Effects of electric field screening induced by photogenerated carriers on terahertz wave measurement in a GaAs epitaxial structure

In this work we investigate the effects of electric field screening induced by photogenerated carriers on terahertz wave radiation originating from transient phenomena in the electric field region of a GaAs epitaxial structure. The transient-phenomena excitation and timing of the screening are individually controlled by a pair of ultrashort optical pulses separated by a time interval. Under the condition that the preceding pulse is intensity-modulated by an optical chopper and is irradiated to the sample, the amplitude of the terahertz wave generated by the subsequent pulse is modulated. This result originates from electric field modulation by photogenerated carriers in the preceding pulse.


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n semiconductor crystals under irradiation by ultrashort laser pulses with a pulse duration of the order of femtoseconds, transient phenomena in electronic and lattice dynamics are excited in the sub-picosecond range. 1)][19][20][21] Hence, the terahertz waveform depends on the electric field strength.Therefore, observation of the electric field dependence of the terahertz wave radiation is important for a comprehensive assessment of transient dynamics.In general, the internal electric field is controlled by an external bias voltage and additional structures of doped layers and electrodes.The internal electric field can be continuously controlled by the structure, but the doped layer on the surface side has to be as thin as possible to decrease the absorption of the terahertz waves generated in the intrinsic layer. 22)Cho et al. investigated the electric field dependence of coherent optical phonon oscillations in bulk GaAs by utilizing the phenomenon whereby electron-hole pairs generated near the surface screen the electric field. 23)This method does not require the fabrication of complex semiconductor structures so the problem associated with the absorption of terahertz waves described above does not arise.Moreover, the terahertz wave radiation with electric field dependence can be selectively observed with a lock-in amplifier if the electric field strength is periodically changed by photogenerated carriers.Such a measurement technique is widely used in modulation spectroscopy. 24)Up to now, however, there are no reports on the utilization of the electric field modulation induced by photogenerated carriers in the measurement of the terahertz wave radiation from transient phenomena.
In this letter we report the effects of the electric field screening induced by photogenerated carriers on the terahertz wave measurement in a GaAs epitaxial structure.The transient phenomena excitation and the timing of the electric field screening were individually controlled using a pair of optical pulses separated by a time interval.Under the condition that the preceding pulse is intensity modulated by an optical chopper and irradiated to the sample, the amplitude of the terahertz wave generated by the subsequent pulse is modulated in synchrony with the modulation frequency of the preceding pulse.This result originates from the electric field modulation induced by photogenerated carriers of the preceding pulse.
The sample used in this work was an epitaxial structure of undoped GaAs (i-GaAs) and n-type GaAs (n-GaAs) layers grown on a (001) semi-insulating GaAs substrate by metal organic vapor phase epitaxy.The thicknesses of the i-GaAs and n-GaAs layers were 200 nm and 3 μm, respectively.The electron concentration of the n-GaAs layer was 3 × 10 18 cm −3 .The i-GaAs top layer has a uniform built-in electric field due to Fermi-level pinning. 25)The electric field strength depends on the thickness of the i-GaAs layer and the temperature. 26,27)The electric field strength in the present sample is 30 kV/cm at 300 K, estimated from photoreflectance spectroscopy with an analysis of Franz-Keldysh oscillations. 28)The characteristics of the terahertz wave radiation in the GaAs epitaxial structures were investigated in our previous studies.The terahertz wave radiation originates from the accelerated motion of photogenerated electrons in the i-GaAs layer. 12)In addition, we clarified that the amplitude of the terahertz wave is almost proportional to the electric field strength in the i-GaAs layer. 27)Figure 1 shows a schematic diagram of the measurement system for the timedomain terahertz waveform.The light source was a modelocked Ti:sapphire laser with a pulse duration of ∼80 fs and a repetition rate of ∼76 MHz.The pump pulse was divided into a pair of optical pulses, with the average power of each optical pulse being 10 mW.One of the optical pulses passes through a retroreflector in a fixed position (hereafter this pulse is referred to as the FIX pulse) and the other passes through a retroreflector that has translational motion by a piezoelectric element (hereafter this pulse is referred as the VAL pulse).The translational motion leads to a change in the optical path length for the VAL pulse, so the time interval between the FIX and VAL pulses τ is changed (τ is defined as positive when the FIX pulse precedes the VAL pulse).In terahertz wave measurements, a conventional electro-optic sampling technique with a 100 μm thick (110) ZnTe crystal was used. 29)The detection bandwidth was from 0 to about 4 THz 30) and the measurable time range restricted by the multiple reflections of the terahertz wave pulse inside the electro-optic crystal was about 2 ps. 31)The present electrooptic sampling is sufficient to observe the terahertz wave radiation from the sample. 27)An optical chopper that is the intensity modulator for lock-in detection was placed at position A or B, and the modulation frequency was 2 kHz.At position A, the FIX and VAL pulses are modulated whereas only the FIX pulse is modulated at position B. All the terahertz wave measurements were performed at room temperature under a dry air purge.
First, we discuss the measured results for the terahertz waveforms under the condition that both optical pulses are modulated by the optical chopper at position A. Figure 2(a) shows the terahertz waveforms at different τ from −1.00 ps to 1.00 ps with a step of 0.25 ps, where the irradiation timings of each optical pulse are approximately indicated by the color bars as a guide to the eye.The terahertz waves generated by the FIX and VAL pulses were observed simultaneously because the amplitudes of both terahertz waves were modulated.Figure 2(b) plots the amplitudes of the terahertz waveforms at the irradiation times of each optical pulse as a function of τ, where the solid circle (solid triangle) indicates the amplitude for the FIX (VAL) pulse.The values are normalized by the terahertz wave amplitude under the single-pulse excitation.The oscillatory shape around τ = 0 originates from the superposition of the terahertz waves generated by the FIX and VAL pulses.Under the condition |τ| > 0.50 ps the amplitudes are clearly different; namely, the amplitude of the terahertz wave generated by the VAL pulse is smaller than that under the single-pulse excitation condition when the FIX pulse precedes the VAL pulse (positive τ) and the relationship is opposite when the VAL pulse precedes the FIX pulse.As demonstrated in earlier work, carriers photogenerated by ultrashort optical pulses rapidly screen the surface electric field in bulk GaAs, as reflected in the dynamics of the coherent optical phonons excited by the subsequent optical pulse. 23)In the present sample, the amplitude of the terahertz wave is almost proportional to the electric field strength in the i-GaAs layer. 27)Accordingly, the amplitude of the subsequent optical pulse decreases with decreasing electric field strength in the i-GaAs layer.The stable behavior at |τ| > 0.50 ps in Fig. 2(b) indicates that electric field screening remains stable for at least 1 ps.At |τ| > 0.50 ps, the amplitude of the terahertz wave generated by the subsequent optical pulse is down to about 70%.As mentioned above, the amplitude of the terahertz waveform varies in proportion to the electric field strength.Assuming that the relationship holds for the dynamical electric field change shown in Fig. 2, the electric field strength in the i-GaAs layer is estimated to change from 30 kV/cm to 21 kV/ cm.
Here, we briefly mention the duration of the electric field screening.The carrier lifetime is one of the factors that determines the duration; the carrier lifetime in GaAs is reported to be of the order of 1 ns. 32)Moreover, the diffusion of photogenerated carriers in the in-plane direction, which   051006-2 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd evolves in tens of picoseconds, could also be related to the duration. 33)The time scales of these are much longer than the time range in Fig. 2. Therefore, the recovery of the electric field strength is negligible in the present discussion.
Next, we discuss the measured results for the terahertz waveforms under the condition that only the FIX pulse is modulated by the optical chopper at position B. Figure 3 shows the terahertz waveforms at different τ from τ = −1.00ps to 1.00 ps with a step of 0.25 ps.The terahertz wave generated by the FIX pulse is observed at all τ, whereas the amplitudes are decreased at negative τ.This behavior originates from the screening of the electric field strength induced by photogenerated carriers of the VAL pulse, as shown in Fig. 2. It is noted that another signal is observed around the timing of VAL pulse irradiation at positive τ. Figure 4(a) shows the terahertz waveforms measured under different chopper positions at τ = 0.75 ps.The signals around 0.8 ps are clearly different.The 0.8 ps signal for chopper position B originates from a different modulation mechanism than that for the chopper at position A because the VAL pulse is not intensity modulated.As discussed for Fig. 2(a), the electric field strength in the positive time delay is decreased by photogenerated carriers of the preceding FIX pulse.From another perspective, the electric field strength changes in real time in synchrony with the modulation frequency of the FIX pulse.Accordingly, the amplitude of the terahertz wave generated by the subsequent VAL pulse can be modulated in synchrony with the electric field change.The difference in the 0.8 ps signals is attributed to the difference in the responses for the preceding FIX pulse irradiation, as schematically shown in Fig. 4(b).The FIX pulse irradiation leads to the radiation of terahertz waves and to screening of the electric field strength.At chopper position B, the amplitude of the terahertz waves generated by the VAL pulse decreases under the FIX pulse irradiation conditions due to electric field screening.Phase adjustment of the lock-in amplifier was performed using the terahertz wave signal generated by the FIX pulse, so the phase of the modulation of the terahertz wave generated by the VAL pulse is the reverse.
In order to verify the above interpretation, we calculated the terahertz waveform at τ = 0.75 ps under chopper position B. The amplitude of the terahertz wave in the present sample varies in proportion to the electric field strength, whereas the frequency change is small for electric field changes of a few kV/cm. 27)Hence, for simplification only the electric field dependence of the amplitude is considered.The red curve in Fig. 5(a) is the measured terahertz waveform (A 0 ) under the condition that only the FIX pulse is irradiated.The blue curve is the terahertz waveform (A s ) calculated by A s = A 0 × k, where k is smaller than 1.A s means that the electric field screening decreases the amplitude of the terahertz wave by a factor k. The green curve is the calculated result for A s − A 0 .The value of A s − A 0 corresponds to the amount of change in the terahertz wave amplitude due to the change in the electric field, which can be measured under lock-in For comparison with the experimental result, A s − A 0 is shifted by 0.75 ps and subsequently added to A 0 .The open circles and green curve in Fig. 5(b) indicate the experimental and calculated results, respectively, where k = 0.65.The experimental result is well reproduced by the calculation.The value of k is reasonable because it almost corresponds to the decrease in the amplitude due to electric field screening, as shown in Fig. 2(b).The result demonstrates that the modulation signal observed at the irradiation timing of the VAL pulse originates from the modulation of the electric field strength in the i-GaAs layer by photogenerated carriers.Thus, the terahertz wave component that depends on the In conclusion, the irradiation of intensity-modulated optical pulses periodically changes the electric field strength, which leads to the modulation of the amplitude of the terahertz waves generated by subsequent pulses.This behavior is applied to the selective measurement of terahertz wave components with electric field responsivity.This finding can contribute to comprehensive investigations of transient dynamics and is also useful for the assessment of physical properties through terahertz wave observations.

Fig. 1 .
Fig.1.Schematic diagram of the optical setup for terahertz waveform measurement using a pair of ultrashort optical pulses.One of the optical pulses passes through a retroreflector in a fixed position (FIX pulse) and the other passes through a retroreflector with translational motion by a piezoelectric element (VAL pulse).The time interval between the FIX and VAL pulses is defined as positive when the FIX pulse precedes the VAL pulse.The optical chopper was placed at position A or B.

Fig. 2 .
Fig. 2. (a) Measured terahertz waveforms at different time intervals between the FIX and VAL pulses from -1.00 ps to 1.00 ps with a step of 0.25 ps under the condition that both optical pulses are modulated by the optical chopper.The irradiation timings of each optical pulse are approximately indicated by the color bars as a guide to the eye.(b) Normalized amplitudes of the terahertz waveforms at the irradiation timings as a function of the time interval, where the solid circles (solid triangles) indicate the amplitude due to the FIX (VAL) pulse.The values are normalized by the terahertz wave amplitude under the single-pulse excitation.

Fig. 3 .Fig. 4 . 3 ©
Fig.3.Measured terahertz waveforms at different time intervals between the FIX and VAL pulses from -1.00 ps to 1.00 ps with a step of 0.25 ps under the condition that only the FIX pulse is modulated by the optical chopper.