Weyl semimetals are fascinate material in the world of material science. Weyl semimetals are Basically 2D materials with Intersecting band structure like graphene . The point in which conduction band and valence band intersect is called weyl point. Also the property of band is they are “linearly dispersing”, which means the second derivative of the Energy with Wave vector(k) is 0. So the effective mass of the electron is ZERO!
In many condensed matter systems, there are many phenomenon which persist for a short time in semimetals as well as other forms of matter. To analyze these events one of the useful method is pump probe analysis. Basically in pump probe analysis we use a femtosecond laser which produce laser pulses with femtosecond pulses. We split them as pump and probe, then one of them is sent to sample and another sent to detector. When the laser hits the sample(here TaAs the weyl semimetal), there will generate some transient photocurrent. As a result of this, terahertz radiations are generated . Here these terahertz is collected using the arrangement and analyzed with pump pulse and retrace the waveform of the emitted terahertz. By analyzing the Polarization, phase, Intensity, frequency, we will get many hidden transient phenomenon inside the material.
We can see a clear polarity reversal of the emitted THz waveform polarized along the axis, [1,-1,0] occurring as the helicity of the optical generation pulse is tuned from left circular to right circular polarization.
The peak-to-peak amplitude of the emitted THz electric (E) field while rotating the λ/4 wave plate (QWP) over a full 360° reveals a sinusoidal dependence whose periodicity matches a change in helicity of the incident light. Fitting with a general expression for the polarization dependence of the photocurrent
THz emission polarized along the [1,1,-1] direction, obtained under the same excitation conditions as above, was found to be largely insensitive to the polarization of incident light, ie, The dominant contribution from “D”.
Despite this polarization insensitivity, the emitted THz radiation is linearly polarized along the axis and exhibits a well-defined directionality. This is illustrated by both an azimuthal dependence that shows the amplitude of the helicity-independent THz waveform to peak along this high symmetry direction.
Here we can clearly say that THz is highly directional
THz pulses emitted along both high symmetry axes of the (112) surface are found to scale linearly with laser fluence and exhibit no change in waveform or frequency content as higher excitation fluences
By Fourier transforming the THz time-domain traces shown above, one finds the spectral weight of the THz intensity power spectrum along [1,1,-1] to be shifted towards lower frequencies (∼1.0 THz)
To provide a better estimate for the bandwidth of the helicity-dependent THz pulse polarized along [1,-1,0] we used TFISH
- For a pulsed excitation, the decay of the helicity-dependent photocurrent will be determined by either the momentum or spin relaxation time, depending upon whether it originates from the CPGE or the SGE
- Compared to the axis, the fundamental mechanism underlying THz emission polarized along is distinct. Since this axis contains a projection of the inversion symmetry-broken c axis, both the polarization independence and the well-defined directionality of the photocurrent suggest an underlying mechanism rooted in broken inversion symmetry.
- Like an optical excitation producing electron-hole pairs, regardless of polarization!
- these findings suggest that under optical excitation these transient photocurrents are intrinsic to the underlying crystal symmetry of TaAs, whose C4v symmetry belongs to the gyrotropic crystal class
Reference
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