Other Blog: Naist 茶道会 – 奈良先端科学技術大学院大学　認定課外活動団体 (xrea.com)

The following is an introduction to my current research.

If you have any comments or corrections, please contact me.

Address: ueki.honoka.uj6[at]ms.naist.jp 

Goals

Semiconductors, which are indispensable for the manufacture of electrical appliances, have electrical characteristics that vary depending on the width of the band gap. In current device development, the band gap width is changed by processing semiconductors to suit the application, but this requires many processing steps. Exciton polaritons have attracted attention as a physical phenomenon that could solve this problem. Using this phenomenon, the intrinsic band gap of a material can be changed by coupling it with the cavity mode of light. In this study, we investigate the dynamics of exciton polaritons and explore their application to devices.

Exciton polariton

Exciton polariton is the energy state of an exciton, which is a Coulomb force coupling of an electron and a hole in a semiconductor, coupled with the energy state of a cavity photon. The newly generated polariton level splits into two levels, LP (lower polariton) and UP (upper polariton), and it is possible to arbitrarily create new energy levels that differ from those of bulk materials. The energy level diagram is shown in Figure 1.

Figure 1. 　Energy level diagram of Exciton-polariton

The classical interpretation of this state is that at one instant the energy of light is converted into the energy of excitons of matter, and at the next instant that energy is expelled again to return to the energy state of light, an event that is repeated at high speed. In quantum terms, this is a superposition of the states of matter and light.

Exciton polaritons can create energy levels by coupling that excitons do not originally have. For example, if the bottom level of the electronically excited state is optically inert, the luminescence properties will improve dramatically if the optically active LP level can be made more stable through the formation of polaritons. Thus, it is hoped that it will be possible to control the properties of semiconductors.

Polariton Observation with Transition Metal Dichalcogenides

In ordinary semiconductors, excitons cannot be observed at room temperature. In this study, we focus on two-dimensional materials called Transition Metal Dichalcogenide;TMDC as a new material that can solve this problem.

TMDC has a layered structure in which one transition metal layer is sandwiched by two chalcogen atom layers, each layer bound together by van der Waals forces. I am focusing on MoSe₂, which exhibits semiconducting properties. Excitons formed within a single layer are not shielded from other layers and can form excitons with very strong binding energy.

Another characteristic feature of TMDCs is that their physical properties change with the number of layers: in MoSe₂, the electronic transition changes from an indirect transition to a direct transition when the layers become monolayers. Therefore, the absorbed energy is directly used for light emission. In order to form polariton states, a material with high light absorption and emission efficiency is required, and MoSe₂ is considered suitable for exciton polariton formation at room temperature in this respect.

Figure 2. Structural diagram and band gap of TMDC

(a) Layers of TMDC (b) Dependence of the band gap of TMDC on the number of layers [1].

[1]Splendiani. A, et al., Nano Lett., 2010, 10,  1271-1275

Monolayer Preparation

Currently, we are creating a single layer using the Scotch tape method, which physically creates a single layer by peeling. As mentioned above, the layers of TMDC are bound together by van der Waals forces, so the layers can be peeled off by simply applying and removing the tape. By performing the peeling several times, a single layer can be created. Figure 3 shows an actual sample peeled off using Scotch tape.

Figure 3. TMDC thin layer sample peeled off using scotch tape

 Evaluation of Layer Number by Raman Spectroscopy

The number of layers is evaluated by Raman spectroscopy, which looks at molecular vibrations in terms of scattered light. This results in a higher energy of each molecular vibration. In MoSe₂, the peak in the A1g mode around 240 cm-1 shifts to shorter wavelengths as the number of layers decreases, and the disappearance of the single-layer peak in the B2g mode around 355 cm-1 is observed. We have now succeeded in producing a single layer of a few microns in size, as shown in Figure 4. The Raman spectra results for this sample are shown in Figure 5(a)(b). In the A1g mode shown in Figure 5(a), the peak shifts to the short wavelength side as one moves from the bulk to the monolayer. In the B2g mode shown in Figure 5(b), the peak disappears in the bulk and in the monolayer. As a future prospect, we will work on the fabrication of monolayers with a size larger than 10 micrometers, which is easier to perform pump-probe spectroscopy. 

Figure 4. TMDC single layer observed with a 100x OBJ lens.

The white circle indicates a single layer with a width of about 5 µm.

Figure 5. Raman spectra (red: bulk, purple: single layer)

(a)A_1g mode on the left shows a shift to the low wavenumber side,

(b)B_2g mode on the right shows the disappearance of the bulk and monolayer peaks.

Lifetime evaluation of exciton polariton by pump-probe spectroscopy

In order to apply exciton polaritons as devices, it is necessary to understand the process by which the energy propagates. Exciton polaritons occur in excited states. Since excitation and relaxation of materials occur in the ultrashort time region of femtoseconds and picoseconds, spectroscopy with time resolution of that scale can observe the dynamics of the relaxation of exciton polaritons generated in the excited state.

We have used femtosecond laser pump-probe spectroscopy to observe the time evolution of the energy relaxation process. We irradiate a sample with two beams of light, a pump light and a probe light, and measure the changes in the material excited by the pump light as the reflected spectrum of the probe light. By delaying the time of pump light incidence, the excitation and relaxation processes can be followed like an animation. Using this experimental technique, we observe the energy relaxation process of exciton polaritons formed in the cavity.

Figure 6. Schematic of pump-probe spectroscopy

Simulation using FDTD method

Pump-probe spectroscopy measures the excitation and relaxation processes of a material as a reflection spectrum, but the measurement results include signals unrelated to the dynamics of the material, such as interference of light within a cavity. In order to account for those effects, the light behavior is reproduced by simulation.

Finite-difference time-domain; FDTD method is used for the simulation. A schematic of FDTD method is shown in Figure 7. By calculating the simulation space at each time, the time variation of the electromagnetic field can be observed. Currently, simulations have been successfully performed for a model in which pulses are irradiated to a sample using a cavity. As a future prospect, we plan to conduct simulations for double-pulse irradiation assuming pump-probe spectroscopy.

Figure 7. Schematic diagram of the FDTD method

The area enclosed by the gray square in the figure is the simulation space, with a grid of black lines separating the simulation space.