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decay into fundamental ones, whichthen shift deeper and deeper into theinfrared with further propagation. For
ideal conditions, this continuous shift
will only be terminated by the onset ofabsorption as dictated by the fibre’s corematerial. Meeting the optimum conditionsof soliton fission typically requires a
substantial adjustment of the zerodispersion wavelength of the core material
by means of suitably designed waveguidedispersion. In the case of AsSe, a shift of
1–2 μm was necessary to match the fibreto the employed pump wavelengths in themid-infrared. This requirement puts tightconstraints on the core diameter of thefibre, which then inadvertently results ina second problem — the appearance of
higher spatial modes and the breakdownof the single-mode guidance of the fibre.
Therefore, it seems quite remarkable that
all these constraints could be managed toenable a supercontinuum with a bandwidth
greater than three octaves.
The demonstrated supercontinua
substantially enhance the capabilitiesof spectroscopy in the mid-infrared, aspectral region that has also been dubbed
the molecular fingerprint range. This
wavelength range is rich with characteristic
vibrational spectra of molecules and thus
enables the unambiguous identificationof various species using transmission
spectroscopy (Fig. 1). The greatly increased
luminous intensity of laser-generated
supercontinua, when compared with otherbroadband sources, immediately translates

into a welcome gain in sensitivity andwavelength selectivity.
Although the immediate benefits forconventional spectroscopy are clear, it
may also be possible to use these sources
for frequency comb spectroscopy. In
particular, it seems appealing to extend
the use of mid-infrared continua todual-comb or Vernier spectroscopy 7.
This method requires the use of more
complex laser oscillators with repetition
rates on the order of megahertz, but asa benefit it promises virtually unlimited
spectral resolution for spectroscopy.
Such advanced methods may require
further development and refinement ofchalcogenide fibres. For example, microstructuring the fibre geometry may leadto greater optical confinement and thus
lower the pulse energy requirements ofthe pump laser, similar to what has been
done with photonic crystal fibres8. In
these structures, much tighter guiding isachieved by surrounding the core with
air-filled channels.
In conclusion, it is expected that theavailability of multi-octave mid-infrared
supercontinua will open a new avenue
for spectroscopy in this regime. Higher
selectivity and an enhanced sensitivity may
help enable the unambiguous detectionof even the faintest molecular traces. It isconceivable that the resulting technology
may even outperform the nose of a dog,
which still sets the gold standard for arange of applications. Highly sensitive
mid-infrared spectroscopy may be used

in a wide range of applications, including
the remote detection of explosives
and the detection of certain diseases
from the analysis of a patient’s breath.
It may also be employed for standoffdetection in environmental studies. Other
applications will certainly be in the areaof fundamental scientific studies, and, if
pumped by a simple oscillator, mid-infrared
supercontinua could be interesting forseeding parametric conversion schemes.
Finally, it needs to be mentioned thatthe achieved limit of 13.3 μm does notconstitute the absolute limit for further
extension of supercontinua into theinfrared. Using tellurite-based chalcogenide
glasses, it may be possible to push the cutoff beyond 20 μm — so, there is still muchwork to be done.
❐Günter Steinmeyer* is at the Max Born Institute,
Max-Born-Straβe 2a, 12489 Berlin, Germany.
Julia S. Skibina is at LLC SPE Nanostructured
Glass Technology, Prospect 50 let Oktyabrya 101,Saratov 410033, Russian Federation.
*e-mail: steinmey@mbi-berlin.deReferences

1. Alfano, R. R. (ed.) The Supercontinuum Laser Source 2nd edn(Springer, 2006).
2. Herrmann, J. et al. Phys. Rev. Lett. 88, 173901 (2002).
3. Qin, G. et al. Appl. Phys. Lett. 95, 161103 (2009).
4. Petersen, C. R. et al. Nature Photon. 8, 830–834 (2014).
5. Pigeon, J. J., Tochitsky, S. Ya., Gong, C. & Joshi, C. Opt. Lett.
39, 3246–3249 (2014).
6. Eggleton, B. J., Luther-Davies, B. & Richardson, K. Nature Photon.
5, 141–148 (2011).
7. Bernhardt, B. et al. Nature Photon. 4, 55–57 (2010).
8. Russell, P. Science 299, 358–362 (2003).

SPECTROSCOPYNanoscale terahertz spectroscopy
The advent of terahertz spectroscopy schemes that offer single-photon sensitivity, femtosecond time resolution
and nanometre spatial resolution is creating new opportunities for investigating ultrafast charge dynamics insemiconductor structures.

Hidemi Shigekawa, Shoji Yoshida and Osamu TakeuchiT

he desire to make semiconductor
devices smaller and faster is a strong
driving force behind developments
in nanoscale science and technology.
Marked progress in device design andfabrication in the past decade meansthat we are now in need of a new set ofterahertz (THz) spectroscopy tools that
can analyse semiconductor structuresand devices on unprecedented spatial
and temporal scales. Semiconductor
devices have now become as small as

10 nm. At this scale, fluctuations inthe spatial distribution of dopants —introduced to control the electronic andoptoelectronic properties of devices — andin the nanostructured interface between
dissimilar materials can directly affectdevice operation.
In addition, there is a need to beable to probe transient charge carrier
dynamics that change on the femtosecond
timescale. Traditional techniques thatrely on temporally-averaged or integrated

NATURE PHOTONICS | VOL 8 | NOVEMBER 2014 | www.nature.com/naturephotonics© 2014 Macmillan Publishers Limited. All rights reserved

data are often too slow to provide
this level of resolution. Now, as theyreport in Nature Photonics, Max Eisele
and colleagues1 have developed amicroscopy technique to simultaneously
meet these demanding spatial andtemporal requirements.
The recent development of quantum
optics has provided techniques forobserving ultrafast phenomena that
occur on femtosecond to attosecond
timescales. For example, snapshots of815news & views

816

IR gate
AFM tip
THz probe
ScatteredIR pump

Electric fieldultrafast dynamics can be captured using
an ultrashort pulse laser and the pump–probe method2,3, allowing time-resolved
analysis to be performed. However, thespatial resolution is generally limited by
the wavelength, which dictates the size ofthe light spot. On the other hand, scanningtunnelling microscopy (STM), which wasinvented in 1981, has enabled the analysis
of local electronic structures with atomicresolution. Many valuable results havebeen obtained using STM and the related
technique of scanning probe microscopy(SPM). However, the temporal resolution
is limited, in general, to millisecond tomicrosecond timescales.
Since the invention of STM, combining
it with the techniques of quantum optics
has been a sought after goal. To achieve
this objective, several microscopy
techniques have been realized4–8. Nearfield optical microscopy (NSOM) using
an aperture has achieved a temporal
resolution below 20 fs (ref. 4). The
modulation of STM bias-voltage using THz
pulses has enabled differential-conductanceimaging with simultaneous sub-picosecond
time resolution and nanometre spatialresolution6. The ultrafast spin dynamics
of carriers confined in a quantum wellhave also been probed by STM withcircularly polarized light 8. However, greater
understanding of the carrier dynamics
of nanoscale materials requires thedevelopment of time-resolved spectroscopy
schemes based on new concepts. In
particular, the development of timeresolved THz spectroscopy with nanometre
spatial resolution is highly desirable9.
Eisele et al.1 have combined sub-cycle,
field-resolved detection of THz waves(with a temporal resolution below 10 fs)
with scattering-type NSOM (s-NSOM)10,11.
The result is ultrabroadband time-resolved
THz spectroscopy (in the 20–50 THz
frequency range) on the sub-nanoparticle
scale (less than 10 nm). For the first time,
the scheme reveals information about thecarrier dynamics in a single InAs nanowire
using three-dimensional probing of thetime-dependent dielectric function.
A schematic of the experimental set-up
used by Eisele et al.1 is shown in Fig. 1.
Atomic force microscopy (AFM) was usedto obtain a high spatial resolution and ahigh signal-to-noise ratio. To obtain hightemporal resolution, a 10 fs pulse wasused to control the gate of the detector.
The sample surface below the AFM tip
was excited by a pump pulse and its effecton the dielectric function of the sample
was measured through the change in the
shape of the scattered electric field of thesubsequent probe pulse.

Time (fs)Figure 1 | State-of-the-art implementation of transient terahertz (THz) time-domain spectroscopy witha single-photon-level signal-to-noise ratio, which allows the THz response from a nanoscale object — anInAs nanowire (blue) on a diamond substrate (green) — to be measured. The field enhancement effectof a sharp atomic force microscopy (AFM) tip confines the probed volume (red) in the sample to around
10 nm. Using electro-optic sampling, the scattered electric field of the probe pulse was directly traced
with the 10 fs infrared (IR) gate pulse.

A PtIr-coated AFM tip strongly
enhances the p-polarized componentof the electric field of excitation, whichenables local probing of the signal below
the AFM tip in a space much smaller
than the applied light spot. To detecta weak signal, the AFM was operated
in the tapping mode to enable the useof highly sensitive lock-in detection.
The AFM cantilever was oscillated ata fixed frequency and the separation
between the AFM tip apex and thesample was sinusoidally varied between
0 and, for example, 40 nm. The third
harmonic of the tapping frequency wasmeasured in the signal to reduce the large
nonlocal background.
To perform electro-optic sampling withhigh temporal resolution, the scatteredelectric field of the probe pulse wasdirectly traced with the 10 fs gate pulse.
An instantaneous THz electric field andthe waveform were recorded with subcycle time resolution by scanning the delay
between the THz pulse and the pumppulse while keeping the delay of the pumpgate fixed. In this way, information about
the absolute amplitude and phase wasobtained, thus providing the evolutionof the dielectric function of the sample.
From the analysis of the plasma frequency,
included in the dielectric function,
the local carrier density following
photoexcitation was directly tracked.

Eisele et al.1 applied this microscopy
technique to an InAs nanowire. Highsensitivity detection made it possible tooperate with a signal corresponding to only
50 photons per THz pulse. Therefore, thedetection of electric field transients froma part of a single nanowire was carriedout with a noise floor of less than one
coherent photon per pulse by optimizing
electro-optic sampling.
Two components of the carrier density
decay — an ultrafast decay (less than 50 fs)
and decay with a constant of 2 ps — wereobserved. Interestingly, the decay wasdependent on the AFM tapping amplitude,
suggesting the possibility of controlling
the probing depth in the sample when thetip approaches it. From the measurementof the tapping amplitude dependence ofthe signal, the fast component, which
dominates for small tapping amplitudes,
was attributed to the formation of adepletion layer. In contrast, the slowcomponent, dominant for large tapping
amplitudes, was attributed to carrier
trapping in defect states. A more accurate
depth profile can be obtained with a deeper
understanding of the physical origin of
the tapping amplitude dependence. Sucha three-dimensional analysis is expected
to become a powerful technique forinvestigating nanoscale dynamics. Varying
the light polarizations of both pump andprobe pulses, thus changing the condition

NATURE PHOTONICS | VOL 8 | NOVEMBER 2014 | www.nature.com/naturephotonics© 2014 Macmillan Publishers Limited. All rights reservednews & viewsof tip-induced electric field enhancement,
may provide further information on local
carrier dynamics. Development of thesystem so that it can operate in ultra-highvacuum may make it suitable for use with awider variety of samples.
The microscopy technique demonstratedby Eisele and colleagues is expected to play
an important role in advancing nanoscalescience and technology. As is discussed
above, a variety of time-resolved SPM
techniques have already been realized4–8. In
addition, by combining a spin-polarized tip
and pump–probe pulse using a bias voltage,the investigation of inelastic tunnelling has
provided information on the spin dynamics
in single atoms12. Because the nanoscale
properties of materials differ from those inbulk structures due to various factors suchas the morphology, temperature, band gap,
dielectric function and electron-phonon
coupling strength, the complementary useof these microscopy techniques is expected
to be important for exploring the carrierand spin dynamics in nanoscale structures.
In particular, combining microscopy
techniques with local excitation using
plasmonic superfocusing 13 may beinteresting in the future. The successive
development of novel microscopy
techniques based on new ideas will greatly
aid the further development of nanoscalescience and technology.
❐Hidemi Shigekawa, Shoji Yoshida andOsamu Takeuchi are at the Faculty of Pure andApplied Sciences, University of Tsukuba, Tsukuba
305-8571, Japan.
e-mail: hidemi@ims.tsukuba.ac.jp
References

1. Eisele, L. et al. Nature Photon. 8, 841–845 (2014).
2. Shah, J. Ultrafast Spectroscopy of Semiconductors andSemiconductor Nanostructures (Springer, 1999).
3. Krausz, F. & Ivanov, M. Rev. Mod. Phys. 81, 163–234 (2009).
4. Wu, H. J., Nishiyama, Y., Narushima, T., Imura, K. & Okamoto, H.
Appl. Phys. Express 5, 062002 (2012).
5. Wu, S. W. & Ho, W. Phys. Rev. B. 82, 085444 (2010).
6. Cocker, T. L. et al. Nature Photon. 7, 620–625 (2013).
7. Terada, Y., Yoshida, S., Takeuchi, O. & Shigekawa, H.
Nature Photon. 4, 869–874 (2010).
8. Yoshida, S. et al. Nature Nanotech. 9, 588–593 (2014).
9. Ulbricht, R., Hendry, E., Shan, J., Heinz, T. F. & Mischa, B.
Rev. Mod. Phys. 83, 543–586 (2011).
10. Lucas, M. & Riedo, E. Rev. Sci. Instrum. 83, 061101 (2012).
11. Zhang, R. et al. Nature 498, 82–86 (2013).
12. Loth, S., Etzkorn, M., Lutz, C. P., Eigler, D. M. & Heinrich, A. J.
Science 329, 1628–1630 (2010).
13. Neacsu, C. C. et al. Nano Lett. 10, 592–596 (2010).

TRANSPARENT ELECTRODES

Fractal future
Hilbert fractal

Peano fractal

a silicon photodetector. The investigatedstructures included linear gratings, grids
and Hilbert and Peano fractal geometries.
The wavelength- and polarization-resolved
photocurrent responses of the detector
revealed broadband responsivity in the450–750 nm range without any polarization
dependence apart from the case of thegrating. Importantly, detectors featuring theHilbert and Peano fractal shaped electrodes(pictured) benefited from significantly
improved responsivity: they were about 40%
and 34% better, respectively, compared withthe grid, whereas the increase was 7% and2%, respectively, compared with the grating.
Breaking down the fractal structures
into regions where the metallic wires

are horizontally and vertically parallel
helps elucidate the physics behind theobservations. Horizontal gratings transmit
primarily vertically polarized light, whereas
the opposite is true for vertical gratings. As
a result, reflection imaging reveals various
bright and dark areas along the surface ofthe fractal, corresponding to areas of low
and high transmission, respectively.
Following further research such
fractal electrodes could prove useful incameras and displays as well as solarcells. In the latter case, the properties offractal networks may also be helpful for
light trapping.

© 2014 AMERICAN CHEMICAL SOCIETY

Fractals are beautiful patterns basedon the principle of self-similarity. Wellknown examples include the branchingarrangements of trees, river networks
or blood vessels. Now researchers havediscovered that fractals can be put to gooduse in photonics by creating transparent
ultrathin metallic electrodes with superior
optoelectronic properties.
Transparent electrodes based on
conducting oxides such as indium tinoxide or zinc oxide play a crucial role indevices like solar cells, LEDs and displays.
However their performance is far fromideal; they are costly, brittle and not well
suited to use with some organic materials.
As a result, much research is devoted tofinding alternatives. One option being
explored is the use of ultrathin metallayers. Farzaneh Afshinmanesh andco-workers at the Geballe Laboratory for
Advanced Materials in Stanford, USA as
well as the Institute of Photonic Sciences
and the Catalan Institution for Researchand Advanced Studies in Barcelona,
Spain have demonstrated that ultrathin
aluminium electrodes formed with a fractal
geometry have optoelectronic properties
superior to their metal counterparts in a
grating or grid geometry (Nano Lett. 14,5068–5074; 2014).
When it comes to transparent electrodedesign, a high optical transmission is a key
figure of merit. The authors studied theperformance of aluminium electrodes ofvarious designs that were integrated intoNATURE PHOTONICS | VOL 8 | NOVEMBER 2014 | www.nature.com/naturephotonics© 2014 Macmillan Publishers Limited. All rights reserved

MARIA MARAGKOU817