Japanese Journal of Applied Physics
Vol. 44, No. 4A, 2005, pp. 1777–1780#2005 The Japan Society of Applied Physics

Dependence of Terahertz Electric Fields on Electric Bias and Modulation Frequency
in Pulsed Terahertz Emissions from Electrically-Modulated Photoconductive Antenna
Detected with Free-Space Electro-Optic Sampling
Takeshi Y ASUI and Tsutomu A RAKI
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan(Received October 25, 2004; revised December 22, 2004; accepted January 11, 2005; published April 8, 2005)

We investigated the dependence of terahertz (THz) electric ﬁelds on electric bias and modulation frequency in the pulsed THz
emissions from electrically modulated photoconductive antennas detected with free-space electro-optic sampling. The linear
dependence of the THz electric ﬁeld on the electric bias achieves distortion-free modulation of the THz pulse even for deepmodulation. We conﬁrmed that the signal-to-noise ratio in free-space electro-optic sampling of the THz pulse is strongly
aﬀected by the 1/f noise characteristics of the mode-locked laser, and is improved eﬀectively by increasing the modulation
frequency. The resulting signal-to-noise ratio was 4,900 in THz radiation power at 0.1 THz using an audio-frequency lock-in
ampliﬁer with a time constant of 1 ms and modulation frequency of 100 kHz. The proposed method will be a powerful tool for
real-time or highly sensitive THz measurements. [DOI: 10.1143/JJAP.44.1777]
KEYWORDS: terahertz, photoconductive antenna, laser noise, amplitude modulation, free-space electro-optic sampling

1.

Introduction

Recent advances in the area of coherent terahertz electromagnetic pulses (THz pulses) have opened the door for newschemes of nondestructive material tests,1) biomedicalmeasurements2) and spectroscopy.3) A typical setup forTHz measurements, such as THz time-domain spectroscopy(THz-TDS) and THz imaging, employs a mode-locked
Ti:Sapphire laser oscillator and a photoconductive (PC)antenna as the driving source and THz emitter (PC-THzemitter),4) respectively. In the setup, a free-space electrooptic sampling (FSEOS) method is often applied to detect
the THz signal.5) The FSEOS scheme is attractive for thecoherent detection of freely propagating THz pulses becauseof its ultrabroad detection bandwidth, high stability, ease ofuse, low probe power and real-time two-dimensional imaging capability.
However, the FSEOS method is much more sensitive tolaser noise than conventional PC detection (using a PC
antenna as the THz detector), because the FSEOS method
detects a THz electric ﬁeld in the form of very small changes
in polarization (typically, 104 –105 ) in the probe light,
whereas the PC detection directly detects a photocurrent
induced by the THz ﬁeld. Consequently, the FSEOS method
is not superior to PC detection with respect to signal-to-noise
ratio (SNR) when the phase-sensitive lock-in detection withlow-frequency ( a few kHz) amplitude modulation (AM) isemployed. In general, mode-locked lasers exhibit a highnoise level in the audio frequency region (AF: <100 kHz),with a 1/f rolloﬀ toward the radio frequency region (RF:>100 kHz).6) In the FSEOS, such laser noise inﬂuence canbe decreased by modulating the THz beam at the RF,
resulting in an improved SNR with RF lock-in detection. On
the other hand, the AF lock-in detection is usually applied tothe PC detection because a poor noise characteristic of theRF region in a transimpedance preampliﬁer, following thePC antenna, makes RF modulation unattractive for PC
detection. Hence, the RF-AM of the THz beam is crucial for
FSEOS to achieve a SNR comparable to PC detection with

E-mail address: t-yasui@me.es.osaka-u.ac.jp

AF modulation.7) Such RF-AM can be achieved bymodulating a pump light with an acousto-optic modulator(AOM) or an eletro-optic modulator (EOM). However, thesemodulators induce the following problems: (a) temporal
broadening due to group velocity dispersion in a thickmodulator crystal; (b) modulation distortion due to nonlinear
response in the case of deep modulation; (c) free-spacepropagating electromagnetic-induced noise from the highpower modulation driver; and (d) expensive and complicatedapparatus. Furthermore, (e) spatial dispersion of the modulated laser beam causes another problem in the AOM
method. When we attempt to compensate for (a) and (e),additional procedures such as temporal and spatial prechirping are required.8) There is a need, therefore, for a newtechnique that can modulate the THz beam directly without
the need for an AOM or EOM. Recently, the electric bias
modulation of the PC-THz emitter is often been applied asanother technique for high-frequency AM of the THzpulse,9–11) and it has been implemented in commerciallyavailable system worldwide. The bias modulation technique
is simple and does not require any special or expensive
apparatus such as an AOM or EOM. Other advantages ofthis technique over AOM and EOM include no temporal/
spatial broadening of the pump pulse and no electromagnetic-induced noise. However, the detailed investigations of the dependence of the pulsed THz emission onelectric bias and modulation frequency for this technique arelacking.
Here, we evaluate the dependence of a THz electric ﬁeld
on the electric bias in a pulsed THz emission from anelectrically modulated PC-THz emitter detected using theFSEOS method. We further investigate the dependence ofmodulation frequency on the SNR, and demonstrate animproved SNR based on the resultant frequency dependence.
Furthermore, we show the feasibility of real-time THz
measurements using a FSEOS setup with PC bias modulation.
2.

Experimental Setup

Figure 1 shows a schematic diagram of the experimental
setup. We used a typical THz-FSEOS setup comprising a1777

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T. YASUI and T. ARAKI

Fig. 1. Experimental setup. ML-Ti:S laser: mode-locked Ti:Sapphirelaser, BS: beam splitter, CRs: corner reﬂectors, PC-THz emitter:
photoconductive THz emitter, Si lens: silicon lens, OAP: oﬀ-axis
parabolic mirror, P: polarizer, ZnTe: zinc telluride crystal, =4:
quarter-wave plate, RP: Rochon prism, auto-balanced PD: auto-balanced
photodetector, AMP: ampliﬁer, OSC: oscillator.

mode-locked Ti:Sapphire laser oscillator, a PC antenna for
THz generation (PC-THz emitter) and an electro-optic
crystal for FSEOS. The laser is an 87 MHz Kerr-lensmode-locked Ti:Sapphire laser (Avesta Project, Ltd., TiFKit-100) pumped by a 5 W frequency-doubled Nd:YVO4laser (Showa Optronics, JUNO5000) running at 532 nm. The
laser pulse has a duration of 80 fs and an average power of300 mW at 808 nm. The PC antenna was fabricated on lowtemperature-grown GaAs with a bowtie length of 1 mm anda 5 mm gap. A 1-mm-thick (110) ZnTe crystal was used as aTHz sensor for FSEOS. The probe signal is detected with anauto-balanced photodetecor (New Focus Inc., 2007M,
bandwidth = 125 kHz) after passing through a quarter-wave
plate and a Rochon prism.
For electric bias modulation of the PC-THz emitter, weprepared an oscillator (OSC) and an ampliﬁer (AMP). The
electric bias of the PC-THz emitter was modulated in aunipolar sinusoidal wave with an appropriate oﬀset (max.
freq. = 1 MHz, max. amplitude = 30 Vp-p ). The probe signal inﬂuenced by the modulated THz electric ﬁeld wasmeasured with a lock-in ampliﬁer (NF Corporation, 5610B,bandwidth = 200 kHz) referred to the OSC.
3.

Results

Two major methods are used for modulating a THz pulse
from a PC-THz emitter: (1) modulation of the incident laser
power, and (2) the modulation of electric bias voltage.
Working from this viewpoint, we ﬁrst investigated theeﬀects of both the modulation methods on THz radiation.
Figure 2(a) shows the dependence of the maximum THz
electric ﬁeld in the temporal domain on the PC electric bias.
To evaluate the distortion of the temporal waveform of theTHz pulse with an increase in the electric bias, we also show
the dependence of THz amplitude at several diﬀerent
frequencies (0.25, 0.5, 0.75, 1, 1.25 and 1.5 THz) on electricbias, obtained by THz time-domain spectroscopy (THzTDS), in Fig. 2(b). From these results, we conﬁrm that theFig. 2. Dependence of THz radiation on electric bias: (a) maximum THz
electric ﬁeld in temporal domain and (b) THz amplitude at several
diﬀerent frequencies in frequency domain. THz amplitude is normalized
to its maximum at each frequency.

THz electric ﬁeld and THz amplitude at each frequency
increase almost proportionally with an increase in the bias
voltage. Hence, we can adjust THz radiation without amarked waveform distortion using the bias voltage in thePC-THz emitter. The linear relationship between PC-THz
emitter bias and THz radiation oﬀers the possibility of
distortion-free modulation of the THz pulse even when deep
modulation is required. However, some distortion of the THz
waveform (for example, perturbation on the negative part ofthe bipolar waveform) must be considered under themodulation of higher bias voltages because low-temperature–grown GaAs PC antennas indicate a tendency toincrease response time with bias voltage.12) We next
investigated the dependence of (a) the maximum THz
electric ﬁeld and (b) the THz amplitude at several diﬀerent
frequencies (0.25, 0.5, 0.75, 1, 1.25 and 1.5 THz) on thepump power as shown in Fig. 3. The THz electric ﬁeld andamplitude of the THz pulse increase with an increase inpump power but show a tendency toward saturation in thehigh-pump-power region. In general, the saturation eﬀect isattributed to a screening of the bias ﬁeld by excited
photocarriers.13) Furthermore, the absorption may start tosaturate at the power levels under investigation. Such anonlinear proﬁle in pump power causes a secondary
modulation distortion of the THz radiation when using an
AOM or EOM.
The SNR in FSEOS is strongly aﬀected by the noisecharacteristics of the mode-locked Ti:Sapphire laser, whichJpn. J. Appl. Phys., Vol. 44, No. 4A (2005)

T. YASUI and T. ARAKI

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Fig. 5. Comparison of amplitude in the THz signal and background inFSEOS with respect to modulation frequency.

Fig. 3. Dependence of THz radiation on pump power: (a) maximum THz
electric ﬁeld in temporal domain and (b) THz amplitude at several
diﬀerent frequencies in frequency domain. THz amplitude is normalized
to its maximum at each frequency.

is an essential component for generating and detecting theTHz pulse. It is therefore necessary to determine an optimum
modulation frequency from the viewpoint of laser noisecharacteristics; hence, we evaluated the noise characteristics
of our laser. The laser pulse was detected with a Si-PIN
photodiode (Hamamatsu, S5973, cutoﬀ freq. = 1.5 GHz)and the resultant noise was determined with a spectrum
analyzer (Agilent Technologies, 4395A, RBW = 1 Hz).
Figure 4 shows the noise-power spectrum of our laser inthe AF region between 0.1 and 100 kHz. The background,Fig. 4. Noise power spectrum of the mode-locked Ti:Sapphire laser. The
background is the noise level of the spectrum analyzer and photodiodewhen the laser beam is blocked. The frequency resolution of the spectrum
analyzer is 1 Hz.

i.e., the instrumental noise of the spectrum analyzer andphotodiode under no incidence of the laser beam, isapproximately 120 dBm. As can be seen in Fig. 4, the laser
noise spectrum shows 1/f characteristics with a peak at
2 kHz and becomes comparable to the background noiselevel at 100 kHz. This indicates that a modulation at 100 kHz
yields a suﬃcient improvement in the laser noise characteristics. At a modulation of 100 kHz, laser noise is only 1% ofthat at 1 kHz.
We next investigated the relationship between modulation
frequency and SNR in FSEOS. In the experiment, weapplied a modulated voltage signal (amplitude = 20 Vp-p ,oﬀset = 10 V, and freq. = 100 Hz–100 kHz) to the PC-THz
emitter as an electric bias at a ﬁxed pump power (¼ 50 mW).
A modulated voltage signal such as this is easily generated
by a conventional audio-frequency ampliﬁer (see Fig. 1), forwhich the price is much less than that of an AOM or EOM.
Figure 5 compares the amplitudes of the THz signal andbackground at 0.37 THz (determined by the THz-TDS
method) with respect to the modulation frequency. The
time constant of the lock-in ampliﬁer is 100 ms. The
background corresponds to the signal level of the probe
light when the THz beam is blocked. The background also
conforms to a 1/f trend; while the signal level is almost
constant over the modulation frequency range, such that theSNR improves with an increase in modulation frequency (forexample, an SNR of 1,000 in the THz electric ﬁeld at
100 kHz). The 1/f characteristic of the observed laser noise(see Fig. 4) strongly contributes to the SNR improvement in
the high-frequency region. A 100-kHz modulation results ina practically suﬃcient improvement of SNR in FSEOS,
although the bandwidth of the balanced photodetector(125 kHz) limits the range of the modulation frequency inthis THz-TDS system. If the bandwidth of the signal
detection system is markedly expanded, the upper limit of
the possible modulation frequency is then determined by thefrequency response characteristic, that is the RC time
constant, of the PC-THz emitter and peripheral electronics
such as the electric signal cable and connector. The RC time
constant for the same type of PC antenna has been estimatedto be approximately 0.2 ps.14) Hence, the PC modulating
method oﬀers the possibility of RF modulation (>100 kHz)beyond the AF range (<100 kHz) if the capacitance of theperipheral electronics is negligible. Such RF modulation,
however, is not practical because of the poor performance of

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T. YASUI and T. ARAKI

radiation power) at 0.1 THz for a single scan of the timedelay. This result suggests the applicability of this techniqueto fast-delay-scan, single-pixel THz imaging.
4.

Conclusions

We evaluated the dependence of THz electric ﬁeld onelectric bias and modulation frequency in the pulsed THz
emission from electrically modulated PC-THz emitter
detected using the FSEOS method. The linear dependenceof the THz pulse on electric bias results in the distortion-freemodulation of the THz pulse even when deep modulation isrequired. The SNR based on the FSEOS setup with PC bias
modulation was eﬀectively improved with an increase inmodulation frequency, which is consistent with the 1/f noisecharacteristics of the actual mode-locked laser. The resultant
SNR was 4,900 in THz radiation power at 0.1 THz using an
AF lock-in ampliﬁer with a time constant of 1 ms and a
modulation frequency of 100 kHz. We evaluated the feasibility of real-time THz measurements based on the presentsetup. This technique is also useful for highly sensitive THzTDS by combining the fast bias modulation and doublemodulation techniques.16)
Acknowledgement

Fig. 6. (a) Temporal waveform of the THz pulse and (b) comparison of
amplitude spectra in THz signal and background. The modulation
frequency of the PC electric bias and the time constant of the lock-in
ampliﬁer are 100 kHz and 1 ms, respectively.

the current RF lock-in ampliﬁers and fast balanced photodetectors. For example, the internal noise level of RF lock-in
ampliﬁers is approximately 100 times higher than that of AF
lock-in ampliﬁers.15) A similar improvement in SNR in
FSEOS (see Fig. 5) can be achieved by the conventional
EOM and AOM methods. It is important to emphasize,
however, that the PC bias modulation method achieves theseresults with no temporal or spatial broadening of the pumppulse, no modulation distortion, no free-space-propagating
electromagnetic-induced noise and, most importantly, noneed for an expensive apparatus.
To evaluate the performance of the proposed modulation
method in real-time THz measurements, we carried out rapid
THz measurements using an AF lock-in ampliﬁer with atime constant of 1 ms, in which a modulated voltage signal(amplitude = 30 Vp-p , oﬀset = 15 V, and freq. = 100 kHz)is applied to the PC-THz emitter bias. Figures 6(a) and 6(b)show the resulting temporal waveform and amplitude
spectrum of THz pulse, respectively. For comparison, thebackground signal due to the instrumental noise is also
shown in Fig. 6(b). From this result, we conﬁrmed a SNR ofapproximately 70 in the THz electric ﬁeld (4,900 in THz

This work was supported by the Industrial Technology
Research Grant Program 2004 from the New Energy andIndustrial Technology Development Organization (NEDO)of Japan.
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