Coherent terahertz photonics

: We present a review of recent developments in THz coherent systems based on photonic local oscillators. We show that such techniques can enable the creation of highly coherent, thus highly sensitive, systems for frequencies ranging from 100 GHz to 5 THz, within an energy efficient integrated platform. We suggest that such systems could enable the THz spectrum to realize its full applications potential. To demonstrate how photonics-enabled THz systems can be realized, we review the performance of key components, show recent demonstrations of integrated platforms, and give examples of applications.


Introduction
The terahertz (THz) frequency region of the electromagnetic spectrum (0.1 THz -10 THz) has shown enormous potential for a broad range of applications from health care to security, with communications, high resolution imaging (e.g. in the medical and pharmaceutical sectors), spectroscopic materials analysis and atmospheric sensing of special importance [1]. However, to date, realization of this potential has been severely constrained by the technologies available for the generation and detection of THz signals, which are limited in terms of cost, compactness, energy consumption and room temperature operation. The most widely available commercial systems are based on broadband THz generation using short pulse (fs) near-infrared lasers and photoconductive detector technology resulting in large, power hungry (kW) systems that have nonetheless helped in demonstrating the potential of THz radiation for spectroscopy and high resolution 3D imaging, thanks to the time discrimination enabled by the short pulse system. However the realisation of the full potential of the THz part of the electromagnetic spectrum will require the use of more agile sources in combination with coherent techniques. As a comparable example, the development of the radio frequency (RF) and microwave spectrum was made possible by the compactness, precision, spectral efficiency and sensitivity enabled by coherent techniques. In recent years coherent technology has also found renewed application in optical communication systems, where it permits the increase in spectral efficiency required to accommodate the rapid growth in internet traffic within the limited optical bandwidth available with current optical amplifiers [2]. We assert that the introduction of coherent photonic technology, based on that used for optical communications, into THz systems will unlock the potential of this part of the frequency spectrum for both scientific and commercial applications. Such a development has, however, long been prevented by the lack of cheap, compact, spectrally pure and power efficient continuous wave sources. Semiconductor electronic sources above 300 GHz currently offer only very low (few μW) output powers, the current room temperature operation record being 86 μW at 479 GHz from a third-harmonic mode InP Gunn effect device with a power consumption of up to 20 W [3], although lower frequency sources followed by diode multipliers have achieved 1 mW output power at 625 GHz [4].
By contrast, the use of photonic technologies would enable the creation of compact, optical fibre interfaced, coherent continuous wave (CW) THz sources, on-chip sensors and receivers, offering unprecedentedly narrow linewidth and concommitantly improved detection sensitivity, with power consumption below 1 W, two orders of magnitude lower than current electronic solutions. Photonic THz sources are based around component technologies such as waveguide Uni-Travelling Carrier Photo-Diodes (UTC-PDs), providing world record, room temperature, CW output powers at THz frequencies [5,6] and Quantum Cascade Lasers (QCLs) capable of CW output powers of >100 mW [7]. In order to achieve the required coherence these elements must be driven by a photonic heterodyne generator used as the common local oscillator [8]. For reception, detectors can be driven by a similar photonic local oscillator, providing the required coherence for the system, and thus enabling high sensitivity detection. In this paper we will describe in detail the concept of the system as well as its expected performance. We will also show the recent progress made for each of the components to permit the agility, signal power and detection sensitivity that are necessary for system realisation. Finally, we will discuss a few examples of applications and give some recent results demonstrating that the system philosophy presented in the paper has the potential to enable major growth in THz applications.

System description
The basic philosophy of the system is shown in Fig. 1. At the heart of the system, a highpurity photonic local oscillator based on optical comb generation and locking of two slave lasers generates an optical heterodyne signal at frequencies ranging from 100 GHz to 5 THz. Such a source enables heterodyne linewidths of considerably less than 10 Hz [8]. The heterodyne signal can then be detected directly in an UTC-PD or an InP-based photoconductive switch [9] to generate a high purity THz signal. Such a system will also enable linewidths five orders of magnitude smaller than the heterodyne systems that have already been used successfully for spectroscopy [10], thus enabling a system with a 50 dB higher signal-to-noise ratio (SNR) if using the lowest possible detection bandwidth.
For frequencies above 2 THz, where, despite the enhanced detection sensitivity, the power emitted (tens of nW) will come close to the noise floor, we propose to use QCLs as emitters as they are able to offer much higher powers (mW). QCLs can then be locked to the heterodyne generator by an approach similar to that described in [11] or [12], enabling the same coherence as with the photodetection approach together with the enhanced detection described previously.
Such a system is highly dependent on the performance of each of its components. This is reviewed in the next section and the effect on overall system performance described. Fig. 1. Schematic of the THz coherent system including the local oscillator, emitter and receiver.

Current component performance
The first essential component for the photonic oscillator system is an optical frequency comb generator (OFCG) used as phase reference. Several techniques have been developed over recent years and compact sources are now available [13][14][15][16], either fiber or semiconductor based. For example, a system based on the fiber recirculating loop can offer over a hundred highly coherent lines, which are equally spaced in frequency, as determined by an external synthesizer. Figure 2 shows optical spectra of an OFCG and two semiconductor lasers for selection of two comb lines (spaced by the frequency of interest). The comb optical lines are defined within a 7dB power envelope across 2 THz range and could be further amplified if necessary. The best performing OFCG of that type, as proposed in [17], should offer up to 10 THz in span, using parametric methods, with several mW of total output power, while short pulse mode locked lasers have achieved similar performances. Compact sources have achieved up to 2 THz span [13] with similar power. Current performance is therefore close to what will be needed for the system, as only a few mW will be required to lock the slave lasers and a span of 5THz will be sufficient to access most applications.
The second important part of the system is the dual-laser locked filter, whose principal function is to select and amplify the comb lines of interest. This can be realized using methods such as optical injection, optical phase lock loops (OPLL) and optical injection phase lock loops (OIPLL) [18][19][20]. Figure 3 presents the block diagram of the heterodyne OPLL system and the operating principle is broadly discussed in [21, 22]. The photonic local oscillator based on OPLL enables heterodyne linewidth of less than 10 Hz and phase noise performance better than −80 dBc/Hz at 10 kHz offset across the frequency range, as shown in Fig. 4  Integrating the high-speed photodetector used for THz generation with the dual laser source is an important goal, as it avoids fiber coupling between separate modules, simplifying fabrication and reducing cost. However, it is technically challenging, as the epitaxial structures required for the laser and photodetector sections are significantly different. We have recently integrated dual laser sources with UTC-PDs achieving emission at frequencies exceeding 60 GHz [27] as shown in Fig. 5, which also emphasizes the importance of the correct selection of substrate doping. For the results shown in Fig. 5, the maximum frequency was limited by the measurement probe, but discrete photodetectors with the same structure exhibited bandwidths up to 170 GHz [28]. Conversion of the photocurrent in photodiodes into free-space THz radiation is typically realized by integrating the UTC-PDs with a planar antenna, such as a dipole, bow-tie, spiral or log-periodic antenna, and a silicon lens. In order to exploit the tuneability of photonic THz sources, the integrated THz antenna must have a wide bandwidth, both in terms of the efficiency of energy coupling from the driving source and in terms of the radiation pattern and radiation efficiency. Assessment of the reflection coefficient between the driving source (typically a UTC-PD) and the antenna is complex and needs to be carried out case by case depending on the source employed. Conversely, the radiation pattern and radiation efficiency are intrinsic properties of antennas and can be evaluated independently from the source.
Emission pattern and radiation efficiency of planar THz antennas tend to change dramatically over the frequency range of UTC-PDs. Therefore silicon lenses are typically employed to improve the beam shape and radiation efficiency. Figure 6 illustrates the effect of the silicon lens attached to the antenna substrate on the radiation pattern. When the lens is not present, the substrate modes and reflections occurring within the substrate cause the emission pattern to change almost unpredictably as a function of frequency. The integration of the lens provides good and broadband focusing properties that produce several dBi in directivity over a wide frequency range. Figure 7 shows the radiation efficiency (in dB) calculated in the conical solid angle centered on the axis origin and defined by an angle of 30 degrees with respect to the z negative semi axis (which corresponds to the optical axis of the lens); above about 0.250 THz, the radiation efficiency of the system with lens is greater than or at least equal to the maximum radiation efficiency of the system without lens at 0.5 THz. The peak of the radiation efficiency without lens occurring at 0.5 THz is probably due to a fortunate favorable recombination of the reflections within the substrate and the antenna response. The fact that the radiation efficiency with the lens tends on average to improve as the frequency increases can be explained considering that THz lenses are usually designed and optimized using ray optics (geometrical optics); the ray optics approximation becomes more accurate as the frequency goes up (smaller wavelength) therefore the lens design and optimization is more accurate at higher frequencies. The chip-lens configuration that we have simulated can be considered as a collimating hemisphere design where the distance from the emitter to the tip of the lens is about 1.4 times the lens radius [29].
Antennas are also used for detection of THz radiation. To improve the detection efficiency, the antenna is used to couple the incoming THz wave to the small area of the photodiode, the active area of which is significantly smaller than the wavelength. As in the case of THz emitters, the antenna design affects the conversion efficiency. In addition to numerical modelling, the antenna performance can be evaluated experimentally using nearfield THz imaging [30,31]. Comparing the UTC-PD with other photodetectors used as photomixers, as shown in Fig. 9, it is evident that UTC-PDs offer the highest power emission performance below 100 GHz, while further analysis shows that higher power extraction has also been achieved at THz frequency [6,[30][31][32][33][34][35][36][37][38]. They are therefore attractive THz sources for systems applications.    Meanwhile progress on QCL sources has been considerable over the past ten years and power output is now >100 mW [7], with emission frequencies ranging between 1 and 5 THz having been demonstrated [39]. Central to these advances has been the design and engineering of a range of active region schemes, most notably the bound-to-continuum (BTC) [40] and resonant-phonon (RP) [41,42] designs. The latter exploits fast electron-opticalphonon scattering to depopulate the lower lasing level -an approach that enables a population inversion to be maintained up to high operating temperatures, at which thermal backfilling [43] of the lower lasing state prevents operation of alternative active region schemes. As such the temperature of operation of QCL devices has increased to approaching 200K [44]. Nevertheless, owing to the requirement to drop > 36 meV (the LO phonon energy in GaAs) across each individual module of the RP structure, large applied biases are required at threshold. This leads to large electrical power dissipation in RP QCLs, making continuous wave operation challenging. The BTC design, which typically exhibits low threshold current densities, is therefore most frequently employed for applications requiring continuous wave operation. Lasers based on such a design have also recently been shown to exhibit intrinsic linewidth values approaching the quantum limit [45], and as such are well-suited to the development of coherent systems. Tuneable QCL devices have been demonstrated using a range of approaches [46] together with electronic control using multi-contact lasers [47].
Devices for coherent THz detection have also shown significant progress in recent years. Both Schottky mixer and LT-GaAs photoconductors are enabling good performance over different parts of the frequency range. For compatibility with the photonic components emitting at a wavelength of 1.5 µm, implanted InGaAs switches [48] and quantum well based switches [9] show great promise to be used as a detector in the system. Similarly, considering the integration potential, recent results on optically pumped mixing in UTC-PDs offer an interesting solution, despite their current relatively high conversion loss [49].

Demonstration
As mentioned in the introduction, a number of applications have been envisaged for THz technology. In this section we concentrate on a few examples where the development of the proposed system would help realize the potential of the THz spectrum.
First, one can look at wireless communication where current technology at lower carrier frequencies is limited by the available licensed bandwidth. However, in the unlicensed spectrum above 300 GHz the bandwidth available would easily allow for a transmission rate of more than 10 Gb/s. Recent results for THz wireless links employing photonic carrier generation at frequencies ranging from 200 GHz to 400 GHz have indeed demonstrated the possibility of transmitting wireless data at more than 1 Gb/s. Above 250 GHz, detection has been limited to incoherent envelope detection using Schottky barrier diode (SBD) detectors [50][51][52], but the demonstration of data transmission at up to 28 Gb/s [52] clearly shows the potential for high bandwidth wireless transmission at these carrier frequencies, although, due to transmission impairments and the low signal powers available, a compromise will need to be found between carrier frequency and spectral efficiency.
The coherent system we propose enables the application of classical coherent radio modulation techniques that allow for higher sensitivity and the use of more spectrally efficient modulation formats. Heterodyne detection of a photonically generated 1 Gb/s on-off keying (OOK) modulated signal at 200 GHz has been demonstrated, using a subharmonic SBD mixer for down conversion, and envelope detection of the intermediate frequency (IF) signal [53]. In a similar experiment to demonstrate the key elements of our proposed approach, we used a real-time oscilloscope (RTO) to record samples of the IF signal, and offline processing enabling envelope [54] and synchronous detection to be emulated (Fig. 10). Recently, a wireless link using a photonic heterodyne transmitter operating at a carrier frequency of 237.5 GHz has been demonstrated with QPSK, 8PSK and 16QAM modulation formats at 25 Gbaud, giving up to 100 Gb/s data transmission rates [55]. A monolithic microwave IC (MMIC) I/Q receiver was used to down convert to baseband, followed by analogue-to-digital conversion using a RTO and offline digital signal processing similar to that used in coherent optical receivers. These demonstrations of coherent detection of wireless communications at THz frequencies all employed electronic local oscillators (LOs) generated by frequency multiplication of an RF signal, but in the future the LO could be generated photonically, especially if efficient optoelectronic mixers can be developed. However, the benefits of photonic-enabled THz wireless communications will need to be evaluated in comparison to a fully electronic approach. Already, both transmitter and receiver MMICs supporting data rates up to 20 Gb/s and operating at 220 GHz have been demonstrated [56], while 2.5 Gb/s transmission at 625 GHz has been achieved using electronic frequency multiplication [4]. A second application would be the development of a simple imaging system based most likely on the QCL source as it offers more power. In this application the coherence of the signal allows the use of interferometric techniques, enabling phase-sensitive and 3D imaging. Recent work on QCL coherent imaging, as illustrated in Fig. 11, has shown encouraging results in imaging different reflective elements [57], showing clear interferometric fringes indicating changes in depth. The experimental system was based on the coherence of the QCL which is typically 5 orders of magnitude worse than the system proposed. In this application an enhanced coherence will allow for better fringe definition and higher resolution as well as higher sensitivity, enabling longer distances for imaging. Indeed, imaging has previously been demonstrated, using this approach, at round-trip distances exceeding 20m through air [58]. Fig. 11. Coherent imaging of a set of objects using a coherent QCL based system [57,58].
Finally, we would also like to discuss the potential of THz techniques for the measurement and manipulation of quantum states. Typically, microwave measurement and manipulation of quantum states has been achieved using multiple controlled pump pulses and coherent reemission, which is monitored as a function of pulse length, arrival times, and amplitudes [59]. However, because of the lack of appropriate sources, very few experiments of this type have been performed in the THz regime, though recent experiments using a free electron laser and high frequency microwave sources have demonstrated the potential of THz radiation for quantum control and measurement, with results already achieved for the Rydberg states of Si:P [60].
The use of a small source which can be inserted directly into a dilution refrigerator will allow the study of transitions between electron interaction induced states such as those observed in the Coulomb Blockade regime. Other aspects of the interaction which we can study include the transition between a single line of electrons and two separate rows which form a many body ground state [61].
Using the coherent THz system we envisage, there is potential for introducing a compact lab-on-chip THz system that will enable control using optical communications-based modulation techniques, offering unprecedented opportunities to study a far wider range of coherent electronic and lattice excitations.

Discussion
We have described an approach to the development of a coherent THz system technology, underpinned by photonic components, that would enable access to most applications for frequencies ranging from 100 GHz to 5 THz. Reviewing recent developments in device technology we have shown that the photonic sources necessary to generate a tuneable local oscillator are becoming available, while photonic THz emitter technology has already reached an encouraging state of development, with both UTC-PDs and QCLs offering good performance. We have shown the potential of coherent THz systems to achieve 5 orders of magnitude enhancement in sensitivity and 2 orders of magnitude enhancement in energy consumption relative to pulsed sources, while offering good potential for integration. We have also discussed potential applications for such systems, as well as the potential improvements they enable, allowing for over 100 Gb/s wireless transmission, compact 3D imaging and quantum state manipulation with a lab-on-chip system.