In this chapter, we will briefly describe our femto-second laser system in section
3.1 and introduce the THz time-domain spectroscopy (THz-TDS) systems in section
3.2.
3-1 Introduction of Femtosecond Laser System
Our femtosecond later system is shown in Fig 3-1. We use the Ti:Sapphire laser as
the seeding laser which is then directed into the Ti:Sapphire regenerative amplifier
(Spitfire, Spectra-Physics) for amplification. The pump laser of Spectra Physics
Tsunami laser is a 5W frequency doubled diode-pumped Nd:YLF laser (Millennia V,
Spectra-Physics) with a wavelength λ=532 nm. The Ti:Sapphire laser provides an output trace of intense 35fs pulses with wavelengths ranging from 750nm to 850nm.
The pulse repetition rate is ~82 MHz and the output power can up to 0.4W. The
properties of these two laser systems are shown in Table 3-1. Properties of Tsunami
laser and Ti:Sapphire regenerative amplifier..
The pump laser for the amplification process in Spitfire is Q-switched Nd:YLF
laser which delivers a high power output of 20W at 527 nm. The Spitfire amplifies the
seeding pulses by a million times from 6 nJ of energy per pulse to 2 mJ per pulse. The
pulse repetition rate is 1 kHz and the output power is about 2W.
Fig 3-1. Femtosecond laser system includes Tsunami, Spitfire and two pump laser
(Millennia V and Empower).
Tsunami laser Ti:Sapphire regenerative amplifier
Wavelength 800 nm 800nm
Pulse width 35 fs 50fs
Repetition rate 82 MHz 1kHz
Energy 0.6 nJ 2mJ
Polarization Vertical, linear Horizontal, linear
Table 3-1. Properties of Tsunami laser and Ti:Sapphire regenerative amplifier.
Ti:Sapphire regenerative amplifier
Ti:Sapphire laser for seed beam (Tsunami, Spectra-Physics) Millennia V
Empower
35fs ,82 MHz 200mW,λ= 800nm 50fs ,1 KHz
2W, λ= 800nm
20W , λ= 527 nm
3-2 Electro-Optic THz System
The optical setup of the Electro-Optic THz system is shown in Fig 3-2. An
amplified Ti:Sapphire laser providing 50fs, 800nm, 2mJ pulsed at repetition rate of
1kHz is used to drive this system. The linearly s-polarized incident beam is divided into
two separated beams by a beam splitter. The transmitted beam from the beam splitter is
used as pump beam to excite carrier in our samples and generate terahertz pulse. The
other beam, reflected beam, is used as the probe beam to detect terahertz pulse signal.
There is a half-wave plate in order to rotate the polarization of the pump beam to
linearly p-polarized. Therefore, we could generate linearly p-polarized THz pulsed in a
semiconductor surface emitter such as InAs at the incident angle of 70 degrees to the
surface normal which is close to the Brewster angle. We use a teflon sheet which has a
high transmissive characteristic in the terahertz region to block any reflected laser beam
from the emitter.
The generated THz radiation is collimated and focused onto the sample by a pair
of gold-coated off-axis parabolic mirrors with focal lengths of 3 and 6 inches
respectively. The transmitted THz radiation is again collimated and focused onto a
2-mm-thick (110) ZnTe crystal for free space electro-optic sampling by another pair of
parabolic mirrors with the same focal lengths with previous pair. A pellicle beam
splitter which is transparent to the THz beam and has a reflectivity of 5% for 800nm
light is used to make the probe beam collinear with the THz beam in the ZnTe crystal.
The time delay of the probe beam, which can be tuned by the motor stage, is guided to
the ZnTe crystal and the terahertz pulse collinearly impinged on it. The linear
polarization of the probe beam is perpendicular to the polarization of the THz beam and
we adjust the azimuth angle of the ZnTe crystal to achieve the highest modulation
efficiency.
Polarization of the probe beam modulated by the THz radiation is converted to
ellipsoid polarization by a quarter-wave plate. The transmitted laser pulse with
polarization changed by electro-optical effect is separated into two beams with
orthogonal polarizations by Wollaston beam splitter. These two beams are coupled to a
balanced detector with two silicon photodiodes which is used to detect the differential
signal between two individual probe beams and the signal is proportional to the THz
electric field. A motor stage within the probe beam path is used to scan the delay time
between the probe pulse and the THz pulse imposing on the ZnTe crystal to obtain the
entire THz time-domain waveform. Connecting signal from the balance detector to a
lock-in amplifier, the signal can be easily analyzed by a computer.
In order to reduce the water vapor absorption and increase the signal to noise
ratio, an optical chopper and a lock-in amplifier are used. Otherwise, the entire THz
terahertz pulse with its corresponding spectrum under humidity of 55% and 5%
generated by this setup is shown in Fig 3-3 and Fig 3-4
Fig 3-2. Electro-Optic THz system
Emitter
ZnTe
λ/4 Plate Wollaston Prism
Pellicle
Balanced Detector Motor
λ/2 Plate
Chopper 50fs ,1 KHz ,2W
λ= 800nm
Nd Filter
Nd Filter
0 2 4 6
Fig 3-3. THz time-domain (a) waveform and (b) its corresponding spectrum generated by the electro-optic THz system using a-plane InN as emitter under the humidity of
Fig 3-4. THz time-domain (a) waveform and (b) its corresponding spectrum generated by the electro-optic THz system using a-plane InN as emitter under the humidity of 5%