A simple solution for an intense terahertz emitter
Terahertz (THz, in the far IR region) imaging and spectroscopy offers the ability to discover chemical composition noninvasively. Applications include medical imaging, security screening, and pharmaceutical quality control. Semiconductors can produce pulsed THz radiation when their surface is illuminated by a sub-picosecond laser pulse, rather than continuous wave sources, such as a quantum cascade laser. Yet despite several technical advances, the output power is low. Thus the development of higher power semiconductor THz emitters is of considerable practical significance.1
THz generation occurs when a semiconductor is illuminated by an ultrafast laser pulse with photon energy greater than the semiconductor bandgap. A large number of electron and hole pairs are created and accelerated in opposite directions by the electric field. The resulting charge separation forms a dipole that emits a coherent THz pulse. THz radiation generated within a material of high refractive index n has an extraction problem: at the semiconductor surface, all rays outside an ‘emission cone’ of half-angle
to the surface normal (shown as a grey cone in Figure 1) suffer total internal reflection and do not escape from the device. The fraction of power extracted depends on the orientation of the dipole axis relative to the surface.
Several methods exist to increase the extraction efficiency, such as applying an external magnetic field,2 using a coupling prism,3 or fabrication of low-dimensional nanostructures.4 For example, under a magnetic field the Lorentz force—the force on a point charge due to electromagnetic fields—can rotate the orientation of the dipole to coincide with the emission cone as shown in Figure 1(a). For the extreme case of a dipole parallel to the surface, the emitted power would increase by more than a factor of 20. But these methods require cumbersome equipment, such as a strong magnet, or special sample growth techniques. We simply changed the growth orientation of the semiconductor to a non-polar direction and obtained a 100-fold increase in THz emission.5
Recently, interest has grown in crystalline indium nitride (InN) as an efficient THz emitter because of its narrow bandgap (~0.6eV), high electron mobility, and remarkably large energy-gap (2.8eV) to local minimum. The wurtzite (hexagonal crystal) form of InN can be described using three axes (a, c and m), as shown in Figure 1. It is known that the performance of short-wavelength optoelectronic devices realized by growing nitrides along the a- or m-axis direction can be significantly improved by reducing the strain-dependent piezoelectric polarization. For c-plane InN, the dipole axis is oriented perpendicular to the surface so that little power (much less than 1%) is radiated in the emission cone. We reasoned that if we reoriented the dipole, we could anticipate a drastic increase in the emitted power.
We grew an a-plane InN epitaxial film on an r-plane sapphire wafer and a c-plane InN film on silicon(111), in both cases by plasma-assisted molecular beam epitaxy.6 Unintentionally doped n-type carrier concentrations of 7.0×1018 and 3.1×1018cm−3 and electron mobilities of 298 and 1036cm2/Vs were determined by room-temperature Hall effect measurements for a- and c-plane InN films, respectively. We investigated THz emission from the InN epilayer using a titanium:sapphire regenerative amplifier laser system, which delivers ~50fs optical pulses at a center wavelength of 800nm with a repetition rate of 1kHz. We used free-space electro-optic sampling as a function of delay time with respect to the optical pump pulse to detect THz pulses that followed the specular direction of an obliquely incident laser beam.
Figure 2 shows the maximum amplitude of THz field from the a-plane InN film, compared to that from the c-plane InN film and an n-type InAs film. The amplitude of THz field from a-plane InN is at least 10 times (100 times in intensity) stronger than that from c-plane InN. This enhancement rate is even higher than that obtained by applying an external magnetic field to c-plane InN. As shown in Figure 3, the atomic stacking sequence of c-plane InN is ABABAB⋯ (A and B can be either In or N) along the wurtzite c-axis direction so that the surface layers have either an In- or an N-terminated surface. Therefore, the electric field generated by these In-N bilayers directs perpendicular to the surface and the resultant out-of-surface radiation can be significantly limited by the total internal reflection.
In contrast, the layers of a-plane InN have the same number of In and N atoms in a plane and these in-plane In-N pairs form an intrinsic electric field perpendicular to the a-axis. The highly photoexcited carriers couple efficiently to the in-plane electric field so that more THz radiation lies within the emission cone, similar to the rotated dipole radiation by the magnetic field in Figure 1(a). The power enhancement from a-plane InN depends only on the growth direction of the sample, so that it can be a universal phenomenon for any semiconductor grown along the same direction. In addition, it provides an experimentally simple, efficient method.
In summary, we have demonstrated that THz pulses radiated by a-plane InN film are about 100 times more intense than those radiated by c-plane InN film. This can be applied to any non-polar semiconductors and thus provides a promising solution to the search for effective THz emitters. However, the carrier density of both c- and a-plane InN are high (>3×1018cm−3) and the high carrier density hampers the build-up of the stronger electric field. Doping InN with magnesium (Mg) as an acceptor reduces the carrier density, and preliminary results from Mg-doped InN shows even higher THz power enhancement than a-plane InN.
We would like to thank the National Science Council, Taiwan for financial support.
Hyeyoung Ahn is an assistant professor of photonics. Her areas of research interest include ultrafast optical/terahertz spectroscopy of semiconductors and their nanostructures.
Shangjr Gwo is a professor of physics. His research interests include growth, fundamental studies, and applications of III-nitride nanostructures and heterostructures.
Ci-Ling Pan is a professor of physics and Tsinghua Chair, with joint appointment at the Institute of Photonics Technologies and the Department of Photonics. His research interests are laser science and applications, particularly ultrafast and THz photonics.
Ci-Ling Pan is a professor of physics and Tsinghua Chair, with joint appointment at the Institute of Photonics Technologies and the Department of Photonics. His research interests are laser science and applications, particularly ultrafast and THz photonics.
