12 (2012): 1-7. Print.
  3. Suzuki, Masato, Masayoshi Tonouchi, Ken-ichi Fujii, Hideyuki Ohtake, and Tomoya Hirosumi. "Excitation Wavelength Dependence of Terahertz Emission from Semiconductor Surface." Applied Physics Letters 89.9 (2006): 091111. Print.

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