interview

Ultrafast nanoprobing■■ Tell us about the motivation behind
your research.
The advancement of ultrashort-pulse laser
technology has opened the door to theworld of ultrafast phenomena. Optical
pump–probe measurements, for example,
have made it possible to observe ultrafast
dynamics in the femtosecond range, butwith spatial resolutions limited by thewavelength of light used. In contrast,
scanning tunnelling microscopy (STM)
can directly image atomic-scale structures,
but with a low temporal resolution ofless than 100 kHz (10 μs). Thus, sincethe invention of STM in 1981, the ideaof combining time-resolved tunnellingcurrent measurement with ultrashort-pulse
laser technology to simultaneously obtain
ultimate spatial and temporal resolution
has been a very attractive prospect.
■■ What were the challenges incombining STM and photo-excitation?
Lock-in detection — a technique that
improves the signal-to-noise ratio bymeasuring the tunnelling current only
at a particular frequency — is generally
employed while the laser used for thephoto-excitation is modulated. The
problem is that when the laser intensity
is modulated, the displacement current
induced by the thermal expansion andshrinking of the STM tip and sample
significantly increases the backgroundnoise. A distance variation of just 0.1 nmchanges the tunnelling current by one
order of magnitude. Furthermore, the large
background fluctuates with the unavoidable
small fluctuations in laser intensity that
occur at low frequencies.
■■ How did you get around the problem?
We adopted delay-time modulation as
the reference signal for lock-in detection,
instead of laser-intensity modulation, tosolve the noise problem. The delay-time
modulation is controlled by a pulse-picker
consisting of an ultrafast Pockels cellthat selectively transmits pulses from asequence of laser pulses. In essence, 800 nm
ultrashort laser pulses with pulse widths of
140 fs are divided into two with a certain
time delay to form a train of pulse pairs.
882

Univ. Tsukuba

Imaging the transient carrier dynamics in semiconductors at both high temporal and spatial resolution
has long been a goal for solid-state scientists. Hidemi Shigekawa from the University of Tsukuba in Japan
told Nature Photonics how his team accomplished this feat.

From left to right: Shoji Yoshida, Yasuhiko Terada, Hidemi Shigekawa and Osamu Takeuchi, with theirexperimental set-up.

This paired-pulse train is then used to excite
the sample below the STM tip. The opticalpulses create an excited state in the sampleand result in a rapid change in a physicalquantity such as the photocarrier density,
which often gives rise to changes in the raw
tunnelling current. Depending on the timedelay, the second excitation in the physicalquantity will have a different intensity, andhence a different tunnelling current. Thus,by measuring the tunnelling current as afunction of the delay time, we demonstrated
that the relaxation dynamics of the excitedstate in the target material can be probed by
STM at the resolution of the pulse width.
■■ What have you achieved?
The control of delay-time modulation using
a pulse-picking technique has enabled thevisualization of transient carrier dynamics
in nanometre-scale structures with awide range of lifetimes — a techniquethat has long been desired. We haveobserved the carrier dynamics of varioussemiconductor samples such as GaNAs,
GaAs and low-temperature-grown GaAs.
The obtained results are consistent withthe recombination lifetimes determinedfrom conventional optical pump–probe
measurements. We have also directly
analysed the recombination of excited
carriers via the gap states associated witha cobalt nanoparticle/GaAs structure in

real space. Through the site dependence ofthe decay time on the tunnelling current
injected from the STM tip, we were able
to image the hole capture rate at the gapstates on the nanoscale for the first time.
Electrically controlling the delay timeallows a large and rapid lock-in modulation
to be achieved, providing a signal-tonoise ratio that is 100 times better than if
mechanical mirrors were used.
■■ What are the implications ofyour findings?
As structures shrink in size, small differencesin electronic properties have an increasingly
important effect on their macroscopic
behaviour. For instance, fluctuations in
the distribution of a few dopant materials
can govern the properties of macroscopic
functions. The ability to directly observeand control ultrafast dynamics on thenanoscale is of great importance for thecontinued advancement of this field. For
further advances, a method of exploring thetransient dynamics of the local quantum
functions in organized small structures iseagerly desired. Our microscopy method isexpected to be useful in this respect.

Interview by Rachel Won
Hidemi Shigekawa and co-workers have an
Article on nanoscale pump–probe microscopy
on page 869 of this issue.

nature photonics | VOL 4 | DECEMBER 2010 | www.nature.com/naturephotonics© 2010 Macmillan Publishers Limited. All rights reserved.