Generation of continuous terahertz wave by differential-frequency-mixing in a GaAs/AlAs multiple quantum well

I. Introduction As continuous wave (CW) terahertz (THz) sources, the differential-frequency-mixing (DFM) has an advantage for the frequency tenability by changing the energy separation of the two lasers. Considering the... [ view full abstract ]

I. Introduction

As continuous wave (CW) terahertz (THz) sources, the differential-frequency-mixing (DFM) has an advantage for the frequency tenability by changing the energy separation of the two lasers. Considering the inhomogeneous width of the quantum confinement systems, use of the excitons enables wide frequency tuning. The THz sources with the narrow bandwidth and wide frequency tunability will be applied to the high resolution THz spectroscopy. Therefore, in this work, we show the CW-THz wave generation by DFM under the exciton excitation conditions in a GaAs/AlAs multiple quantum well (MQW).

II. Methods 

We used an undoped GaAs/AlAs MQW embedded in a p-i-n structure on a (001) n⁺-GaAs substrate. The thickness of GaAs and AlAs layer is 7.5 nm. The measurements of THz wave were carried out at 297 K. As the laser sources, a semiconductor laser and a CW-mode Ti:sapphire laser to change the excitation energy were used. The schematic of the experimental setup is shown in Fig. 1. The two beams were spatially combined on the half mirror in free space with the same polarization, and the combined beam is focused on the sample surface.

III. Results and Discussion

Figure 2 shows the dependence of the signal intensity on the energy of the Ti:sapphire laser. The energy of the semiconductor laser is indicated by an arrow. The dotted lines indicate the exciton energies. While the excitation power of semiconductor laser was kept at 3.40 kW/cm², that of the Ti:sapphire laser was variously changed. When the Ti:sapphire-laser power increases up to 17.0 kW/cm², the peak structure appears in the exciton energy region as indicated by the open circles. In the excitation-power dependence measurement, the signal intensity shows the square dependence on the excitation power. Essentially, the excitons in the zincblende structure semiconductors does not show the second optical nonlinearity because of the inversion symmetry. However, the built-in-field by p-i-n structure breaks the symmetry of the electron and hole envelope functions. This symmetry breaking creates the second optical nonlinear polarization. Therefore, our results demonstrate the possibility of the CW tunable THz source based on the exciton effects.