Chapter 1
Introduction
1.1 Overview of Terahertz Technology
The terahertz (THz) radiation (0.1 ∼ 3 0 THz) is categorized between
millimeter-wave (mm-wave) and infr ared light wave [16]. Recently, a great
deal of attention has been paid to the THz spectroscopy and imaging system
due to the mo de rate wavelength of the T Hz wave that can leverage the advan-
tages of both millimeter-waves (mm-waves) and light waves [17, 16, 1 8]. Like
mm-wave, THz wave has deep penetration to dielectric substances such as
ceramics, plastics, powders and food; like light wave, THz images with high
spatial resolution can be obtained by a 2-dimensional (2D) detection with
THz s ensor array [19]. On the other hand, high-data-rate THz wireless com-
munications have a lso come into view due to the abundance of undeveloped
bandwidth resources [20].
1.1.1 Terahertz Applications
1.1.1.1 Terahertz Spectroscopy and Imaging
Spec troscopy is a very mature method to identify substance comp osition by
fingerprint analysis in physical and analytical chemistry. A comparison of g en-
eral s pectrosc opy and imaging technologies is shown in Table 1.1. The c om-
monly used spectroscopy methods are Fourier transform infrared spectroscopy
(FTIR), non-dispersive infrared spe ctroscopy (NDIR), Ramen spectroscopy,
and X-ray spectros copy. THz spectroscopy not only shows the unique spectral
fingerprints for many substances [21], but also has several distinct advantages
when compared to these conventional spectroscopy methods. Firstly, unlike
X-ray, THz radiation is s afe to the tissue under test as well as the people
3
4 Design of CMOS Millimeter-Wave and Terahertz Integrated Circuits
Table 1.1: Comparison of General Spectroscopy and Imagi ng Technologies
Spec ific ations Ultrasound MRI THz
FTIR, NDIR,
Ramen
CM, OCT X-rays
Applications Imaging Imaging
Spec troscopy &
Imaging
Spec troscopy Imaging
Spec troscopy &
Imaging
Radiation
Mechanical
wave
Magnetic
field
EM wave
(Non-ionizing)
EM wave
(Non-ionizing)
EM wave
(Non-ionizing)
EM wave
(Ionizing)
Frequency (Hz) 10
6
-10
7
10
7
-10
8
10
11
-10
13
10
13
-10
15
∼10
15
10
16
-10
19
Molecular
interactions
No No Yes No No No
Image resolu-
tion
∼2mm ∼1mm ∼200µm — 0.1-10µm ∼15nm
Penetration
depth to hu-
man body
200-300mm >1m 1-3mm 0.5-5µm 0.2-0.8µm >1m
Introduction 5
Figure 1.1: Application examples of THz s pe ctroscopy and imagi ng.
(a) non-destructive detection of crack initiation in a film-coated layer
on a swelling tablet [1], (b) hydration state characterization in so-
lution [2], (c) in-vitro breast cancer diagnosis [3], and (d) in-vivo skin
cancer diagnosis [4].
who are c onducting the mea surement due to its longer wavelength and non-
ionizing nature [16]. Secondly, many materials that cannot be p enetrated by
infrared are transpa rent to THz, w hich e nables a non-destructive analysis to
the coated substance. For example, THz-based inner layer reflectance analy-
sis can be applied in the thin-film coating analysis of drug tablets as shown
in Figure 1.1(a) [1]. Thirdly, T Hz radiation is highly sensitive to the water
hydration state, which can be utilized in the concentration analysis of disac-
charide water solutions as shown in Figure 1.1(b) [2]. Finally, THz radiation is
more sensitive to the vibra tion and interaction of molecules such as protein or
polymer. Therefo re, it can be utilized for the in-vitro c ancer diagnosis (Figure
1.1(c)) [3] as well as explosive detection [22].
6 Design of CMOS Millimeter-Wave and Terahertz Integrated Circuits
THz imaging also has several distinct advantages compared to other imag-
ing techniques, such as ultra sound scan, ma gnetic resonance imaging (MRI),
confocal microsco py (CM) and optical co he rence to mography (OCT). Firstly,
THz imaging has higher resolution than ultrasound scan or MRI due to its
much shor ter wavelength. It is also more sensitive to the thin tissues due to
a stronger reflection and attenuation in the water content. Secondly, com-
pared to the existing optics-based imaging methods such as CM and OCT,
even though the THz imaging system has lower resolution, it has much higher
pene tration depth due to its much longer wavelength. Recently, with remark-
able contrast in skin and breast cancer demonstrated in THz images as shown
in Figure 1.1(d) [3, 4], the THz imaging system has been used as an intra-
operative tool during breast cancer surgery in Guys hospital in London [23].
1.1.1.2 Terahertz High-Data-Rate Wireless Communication
Ever since the invention of radio in the late 1800s, the pace of development
of wireless communication has never stopped. In the past half c entury, the
carrier frequency o f communication systems has r apidly increased from several
megahertz (MHz) into multi-gigahertz (GHz) ranges to satisfy the growing
bandwidth requirement. The recently developed 60GHz systems with 5 ∼
9GHz license-free band are able to provide a transfer speed up to 10 Gb/s for
short distance data co mmunication [5]. In order to further enhance the data
transfer speed to multiple tens or hundreds o f Gb/s for vario us applications
such as ultra-high definition TV in the near future, we have to develop the
communication s ystems in THz regime with abundant bandwidth resources.
The application of THz communication systems can be mainly categorized
into in-door data-link s and system level data-transfer as shown in Figure 1.2.
For the in-door data-links application, one THz wireless data transmis-
sion was initially demonstrated in 2009 at 30 0GHz with a photonics-based
transmitter; this system is able to achieve a data rate of 12 .5 Gb/s over 0.5-m
distance [24]. The la b scale communication was also demonstrated at 6 25 GHz
with a data rate of 2.5Gb/s in 201 1 [25]. Most recent developments in semicon-
ductor technologies have demonstrated a very clear potential of highe r-level
integration with wireless I/Os fo r inter-chip or intra-chip communication [26],
which is very likely to be achieved in THz. Recently, an integrated millimeter-
wave integrated c ircuits (MMICs) THz transceiver has been demonstrated in
50nm mHEMT technology with a data rate of 25 Gb/s at 220 GHz [27]. Po-
tentially, it is very promising for inter-chip or intra- chip communication in
THz regime with high data rate and energy efficiency [28, 29].
1.1.2 Optics-Based Te rahertz System
The current o ptics -based THz imag ing system is developed by the well-known
electro-optic sampling technique [30] as shown in Figure 1.3. When an ultra-
short optical pulse (∼50 femtoseconds) illuminates a non-linear semiconductor
Introduction 7
Figure 1 .2: Application examples of THz communication. (a) high-
definition multimedia interface (HDMI) provided by WiGiga and
Wireless-HD [5], and (b) THz intra-chip high spe ed data link be-
tween core and memory, and THz inter-chip high speed data link
between core and core.
Figure 1 .3: Schematic of a THz spectroscopy and imaging system by
electro-optic sampling technique in transmission type.
material such as Zinc Telluride (ZnTe), a very s hort electric pulse is generated
at the input of the dipole antenna with THz power spectrum. T he average
power level of the THz signal generated in this way is in the order of nanowatts,
The resulting T Hz pulse usually has a very wide bandwidth, of which the
upper and lower frequency boundaries are determined by the charge ca rrier’s
accelera tion in the semiconductor material and the antenna cut-off frequency,
respectively. After penetrating thro ugh the sample under test, the re sulting
THz radiation is coherently detected in the time domain with both intensity
and phase information, which can be further utilized in the non-destructive
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