Generation, transmission, and detection of terahertz photons on an electrically drivensingle chipKenji Ikushima, Atsushi Ito, and Shun Okano Citation: Applied Physics Letters 104, 052112 (2014); doi: 10.1063/1.4864168 View online: http://dx.doi.org/10.1063/1.4864168 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/5?ver=pdfcov Published by the AIP Publishing

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Generation, transmission, and detection of terahertz photonson an electrically driven single chip

Kenji Ikushima, Atsushi Ito, and Shun OkanoDepartment of Applied Physics, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei,Tokyo 184-8588, Japan

(Received 25 December 2013; accepted 22 January 2014; published online 7 February 2014)

We demonstrate single photon counting of terahertz (THz) waves transmitted from a local THz

point source through a coplanar two-wire waveguide on a GaAs/AlGaAs single heterostructure

crystal. In the electrically driven all-in-one chip, quantum Hall edge transport is used to achieve a

noiseless injection current for a monochromatic point source of THz fields. The local THz fields

are coupled to a coplanar two-wire metal waveguide and transmitted over a macroscopic scale

greater than the wavelength (38 lm in GaAs). THz waves propagating on the waveguide are

counted as individual photons by a quantum-dot single-electron transistor on the same chip. Photon

counting on integrated high-frequency circuits will open the possibilities for on-chip quantum

optical experiments. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4864168]

Integrated quantum optical circuits are particularly

promising for future quantum information technology. The

transmission and confinement of microwaves can be

achieved by standard lithographic techniques, and strong

light-matter coupling has been demonstrated in superconduc-

tor and semiconductor circuits.1–4 However, single-photon

detection that directly measures the quantum properties of

light is extremely difficult because of the small photon

energy. Therefore, measurements have only been possible by

microwave spectral analysis with external equipment. For

visible light, integrated circuits with sub-wavelength optical

elements are still a challenge, although single-photon count-

ing measurements are more feasible for incident light from

free space. Given the trade-off between photon energy and

wavelength, the energy and spatial scale in the THz region

(wavelengths of 100 lm, photon energies of 10 meV)

may be an appropriate frequency range for manipulating

both aspects of the wave-particle duality of electromagnetic

fields on solid-state devices. Indeed, THz single-photon

detection is possible with quantum dot (QD) detectors5 and

extreme THz confinement can be achieved in a microcavity

laser based on a planar subwavelength resonant circuit.6

A two-dimensional electron system (2DES) in a strong

magnetic field is a possible platform for on-chip photon

manipulation. This is because a gate-controlled QD in a

strong magnetic field serves as a single THz-photon coun-

ter.5 Furthermore, in the energy-dissipationless quantum

Hall effect (QHE) regime, the electron system can be a well-

localized THz generation source originating from the inter-

Landau level (LL) optical transition.7 Because the electrons

are carried by noiseless edge channels without backscatter-

ing, the LL emission can become a sub-Poissonian THz gen-

eration source. These sub-wavelength THz components can

be embedded on a single substrate (GaAs/AlGaAs single het-

erojunction) with fully suppressed background radiation.

In this work, we demonstrate the generation of THz

fields, transmission of THz waves, and counting of THz pho-

tons on a single chip (Figs. 1(a) and 1(b)). In the all-in-one

chip, THz fields are locally generated by applying a direct

current (DC). The fields are coupled to a coplanar two-wire

line and transmitted over a macroscopic scale larger than the

wavelength. THz waves propagating on the waveguide are

counted as individual photons by a QD on the same chip.

The DC-driven local THz fields are finally converted to low-

frequency single-photon-counting signals.

The device consists of a LL THz diode and a gate-

controlled QD that are coupled by a coplanar two-wire

waveguide (Fig. 1). The electromagnetic propagation and

nanofocusing with the two-wire waveguide has been recently

demonstrated in mid-infrared optical devices, where the

design has been adapted from microwave technology.8 The

transmission efficiency is mainly determined by two factors:

impedance matching to the loads (emitter and detector), and

radiation loss. The characteristic impedance Z0 of this

waveguide is calculated to be 76 X for a wavelength of kd

¼ 38lm in GaAs. Using the impedance of the QD,

ZQD � 30 X,9 the transmission coefficient is calculated as

1� ðZ0 � ZQdÞ2=ðZ0 þ ZQDÞ2 ¼ 0:81. Assuming that the im-

pedance of the emission spot in 2DES is comparable to that

of the QD, the power transmission efficiency is estimated to

be ð0:81Þ2. In addition, the radiation attenuation coefficient

for the two-wire waveguide on the GaAs substrate is calcu-

lated to be 1.15 dB/L,10 leading to a radiation loss of 23% for

L ¼ 0:5 mm. Therefore, including the impedance matching,

the total transmission coefficient reaches about 50% for the

THz device based on microwave technology.

The QD serves as a single THz photon counter in a

strong magnetic field. When a THz photon is absorbed by

the QD through plasma-shifted cyclotron resonance, an elec-

tron is created in the upper LL and internal polarization takes

place. The single-photon absorption events give rise to con-

ductance switches with a typical photoexcited lifetime of

30 ls to 0.1 s at 300 mK. In a magnetic field of 5.2 T, the

plasma-shifted cyclotron energy of the QD, �hxQD

¼ �hffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðxc=2Þ2 þ x2

0

qþ �hxc=2, is estimated to be �hxQD

¼ 9:16 meV, where the plasma frequency observed in a sim-

ilar QD, x0 ¼ 1:98 meV, is used.11 The resonance linewidth

0003-6951/2014/104(5)/052112/4/$30.00 VC 2014 AIP Publishing LLC104, 052112-1

APPLIED PHYSICS LETTERS 104, 052112 (2014)

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of about 5% was observed in the earlier study.11 The dark

switching rate is about 2 counts/s in this study, implying that

a THz power of 10�20 W can be detected in the integrated

circuit with a transmission efficiency of 50%. The QD cur-

rents are measured by a direct coupled mode during photon

counting experiments.12

Although it is easy to generate THz fields by using black

body radiation from a heat source, heat diffusion in solid-

state devices makes it difficult to ensure a localized THz

source. Therefore, we use LL emission from nonequilibrium

edge channels in the dissipationless QHE states.7 The ring-

shaped mesa structure of the Corbino geometry restricts the

energy-dissipative spot to the confluence of the edge channels

in the QHE plateau regime and exhibits diode-like behavior

in transport measurements.13 In the all-in-one chip, we use

the LL diode as a THz point source without background ther-

mal radiation. Fig. 2(a) shows a schematic of a LL diode. In

order to avoid cyclotron absorption in the surrounding 2DES,

the emission spot (the confluence of edge channels) is located

on a protruding portion of the ring-shaped mesa.

In nonlinear diode behavior caused by the LL splitting,

the finite current flowing between the inner and outer con-

tacts indicates the presence of electron transfer between adja-

cent edge channels in the ungated region. A distinct onset

voltage (Vonset ¼ �6:8 mV) for the current flowing between

two contacts (7 and 10) is observed at an LL filling factor of

� ¼ 3 on the negative side of the source-drain voltage,

V7;10 ¼ �ðl7 � l10Þ=e (Fig. 2(b)), where the filling factor

beneath the top gate, �g, is set to 2. The critical threshold

indicates the strong promotion of the electron transfer

between edge channels. From the Landauer-B€uttiker formal-

ism in the present geometry,14 the resistance can be

expressed by R7;10 ¼ he2

��g�g

at the strong scattering limit.

The slope in the current-voltage (I–V) characteristics at

V7;10 < Vonset corresponds to R7;10 ¼ 32

he2 ¼ 38:7 kX for con-

ditions where ð�; �gÞ ¼ ð3; 2Þ, indicating that the � ¼ 3 edge

channel is fully equilibrated with the � ¼ 2 edge channels.

The deviation of the onset voltage (Vonset ¼ 6:8 meV) from

the cyclotron energy (�hxc ¼ 8:7 meV) will be attributed to

the exchange enhanced spin-splitting, g�lBB.13

We measured photon-counting rates as a function of the

source-drain voltage with the QD photon counter embedded

on the same chip (Fig. 2(b)). THz photons are emitted on the

FIG. 1. Integrated circuit for on-chip THz photon transmission. (a) Schematic

of the device for transmitting THz waves between quantum transport channels

in strong magnetic fields. (b) The device is fabricated on a GaAs/AlGaAs sin-

gle heterojunction with an electron sheet density of ns ¼ 3:8� 1015 m�2 and

an electron mobility of l ¼ 56 m2/Vs using electron beam lithography. The

coplanar two-wire waveguide is formed from a 200-nm-thick NiCr/Au layer.

The total width of the line is 6.0 lm with a separation of 1.2lm, resulting in a

wave impedance of Z ¼ 76 X for the structure at a wavelength of kd ¼ 38 lm

in GaAs. The length of the waveguide is 0.5 mm. The waveguide also serves

as gate lines for forming the QD (see (d)). To minimize the influence on THz

transmission properties, thin gate lines for supplying the DC voltages are

orthogonally connected from bonding pads to the waveguide (thickness:

30 nm). (c) False colour electron micrograph of a LL THz diode with a

30-nm-thick top gate (yellow) coupled to the two-wire waveguide (red). The

dotted circle shows the confluence of adjacent edge channels (the emission

spot). (d) Electron micrograph of the gate controlled QD THz photon counter.

FIG. 2. LL THz diode. (a) Schematic of a quasi-Corbino geometry that func-

tions as a THz generation source. The filling factor is � ¼ 3 at 5.2 T and the

filling factor beneath the top gate is set to �g ¼ 2. The outer (lowest LL,

N¼ 0) and inner (first excited LL, N¼ 1) edge channels are shown in this

schematic. For simplicity, spin splitting is neglected. Energy dissipation is

restricted to the confluence of unequally populated edge channels.13 (b) The

I–V characteristics and the emission behavior of the LL diode at 300 mK.

The photon counting rates are saturated above 50 s�1 due to the limit of the

detection speed of the QD currents.

052112-2 Ikushima, Ito, and Okano Appl. Phys. Lett. 104, 052112 (2014)

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negative V7;10 side with a clear onset, where the electrochem-

ical potential of the outer � ¼ 2 edge channels is lower than

that of the inner � ¼ 3 edge channels. This can be interpreted

with the reconstructed energy structures of the edge channels

satisfying the necessary conditions for a vertical LL optical

transition as discussed in the Hall-bar geometry (Fig. 3(c)).7

In order to investigate the contribution of thermal radiation,

we also measured photon-counting rate as a function of

the top-gate voltage (Fig. 3). The applied electric power

PJoulð�; �gÞ under the conditions ð�; �gÞ is evaluated as

PJoulð3;1Þ�4PJoulð3;2Þ�1:2�10�9 W at V7;10¼�8:0mV.

Assuming that the applied power is mainly converted to

thermal energy through acoustical phonon emission,7 the

inter-edge scattering region will be hotter when ð�;�gÞ¼ð3;1Þ. No definite photon emission was observed for

ð�;�gÞ¼ð3;1Þ, which suggests that thermal radiation is negli-

gible in the present experimental conditions.

We also controlled the THz-field coupling between the

emission spot and the two-wire waveguide (Fig. 4). The

direction of travel of the edge channels is reversed when the

magnetic field polarity is reversed, which shifts the position

of the emission spot. We recorded the photon counting sig-

nals of the QD for both magnetic-field polarities. When the

end of the waveguide is positioned in the near-field region

of the emission spot, at a distance of r � kd, the photon

counting rate exceeds the dark switching rates. Therefore,

the experimental results exclude the contribution of photons

straying through the substrate or the vacuum of space, pro-

viding evidence that high-frequency THz signals propagat-

ing along the coplanar line are counted as individual

photons.

In the all-in-one chip, the total energy conversion effi-

ciency from the applied DC electric power to the detected

THz power is on the order of 10�9 around V7;10 ¼ �8:0< �hxc=e mV. The small efficiency is mainly caused by the

photon emission probability of the LL diode. Because of the

limited detection speed of QD currents in the present setup,

the photon counting measurements were restricted near the

onset voltage of emission. At V7;10 < �hxc=e ¼ 8:7 mV, the

inter-edge-state scattering will be strongly suppressed by the

requirements of the spin-flip process.15 Controlling the spin

states of the edge channels through electron spin resonance

or spin-orbit interactions will dramatically affect the photon

emission probability.

The photon counting of THz waves confined in metal

structures on a solid-state chip surface enables quantum opti-

cal experiments on high-frequency circuits. The system can

be also combined with an external THz source. Possible

experiments include on-chip Hanbury Brown and Twiss

FIG. 4. Control of THz-field coupling

with a coplanar waveguide. When the

direction of the external magnetic field

is reversed, the distance between the

THz emission spot and the waveguide,

r, changes from the near-field region

(r � kd) to the radiative region

(r > 2kd). Photon-counting signals are

observed when the THz fields are

capacitively coupled with the metal

wires in the near-field region.

FIG. 3. Gate characteristics of the LL THz diode at V7;10 ¼ �8:0 mV. (a)

Photon counting rates and the corresponding current through the inter-edge

state scattering. (b) Energy structures of the edge channels for ð�; �gÞ¼ ð3; 1Þ. The spin-flip scattering is promoted but no THz photon is gener-

ated. (c) Energy structures of the edge channels for ð�; �gÞ ¼ ð3; 2Þ. The

inter-edge optical transition is expected.

052112-3 Ikushima, Ito, and Okano Appl. Phys. Lett. 104, 052112 (2014)

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experiments on a Y-junction THz power divider, cavity

quantum electrodynamics with a planar THz resonator, or

studies of quantum plasmonics in low-dimensional semicon-

ductors or graphenes

We thank Professor S. Komiyama of University of

Tokyo for helpful discussion. This work was supported by

PRESTO of the Japan Science and Technology Corporation

(JST) and also by JSPS KAKENHI Grant Nos. 24654085

and 25287071.

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