Abstract-Parasitic transport paths in two-well scattering-assisted terahertz quantum cascade lasers

Li Wang, Tsung-Tse Lin, Ke Wang,  Hideki Hirayama,

https://iopscience.iop.org/article/10.7567/1882-0786/ab2b56

Using nonequilibrium Green's functions, possible parasitic paths are identified in two-well scattering-assisted terahertz quantum cascade lasers operating at 3.5 THz. The majority of electrons in the upper laser state can escape through these paths, causing a 66% loss of population inversion at 50 K. Three types of paths are clarified: one is responsible for non-selective injection via LO-phonon scattering, the other two lead to leakages via high-lying states to downstream periods by sequential tunneling. Finally, several ways of suppressing these paths are suggested by having small oscillator strength (<0.3), or employing asymmetric structure.

Abstract-Controlling loss of waveguides for potential GaN terahertz quantum cascade lasers by tuning the plasma frequency of doped layers

Ke Wang, Tsung-Tse Lin, Li Wang, Wataru Terashima and Hideki Hirayama

http://iopscience.iop.org/article/10.7567/JJAP.57.081001

We have analyzed the waveguide loss originating from various doping layers in double metal waveguides for potential GaN-based terahertz quantum cascade lasers (THz QCLs) by theoretical calculations. The optical field can be very well confined in the active QCL region. Average electron densities in the QCL active region and n+ contact layers should be controlled carefully. The loss in the low-frequency range (3 THz) can be minimized by decreasing the electron density in the QCL layer. In the middle- and high-THz-frequency ranges (~3 < f < 15 THz), the absorption by the heavily doped n+ contact layer dominates the waveguide loss. Consequently, the bulk plasma frequency, which is determined by electron density and shows a strong absorption peak, must be tuned to deviate from the target QCL frequency. The waveguide loss plus the cavity mirror loss can be controlled to be as low as ~24 cm−1.

Abstract-Recent progress of THz-quantum cascade lasers using nitride-based materials

Hideki Hirayama; Wataru Terashima

http://spie.org/Publications/Proceedings/Paper/10.1117/12.2188109

Nitride semiconductor is a material having potentials for realizing wide frequency range of quantum-cascade lasers (QCLs), i.e., 3～20 THz and 1～8 μm, including an unexplored terahertz frequency range from 5 to 12 THz, as well as realizing room temperature operation of THz-QCL. The merit of using an AlGaN-based semiconductor is that it has much higher longitudinal optical phonon energies (ELO> 90meV) than those of GaAs-based semiconductors (~ 36 meV). In this study, we demonstrate the first lasing action of GaN-based QCLs. We introduced an unique quantum design active region, i.e., “pure 3-level system design”, which is consisting of 2 quantum wells (QWs) per one period. We grew GaN/AlGaN QC structures by using molecular beam epitaxy (MBE). The layer structure of the GaN/AlGaN QCL was consisting of 100~200 periods of QC active layers sandwiched by Si-doped (Al)GaN upper and lower contact layers, which were grown on a high-quality AlGaN/AlN template grown on a c-plane sapphire substrate. After the crystal growth, we fabricated QCL sample with single metal plasmon waveguide structure. Lasing spectrum was obtained at 5.39 THz measured under pulsed current injection at 5.8K. The threshold current density Jth and the threshold voltage Vth were 1.75 kA/cm2 and 14.5 V, respectively. We also fabricated similar design GaN/AlGaN QCL by metal organic chemical vapor deposition (MOCVD), and obtained lasing at 6.97 THz. The Jth and Vth of the MOCVD grown QCL were 0.75 kA/cm2 and 27 V, respectively, measured at 5.2 K.

Terahertz frequency emission with novel quantum cascade laser designs

Wataru Terashima and Hideki Hirayama

http://spie.org/x115009.xml

Group-III nitride semiconductor materials and a unique quantum design for active regions are used to realize compact lasers that operate in an unexplored frequency range.

14 August 2015, SPIE Newsroom. DOI: 10.1117/2.1201507.006058

Terahertz (THz) waves (i.e., with frequencies of 0.3–30THz and wavelengths of about 1mm to 10μm) have several potential applications, such as for cancer treatments,1 weapon and drug security screenings,2as well as high-speed and high-capacity optical wireless communications.3 Quantum cascade lasers (QCLs) are the only solid-state sources that emit light at THz frequencies with power outputs of more than 1W.4 Light emission in QCLs is achieved with the use of optical transitions that occur between subband (i.e., subdivisions of a band) levels in semiconductor superlattices. QCLs can therefore potentially be used for the realization of compact, high-power, and narrow linewidth devices that have continuous wave (CW) operability. Although THz QCLs were first demonstrated in 2002,5 there is still ongoing work to improve their operation and usage.

Since their first introduction, THz QCL devices based on gallium arsenide (GaAs) have developed rapidly. With the design of active regions and metal waveguide structures, the lasing frequency range of the devices has been extended from 1.2 to 5.2THz,6, 7and the maximum operating temperatures have increased to 199.5K (pulsed)5 and 117K (CW).8 The frequency range from 5.3 to 12THz, however, remains unexplored and unused. The impact of THz applications and the potential of THz QCLs are therefore substantially reduced. It is thought that THz waves with frequencies of 5–12THz can be used to identify materials such as water, pigments, and semiconductors (as these substances exhibit unique absorption spectra between 5 and 25THz).9, 10 In the 5–12THz range, however, GaAs materials have strong light absorption characteristics. This is because of electron-longitudinal optical (LO) phonon interactions. It is therefore difficult to operate GaAs-based devices in this frequency range.

We have therefore examined group III-nitride semiconductors as potential materials for THz QCL devices.11 Materials such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), and aluminum nitride (AlN) have much larger LO phonon energies (more than 18THz) than GaAs-based materials. It should therefore be possible, with these substances, to realize THz QCLs that cover the 5–12THz frequency range. We have also introduced a novel active region with a ‘pure three-level’ laser design in our test devices.11 This quantum cascade (QC) design is limited only to the three subband levels that contribute to the emission scheme. This means that we can achieve good control of a complicated band in the presence of an internal electric field.

A conduction band profile (and the associated square wave functions) for a GaN/AlGaN-based QC structure with a pure three-level design is shown in Figure 1. Electrons that are injected into subband level 3 (red line) make an optical transition to subband level 2 (green line). The resultant emission frequency from this transition is 5.37THz. Meanwhile, electrons that are injected into subband level 2 are non-radiatively scattered into subband level 1 (blue line) via electron–electron interactions. Those electrons that are scattered into subband level 1 are then depopulated and injected resonantly by LO phonons into subband level 3' (i.e., a level with the same wave function shape as level 3, but that differs in position). This occurs because the intersubband transition energy between levels 1 and 3' (about 120meV) is above the LO phonon energy of the materials (about 90meV).

 

Figure 1. Conduction band profile (solid black line) and square wave functions of a pure three-level (two quantum well) design for a gallium nitride/aluminum gallium nitride (GaN/AlGaN) terahertz quantum cascade laser (THz QCL). The thickness of each layer in this sequence is given in angstroms. Dashed black lines indicate the mean wave functions (subband levels) of the second level that exists in the quantum wells. Solid colored lines represent different subband levels (level 1: blue, level 2: green, level 3: red). The line labeled 3' indicates the wave function with the same shape as level 3, but which has a different position (i.e., from a different period of the quantum cascade structure). LO: Electron-longitudinal optical.

We have used a variety of techniques to construct our new QCL devices (see Figure 2). As shown in Figure 2(a), the layered GaN/AlGaN QC structures are sandwiched by silicon-doped GaN and AlGaN contact layers. We used a radio-frequency molecular beam epitaxy technique to grow the GaN/AlGaN QC structures. In addition, we grew the contact layers on a high-quality AlGaN/AlN template that itself was grown, via metal organic chemical vapor deposition (MOCVD), on a c-plane—i.e., the (0001) Miller index plane—sapphire substrate. To encourage the scattering of electrons by LO phonons and to achieve the population inversion condition, we silicon-doped the 60Å-thick GaN wells within the QC structures by 5×1017cm−3. The laser ridge structure in our devices has a resonant length of 1.14mm and a width of 120μm. We used a single metal waveguide structure for the fabrication of our devices. In the final step, we wire-bonded and mounted the devices onto a gold-nickel heat sink, as illustrated in Figure 2(b).

 

Figure 2. (a) Schematic diagram for the overall structure of a GaN-based THz QCL. (b) Photograph of the THz QCL samples mounted on a gold-nickel heat sink. Ti: Titanium. Al: Aluminum. Au: Gold. n+: N-type semiconductor with high impurity density. MOCVD: Metal organic chemical vapor deposition.

We have obtained a frequency spectrum (see Figure 3) from our GaN-based THz QCL device that has the quantum design shown in Figure 1. We used pulses that were 200ns wide, and repeated at 980Hz, to obtain this spectrum at a temperature of 5.6K. As can be seen in Figure 3, we observed the lasing spectrum with a peak frequency of 5.47THz. The threshold current (Ith), threshold current density (Jth), and the threshold voltage (Vth) of this sample were 3.5A, 2.75kA/cm2, and 16.5V, respectively. We also used MOCVD to grow a similar pure three-level design for a GaN/AlGaN QCL structure. We achieved lasing at 6.97THz for this device, which had Ith, Jth, and Vth values of 1.0A, 0.75kA/cm2, and 27.0V, respectively (measured at 5.2K).

 

Figure 3. Fourier transform IR frequency spectrum obtained from a GaN-based THz QCL with a pure three-level design. The lasing spectrum has a peak frequency of 5.47THz and wavelength of 55μm. T: Temperature. Ith: Threshold current. Jth: Threshold current density. Vth: Threshold voltage. arb: Arbitrary.

We have successfully grown GaN-based THz QCLs and have achieved lasing with such devices for the first time. With this lasing action, we are able to produce light emission at 5.47 and 6.97THz from our devices. As part of our work, we have introduced a unique quantum design of a pure three-level laser system. We now plan to improve the properties of our GaN-based devices. We will improve the crystal quality of our QC structures and increase the number of active layers (to increase the optical confinement factor).

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Wataru Terashima, Hideki Hirayama

Quantum Optodevice Laboratory
RIKEN

Wako, Japan

Wataru Terashima has a PhD in engineering. He specializes in developing group-III nitride semiconductor materials. He also works on developing semiconductor lasers that operate in unexplored wavelengths between near-IR and THz regions.

Hideki Hirayama is a chief scientist and an optical device engineer. He specializes in developing deep-UV LEDs with metal organic chemical vapor deposition growth methods. He also works on using quantum dots and photonic crystals to develop unique optical devices.

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11. H. Hirayama, W. Terashima, Recent progress of THz-quantum cascade lasers using nitride-based materials. Presented at SPIE Optics + Photonics 2015.