microwave photonics || terahertz sources

5

Terahertz Sources

R. E. Miles and M. Naftaly

5.1 Introduction

The terahertz (THz) region of the electromagnetic spectrum is commonly defined as that

lying at frequencies between 0.1 and10 THz. The frequently-heard expression ‘the THz gap’

refers to the fact that at these frequencies generation and detection of radiation becomes very

difficult using either electronic or optical means. Nevertheless, with the growth of interest in

THz research and applications and a great expansion of activities in this area, a large number

and variety of sources have been developed and have becomewidely available. These include

THz lasers, laser-activated emitters and electronic devices. Broadly speaking, in applications

requiring a continuous-wave narrow-line signal at frequencies below 1 THz, electronic

sources predominate. Examples include local oscillators for astronomical instruments and

security scanners. On the other hand, lasers and laser-activated emitters are mainly used for

broadband THz spectroscopy covering a bandwidth of several THz. This division of

functions is likely to persist since electronic THz sources are, by their nature, single-

frequency devices, whereas laser-based sources are either inherently broadband or widely

tuneable. Nevertheless, both types of THz emitting devices have found numerous applica-

tions, and will be described here.

5.2 Terahertz Generation from Laser Sources

The recent rapid expansion of the field of terahertz research and applications owes much of its

existence to the development of suitable laser sources and methods of THz generation. These

developments fall into two categories: direct emission from far-infrared lasers and optical

down-conversion of near-infrared lasers. Owing to the flexibility of the systems and the broad

tuning range, down-conversion greatly predominates as the technique of choice, and accounts

for the vast majority of work in the THz field.

Microwave Photonics: Devices and Applications Edited by Stavros Iezekiel

© 2009 John Wiley & Sons, Ltd. ISBN: 978-0-470-84854-8

5.2.1 Far-Infrared Lasers

These include molecular lasers, p-Ge lasers, and free-electron lasers (FELs).

5.2.1.1 Molecular Lasers

Molecular lasers [1] use as their active media gases which have a permanent dipole moment.

THz (far-infrared) emitting transitions take place between adjacent rotational levels of the

same vibrational state. Excitation is via optical pumping, typically by a high-power (>10W)

CO2 laser. Depending on the gas used, several different lines can be generated, although

the laser is not continuously tuneable. Narrow linewidths of a few GHz are obtainable because

the gases are used at low pressures in order to reduce collisional relaxations. Avariety of gases

are used in molecular lasers, including CH4, CH3OH, CH3NH2 and C2H2F2, producing lines in

the range of 40–1200mm (7.5–0.25 THz). CW laser powers of the order of several mW are

generated. The limitation of these sources is that they offer a discrete set of lines with no

tuneability, and so are unsuitable for the majority of spectroscopic applications.

5.2.1.2 p-Ge Lasers

A p-Ge laser consists of a p-doped Ge crystal cryogenically cooled to 4K and mounted in a

magnet [2]. Such lasers operate in pulsedmode,with a repetition rate of tens ofHz. The average

THz power produced is 10–100mW, with peak powers of several watts. The emission

frequency can be tuned by varying the magnetic field: the typical range is 1.5–4.5 THz. The

magnetic field required by the laser can be of several Tesla, and therefore necessitates a

superconducting magnet. p-Ge lasers are useful for THz spectroscopy, especially where high

incident THz power is required, but their applications are limited owing to the size and

complexity of the auxiliary systems.

5.2.1.3 Free-electron Lasers

Free-electron lasers (FELs) or synchrotron sources [3] are continuously tuneable over a range of

tensofTHzandcanproduceseveralwattsofcontinuous-wavenarrow-bandradiation.Althoughan

FEL is in many respects an ideal THz source, it is in fact a type of high-energy electron beam

accelerator.Itisthereforealarge-scaleinstallation,whichseverelylimitsitspracticalapplicability.

5.2.2 Optical Down-Conversion

THz generation by optical down-conversion falls into two types: broadband emission employ-

ing ultrafast lasers and single-frequency tuneable emission employing difference-frequency-

generation. Of these, broadband generation is by far the most widely used method, largely

owing to the advantages of coherent detection available in a pump-probe configuration (these

include high sensitivity and a large dynamic range).

5.2.2.1 Broadband THz

By far the most widespread method of THz generation involves employing ultrafast lasers.

These aremode-locked lasers whose pulse length is typically less than 200 fs, andmore usually

112 Microwave Photonics: Devices and Applications

well under 100 fs. The most commonly used are Ti–sapphire lasers which typically deliver

pulse lengths of 50–100 fs, and can be as short as 10 fs. Ytterbium and erbium solid-state or

fibre lasers have longer pulse lengths of 100–300 fs. The repetition rate of all such lasers is of

the order of 50–100MHz; the average power is typically in the range 0.2–2W. Consequently,

the peak optical pulse power is of the order of 108W.

There are two types of broadband THz emitter in common use: optical rectification by

electro-optic crystals and biased photoconductive emitters (also known as Auston switches).

Both of these produce a single-cycle THz pulse whose amplitude is proportional to the time

derivative of the optical intensity of the laser pulse, as seen in Figure 5.1. It follows therefore

that a shorter laser pulse will produce a correspondingly shorter THz pulse, with a consequent

broader spectral content.

5.2.2.2 Optical Rectification

Optical rectification in electro-optic (EO) crystals first emerged in the late 1980s as a means of

producing broadband THz radiation from ultrafast laser sources. Typically, zinc-blende semi-

conductors such as ZnTe, GaP and GaSe, or organic crystals (e.g. 4-(4-Dimethylaminostyryl)-

1-methylpyridinium tosylate (DAST)) are used for this purpose, all having bandgaps in

the 2–2.5 eV range. In order to avoid absorption of the incident radiation, the laser wavelength

must be longer than the bandgap of the material, that is Ephoton¼ hc/l<Eg, making 800 nm

Ti-sapphire lasers suitable for this purpose [4]. Optical rectification arises as a result of the

transient polarization which occurs when a short, high-intensity laser pulse interacts with the

electro-optic medium. The THz power generated is proportional to the square of the optical

power, is determined by the second-order optical nonlinear coefficient (x(2)), and varieswith therelative orientation of the laser polarization and the crystallographic axes.

An important issue is phase matching, which limits the interaction length in the crystal and

restricts the usable crystal length. Optical rectification can be viewed as a mixing of different

(c)

TH

zam

plitu

de

Frequency

(b)

Time

TH

zam

plitu

deTime

Lase

rin

tens

ity

(a)

Figure 5.1 (a) Laser pulse, (b) THz pulse and (c) THz spectrum

Terahertz Sources 113

spectral components of the laser pulse separated by the THz frequency: vopt and vopt þ vTHz.

This also serves to explain why a shorter laser pulse containing a broader spectral bandwidth

produces a broader THz spectrum (via Dn/Dt� 1). The phase-matching condition for thewave

vector k is then [5]:

Dk ¼ kðvopt þvTHzÞ� kðvoptÞ� kðvTHzÞ ¼ 0 ð5:1ÞNeglecting optical dispersion, the coherence length lc for phase matching is given by:

lc ¼ p=Dk ¼ pðvTHzjnopt � nTHzjÞ ð5:2Þwhere n is the refractive index.

For bandwidths of up to 3 THz in commonly used EO crystals lc is of the order of 0.5mm.

However, owing to dispersion, it is reduced to < 50 mm when ultrabroadband generation

( > 3 THz) is required.Typical THz bandwidths obtained from optical rectification in EO crystals are 0.1–3 THz,

with a total average THz power of a few mW. However, much broader THz emission has been

achieved. In both ZnTe and GaSe bandwidths of up to 40 THz have been demonstrated, as seen

in Figure 5.2 [6].

5.2.2.3 Biased Photoconductive Emitters

Broadband THz generation using biased photoconductive emitters activated by ultrafast lasers

was first explored in the 1980s, and has since become the most commonly used method in both

research and commercial THz systems. An emitter consists of a semiconductor wafer with bias

electrodes deposited on its surface (Figure 5.3) [4]. In contrast to optical rectification, a

photoconductive emitter must be excited by a laser wavelength that is shorter than the bandgap

Figure 5.2 Temporal waveform (upper figure) and spectrum (lower figure) of THz waves measured by

the GaSe (solid line) crystal and the ZnTe (dotted line) crystal, respectively. Reprinted with permission

fromKai Liu, Jingzhou Xu and X.-C. Zhang, Applied Physics Letters, 85, 863 (2004).� 2004, American

Institute of Physics


Gold electrodes

Laser beam

GaAs substrate

Stripline gap: 30-100 µm Line thickness: 10-50 µm Line length: 1-2 mm Electrode gap: 5-20 µm

Large-gap emitter Small-gap emitter / antenna

Laser beam

Gold electrodes

GaAs substrate

Electrode length/width: 3-10 mm Electrode gap: 0.5-1 mm

Figure 5.3 Schematic drawings of typical biased photoconductive emitters

of the material, that isEphoton¼ hc/l > Eg.When a laser beam is incident on the semiconductor,

the absorbed photons generate photocarriers, electrons in the conduction band and holes in the

valence band. These are accelerated by the bias field,while simultaneously their density changes

under the varying laser intensity. As a result, ultrashort high-peak currents are generated in the

semiconductor, which radiate into free-space at THz frequencies.

In order for the process to be efficient, the carrier lifetime of the semiconductormaterialmust

be short on the timescale of the laser pulse.Most THz emitters employGaAs, either in its semi-

insulating (SI) form or low-temperature grown (LT), owing to its short carrier lifetime and its

bandgap of 1.42 eV allowing absorption of the 800 nm radiation from Ti-sapphire lasers. LT-

GaAs has a shorter photocarrier lifetime (< 0.5 ps) than SI-GaAs (�100 ps), which improves its

THz-emitting performance. The gap between the electrodes can vary from a few microns to

severalmillimetres. In large-gap emitters ( > 0.1mm) the shape of the electrodes has little effect

on THz generation. However, in small-gap emitters the bias electrodes also act as a radiating

antenna, and are designed accordingly.

Owing to the additional energy supplied by the bias current, photoconductive emitters

produce an average THz power of tens of mW, that is an order ofmagnitude higher than electro-

optic crystals. It may be assumed, especially in the case of large-gap emitters, that the transient

photocurrent J(t) is not affected by the electrodes, and is directly proportional to the induced

conductivity s(t) and the applied bias field E [7]:

JðtÞ � sðtÞ E: ð5:3ÞThis then leads to the conclusion that the average THz power Pave produced by the emitter

scales with the square of the optical power Popt and the square of the applied voltage E:

Pave � P2optE

2: ð5:4Þ


The THz spectral content delivered by photoconductive emitters is significantly narrower

than that obtainable from optical rectification. Figure 5.4 shows a typical THz spectrum

generated by an emitter which was fabricated on SI-GaAswith an electrode spacing of 0.5mm,

biased at 200Vand excited by 0.8Waverage power from a 60 fs Ti-sapphire laser. The usable

THz spectrum extends to nearly 4 THz; however, the power decreases exponentially with

frequency as explained in Figure 5.1 above.

Small-gap LT-GaAs emitters employing short-pulse lasers of �10 fs are capable of

generating much larger THz bandwidths, although at the cost of greatly reduced total power.

A usable THz spectrum of up to 20 THz has been demonstrated as shown in Figure 5.5 [8].

Note, however, that the spectrum suffers from substrate absorption at 5.2 and 8.0 THz.

5.2.3 Difference Frequency Generation

There are two types of THz source operating by difference frequency generation (DFG) and

using near-infrared lasers. Both employ two laser beams which are detuned by the desired

4.03.53.02.52.01.51.00.50.0

1E-3

0.01

0.1

1

TH

z am

plitu

de (

a.u.

)

Frequency (THz)

Figure 5.4 A typical THz spectrum from a large-gap biased photoconductive emitter

Figure 5.5 The Fourier transform amplitude spectrum (upper trace, solid line) together with the

spectrummeasured in the presence of PTFE sample (lower trace, dotted line). Reprinted with permission

fromY. C. Shen, P. C. Upadhya, E. H. Linfield, H. E. Beere andA. G. Davies,Applied Physics Letters, 83,

3117 (2003). � 2003, American Institute of Physics


THz frequency, and rely on frequency mixing to generate the desired radiation. In both

cases the THz frequency can be tuned continuously by varying the wavelength of one of the

lasers. The first type employs biased photoconductive emitters and the second electro-optic

crystals.

5.2.3.1 CW THz DFG from Photoconductive Emitters

The principle of operation is similar to that of broadband emission; however, in this case two

CW lasers are used instead of a single pulsed source. THz frequencies are generated in the

biased photoconductor due to the fluctuations in carrier density, which produces a signal via the

heterodyne process [9]:

½E1cosv1tþE2cosv2t�2 ¼ constþE1E2cosðv1 �v2Þt ð5:5Þ

where E1 and E2 are the respective field amplitudes of the two sources, and v1 and v2 are their

respective frequencies.

As in the broadband case, the THz power produced scales with the square of the optical

power and the applied voltage. However, it also depends crucially on the carrier lifetime of the

material and on the efficiency of the antenna. Avery short carrier lifetime (< 200 fs) is requiredfor successful THz generation. The typical THz power generated is of the order of 1mW [10]

and the bandwidth is limited to approximately 1.5 THz. Owing to these limitations, such

emitters have not found wide application.

5.2.3.2 Pulsed THz DFG from Electro-optic Crystals

Recently there has been growing interest in difference-frequency THz generation using

electro-optic crystals activated by pulsed lasers. A typical set-up employs a Q-switched

Nd-YAG laser pumping an optical parametric oscillator (OPO),which together provide the two

laser wavelengths detuned by the desired THz frequency. These are focused onto an EO crystal,

where the THz difference frequency is generated via three-wave mixing.

As with optical rectification, DFG is a parametric process which arises from transient

polarization whose efficiency is dependent on the second-order optical nonlinear coefficient

x(2). The emitted THz power at the frequencyvTHz from a crystal of length L and using exciting

beams of power P1 and P2 is [11]:

PTHz � v2THzðcð2ÞÞ2L2P1P2: ð5:6Þ

As a consequence of the dependence on vTHz, the efficiency of DFG and the obtained THz

power tend to increase with THz frequency. This is the reverse of what happens in broadband

generation, where THz power decreases with frequency (Figure 5.1). Since the THz power is

proportional to the product of powers of the two mixing beams, pulsed lasers with high peak

powers are necessary to activate the process. The laser wavelength must be well below the

bandgap of the crystal, to avoid absorption.

The phasematching condition for thewavevectors k is similar to that for optical rectification

(Equation (5.1)):

Dk ¼ kðvopt þvTHzÞ� kðvoptÞ� kðvTHzÞ ¼ 0: ð5:7Þ


However, in this case noncollinear phase matching is possible, so that the vector form is

retained. Owing to dispersion, the crystal must be rotated as the THz frequency is tuned, in

order to preserve the relationship between the three wave vectors. This allows the phase

matching condition to be maintained in crystals over a length of several millimetres, thus

providing a longer interaction length and increasing the THz power produced.

Using DFG, THz peak powers of the order of hundreds of milliwatts have been demon-

strated. Other advantages of THz DFG include the availability of THz frequencies above

3 THz, the ability to produce a tuneable narrow-line single frequency (instead of having to

generate and analyse a complete broad spectrum) and the simplicity and compactness of the

systems. A number of THz DFG systems have been demonstrated. Tuning over 0.5–7 THz has

been obtained in a GaP crystal, with peak THz powers of around 100mW [12]. In an organic

DAST crystal continuous tuning between 2 and20 THz has been achieved, with peak powers of

> 10W (Figure 5.6) [13].

5.3 Electronic Sources

It is true to say that the progress that has occurred in terahertz technology over the last 15 years

hasmainly resulted from the development of theAuston switch [14] as discussed in Section 5.2.

Previous to this, terahertz waves were generated electronically, but in devices based on

vacuum-tube technology. Such devices as the backward wave oscillator (BWO) can deliver

THz power in the tens of milliwatt range but they have a relatively short lifetime (in modern

electronic terms) and are not always readily available.

At the time ofwriting,modern semiconductor based electronic sourceswhich can oscillate at

frequencies in the range of� 0.7 THz to 2 THz are still not available, but steady development,

as will be discussed below, sees themaximum frequencies of operationmoving slowly upward.

THz power can be produced from below the THz gap by multiplication up from lower

frequencies and such sources [15,16] have been used extensively in space science for remote

sensing. On the other (higher frequency) side of the THz gap the frequencies delivered by

Figure 5.6 THz output energy as a function of the THz frequency with 1 and 0.5mm thick DAST

crystals: solid line, 1mm DAST; broken line, 0.5mm DAST. Reused with permission from T. Taniuchi,

S. Okada and H. Nakanishi, Journal of Applied Physics, 95, 5984 (2004), � 2004, American Institute

of Physics


quantum cascade lasers (QCLs) are slowly moving downward and have in fact reached the

�1THz region, albeit with the added complexity of requiring an applied magnetic field.

Although QCLs can deliver significant amounts of power (tens of mW) they do suffer from the

disadvantage of having to operate at cryogenic temperatures and are not readily tuneable.

Whilst it is clear that THz frequencies have significant uses, mainly in scientific areas such

as spectroscopy where the cost and size of femtosecond systems are less of an issue, it can be

argued that the technology will not become ubiquitous until inexpensive and robust

electronic sources are available. The situation today can be compared with that of the

semiconductor diode laser 50 years ago where, at that time, they could only operate at

cryogenic temperatures. Who then would have predicted that every household would own a

diode laser (in CD players), that we would be using them as pointers in the class room and

buying them fromDIY stores to replace the humble spirit level to produce horizontal lines on

a wall to act as a tiling guide?

Many uses for terahertz waves have been suggested, for example molecular recognition,

flame tomography, gas sensing, damp detection, archaeology and various medical and security

applications, but none of these has taken off because of the expense, large physical size and

fragility of the femtosecond systems. Given an inexpensive electronic source, THz technology

would be in a much better position to compete with existing techniques. The need is also

becoming more urgent as processing speeds in integrated circuits move up into the THz region

and greater bandwidths (and therefore higher frequencies) are required in communications.

The section will discuss the state of the art in THz electronic sources.

5.3.1 Two-terminal Devices

Gunn diodes (sometimes known as transferred electron devices), IMPATT (impact avalanche

transit time) diodes and TUNNETT (tunnel injection transit time) diodes [17] are well-

established sources atmicrowave andmm-wave frequencies. In each case, the device structures

are designed to generate a negative slope in the current–voltage characteristic, giving rise to a

region of negative differential resistance (NDR) as illustrated in Figure 5.7.

When a DC voltage large enough to bias the device into the NDR is applied, it creates an

inherently unstable state and the device goes into oscillation. Frequencies up into themm-wave

region can be generated but the specific mechanisms giving rise to the NDR impose an upper

Figure 5.7 Current–voltage curve showing the negative differential resistance region (shaded)


limit to the oscillation frequency. As the name ‘transferred electron device’ suggests, the cause

of the NDR in Gunn diodes is the transfer of electrons from the light to the heavy mass

conduction bands. This process takes a finite time, which in turn limits the highest frequency

available. Similarly, in the IMPATT diode a finite amount of time is needed to build up to the

avalanche condition.

If the negative resistance instability can be produced by another and faster mechanism then

the way would be open for THz generation. Modern molecular beam epitaxy (MBE) growth

techniques have resulted in TUNNETTdiodeswith very short transit time layerswhich are thus

capable of oscillating at over 600GHz [18], but with very low power levels. Two other types of

device which can potentially exhibit NDR at high enough frequencies are possible candidates

for THz generation. They are the resonant tunnelling diode (RTD) and the superlattice electron

device (SLED). These devices are approaching the THz gap from the lower frequency side.

The quantum cascade laser (QCL) which is approaching the 1 THz region from above is also

a two-terminal device but relies on laser action rather than NDR. A common feature of all of

these devices is that they take advantage of semiconductor bandgap engineering to produce the

desired electrical characteristics.

5.3.1.1 Resonant Tunnelling Diodes

Resonant tunnelling diodes have a double barrier structure as shown (under zero bias) in

Figure 5.8(a). In the narrow space between the barriers the electron energies are quantized into

discrete levels. At a particular DC bias (Figure 5.8(b)) the energy levels at the bottom of the

conduction band on the left-hand side align with the lowest level in the well giving rise to a

resonant condition whereby electrons can tunnel through the barriers. As the bias is increased

further (Figure 5.8(c)), the resonant condition no longer holds and the current falls as the

applied bias voltage increases.

At even higher applied voltages the current once more increases under the combined action

of electron excitation over, and tunnelling through, the barriers. This results in an I–V curve

which has an NDR region and hence can lead to oscillations.

Tunnelling times of about 10�14 s are expected, holding out the prospect of operation into the

THz region and beyond. In practice, the highest frequency demonstrated to date is 712GHz, a

result that dates back to 1991 [19]. The THz power generated was in the mWregion – a level too

low formost applications. The reason for this low power can be explained from the I-V curve as

IP

EF

qV = 0

EF

qVP

EF

qVV

(a) (b) (c)

Figure 5.8 Double barrier resonant tunnelling diode (DBRTD) conduction band edge diagrams: (a) zero

bias, (b) at resonance and (c) in the NDR region


depicted in Figure 5.9 where it can be seen that the region of NDR is rather narrow along the

voltage axis. The power generated is determined by the product DI�DV where the terms

represent the current and voltage swings so a smallDV results in lowpower. The 712GHzfigure

was achieved many years ago and has not been improved upon. However, research continues

using materials other than GaAs/AlGaAs heterostructures and perhaps involving quantum

dots.

5.3.1.2 Superlattice Electron Devices (SLEDs)

Asmentioned above, Gunn diodes rely on interband electron transfer for their NDR. However,

if the material band structure is such that the electrons gain extra mass without transferring to

another band then an NDR effect could occur which would not be transfer-time limited. The

required band structure does in fact exist as illustrated in the typical semiconductor energy/

momentum (or E–k) curve of Figure 5.10(a). As the momentum increases, the curve becomes

less steep and this is equivalent to an increase in effectivemass for the electrons. At high enough

bias, the electrons will therefore slow down producing an NDR. The trouble is that in natural

semiconductors, the electrons either transfer, or begin to transfer, to another energy band before

they reach this region. Superlattice devices, because of the periodicity of the artificial structure

(Figure 5.10(b)), allow us to engineer a material which eliminates the band-to-band transition.

A wide range of component materials can be used in SLED structures and of course the

periodicity can also be optimized.

At the time of writing SLEDs have demonstrated power generation but only up to around

200GHz and then only at very low sub-microwatt power levels. However, continuous steady

progress, especially in heat dissipation, is being made and 1mW at 1 THz is predicted.

Another motivation for the development of SLEDs is to produce a THz frequency Bloch

oscillator.With reference to Figure 5.10(a), as an electron is accelerated from rest (k¼ 0) by an

externally applied electric field in the positive k direction up the (lower) E–k curve, it

eventually reaches the edge of the Brillouin zone at k¼ p/a, At this point its momentum is

reversed and it proceeds back upon itself to k¼�p/a where a further momentum reversal

∆I

Voltage (V )

Cur

rent

(I)

∆V

Power ≈ ∆I∆V

Figure 5.9 I–V characteristic of a resonant tunnelling diode showing the very steep negative differential

resistance region


occurs. In the ideal case this process repeats itself continuously giving rise to an oscillatory

motion.Much has been published on this phenomenon but it is not clear that Bloch oscillations

have actually been observed. However, from the results mentioned above, it may well be that

the future of SLEDTHz oscillators lies in the optimization of domain formation resulting from

the Bloch effect.

5.3.1.3 Quantum Cascade Lasers (QCLs)

Quantum cascade lasers (QCLs) are also superlattice devices but in this case they have been

engineered to emit THz radiation directly by laser action. The problem with the THz laser is

that, at this frequency, the photons have energies of �4meV. With room temperature

corresponding to 25meV normal laser action tends to be wiped out because of thermal

excitation of electrons between the laser energy levels. In the superlattice structure the laser

levels can be separated in space as well as energy, making laser action more likely, but for

operation in the THz gap, the devices still have to be cooled to near cryogenic temperatures for

useful operation. Power is not a problem with current performance figures of � 0.6mW at

1 THz rising quickly to about 10mWat 2 THz. However, the figures at 1 THz have only been

achieved with the addition of a magnetic field which of course increases the complexity, cost,

power consumption and physical size. In general, the higher powers are achieved under pulsed

biasing conditions and the lower the frequency the lower must be the operating temperature –

temperatures as low as 40K are needed at 1 THz.

QCLs are not conveniently tuneable by, for example changing the bias voltage. This

may not be a serious disadvantage, except perhaps in spectroscopy, but this has been

overcome for mid-IR frequencies by Benjamin Lee et al. [20] who, on a single chip,

fabricated an array of single mode distributed feedback QCLs, each emitting at a different

frequency. Steady progress is being made in the performance of QCLs as the technology

improves [21].

(b) (a)

E

k

-π/a π/a Reduced k space

EC

Figure 5.10 (a) Reduced energy/momentum (E–k) curves for a periodic structure; (b) superlattice

structure with conduction band profile


5.3.1.4 Josephson Plasma Sources

When a DC bias V is applied across a Josephson junction an AC signal of frequency f is

produced given by:

f ¼ 2qV=h ð5:8Þ

where q is the charge on the electron and h is Planck�s constant.For a typical DC bias of 1mV the frequency of the AC current produced is � 0.5 THz. A

single junction will supply very little power but, if the currents produced by a number of

interconnected junctions can be combined coherently, then a useful amount of power in themW

rangemay be possible. This possibility was predicted by Tilley as long ago as 1970 [22] but it is

only recently (2007) that a practical device has been reported by Ozyuzer et al. [23].

Using crystals of the high-temperature superconductor Bi2Sr2CaCu2O8 (BSCCO), Ozyuzer

et al. formed a stack of Josephson junctionsmade up of superconductingCuO2 layers separated

by insulating Br–Sr–0 layers in a mesa structure about 1mm high as shown in Figure 5.11. The

vertical side walls of the mesa reflect the waves generated at the junctions backwards and

forwards as in a laser bringing all of the components of the radiation into phase. The THz power

output of the device scaled as n2 – where n is the number of junctions – but with too many

junctions the device would not operate because the temperature would then rise above the

critical value for superconductivity.

At a temperature of 50K the authors recorded a power of � 0.5mW at a frequency of

0.85 THz but predict that up to 0.5mW could be generated in the 0.5–1.5 THz range. The

frequency capability of these devices nicely fills the remaining THz gap between transit-time

devices and QCLs but the power levels may still be a bit on the low side for practical

applications – and of course, they do need to be cooled down to low temperatures. In principle,

tuning could be achieved by varying the applied DC voltage but the length of the cavity also

determines the frequency of the emitted signal.

Figure 5.11 Josephson Plasma Source using the high-temperature superconductor Bi2Sr2CaCu2O8

(BSCCO) sandwiched around nonsuperconducting insulators. (From Science, 318, No. 23, Nov 2007.

Reprinted with permission of AAAS)


5.3.2 Three-terminal Devices

In electronics, conventional three-terminal devices are based on the flow of current over a

defined distance from one region to another. For bipolar transistors the current carriers make a

transition across the base region; in field effect devices the carriers must flow from source to

drain (Figure 5.12).

The time taken for current carriers to flow across a device – that is the ‘transit time’

determines the maximum frequency of operation. The shorter the transit length (L) and the

higher the carrier velocity (v), the shorter will be the transit time and the higher will be the

maximum frequency. However, current carriers (usually electrons) in semiconductors have a

maximum speed (or saturation velocity vSAT – but see below) in the region of 105ms�1.

(Beyond this speed they quickly give up their kinetic energy to the semiconductor lattice.)

Therefore, the only way to make the devices faster is to reduce the transit distance. This in turn

leads to difficulties because at small separations (<1micron) electrical breakdown can easily

occur. So, cut-off frequency, fT� v/L but v has an upper limit �105ms�1. Therefore for

operation at 1 THz, L� 0.1mm. The maximum voltage VMAX that can be applied across a

device is limited by electrical breakdown, that is

VMAX¼EBL

where EB is the breakdown field and the power generated

P / V2MAX

which can then be written as

P / E2Bn

2SAT=f

2T ;

that is the power falls off as the inverse square of the frequency. vSAT depends on thematerial so

with careful choice the power can be maintained at higher frequencies. Also as the dimensions

get smaller, the probability of an electronmaking a collisionwith the lattice in the transit region

reduces and ballistic transport can occur. In this situation there is no speed limit on the electrons

(except of course, for the velocity of light.)

The frequency of operation of ‘conventional’ electronic devices is slowly creeping up

towards the 1 THz figure and the following sections will describe how these improvements are

being brought about in different devices.

5.3.2.1 The Heterojunction Bipolar Transistor (HBT)

Workers at the University of Illinois [24] in the USA have designed, fabricated and tested

double heterojunction bipolar transistors (DHBT) with a cut-off frequency (i.e. unity current

S

GATE

L

D

Figure 5.12 Basic structure of a field effect transistor


gain) fT of 765GHz at room temperature (855GHz at 218K). The first consideration for this

device is the choice ofmaterial which, in this case, was to use the InP/GaAsSb system. This has

a high breakdown field and thermal conductivity but does have the disadvantage of relatively

slow diffusion through the base region. To overcome this difficulty, the composition of the base

regionwas graded to produce a built in electric field so that the electronswere driven through by

a combination of drift and diffusion.

In any electron device the ultimate performance is affected by parasitic circuit effects such as

contact resistance and interlayer capacitance. In the DHBT, the choice of materials served to

reduce these effects but also precision electron beam lithography was used to define the device

geometry and henceminimize unnecessary lengths of parasiticmaterial. The thicknesses of the

graded base and InP collector layers were 25 nm and 65 nm respectively. In practice, the

devices were only characterized up to 50GHz and hence the cut-off frequency is estimated by

extrapolation.

5.3.2.2 Field Effect Transistors (FETs)

As with the DHBT, the capabilities of field effect transistors (FETs) are slowly moving up in

frequency. This movement can again be put down to developments in material growth and

device processing. Again, the favoured material is InP because of its high electron mobility. In

December 2007 at the International Electron Devices Meeting (IEDM), Northrop Grumman�sSpace Technology Sector announced an InP high electron mobility transistor (HEMT) with

fT > 1THz [25]. This device has a sub 50 nmgate length, a 25% increase in electronmobility in

the channel and a similar reduction in resistivity which lowers the contact resistance. The

HEMTswere used in a three-stage common source low-noiseMMIC amplifierwhich exhibited

> 18 dB gain at 300GHz and 15 dB gain at 340GHz. Extrapolation of these results to 0 dB gain

gives the fT > 1THz.

There is also another pressure on the drive to make FETs operate at ever higher

frequencies. This comes from digital technology where the FET, operating as a switch, is

the device of preference in integrated circuits. To keep Moore�s Law on course, ever faster

switches are needed. In 2001 both Intel and IBM announced that they had developed

‘terahertz transistors’ aimed at the 45 nm generation of chip technology (see, for

example, [26, 27]). The Intel THz transistor is fabricated on a silicon substrate, but

unlike the conventional device, is separated from the substrate by a layer of oxide. This

layer reduces leakage between the source and drain thus reducing parasitic power

consumption. For the same reason, a new dielectric layer is used under the gate. Thicker

source and drain layers are introduced to reduce the series parasitic resistance. The gate

length is 45 nm.

Terahertz switching speeds have also been achieved in ‘ballistic deflection transistors’

(BDTs) which have been developed at the University of Rochester [28]. These devices are

fabricated using nanolithography and have dimensions less than the mean free path of

electrons in the material – hence ‘ballistic’ as the motion is collision free.

As illustrated in Figure 5.13, electrons directed into the BDT are deflected to the right

(indicating logic 1) or to the left (logic 0) by the polarity of the potential difference

between the control electrodes. Because of the nanometre dimensions, the parasitic

capacitances are also very small – in the attofarad range resulting in sub-picosecond

response times.


5.3.2.3 Plasma Wave Device

Dyakonov and Shur [29] have proposed and developed a novel application of a three-terminal

device for THzwave generation. Their ‘plasmawave transistor’ looks verymuch like a HEMT

(Figure 5.14) but operates in a very different way. In their device, plasmawaves are set up in the

electron gas in the channel rather like sound waves in a wind instrument – hence their name

‘electronic flute’.With a suitable choice of channel length, a THz frequency standingwave can

be generated between the source and drain, which in turn causes a current at this frequency to

flow in the external circuit.

In 2006, Dyakonova et al. [30] described the emission of radiation in the 0.2–4.5 THz range

from AlGaN/GaN HEMTs operating at room temperature. Power levels of 0.1 mW were

observed.

5.3.3 Multiplication

Terahertz signal generation by multiplication up from a lower frequency is perhaps the longest

established electronic technique – for example, see [31]. The motivation for this technology

stems largely from the astronomy and remote sensing communities where compact and robust

systems are a priority for mounting in space vehicles and cost is not a primary factor.

Awide range of sources are available from companies such as Virginia Diodes (VDI) and an

example of one of their THz sources is shown in Figure 5.15. The starting point for this device is

a low frequency (< 25GHz) signal which is multiplied by a factor of 72 to give an output in the

1 THz range. However, frequency multiplication is a relatively inefficient process so the

Figure 5.13 The ballistic defection transistor: (a) schematic and (b) micrograph of device structure.

(Reproduced by permission of the University of Rochester)

Figure 5.14 Schematic representation of the plasma wave transistor


resulting THz power is only 25mW,whichmay not be sufficient in some of the anticipated THz

applications such as imaging, stand-off detection or communications.

The nonlinear component that is at the heart of the VDI sources is the tried and tested

Schottky diode whose history dates back to the early days of radio communications and is

perhaps the first ever solid-state electronic device. Over the last few years researchers in

Sweden [32] have been developing an alternative nonlinear multiplier know as the hetero-

structure barrier varactor (HBV). TheHBVusesmodern growth techniques to produce a device

which has a symmetrical capacitance–voltage (C–V) characteristic (Figure 5.16) and hence,

when acting as a multiplier, produces only the odd harmonics of the drive signal. This feature

has the advantage of minimizing the number of different frequencies generated and removes

Figure 5.15 ATHz source consisting of a low-frequency coaxial input, an integrated doubler/amplifier,

a quadrupler and two triplers. It has generated up to 25mWin theWR-0.65waveguide band, total length is

6 inches, no mechanical tuners are used. (Reproduced with permission from Virginia Diodes)

0.2 0.2

0.1

-0.2

0

-60 -40 -20 0 20 40 60

Voltage [V]

Cur

rent

[µA

/µm

2 ]

Cap

acita

nce

[fF/µ

m2 ]

© 2007 IEEE

Figure 5.16 Capacitance–voltage and current–voltage measurements for a 12 barrier 700mm2 HBV.

Reproduced by permission of Josip Vukusic, Tomas Bryllert, T. Arezoo Emadi, Mahdad Sadeghi and Jan

Stake, IEEE Electron Device Letters, 28, no. 5, May 2007. (� 2007 IEEE)


the need to terminate the circuit at the even harmonics. Operated as a tripler, the HBV develops

an output power of 240mW at 110GHz with a conversion efficiency of about 20%. In

quintupler mode it develops 20mW at 202GHz at a conversion efficiency of about 3%. This

performance is comparable with that of state-of-the-art Schottky diodes and further develop-

ments already in progress in material systems, heat sinking and circuit design will undoubtedly

lead to significant improvements. However, at the present state of development, the frequencies

achieved are still only just in the THz range!

Ong and Hartnagel [33] have suggested a structure based on quasi-ballistic electron reflec-

tion (Q-BER) which according to their simulations should be superior to the HBVat the higher

frequencies. At the time of writing, results for actual devices have not been published.

5.4 Conclusion

Terahertz technology is a rapidly developing and expanding field. Historically, the utilization

of these frequencies has been limited by the scarcity and low brightness of sources. Currently,

however, a number of different technologies are increasingly bridging what used to be known

as the ‘THz gap’. These include both optical techniques, based on laser down-conversion, and a

variety of electronic technologies. It may therefore be expected that in the next few years

terahertz technology and its applications will become as widespread and well-established as

that of other spectral regions.

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