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We demonstrate a compact, robust, and stable terahertz source based on a novel two section digital distributed feedback laser diode and plasmonic photomixer. Terahertz wave generation is achieved through difference frequency generation by pumping the plasmonic photomixer with two output optical beams of the two section digital distributed feedback laser diode. The laser is designed to offer an adjustable terahertz frequency difference between the emitted wavelengths by varying the applied currents to the laser sections. The plasmonic photomixer is comprised of an ultrafast photoconductor with plasmonic contact electrodes integrated with a logarithmic spiral antenna. We demonstrate terahertz wave generation with 0.15-3 THz frequency tunability, 2 MHz linewidth, and less than 5 MHz frequency stability over 1 minute, at useful power levels for practical imaging and sensing applications.
© 2015 Optical Society of America
1. Introduction
The study of sub-terahertz and terahertz frequency ranges has attracted significant attention over the last decade with an increasing number of potential applications including biomedical imaging, security screening, high speed wireless communication, material identification, and space exploration. Compact, efficient and cost effective terahertz sources that can operate at room temperature are highly in demand to ensure that the applications mentioned above can be fully exploited. In addition to these general attributes, specific applications such as spectroscopy require narrow linewidth terahertz signals which can be readily tuned.
Current terahertz generation technologies can be divided into electronic and optical techniques. In the electronic category, electronic up-converters through frequency multiplication [1] and terahertz oscillators based on impact ionization avalanche transit-time (IMPATT) diodes, Gunn diodes, and resonance tunneling diodes (RTDs) [2] have offered very promising compact terahertz sources. However, this category of sources has limitations in efficiency and frequency tunability. As an example, Schottky multipliers are a commercial solution that can generate terahertz signals approaching 3 THz with narrow linewidths and reasonable power levels (> 100 μW at 1 THz) [3]. However, they are limited in frequency tunability and their efficiency and linewidth/phase noise degrade as their multiplication order increases. Moreover, RTD based terahertz oscillators have demonstrated terahertz generation above 1 THz [4], but with limited frequency tunability and poor radiation linewidth/phase noise. In the optical category, terahertz quantum cascade lasers (QCLs) have shown great promise for direct generation of terahertz signals with very high signal power levels approaching 200 mW in continuous wave (CW) operation [5, 6]. However, they seldom operate below 2 THz (the minimum demonstrated frequency is 0.8 THz, achieved by applying external magnetic fields) and require significant cryogenic cooling. In addition, in order to achieve narrow linewidths below 1 MHz, complicated phase-locked loop (PLL) systems are required and limited tunability can only be obtained by mechanical means. Terahertz QCLs based on intracavity difference frequency generation have also shown great promise for room-temperature terahertz wave generation, but with limited frequency tunability and poor efficiency due to the utilized nonlinear process [7]. Another optical technique for terahertz generation is optical down-conversion. As an example, terahertz signal can be generated through difference frequency generation by nonlinear and/or photoconductive mixing of two optical beams with a terahertz frequency difference [8]. This category of sources typically provide lower powers than other solutions but can offer continuous tunability over a broad terahertz frequency range through a compact and potentially integrated platform operating at room temperature. In addition, by use of optical comb based sources with phase correlated optical frequency lines difference frequency generation with photomixers can generate terahertz signals with long term stability and ultra-narrow linewidths (< 1 kHz) [9, 10].
In this paper we demonstrate a terahertz source, employing the combination of a novel optical pump source and plasmonic photomixer, which offers a compact, robust, and stable terahertz source that can be tuned from 0.15 THz to 3 THz with narrow linewidths and useful terahertz power levels for many practical imaging and spectroscopy applications. The employed optical pump source is a novel two section digital distributed feedback laser diode (D-DFB), which is designed to offer an adjustable frequency difference between the emitted wavelengths within 1538-1569 nm range by varying the currents applied to the laser sections. A plasmonic photomixer, comprised of an ultrafast photoconductor with plasmonic contact electrodes integrated with a terahertz antenna is pumped by the optical signal to generate terahertz radiation.
2. Optical source for dual mode wavelength generation
Dual wavelength laser sources are one of the key components required for terahertz generation through photomixing [11, 12]. The wavelength tuning range and tuning speed of semiconductor lasers make them extremely suitable when frequency flexibility is required. If the outputs of two separate laser diodes are coupled together to generate the dual wavelength optical pump signal, the relative wavelength drift and uncorrelated phase noise of the two sources result in terahertz signals with poor long term stability and large phase noise. There have recently been several demonstrations of monolithic and hybrid integration of dual wavelength laser sources to increase compactness and/or correlation between the lasing modes and, thanks to this, low phase noise signal generation has been achieved [13–15]. Some of these sources employ distributed feedback (DFB) laser diode structures originally developed to emit light at fiber optic communication wavelengths between 1300 nm and 1550 nm. One obstacle to employing these DFB structures in integrated terahertz sources has been the relative inflexibility of the technology when it is desired to incorporate DFB devices in photonic integrated circuits, especially for lower volume applications. In this paper we make use of a digital-DFB (D-DFB) technology [16, 17] to develop dual mode laser sources for terahertz generation through photomixing. The D-DFB technology uniquely enables producing low cost, high quality, single mode and dual mode laser devices by using standard InP microelectronics tool kits and processes. This allows process control of high volume InP electronics manufacturing lines to be leveraged for low cost laser production at any volume.
Figure 1(a) illustrates the schematic diagram of a D-DFB laser diode. Starting with a basic Fabry-Perot ridge waveguide laser diode, which exhibits a classic multimode Fabry-Perot emission spectrum, features are etched into the ridge waveguide to select a single lasing mode of the Fabry-Perot and suppress all other modes [16, 17]. This offers a new laser structure type that emits in a dynamically stable single mode spectrum [17]. Importantly, the laser does not use regrowth technology, which results in a highly reliable device. Moreover, the digital nature of the structure etched into the upper waveguide layer allows for excellent control of both the laser emission wavelength and the stability of the chosen single mode. Another key advantage of this technology is its flexibility, allowing the realization of more complex structures such as lasers with tunable output wavelength or bimodal emission with independent tuning of the two modes [18, 19]. The two section D-DFB laser employed in this work is schematically shown in Fig. 1(b). The laser is a standard ridge waveguide laser diode with a ridge width of 2.5 µm. The structure is grown by low pressure metalorganic chemical vapor deposition (MOCVD) on a (100) n-type InP substrate. The laser cavity is 700 µm long and divided into two sections, section 1 and section 2 with lengths of 400 µm and 300 µm, respectively, which are separated by a 2 µm wide etched trench. The output beam is emitted from section 1. In order to achieve dual mode output wavelengths, the pattern of index perturbations in both sections is slightly different. When operated independently and around 25°C, the output wavelength from section 1 can be varied from 1536 nm to 1538.5 nm continuously, and the output wavelength from section 2 can be varied from 1536.5 nm to 1541.3 nm continuously (by varying the injection currents) [16, 19]. The ridge and index perturbations are realized in the ridge upper surface using standard etch techniques used to fabricate Fabry-Perot ridge waveguide lasers. SiO2 hard masks are patterned lithographically to outline the ridge and index perturbation features. The depth of the etched features is slightly less than the height of the ridge. Dry etching using inductively coupled plasma (ICP) is used to reach approximate target depths of the features and trenches and wet etching is used to ensure uniform and accurate depths across the wafer. It should be noted that all surface etched features are in the upper wave-guiding layers and do not extend to the laser active region. The laser active region consists of five compressively strained AlGaInAs quantum wells with a well thickness of 5 nm. The final step in surface processing is a deep ICP etching step to define the trench isolating section 1 from section 2. Since the etched trench acts as a back facet for section 1 and a front facet for section 2, a wet etching step is used to reduce the surface roughness of the trench. Electrical contacts for both sections are formed while using SiO2 as an insulator for contact definition.
Fig. 1 (a) The D-DFB structure showing etched features in the ridge waveguide to achieve single mode lasing. (b) Two section D-DFB structure that offers dual wavelength operation through coupling of the laser cavities.
When both lasers are operated together, the coupling between the two laser cavities results in the generation of light at two wavelengths [20]. In the meantime, the separation between the wavelengths can be tuned by varying the temperature of the overall structure and the currents applied to the laser sections. To explain the operation in more details it should be initially considered that the laser sections have different cavity lengths and different index perturbation patterns, as outlined above. The cavity length of each section sets the Fabry-Perot modes of that cavity. The index perturbations in each section allow the loss spectrum of the Fabry-Perot laser to be manipulated in order to achieve single mode emission in the D-DFB structure [16, 17]. More specifically, the introduction of the index perturbations produces a loss profile with a number of loss minima spaced periodically as a function of wavelength across the gain bandwidth of the material used [18], but a given separation between the features results in just one wavelength for which the loss is minimized and lasing occurs. In the two section device employed in this work, both laser sections have different separations between the features and different cavity lengths, resulting in loss profiles for the two sections with different periodicities [18]. The loss profile of each section can be varied by changing both the current injected into the section and the operating temperature of the device. In addition, since the lasing emission from one section is injected into the other section, and vice versa, light from one section will affect the loss profile of the other section [20]. Thus by varying the currents applied to both sections and the temperature of the device it is possible to achieve a single loss minimum for the entire device (single mode lasing) [18], or indeed two loss minima for the entire device (dual mode lasing), through application of the Vernier effect. In addition, in the single mode lasing or dual mode lasing cases, it is possible to achieve tuning of the output wavelength(s) over the entire gain bandwidth of the material (several THz) by varying the currents applied to the sections and the device operating temperature. The tuning can be continuous over small frequency ranges (several GHz) but because of the interaction between the two sections it is not possible to tune the output wavelengths independently using both sections. The typical output power of the dual wavelength source is around 0 dBm, when both sections are biased at around 50 mA. However, the output power can drop down below −5 dBm as the drive currents to the laser sections are varied to change the frequency spacing between the output wavelengths. Figure 2 shows the output spectrum of the two section D-DFB laser at three specific operating points (Table 1) that achieve spectral separations of 0.15 THz, 1.62 THz and 2.99 THz.
Fig. 2 Optical spectra of the two section D-DFB laser offering two main spectral peaks in the 1550 nm wavelength range with a tunable frequency difference in 0.15-3 THz range.
Table 1. Operation conditions offering different spectral separations for the output of the two section D-DFB laser
It should be noted that spectral separations as low as 32 GHz are also achieved but are not presented here for figure clarity. Optical linewidth measurements for the spectral lines of the dual wavelength laser are carried out using a standard self-heterodyning technique [21] and linewidths of around 1 MHz are measured when the spectral separation between the lines is 1.62 THz, as shown in Fig. 3.
Fig. 3 Spectral measurements for the optical lines from the implemented two section D-DFB laser at frequency separations of 1.62 THz and 0.8 THz. The black curve represents the optical line which is common to the outputs of the laser with 1.62 THz and 0.8 THz separations. The red and blue curves represent the second line from the outputs with 1.62 THz and 0.8 THz separations, respectively.
3. Terahertz wave generation by plasmonic photomixing
In order to achieve high optical-to-terahertz conversion efficiencies, a plasmonic photomixer is used to convert the optical beam from the two section D-DFB laser to terahertz radiation [22, 23]. The plasmonic photomixer is comprised of an ultrafast photoconductor with plasmonic contact electrodes integrated with a terahertz antenna on an ErAs:InGaAs substrate, as shown in Fig. 4(a). A logarithmic spiral antenna is used as the terahertz antenna to achieve broadband resistance of 70-100 Ω and reactance of 0 Ω over 0.15-3 THz frequency range [24, 25]. The plasmonic contact electrodes are formed by metallic gratings with 5/45 nm Ti/Au height, 100 nm width, and 200 nm pitch with a 250 nm-thick Si3N4 anti-reflection coating to allow transmission of more than 70% of a TM-polarized optical pump beam in the 1550 nm wavelength range through the metallic gratings into the ErAs:InGaAs substrate [22, 23]. When the optical beam from the two section D-DFB laser is incident on the anode plasmonic contact electrodes of the plasmonic photomixer, a large fraction of photo-generated carriers is generated in close proximity to the contact electrodes. This is because of excitation of surface plasmon waves along the plasmonic contact electrodes, concentrating a major portion of the incident optical beam near the plasmonic contact electrodes [25–32]. Therefore, a large number of the photo-generated electrons is drifted to the anode plasmonic contact electrodes in a sub-picosecond timescale to efficiently contribute to terahertz radiation. In the meantime, most of the photo-generated holes are recombined in the ErAs:InGaAs substrate along their drift path to the cathode contact electrode. The induced photocurrent, which has the same frequency components as the envelope of the two section D-DFB laser beam intensity, is then fed to the logarithmic spiral antenna to generate terahertz radiation at the beating frequency of the two main spectral peaks of the two section D-DFB laser.
Fig. 4 Schematic diagram and scanning electron microscope (SEM) images of the fabricated ErAs:InGaAs plasmonic photomixer with plasmonic contact electrode gratings are shown in (a) and (b) respectively.
The plasmonic photomixer is fabricated on an ErAs:InGaAs substrate with a carrier lifetime of ~0.85 ps [33]. Figure 4(b) shows the scanning electron microscope (SEM) images of the fabricated plasmonic photomixer. The fabrication process starts with patterning the plasmonic contact electrodes by electron-beam lithography, followed by 5/45 nm Ti/Au deposition and liftoff. The 250 nm-thick Si3N4 anti-reflection coating is then deposited by plasma enhanced chemical vapor deposition (PECVD). Contact vias are then formed by photolithography followed by reactive ion etching (RIE). Finally, the logarithmic spiral antenna and bias lines are formed by photolithography, followed by 10/400 nm Ti/Au deposition and liftoff. The plasmonic photomixer is centered and mounted on a hyper-hemispherical silicon lens to efficiently collect and collimate the generated terahertz radiation from back-side of the substrate.
Figure 5 shows the experimental setup used for characterizing the terahertz source based on the two section D-DFB laser and ErAs:InGaAs plasmonic photomixer. The two section D-DFB laser offers two main spectral peaks in the 1550 nm wavelength range with a tunable frequency difference in the 0.15-3 THz range. The optical power level of the two main spectral peaks is balanced by controlling driving current and temperature of the two section D-DFB laser, while maintaining a side-mode suppression ratio (SMSR) of more than 25 dB. In order to prevent thermal breakdown and achieve high terahertz radiation powers at high optical pump power levels, the laser output is modulated with 2% duty cycle by an acousto-optic modulator (NEOS Technology 15200-.2-1.55-LTD-GaP-FO) and amplified by a pulsed amplifier (Optilab APEDFA-C-10) [22, 23]. A combination of GRIN and aspheric lenses is used to focus the output optical beam onto the anode plasmonic contact electrodes of the plasmonic photomixer with a 12 μm diameter focus spot size. A quarter wave-plate followed by a linear polarizer is used to maintain a linearly polarized optical pump beam orthogonal to the plasmonic gratings. By placing a pellicle along the optical path, the spectrum of the incident optical pump beam on the plasmonic photomixer is monitored simultaneously by an optical spectrum analyzer to accurately measure the frequency of the generated terahertz radiation. Finally, the generated terahertz power is measured by a calibrated Si bolometer from Infrared Laboratories.
Fig. 5 Experimental setup for characterizing the terahertz source based on the two section D-DFB laser and plasmonic photomixer.
Figure 6(a) shows the induced photomixer photocurrent as a function of the average optical pump power and bias voltage, indicating a linear dependence on the optical pump power and bias voltage within 20-100 mW and 0-4 V ranges. Figure 6(b) shows the measured terahertz radiation power as a function of the induced photocurrent at each CW radiation cycle for different optical pump powers and radiation frequencies, indicating a quadratic relation between the radiated power and the induced photocurrent at each radiation frequency. As illustrated in Fig. 6(c), the radiation power drops at higher frequencies due to photomixer parasitics, resulting in a RC roll-off in device frequency response. At an average optical pump power of 100 mW and a bias voltage of 4 V, terahertz radiation powers as high as 1.3 mW, 106 μW, and 12 μW are achieved at each CW radiation cycle at 0.44 THz, 1.20 THz, and 2.85 THz, respectively. Higher terahertz radiation powers are achieved at higher optical pump powers. Figure 6(d) shows the measured radiation power at each CW radiation cycle at 1.62 THz as a function of the average optical pump power and bias voltage. Radiation powers as high as 450 μW are achieved at 1.62 THz at an average optical pump power of 250 mW.
Fig. 6 (a) The induced photomixer photocurrent as a function of the average optical pump power and bias voltage. (b) The radiated terahertz power as a function of the induced photocurrent at each CW radiation cycle at different optical pump powers and radiation frequencies. (c) The radiated terahertz power at each CW radiation cycle as a function of the average optical pump power over a 3 THz range. (d) The radiated terahertz power at each CW radiation cycle as a function of the average optical pump power and bias voltage at 1.62 THz.
Linewidth measurements for the generated signal are carried out at 150 GHz by using an electrical spectrum analyzer incorporating a harmonic GaAs diode mixer [34]. The results demonstrate radiation linewidths of ~2 MHz, which are expected for the employed dual wavelength laser with optical linewidths of ~1 MHz. As the process for terahertz signal generation is the same at higher frequencies, the linewidth of the generated signal will remain in the 2 MHz range at higher terahertz frequencies. In the meantime, external phase locking of the dual wavelength signals could be employed for applications that require lower phase noise levels [35]. Stability measurements for the generated signal are also performed at 150 GHz, exhibiting a frequency stability of less than 5 MHz over a 1 minute time frame.
4. Conclusion
In summary, we demonstrate terahertz wave generation using a novel bimodal laser diode and plasmonic photomixer combination. A high efficiency plasmonic photomixer is used for terahertz generation through difference frequency generation. The frequency of the generated radiation is adjusted by differentially biasing different sections of the bimodal laser diode. The presented experimental configuration has a great potential to offer a fully integrated, compact, reliable, and power efficient tunable terahertz source.
Acknowledgment
The authors gratefully acknowledge the financial support from Presidential Early Career Award for Scientists and Engineers (# N00014-14-1-0573), NSF CAREER Award (# N00014-11-1-0096), ONR Young Investigator Award (# N00014-12-1-0947), ARO Young Investigator Award (# W911NF-12-1-0253), Science Foundation Ireland IPIC program (# 12/RC/2276) and European Space Agency project FIRLO.
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