We propose an experimental technique to generate large amplitude coherent phonons with irradiation of THz-rate pump pulses and to study the dynamics of phase transition in GeTe ferroelectrics. When a single pump pulse irradiates the sample at various pump power densities, the frequency of the soft phonon decreases sub-linearly and saturates at higher pump powers. By contrast, when THz-rate pump pulse sequence irradiates the sample at matched time intervals to forcibly drive the oscillation, a large red-shift of the phonon frequency is observed without saturation effects. After excitation with a four pump pulse sequence, the coherent soft phonon becomes strongly damped leading to a near critical damping condition. This condition indicates that the lattice is driven to a precursor state of the phase transition.
Breaking inversion symmetry in solids plays a central role in nonlinear optics because it can change material properties, such as producing even-order nonlinear optical (NLO) effects. Although a centrosymmetric diamond has been developed as a photonic platform, including waveguides and light sources, the NLO effects in bulk diamonds are limited to the third-order. Thus, exploiting more powerful second-order NLO effects, such as second harmonic generation (SHG), are still challenging. Here we explore symmetry-breaking-induced second-order NLO effects in bulk diamonds using the color center, nitrogen-vacancy center. Exciting with ultrashort laser pulses, SHG and third-harmonic generation (THG) are simultaneously observed at the same time, exhibiting characteristic intensity patterns, depending on both the excitation fluence and the angle of light polarization. We uncovered that SHG serves as the source for THG by the cascading process. Our findings will offer NLO effect-based quantum sensing by diamond color centers, such as imaging of the electromagnetic field by electro-optic effects on the nanofemto scale.
The current generation of quantum sensing technologies using color centers in diamond crystals is primarily based on the principle that the resonant microwave frequency of the luminescence between quantum levels of the nitrogen-vacancy (NV) center varies with temperature, electric and magnetic fields. This principle enables us to measure, for instance, magnetic and electric fields, as well as local temperature with nanometer resolution in conjunction with a scanning probe microscope (SPM). However, the time resolution of conventional quantum sensing technologies has been limited to microseconds due to the limited luminescence lifetime. Here, we investigate ultrafast opto-magnetic effects in diamond crystals containing nitrogen-vacancy NV centers to improve the time resolution of quantum sensing to sub-picosecond time scales. The spin ensemble from diamond NV centers induces an inverse Cotton-Mouton effect (ICME) in the form of a sub-picosecond optical response in a femtosecond pump-probe measurement. The helicity and quadratic power dependence of the ICME can be interpreted as a second-order opto-magnetic effect in which ensembles of NV electron spins act as a source for the ICME. The results provide fundamental guidelines for enabling high-resolution spatial-time quantum sensing technologies when combined with SPM techniques.
Terahertz (THz) time domain emission spectroscopy was performed on bulk single crystal MoSe 2 and WSe 2 . Results show THz emission signals which comprise of a single cycle transient current-driven signal and damped oscillatory signals arising from coherent phonon modes. The damped oscillations have frequencies below 1 THz and are attributed to interlayer shear and breathing modes.
We investigate the effect of nitrogen-vacancy (NV) centers in single crystal diamond on nonlinear optical effects using 40 fs femtosecond laser pulses. The near infrared femtosecond pulses allow us to study purely nonlinear optical effects, such as optical Kerr effect (OKE) and two-photon absorption (TPA), relating to unique optical transitions by electronic structures with NV centers. It is found that both the nonlinear optical effects are enhanced by the introduction of NV centers in the N$^{+}$ dose levels of 2.0$\times$10$^{11}$ and 1.0$\times$10$^{12}$ N$^{+}$/cm$^{2}$. In particular, our data demonstrate that the OKE signal is strongly enhanced for the heavily implanted type-IIa diamond. We suggest that the strong enhancement of the OKE is possibly originated from cascading OKE, where the high-density NV centers effectively break the inversion symmetry near the surface region of diamond.
The authors report femtosecond dynamics of the coherent optical phonon of single crystal diamond. Sub-10fs, 395nm laser pulses excite 40THz coherent phonons with an extremely small damping rate (0.15ps−1). Linear power dependence of the phonon amplitude under off-resonant excitation condition gives a direct evidence for an eletric-field-driven generation mechanism. The coherent phonon generation is noticeably suppressed by doping with nitrogen impurities, in spite of their absorption in the near ultraviolet. The present study demonstrates that a simple pump-probe technique can be a powerful tool for evaluating the ultrafast coherent electronic and lattice dynamics of diamond materials.
We report the optical perturbation of atomic arrangement in the layered in GeTe/Sb2Te3 phase change memory material. To observe the structural change, the coherent A1 mode of GeTe4 local structure was investigated at various polarization angles of femtosecond pump pulses with the fluence at ≤78 μJ/cm2. p-polarization found to be more effective in inducing the A1 frequency shift that can be either reversible or irreversible depending on the pump fluence. The predominant origin of this shift is attributed to rearrangement of Ge atoms driven by anisotropic dissociation of the Ge-Te bonds along the [111] axis after the p-polarized pulse irradiation.
