5350

LetterVol. 44, No. 21 / 1 November 2019 / Optics Letters

Subcycle mid-infrared coherent transientsat 4 MHz repetition rate applicable tolight-wave-driven scanning tunneling microscopy
KATSUMASA YOSHIOKA,1 IPPO IGARASHI,2 SHOJI YOSHIDA,2 YUSUKE ARASHIDA,1,2 IKUFUMI KATAYAMA,1,3
JUN TAKEDA,1,4 AND HIDEMI SHIGEKAWA2,*
1

Department of Physics, Graduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan
Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan3e-mail: katayama@ynu.ac.jp
4e-mail: jun@ynu.ac.jp*Corresponding author: hidemi@ims.tsukuba.ac.jp
2

Received 14 August 2019; revised 9 October 2019; accepted 9 October 2019; posted 9 October 2019 (Doc. ID 375431); published 30 October 2019

We produce subcycle mid-infrared (MIR) pulses at a 4 MHz
repetition rate via the optical rectification (OR) of sub-10 fs
near-infrared pulses delivered by an optical parametricchirped pulse amplifier. The coherent MIR pulses generated in a GaSe crystal under an ultrabroadband phasematching condition contain only 0.58–0.85 oscillation
cycles within the full width at half-maximum of the intensity envelope. The use of OR enables excellent phasestability of 56 mrad over 5.6 h, which is confirmed byfield-resolved detection using electro-optic sampling. An
electromagnetic simulation using a finite integration technique reveals that the peak field strength can easily exceed
10 V/nm owing to the field enhancement resulting fromfocusing MIR pulses onto a tunnel junction. © 2019
Optical Society of America
https://doi.org/10.1364/OL.44.005350

The development of ultrashort phase-locked few-cycle pulses
has been subject to intensive research over a vast region of theelectromagnetic spectrum ranging from terahertz (THz) to ultraviolet [1–11] to reveal novel phenomena in nonlinear lightmatter interactions. In recent years, intense mid-infrared (MIR)
pulses have attracted significant attention owing to their advantageous characteristics of low photon energy, large ponderomotiveenergy, and faster field oscillation than those of typical electron
scattering in solids. Exploiting these advantages, MIR pulses can
induce various exotic quantum phenomena such as FloquetBloch and Volkov states in a topological insulator [12], the acceleration and recollision of quasiparticles in a transition metal
dichalcogenide [13], high-harmonic generation in a semiconductor [14], and bright attosecond pulses from noble gases in thesoft x-ray region [15]. As with THz [16,17] and near-infrared(NIR) [18,19] pulses, the combination of MIR pulses with ananoplasmonic system will further accelerate the ongoing revolution of light wave electronics [13–19] by exploiting locally enhanced near fields. In particular, the use of a tunnel junction is0146-9592/19/215350-04 Journal © 2019 Optical Society of America

the most promising means of producing efficacious near fields
whose field enhancement factor α is expected to be on the orderof λ∕d (λ is the incident wavelength, and d is the nanometerscale gap width) [20].
Recently, light-wave-driven scanning tunneling microscopy(STM) has been realized using carrier-envelope-phase (CEP)stable THz pulses with a huge field enhancement factor of∼105 [21–26], and electron tunneling was coherently manipulated across a tunnel junction. Although THz-field-driven
STM enables ultrafast dynamics to be probed and even controlled with atomic spatial resolution [24,25], the time resolution is limited to the sub-picosecond time scale. The use of
CEP-stable MIR pulses will significantly improve the time
resolution and, therefore, will enable the filming of various
ultrafast processes that are unattainable with THz pulses. MIR
near-field-mediated nonlinear light-matter interactions can also
be studied with unprecedented spatiotemporal resolution,
which will pave the way towards light wave nanoelectronics.
To accurately steer electrons with a light wave in the nonperturbative regime, a single-cycle or even subcycle pulse is indispensable; the inversion-symmetry-broken light field within thepulse can be used to precisely manipulate the motion of electrons. With the field enhancement factor of α ∼ 103 expectedin the MIR region taken into account, a peak field strength of∼100 kV∕cm is required to steer electrons in MIR-field-driven
STM. Although a single-cycle CEP-stable MIR pulse can begenerated by the optical rectification (OR) of broadband
few-cycle NIR pulses [27], the reported peak field strengthis 15 kV/cm, which is limited by the pulse energy of NIR pulses
generated using a Ti:sapphire laser system. Intensive effort has
been made to generate highly intense single or subcycle MIR
pulses by several techniques, such as OR [3], difference frequency generation (DFG) using two optical parametric amplifiers (OPAs) [5], four-wave mixing through filamentation in agas [6], adiabatic DFG [7], and the multiplexing of the signal
and idler pulses from an OPA [8]. However, the repetition rateof these outputs was limited to kHz order. Furthermore, theVol. 44, No. 21 / 1 November 2019 / Optics Letters

Letter

WP1

GaSe
Ge filter

1

10

10

GaSe

10

-1

-2

GaP

10

10
-4
10

Intensity (a.u.)

Intensity (a.u.)

0.01

(b)
0.001

-100

0.6

0.2

8.2 fs

(c)

0.0
700

800

900

Wavelength (nm)

1000

10

-5

0.8

0.4

-40

-20

0

20

-3

-4

-5

-6

PCA

-7

-8

PCA

QWP
1.0

0.1

GaSe
-2

GaP
10

10

-3

WP2
1

-1

0

BPD
EO WP

(b)
10

(a)
DS

0

(a)

Intensity (a.u.)

OPCPA
4.3 W, 4 MHz

The NIR pulses were split into a pump beam and a probe beam
for the generation and field-resolved detection of MIR pulses,
respectively.
The MIR pulses were generated by OR in a z-cut GaSe crystal [27]. To generate subcycle pulses with ultrabroadband phasematching over the entire bandwidth, we used an extremely thincrystal of 20 μm thickness. The crystal was tilted by an angleof 50° with respect to the direction of the pump beam to satisfy
the phase-matching condition. By rotating the emitter crystal
around the optic axis, type-I phase matching was selected. The
generated MIR pulses were then collimated by a gold-coatedoff-axis parabolic mirror of 100 mm focal length and refocusedby a parabolic mirror of 50 mm focal length onto either a
30-μm-thick GaSe or 450-μm-thick GaP crystal for performing
EOS. We also measured MIR pulses using a photoconductive
antenna (PCA, BATOP PCA-44-06-10-1030), which simplified the detection setup.
Figure 2(a) shows the MIR electric field transients measured
by different detection methods: EOS using a GaSe or GaP crystal and the PCA. Each waveform has a strongly asymmetricshape, which is ideal for accurately steering electrons with alight wave. In the case of the EOS trace measured using a
GaSe crystal, the FWHM of the intensity envelope was foundto be 31 fs (determined using a Hilbert transform), which corresponds to 0.85 cycles of the carrier wave at a center frequencyof 27.5 THz. The Fourier-transformed spectra obtained fromthe waveforms are shown in Fig. 2(b). The detection scheme
using a GaSe crystal highlights the highest-frequency component up to 80 THz (3.7 μm). The low-frequency part below
9.5 THz was not detected owing to the Reststrahlen band of

Electric field (a.u.)

long-term CEP stability remains unreliable because of the complexity of the laser systems. Very recently, 1.16 cycle MIR
pulses were generated at a 50 MHz repetition rate by the
OR of 2 μm NIR pulses delivered by a high-power fiber laser
system [28]. Although this technique will provide a scalable
system with a peak field of MV/cm order, its complex building
blocks may reduce the stability and usability of the system.
Besides, the enhanced asymmetry of light waves with fewer
oscillation cycles is indispensable for broadening the range oftargets in light-wave-driven STM. Therefore, future applications for light-wave-driven STM require a straightforward optical setup having subcycle pulses with significantly increased
repetition rates and CEP stability as well as a markedly improved signal-to-noise ratio while maintaining the peak fieldstrength above 100 kV/cm.
Here, we report the generation of CEP-stable subcycle MIR
pulses via OR by employing an optical parametric chirped pulse
amplifier (OPCPA) that delivers sub-10 fs NIR pulses. The
combination of broadband phase matching in a GaSe crystal
and the high average output power from the OPCPA enables
the generation of 0.58–0.85 cycle, 190 kV/cm peak field MIR
pulses at a repetition rate of 4 MHz. Excellent CEP stability
over 5.6 h was demonstrated using electro-optic sampling(EOS). Furthermore, we investigated the MIR near field ina tunnel junction by performing a finite integration simulation.
The enhanced near field above the 82 V/nm peak field is highly
beneficial for driving electron tunneling as well as studying
strong-field light-matter interactions at the nanoscale.
A schematic of the experimental setup is illustrated inFig. 1(a). A custom-built OPCPA system [29] (venteon OPCPA)
was employed to generate subcycle MIR pulses. The repetition
rate of the system can be tuned from 200 kHz to 4 MHz byvarying the built-in pulse picking and amplifier settings. In thecase of a repetition rate of 4 MHz, the maximum output power
was 4.3 W. The spectrum of the NIR laser pulse ranged from 650
to 1050 nm, as shown in Fig. 1(b). Figure 1(c) shows the measured temporal profile of the NIR pulses using a D-scan module[30] (Sphere Ultrafast Photonics). The pulse duration was determined to be 8.2 fs at the full width at half-maximum (FWHM).

5351

40

Time (fs)Fig. 1. (a) Generation of subcycle MIR pulses and field-resolved
detection using electro-optic sampling. DS, delay stage; WP1 and
WP2, wedge plates; EO, electro-optic crystal; QWP, quarter-wave
plate; WP, Wollaston prism; and BPD, balanced photodiode.
(b) Spectrum of the OPCPA output. (c) Temporal profile of the output NIR pulses obtained by the D-scan method.

0100Time (fs)

200

-9

0

20

40

60

80

100

Frequency (THz)

Fig. 2. (a) Field-resolved subcycle MIR pulses detected by different methods: EOS using a 30-μm-thick GaSe crystal with a phasematching angle of 49°, EOS using a 450-μm-thick GaP crystal,and PCA. The black dashed curve shows the Hilbert transform of
the EO trace. The dotted curve shows raw data. For clarity, each waveform is normalized by the maximum peak electric field and offset by 2.
(b) Fourier-transformed spectra obtained from the waveforms in (a).
The different detection schemes reveal that the generated MIR pulses
have an ultrabroadband spectrum spanning the range of 2.4–80 THz(120–3.7 μm).

Vol. 44, No. 21 / 1 November 2019 / Optics Letters

Delay time (fs)
-200

-100

0

100

CEP shift ( rad )
200

0

1

0.1

2
3

1

2

3

4

5

4

0.0

(b)

4

-1

-0.1
0

5

Time (hour)1(a)Electric field (a.u.)

GaSe [27]. On the other hand, the detection scheme using
a GaP crystal is effective for the lowest-frequency componentof 2.4 THz (120 μm) with a dip around 8 THz due to thecancellation of the electronic and lattice contributions to theoptical nonlinearity [31]. The estimated pulse duration wasdetermined to be 27 fs with 0.58 cycles, which is the shortest among the three traces. From the measured pulse energyof 160 pJ [using a calibrated liquid-nitrogen-cooled mercury
cadmium telluride (MCT) detector] and the focal diameterof 100 μm, we obtained a peak electric field strength of
190 kV/cm. To verify the absolute field strength, we also calculated the field strength from the EOS signal of a GaP sensor.
By considering the reflection losses and frequency-dependent
sensitivity of the detector [3,31], we obtained a peak field of
200 kV/cm, which is in good agreement with that from theenergy measurement. In the case of the PCA, we calculated
the time derivative of the measured current response to obtain
the original electric field [32], and the pulse duration was determined to be 47 fs. The simple optical setup of the PCA might behighly beneficial for measuring MIR pulses inside a vacuum
chamber for light-wave-driven STM. Note that the detected
waveforms represent the convolution of the actual electric fieldwith the response function of each detection system [31]. Thus,
the actual pulse duration could be shorter than 27 fs.
During practical experiments on light-wave-driven STM,
measurements with long-term stability over multiple hours
are essential. The extremely high stability of our system wastherefore demonstrated by measuring MIR waveforms over
5.6 h using EOS without any feedback, as shown in Fig. 3(a).
In total, 86 waveforms were taken with a 2 min scan per waveform. Figure 3(b) shows the CEP shift extracted by calculating
the instantaneous phase using the Hilbert transform. The standard deviation from its mean value was determined to be
56 mrad over 5.6 h, which is 1-2 orders of magnitude lower
than that for OPA-based MIR pulses [9,10]. As can be seenin Fig. 3(c), the first and last waveforms are identical except
for the small time drift of less than 2.5 fs. This time drift ismost likely due to the timing jitter between MIR and NIR
probe pulses, which corresponds to the optical path lengthof 750 nm. However, to the best of our knowledge, such extreme long-term stability has not been previously demonstrated
for single or subcycle MIR pulses.
To develop light-wave-driven STM using subcycle MIR
pulses, the estimation of the field enhancement factor α in a tunnel junction is crucial. In the THz spectral range, α was investigated by both experiment and simulation [22,23,25] and wasfound to be approximately 105, which is in good agreement withthe value estimated from λ∕d [20]. However, despite its importance, no previous studies have focused on the field enhancementof MIR pulses in a tunnel junction. We investigated the MIR
near field in a tunnel junction by performing a finite integration
simulation (CST MW STUDIO), which shows a quantitative
agreement with the experiments in THz-STM [23].
Figure 4(a) shows the tip-sample configuration used in thesimulation. The tip was modeled as a cone with a diameterof 0.8 μm and a height of 6.0 μm. The materials used forthe tip and sample were tungsten (W) and gold (Au), respectively, which are typically used in STM experiments. A gap widthof 1.0 nm and a smallest grid size of 2.5 nm × 2.5 nm ×
0.5 nm between the nanotip and the sample were used in
the calculation. The actual MIR far-field waveform measured

LetterElectric field (a.u.)

5352

0 hour
5.6 hour

(c)

2
0
-2
-4
-200

-100

0

100

200

Delay time (fs)Fig. 3. (a) Long-term stability of the CEP-stable MIR pulses. The
MIR waveforms were measured over 5.6 h by EOS using a 30-μmthick GaSe crystal. (b) Overall CEP shift of MIR waveforms measured
over 5.6 h. The black dashed lines indicate the standard deviation of
56 mrad from the mean value. (c) Line plots of the waveforms scanned
at t  0 and 5.6 h.

via EOS was incident from a source plane with p polarization
and an incident angle of 75°. The time evolution of the MIR
electric field was calculated at the midpoint between the nanotip
and the sample, as shown in the inset of Fig. 4(b). The calculated
α was 4,300, which corresponds to a peak voltage of 82 V in thejunction. This α is much higher than that of the isolated resonant
tip [33] because of extremely tight confinement. This extremely
high electric field is higher than those achieved in the field emission regime [16,17] and sufficient for driving real-space electron
tunneling, while the maximum electric field might damage thejunction. Note that there is a notable phase shift between theincident far field and the near field. This phase shift might havebeen due to the antenna effect of the tip [23,26]. In actual experiments, both α and the phase shift differ from tip to tip owing
to variations in geometrical shape. Since complete control of thetip shape is very difficult, the in situ characterization andmanipulation of the MIR near field are highly desirable, similar
to those in the case of THz near fields [23,26].
Our system opens new possibilities for light-wave-driven
STM with sub-10 fs time resolution. The greatly improved
repetition rate and robustness of the system will enable lightwave-driven STM to be combined with additional novel STM
methodologies such as scanning tunneling luminescence spectroscopy [34], optical pump–probe STM [35,36], and lasercombined scanning multiprobe spectroscopy [37]. The enhanced MIR near field of 10 V/nm (100 MV/cm) can easily
access the nonperturbative regime of condensed matter systems. Furthermore, by tuning the repetition rate of the OPCPA

LetterFig. 4. (a) Tip-sample configuration with a gap width of 1.0 nm.
The waveform of far-field MIR electric fields was measured via EOS
with a GaSe crystal [Fig. 2(a)]. (b) Far- and near-field waveforms. The
electric field is normalized by the peak electric field of the far field.
The inset shows a close-up of the tunnel junction.

system to 100 kHz order, the peak field strength reaches the
MV/cm range without the use of a nanoplasmonic system,
making it capable of ultrafast nonlinear spectroscopy [38] inthe subcycle regime.
In summary, by employing a high-power OPCPA systemwith sub-10 fs NIR pulses, we have successfully generated
CEP-stable subcycle MIR pulses at a 4 MHz repetition rate.
The field-resolved detection scheme using EOS and a PCA revealed that the subcycle MIR pulses have an ultrabroadband
spectrum spanning the range of 2.4–80 THz (120–3.7 μm).
The excellent long-term stability of the CEP, the field strength,
and the timing jitter between MIR and NIR probe pulses overmultiple hours was achieved, which resulted from the robust
and straightforward optical setup. From the results of a finite
integration simulation, a field enhancement factor of 4,300
was predicted by focusing the MIR pulses onto the tunnel junction of a scanning tunneling microscope. The enhanced MIR
near field can greatly improve the temporal resolution of lightwave-driven STM. Because of the ultrabroadband spectrum,
high field strength, high stability, and high repetition rate, oursystem is expected to open the door to strong-field physics aswell as ultrafast nanoscale electronics and metrology with anunprecedented signal-to-noise ratio.
Funding. Grants-in-Aid
for
Scientific
Research(17H06088, 16H04001, 18H04288); Japan Society for the
Promotion of Science (17J05234); Ministry of Education,
Culture, Sports, Science and Technology.
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