On-Chip Sub-Diffraction THz Spectroscopy of Materials and Liquids

Abstract

This chapter summarizes the trends in terahertz measurements on the surface of rigid and flexible substrates. It focuses on research incorporating fast photoconductive switches to generate and detect on-chip THz pulses using a femtosecond laser. The chapter aims to review progress toward the study of picosecond dynamics and THz spectroscopy of materials and liquids. We emphasize general sub-diffraction techniques for THz spectroscopy, transmission line and waveguide design considerations, time-domain measurements for studies of material dynamics, and provide a survey of recent research on the THz spectroscopy of materials and liquids on-chip. We conclude with an outlook on the field and highlight promising new directions. This chapter is meant to be an introduction and a general guide to this emerging field for new researchers interested in on-chip THz studies.

1. Introduction

Terahertz (THz) time-domain spectroscopy has led to a deeper understanding of the properties of matter. Developed from early measurements of the optical switching of silicon [1, 2], generation and detection of free-space THz pulses in the late 1980s [3, 4, 5] gave way to a new spectroscopic approach for fundamental science [6]. With both amplitude and phase sensitivity, the technique allows direct extraction of the complex-valued index of refraction. The THz spectrum covers a window of energies (and timescales) that host rich phenomena in solids including: important carrier dynamics [7, 8], charge ordering [9], gapped excitations in superconductors [8, 10], magnetic excitations and dynamics [11], and energy quantization in the quantum Hall effect [12, 13, 14].

THz radiation has a number of appealing properties for biomedical sciences as well [15, 16, 17, 18, 19]. The non-ionizing and non-destructive nature of the radiation makes it particularly promising for non-invasive, in vivo imaging and cancer detection [20, 21, 22]. The time resolution of time-domain measurements opens the door to studying important hydration dynamics in solvated biomolecules and chemical reactions at the picosecond timescale [23]. Moreover, the capacity of this frequency of radiation to resonantly excite intra- and inter-molecular oscillations presents a unique advantage in the detection of low-concentration biomolecules. Highlighting a few notable recent examples, Cheon et al. showed that methylated DNA presents a resonant excitation at 1.6 THz and the degree of methylation can be quantified by the amplitude of the absorption peak [24]. Niessen et al. have reported clear resonant features of lysozyme, E. coli DHFR, and RNA G-quadruplex [25].

While the majority of research involving THz spectroscopy has been done using free-space radiation, there is a growing interest in probing samples smaller than the diffraction limit on the surface of a chip. The diffraction limit puts a fundamental restriction on the smallest size of a probed sample at roughly the half-wavelength ($\lambda /2$) [26, 27]. Since THz radiation has wavelengths on the order of a millimeter, this limit impedes THz spectroscopy of microscopic samples such as high-quality exfoliated two-dimensional (2D) flakes with lateral dimensions on the order of tens of microns. Two-dimensional materials have enjoyed fantastic appeal in material and applied sciences since the exfoliation and isolation of a single layer of carbon atoms [28]. The recent observation of unconventional superconductivity in two layers of graphene precisely rotated by 1.1 degrees [29, 30] has reinvigorated investigations of correlated phenomena in these heterostructures. Near-field techniques have been developed to overcome the diffraction limit using apertures and sharp metal tips, but another promising direction is the use of on-chip waveguides and transmission lines to perform THz measurements on micrometer samples.

Sub-diffraction techniques have also revolutionized biotechnology. The zero mode waveguide presents a canonical example, now allowing researchers to perform single-molecule measurements at biologically relevant concentrations all at exceedingly small sample sizes [31, 32]. This is generally achieved by reducing the observation space to a sub-diffraction aperture of zeptoliter volume, allowing rapid and parallel measurements of single molecules at micromolar concentrations [33]. Researchers are actively trying to extend sub-diffraction techniques to the terahertz region of the electromagnetic spectrum for fingerprint analysis of bioliquids. This is challenging as water leads to strong attenuation of THz signals. For the same reason that THz radiation is appealing for identifying biomolecules, water molecules themselves present strong resonant absorptions. These absorptions mask the underlying fingerprints of the biomolecules present in a liquid sample. In Cheon et al., measurements of methylated DNA are performed at 253 K (−20°C) in order to freeze liquid water and reduce background absorption [24]. In Niessen et al. paraffin oil is used to maintain protein hydration in place of water [25]. Confining small amounts of bioliquids in microfluidic cells on the surface of on-chip THz spectrometers allows a reduction of the interaction volume between the probing field and the target sample and may provide avenues to diagnostic devices.

This chapter is organized into seven sections including this introduction. Section 2 provides a survey of the historical developments in sub-diffraction techniques for THz measurements. Section 3 presents waveguide and transmission line design considerations for on-chip measurements. Section 4 surveys research on time-domain measurements for studies of dynamics in material systems. Section 5 reviews research on THz spectroscopy on-chip and examples of spectroscopy of materials. Section 6 reviews research on THz spectroscopy of liquids. Finally, Section 7 concludes the chapter and provides an outlook on the field.

2. Historical summary of sub-diffraction techniques

Sub-diffraction techniques have been successfully used in THz imaging and microscopy, whereas sub-diffraction has proven to be more challenging [34]. Many interesting physical systems have geometries smaller than the diffraction limit: a scale less than half the wavelength of radiation where light does not interact efficiently with a material. This limit for THz radiation is on the order of 100 μm. In order to probe materials below the diffraction limit, several techniques have been developed which can be grouped into two general areas: apertures and sharp metal tips. See Figure 1 for simple model representations of these two approaches.

Figure 1.

Sub-diffraction terahertz imaging and spectroscopy techniques. (a) Aperture-based techniques limit the probing radiation to a small aperture, thereby increasing the resolution. This is a recent example from Kawano et al. showing an optical image of an aperture on the surface of a GaAs substrate. (b) Model schematic of the aperture device and principle of operation. The THz radiation is enhanced at the near-field probe and detected with the buried 2DEG. Panels (a) and (b) have been adapted from ref. [35]. (c) Near-field imaging and spectroscopy with a sharp metal tip. This is an example from Moon et al. showing the principle of operation. THz radiation is scattered off the metal tip and detected. Panel (c) has been adapted from ref. [36].

Apertures form the earliest examples of sub-diffraction imaging and the idea dates back to suggestions by Synge in 1928 [37]. Aperture techniques for THz radiation were experimentally realized as early as 1993 when Merz et al. displayed an increased resolution of $\lambda /2$ [38]. Researchers have continued to improve signal resolution [39, 40, 41, 42, 43], up to a recent publication by Kawano, et al. where they demonstrated a 9 μm resolution using an aperture on a gallium arsenide-aluminum gallium arsenide (GaAs/AlGaAs) substrate [35]. Figure 1(a-b) display their device. The THz radiation is enhanced by the near-field probe and detected using the buried 2D electron gas (2DEG), as shown schematically in Figure 1(b). The aperture approach is limited by the cut-off effect: as the aperture becomes smaller, and the resolution better, the transmitted signal is reduced and the signal-to-noise ratio (SNR) ultimately suffers [44, 45]. As an inconvenient technicality, the detector must be exceedingly close to the aperture to prevent signal loss from quick divergence of the radiation through the aperture.

A second approach involves the use of a metal tip that enhances the probing field through near-field, plasmonic effects [46, 47, 48, 49]. This was demonstrated in 1985 by Wessel using infrared radiation [46] and subsequently investigated with THz radiation [36, 47, 48, 49]. An example of this technique is shown in Figure 1(c). The confinement at the apex of the metal tip produces strongly enhanced fields and sub-diffraction radiation. The increased resolution is comparable to the tip diameter (tens of nanometers). This technique has enjoyed recent interest in the imaging of 2D materials at THz frequencies [50, 51]. The technique has a few drawbacks. One, a probing field is required which may also perturb the sample under study and, two, the tip itself can lead to unwanted tip-sample interactions. Moreover, most of the scattered signal from the sample surface is background signal and the weak tip enhanced radiation requires long measurement times. Ribbek et al. have reported detection of the near-field part of the scattered signal with an integration time of 19 mins for a scan delay of 7.35 ps (amounting to a frequency resolution of 0.14 THz) [52].

Lastly, on-chip waveguides and transmission lines can be used to confine sub-diffraction radiation [53, 54, 55, 56]. To perform spectroscopy, photoconductive switches can be fabricated near or within the on-chip circuit to generate and sample single cycle pulses using a femtosecond laser. This measurement scheme is similar to the early studies by Auston et al. that were the basis of free-space spectroscopy but that were abandoned due to pulse attenuation from lossy substrates [2]. With careful design, waveguides and transmission lines can be fabricated to achieve reflection-free spectroscopy resulting in long time-domain scans and high frequency resolution.

3. Circuit design considerations

Ultrafast pulse generation and readout on-chip have been accomplished using a number of transmission line and waveguide designs. In the earliest studies from Auston and colleagues, the chips were fabricated with microstrip lines on high-resistivity silicon [1, 57, 58]. Following this, two and three-arm coplanar transmission lines were investigated as well as coplanar waveguides [59, 60, 61, 62, 63, 64, 65, 66]. More recently, single conductor waveguides (Goubau lines) have been used to route THz radiation and single pulses on chip [67, 68, 69, 70, 71]. In most cases, the circuits have been fabricated using conventional metals, but superconductors have also been used [72].

An overview of these different designs is shown in Figure 2. Figure 2(a) shows the original design by Auston [1]. The geometry consists of a continuous ground plane on the back of a silicon substrate and a hundreds-of-microns wide stripline on the surface of the chip. Auston used this design to demonstrate ultrafast (picosecond) switching of electrical signals on-chip with the aid of optical laser pulses to electrically open and close microstrip gaps. Shortly after this, Ketchen et al. measured picosecond pulses in a three arm transmission line [59]. A schematic of this design is shown in Figure 2(b). Aluminum lines (5 microns wide) were patterned using standard photolithographic techniques on top of a silicon-on-sapphire wafer. The silicon was subsequently radiation-damaged to produce short (less than 1 ps) carrier lifetimes required for pulse generation and detection with a femtosecond (80 fs) laser. Using this design, the authors were able to change the relative distance between the locations of the pulse generation and detection to measure pulse dispersion. They found that the pulse width increased roughly 1 ps for a propagation length of 8 mm. Figure 2(c) shows an example of a coplanar waveguide design studied in ref. [70]. Here, a ground plane surrounds a center conductor and electrical lines for connections to the photoconductive switches. The circuit is created using a photolithographically defined mask and deposition of titanium (10 nm) and gold (200 nm) metals. The circuit is patterned on a gallium arsenide substrate with low temperature grown gallium arsenide (LT-GaAs) of 2 microns thickness used as photoconductive switches. The last example is a single conductor Goubau line as shown in Figure 2(d) from ref. [70]. Here, the single conductor from a coplanar design, such as that shown in Figure 2(c), is extended across the chip without an adjacent ground plane. Dazhang et al. fabricated this structure on top of a quartz substrate using LT-GaAs as photonconductive switches.

Figure 2.

Various transmission line and waveguide designs for on-chip THz studies. (a) An example of a microstrip line design. This schematic shows a side view of the device. A single conductor (hundreds of microns wide) is photolithographically patterned on top of the silicon substrate. The back side is coated with metal to function as a ground plane. This panel has been adapted from ref. [1]. (b) An example of a coplanar transmission line design. Two grounded lines surround a center conductor. Readout (sampling) is accomplished with a fourth electrical line at the bottom. This panel has been adapted from ref. [59]. (c) An example of a coplanar waveguide design. A center conductor is surrounded by ground plane patches and readout is performed with additional electrical lines (labeled probe arms). This panel has been adapted from ref. [66]. (d) An example of a Goubau (single conductor) waveguide. A single metallic strip connects two sides of the device used for pulse generation and readout. It also includes a stub filter along the transmission line of length ‘t’ to create a band-stop filter. This panel has been adapted from ref. [70].

All of these designs, in principle, can accommodate a microfluidic cell placed on top of the chip to hold small amounts of bioliquids for study. Several groups have incorporated such cells for studies of liquid analytes [73, 74, 75, 76, 77, 78, 79, 80, 81].

There are a few particularly attractive features of using these types of circuits fabricated on-chip. Photolithographically-defined electrodes allow control over where the radiation is routed. This includes fabricating various bends and branches to guide and even split the radiation along different paths [82]. Moreover, waveguide modes can be selectively excited. Wu et al. has shown that the dominant modes in a coplanar waveguide, slotline and coplanar modes, can be selectively excited using two photoconductive switches and a defocused laser spot [65]. This is advantageous because the field orientations for the two modes are different and one may couple better to a material or bioliquid under study. This was exactly the case in Wu et al.’s study as they found that the coplanar mode coupled better to their sample than the slotline mode. Follow up full wave modeling supported this experimental result, finding that decoupling the gate line from the ground plane better coupled the waveguide modes to the plasmon modes [83]. In such a way, the general electric field orientation can be controlled with careful choice of circuit design. For example, the coplanar transmission line design predominantly displays fields horizontal to the chip surface for materials placed between the lines. The Goubau line presents a vertical electric field for materials placed on top of the conductor. In this way, a preferential direction of the electric field excitation can be chosen.

As can be readily surmised, attenuation and pulse dispersion become significant factors in on-chip studies. As opposed to free space measurements, the pulsed radiation on-chip travels through lossy media. There are three considerations to improve the bandwidth for on-chip measurements. The first and most important is by judicious choice of a low loss substrate. Several materials have been investigated as low loss THz substrates and windows including sapphire, quartz, polyethylene, picarin, polytetrafluoroethylene (PTFE), and polyethylene terephthalate (PET) [63, 84, 85, 86, 87, 88]. On-chip bandwidths greater than 2 THz have been realized using picarin (Tsurupica) [63]. Another method to improve bandwidth is by thinning the substrate so the effective permittivity is lowered [89, 90]. Picarin may not support aggresive thinning ($<100$ μm) required for appreciable improvement but quartz is a promising alternative [89, 90]. The last method is to optimize the pulse generation and detection. The generated electric field of the THz pulse is proportional to the transient photocurrent density in the semiconducting junction [91, 92],

$$E_{\mathit{THz}}\propto \frac{\partial J}{\partial t}=e\left(n\frac{\partial v_n}{\partial t}+v_n\frac{\partial n}{\partial t}t+p\frac{\partial v_p}{\partial t}+v_p\frac{\partial p}{\partial t}\right),$$E1

where $J$ is the photocurrent density, $n$ and $p$ are the respective electron and hole carrier densities, and $v_n$ and $v_p$ are the respective electron and hole average velocity. The rate of change of the densities is related to the relaxation time of the semiconducting material (${\tau }_m$) and the pulse width of the excitation laser ($\mathrm{\Delta }t$),

$$\frac{\mathit{dn}}{\mathit{dt}}=-\frac{n}{{\tau }_m}+n_{0}exp-{\left(t/\mathrm{\Delta }t\right)}^2,$$E2

where $n_0$ is the carrier density at $t=0$ s. The same equation applies for the hole carrier density. The rate of change of the velocity is proportional to the applied electric field ($E$) (the voltage bias applied to the junction) through,

$$\frac{{\mathit{dv}}_{n,p}}{\mathit{dt}}=-\frac{v_{n,p}}{{\tau }_s}+\frac{q_{n,p}}{m_{n,p}}E,$$E3

where ${\tau }_s$ is the momentum relaxation rate and $m_{n,p}$ is the effective mass for electrons and holes respectively. Summarizing these relations, there are two aspects that will improve signal and bandwidth. The first is by using a material with a fast carrier relaxation rate and the second is utilizing a laser with a short pulse width. Radiation damaged silicon has a relaxation time of 560 fs [93] but shorter relaxation rates have been observed in LT-GaAs ($\approx 300$ fs) [94].

4. Utrafast pulse measurements

Besides spectroscopy, ultrafast pulse measurements can be used to study carrier relaxation times, magnetization dynamics, and hydration processes in the time domain. Strictly speaking, these include studies that are not interested in the frequency domain and focus solely on the time-domain data.

Some of the earliest examples harness on-chip pulses to study ballistic transport in clean one-dimensional and two-dimensional systems [95, 96]. Shaner and Lyon studied two-dimensional electron gas (AlGaAs/GaAs) structures connected to an on-chip coplanar waveguide [95]. From their THz time-domain data, shown in Figure 3(a), resonant oscillations appear for positive magnetic fields after a 50 ps time delay. Figure 3(b) displays their full data, showing the evolution of collector voltage as a function of magnetic field and time delay. The ballistic signal is distinguished by a stable signal over time (horizontal streaks), and signatures of magnetoplasmon oscillations in the 2DEG are evident by the weak signal modulation with magnetic field. Zhong et al. performed a similar experiment with carbon nanotubes connected to an on-chip coplanar transmission line [96]. They also observed ballistic carrier resonances in their time-domain data, Figure 3(c), that change with the length ($L_g$) of the ballistic channel.

Figure 3.

Time-domain studies of various materials. (a) Ballistic carrier transport in a two-dimensional electron gas (2DEG). The collector voltage (y-axis) is plotted as a function of perpendicular magnetic field for different time delays between the generation switch and collector. Oscillations corresponding to ballistic transport can be seen for delays of 50 ps and greater. (b) Collector voltage plotted as a function of magnetic field and time delay. Horizontal streaks in the data correspond to ballistic transport and weaker oscillations are from magnetoplasmon excitations. Panels (a) and (b) have been adapted from ref. [95]. (c) Drain current of a carbon nanotube device as a function of time delay between two on-chip THz pulses. Dips in the data correspond to ballistic carrier transport. The curves from top to bottom correspond to ballistic channels with lengths of L_g = 1.5 mm (black), 3.2 mm (red) and 4.9 mm (green). This panel has been adapted from ref. [96]. (d) Readout current as a function of time delay between generation and detection pulses for a graphene junction emission switch. The various curves correspond to different voltage biases applied to the graphene flake. This panel has been adapted from ref. [71]. (e) Helicity-dependent photocurrent as a function of time delay between excitation pulse and readout pulse. The two curves show that the photocurrent changes sign for different orientations of the applied voltage bias, as expected for anomalous hall currents. This panel has been adapted from ref. [97].

The largest category of these investigations includes emission studies of various materials including GaAs [64], InAs nanowires [98], topological insulators [99, 100, 101], silicon [102], graphene [71, 93, 97, 103, 104], molybdenum disulfide [105], carbon nanotubes [106], GaAs nanowires [107], and field emission in nanojunctions [108]. Hunter et al. demonstrated emission of picosecond pulses from graphene flakes attached to an on-chip Goubau waveguide, Figure 3(d) [71]. The amplitude of the pulse could be linearly controlled with the applied bias to the graphene junction (inset of Figure 3(d)). Using circularly polarized light, McIver et al. showed that helicity-dependent picosecond photocurrents could be measured in graphene junctions using on-chip Goubau waveguides. Figure 3(e) displays the data showing that the measured photocurrent changes sign when the bias direction is changed across the graphene.

Ultrafast on-chip pulses have also been used to study picosecond switching dynamics in magnets and strained semiconductors [109, 110, 111, 112, 113, 114, 115, 116]. Of particular note, Gerrits et al. studied magnetization reversal in permalloy films (NiFe) [109]. They showed that by using two photoconductive switches to shape ultrafast pulses to excite the material, they could suppress unwanted ringing effects with a second stop pulse. Yang et al., using a coplanar transmission line, displayed magnetization reversal in GdFeCo films with 10 ps pulses [113]. The mechanism proposed for the switching is heating caused by the electrical pulses. Interestingly, only 4 fJ of energy is required to switch a 20 nm³ magnetic cell.

5. THz spectroscopy on-chip

Using two photonconductive switches fabricated close to a transmission line or waveguide on-chip, it is possible to perform spectroscopy from the time-domain data using either pulsed lasers or continuous wave optical beating. Several research groups have investigated circuit elements that lead to resonant features in the THz spectrum. For example, Dazhang et al. have used a Goubau waveguide with a stub line, shown in Figure 2(d), to create a band-stop filter [70]. Figure 4(a) displays their data showing the measured time-domain scan in the inset and the Fourier transformed data in the main panel. Band-stop response can be seen at the location of the arrows centered at 250 GHz (fundamental mode) and 780 GHz (third harmonic).

Figure 4.

Examples of on-chip THz spectroscopy. (a) Transmission spectra of a Goubau waveguide device with a band-stop filter (a stub line fabricated on the Goubau line shown in Figure 2(d)). The dips with arrows indicate the band stop frequencies of 250 GHz (fundamental mode) and 780 GHz (third harmonic). The inset shows the time domain scan used to form the spectra in the main panel from a Fourier transform. This panel has been adapted from ref. [70]. (b) Amplitude spectra obtained for a coplanar waveguide device with and without a DC block (discontinuity in the central conductor). The inset shows the raw current measurement from which the amplitude spectra is calculated. This panel has been adapted from ref. [117]. (c) Time-domain data at different temperatures of a coplanar device with a 2DEG (AlGaAg/GaAs) mesa integrated within the centerline conductor. (d) Amplitude spectra of the Fourier transformed time-domain data in panel (c). Resonant peaks correspond to plasmon excitations in the gated (132 GHz and 264 GHz) and ungated (311 GHz) regions. Panels (c) and (d) have been adapted from ref. [118]. (e) Real part of the complex conductivity of a graphene heterostructure integrated within a coplanar transmission line device. The colored curves correspond to different Fermi energies labeled in the legend in panel (f). The solid lines are fits to the Drude model. The inset shows the time-domain data for two different Fermi energies. (f) The imaginary part of the complex conductivity of a graphene heterostructure. The solid lines are fits to the Drude model. Panels (e) and (f) have been adapted from ref. [119].

Very recently, Yoshioka et al. have demonstrated on-chip spectroscopy using continuous wave THz radiation created from the optical beating of two near-IR lasers [117]. This technique offers greater frequency resolution (10 MGz vs. 10 Ghz) when compared with the time-domain technique. Figure 4(b) displays data from their work. They investigated devices with and without a DC block (a gap in the centerline conductor) in a coplanar waveguide structure. The data (red) measured with a DC block show dips in the spectra due to interference of even and odd modes of the waveguide. The DC block enhances the bandwidth spectrometer and reduces unwanted DC leakage currents.

On-chip THz spectrometers have also been used to study plasmon excitations in clean 2DEG systems [118, 120, 121]. Wu et al. showed that plasmon excitations in an etched mesa of AlGaAs/GaAs could be electrically gated [118]. Figure 4(c) shows their time-domain data. Very prominent oscillations with different periods can be seen. The Fourier transform of the time-domain data is shown in Figure 4(d). Three peaks are produced at frequencies of 132, 264, and 311 GHz. The lower frequencies coincide with the predicted plasmon frequency of the first and second mode for the gated region of the device. The third resonance agrees with the plasmon frequency expected for the ungated region.

One of the qualities of the on-chip system, as highlighted in the introduction, is the ability to probe materials below the diffraction limit. Clean exfoliated 2D materials are typically tens of microns on a side and are well-suited for investigation with on-chip spectroscopy. Gallagher et al. have probed the THz properties of a graphene heterostructure using a coplanar transmission line and LT-GaAs photoconductive switches [119]. They investigated the predicted quantum critical characteristics of graphene at low doping and its hydrodynamic properties at higher doping. The inset of Figure 4(e) displays their time-domain data at two different dopings. The main panels of Figure 4(e-f) show the optical conductivity (real and imaginary, respectively) calculated from the Fourier transform of the time-domain data for various Fermi energies corresponding to the legend shown in Figure 4(f). The data follow the Drude formula closely, $\sigma =D_{\mathit{gr}}{\pi }^{-1}{\left({\tau }^{-1}-\mathit{i\omega }\right)}^{-1}$ with the known Drude weight of graphene, $D_{\mathit{gr}}=2{\left(e/\mathrm{\hslash }\right)}^2{k}_B{T}_{e}log\left(2cosh\left(m{\left(2k_B{T}_e\right)}^{-1}\right)\right)$ and $\tau$ the only fitting parameter. This data shows the Fermi liquid character of graphene at finite doping. The authors go on to extract the scattering rates for charge neutral graphene, finding support for quantum critical behavior in their linear dependence with temperature.

6. THz spectroscopy of liquids

In addition to solids, several research groups have incorporated reservoirs and microfluidic channels to perform spectroscopy on liquids. This could open up an avenue toward diagnostic devices for the detection of low-concentration biomolecules or investigations of chemical reactions in-situ.

Probably the most straightforward approach to investigating liquids is by fabricating a microfluidic channel or reservoir on the surface of the chip [73, 74, 75, 76, 77, 78, 79, 80, 81]. Swithenbank et al. has shown that a polydimethylsiloxane (PDMS) molded microfluidic channel can be plasma-bonded to the surface of a Goubau waveguide chip [74]. Figure 5(a) shows a model schematic of the device. A thin (6 μm) layer of benzocyclobutene is spun on the chip before the PDMS mold to electrically isolate the Goubau line from the liquid channel. Using this device, they measured the complex permittivity of several liquids in the THz regime including methanol, ethanol, propan-1-ol, butan-1-ol, hexan-1-ol, octan-1-ol, and mixtures of propan-2-ol/DI-H2O. Figure 5(b) displays measurements on mixtures of propan-2-ol/DI-H2O for different concentrations of DI-H2O in 10% increments from red (0%) to purple (100%). The complex permittivity can be seen to gradually increase with the percentage of DI-H2O, as expected. One can also appreciate from the data that the bandwidth of the measurement decreases as the mixture becomes more absorbing. This highlights a design parameter of the spectrometer that is important. If greater bandwidth for a relatively absorbing substance is required, one can reduce the interaction volume between the probing field and the measured liquid by shrinking the volume of the microfluidic channel.

Figure 5.

Examples of on-chip THz spectroscopy of liquids. (a) Model schematic of an on-chip spectrometer for measurements of liquids. A PDMS mold is plasma bonded to the surface of a Goubau line device. (b) Complex permittivity measured for various mixtures of propan-2-ol/DI-H2O for different concentrations of DI-H2O in 10% increments from red (0%) to purple (100%). Panels (a) and (b) have been adapted from ref. [74]. (c) Model schematic of an on-chip spectrometer for measurements of liquids. A silicon microfluidic channel is fabricated on the surface of a Goubau line device. (d) Transmission (S21) measurements of various concentrations of BSA in DI water. Panels (c) and (d) have been adapted from ref. [75]. (e) Model schematic of an on-chip spectrometer for measurements of liquids. A reservoir made from SU-8 photoresist is created on the surface of a chip with a microstrip line. (f) THz pulse delay as a function of sample volumes of single-stranded (ss) and double-stranded DNA. The solid lines are fits to an exponential model. Panels (e) and (f) have been adapted from ref. [80].

Another example of this sort of device is given by Laurette et al. [75]. They also used a Goubau line device but instead of using photoconductive switches pumped with femtosecond lasers, they connected the chip to a high-frequency vector network analyzer with frequency bands of 0–110 GHz and 140–220 GHz. They measured mixtures of bovine Serum Albumin (BSA) in DI water and lysozyme in DI water at different concentrations to study hydration shell structure. Figure 5(c) shows a model schematic of the device consisting of a Goubau line fabricated on a pyrex borosilicate glass substrate. The microfluidic channel is created by deep reactive ion etching of silicon that is bonded to the glass substrate [81]. Figure 5(d) shows the measured transmission (S21) for various concentrations of BSA powders. The transmission amplitude is seen to increase with BSA concentration. They found a lower detectable sensitivity limit of the system of 5 mg/mL² for protein spectroscopy.

Kasai et al. have demonstrated THz pulse measurements of DNA solutions using an on-chip device consisting of a microstrip line fabricated on a silicon substrate, Figure 5(e) [80]. A sample reservoir with a diameter of 400 μm and a height of 6.5 μm was created from SU-8 photoresist. Figure 5(f) displays their results showing that the pulse delay tracks with the amount of single-stranded (ss) and double-stranded (ds) DNA samples. The pulse’s propagation is exponentially delayed with increased sample volume and follows the expected trend of $t=t_{\mathit{sat}}\left[1-exp\left(-d/V\right)\right]$, where $V$ is the sample volume, and $t_{\mathit{sat}}$ and $d$ are fitting parameters. The distinct difference between the single-stranded and double-stranded results paves the way for a diagnostic device for biomedical sciences.

Finally, highlighting the versatility of on-chip spectroscopy, Russel et al. [122] have presented measurements of various alcohols using a Goubau line fabricated on a flexible polyimide substrate wrapped around a quartz capillary (3 mm diameter with 10 μm thick walls). Using this system they were able to measure the time-of-flight permittivities [123] of various alcohols. The measurement only provided relative approximations of the permittivities but the signal-to-noise ratio was found to be better than that obtained from free-space measurements of the same alcohols. The flexible substrate approach provides another degree-of-freedom in the design of on-chip systems.

7. Conclusions

This chapter has reviewed the history and common applications of on-chip THz time-domain spectroscopy. Free-space THz methods struggle to probe samples smaller than the diffraction limit. Still, several techniques have been developed to overcome this limit using apertures, sharp metal tips, and on-chip waveguides. Common designs include coplanar transmission lines and Goubau waveguides. Using photolithography and metallization, the circuit design has many freedoms, useful for signal routing, filtering, and even selecting the propagation mode of the electric field.

Using this technique, researchers have investigated electronic properties of materials in both the time and frequency domain. Time-domain data can show the evolution of a system with picosecond resolution. This has been used to show rapid phenomena, such as magnetoplasmon oscillations in GaAs/AlGaAs 2DEGs, ballistic carrier resonances in carbon nanotubes, and emission and relaxation characteristics of a variety of photoconductive materials impinged by a femtosecond laser pulse. Frequency-domain data is calculated using a simple discrete Fourier transform of the time domain. This absorption spectra have been used to characterize on-chip band filters, assess the efficacy of DC blocks in the transmission line, and directly measure plasmon excitation frequencies in gated GaAs/AlGaAs. Additionally, the frequency-dependent complex conductivity of materials can be extracted from the THz spectra, enabling deep analysis of the scattering rates of electronic systems.

The modularity of chips allows microscopic liquid samples to be probed as well. Various designs for microfluidic cells have been developed, including reservoirs made of PDMS and photoresist, and channels etched into a silicon substrate, each fabricated atop a transmission line. These developments are especially useful for biomedical applications, as minimal sample size is necessary for comprehensive results. Studies include characterization of frequency bandwidth with various volumes of sample, determination of sensitivity to small samples of protein, and distinction between single-stranded and double-stranded DNA samples. The Goubau line design is also capable of fitting on a flexible substrate, demonstrating its versatility.

Terahertz spectroscopy has great potential in probing a realm of physics inaccessible by other techniques and applying it on a small-footprint chip opens up many interesting directions for future research. Spectroscopy on nanomaterials is one clear avenue for further development. THz spectral information could lead to a better understanding of complex ground states in 2D materials and heterostructures. One example is the recently discovered twisted heterostructures that present superconductivity such as twisted bilayer graphene. On-chip THz spectroscopy offers a method to probe these types of samples that are too small for free space measurements. The transmission spectra acquired could provide detailed tracking of spectral weight and sensitive investigation of the onset of interactions and correlations in these systems. Moreover, using on-chip spectroscopy with an optical pump would enable studies of quasiparticle dynamics and information on possible pairing mechanisms that are not well understood in twisted heterostructures.

Given the progress so far, another clear direction of development is the study of bioliquid samples on-chip. Many biomolecular motions present vibrational and rotational excitations in the THz spectrum. These excitations can serve as spectral fingerprints of the species for in-vitro diagnosis. Special attention will be needed on signal attenuation from water but careful adjustment of the interaction volume may lead to the detection of very low-concentration biomolecules in liquids. With advances in compact THz sources and detectors, new diagnostic devices with better sensitivity may be developed.

Acknowledgments

This material is based upon work supported by the National Science Foundation under Grant No. (2047509).

Conflict of interest

The authors declare no conflict of interest.