NAMBE 2024 Session WEG-SaA: Workshop on Epitaxial Growth of Infrared Materials: IR Devices and Applications

Generation of radiation in the terahertz frequency range (100 GHz to 10 THz) is a challenging problem. The lack of powerful and tunable sources in that frequency region can also limit the accuracy and resolution of spectroscopy techniques. In addition, the relevant part of molecules’ rotational spectrum lies within that frequency region. While the ground state rotational spectrum of molecules is easily measured thanks to the large thermal population of lower rotational levels at room temperature, measuring the rotational spectrum of a molecule in the excited state can be much harder. Here we introduce the quantum cascade laser pumped molecular laser (QPML): a widely tunable source that can emit light between 100 GHz up to 10 THz and uses a widely tunable quantum cascade laser to pump ro-vibrational transitions. We first demonstrated the QPML concept using the nitrous oxide molecule [1], where more than 30 lines were measured between 300 GHz and 772 GHz. We subsequently utilized the methyl fluoride and the ammonia molecule to demonstrate the universality of the concept [2], [3].

Compared to many existing THz sources, the QPML operates at room temperature is widely tunable and can be made compact. The first demonstration of the QPML was performed using the nitrous oxide (N₂O) molecule [1]. This molecule has a simple rotational spectrum due to its linear geometry, and a typical energy diagram for the considered vibrational transitions of N2O is shown in Fig. 1(a) of supplemental document. By placing the molecule into a tubular copper cavity and pumping vibrational transitions with a mid-infrared (MIR) QCL, laser emission at THz frequency was obtained (see Fig. 1(b)).

References

[1] P. Chevalier, A. Amirzhan, F. Wang M. Piccardo, A. Amirzhan, S. G. Johnson, F. Capasso and H. O. Everitt, “Widely tunable compact terahertz gas lasers.”, Science 366, 856-860 (2019).

[2] A. Amirzhan, P. Chevalier, J. Rowlette, H. T. Stinson, M. Pushkarsky, T. Day, H. O. Everitt and F. Capasso,“A quantum cascade laser-pumped molecular laser tunable over 1 THz.”, APL Photonics 7, 016107 (2022).

[3] P. Chevalier, A. Amirzhan, J. Rowlette, H. T. Stinson, M. Pushkarsky, T. Day, F. Capasso and H. O. Everitt, “Multi-line lasing in the broadly tunable ammonia quantum cascade laser pumped molecular laser”, Appl. Phys. Lett. 120, 081108 (2022).

Digital alloying has a rich history in the MBE community as a technique for improving key properties including lattice-matching, optical absorption/emission efficiency, phonon transport, compositional grading, and phase stability to name but a few. More recently, it has been found to improve high-field transport leading to the emergence of AlInAsSb on GaSb as the first low-noise III-V alloy family for conventional avalanche photodiodes (APD).The seamless band engineering afforded by digital alloying has also enabled ultra-low-noise staircase APDs, which are the solid-state analog of photomultiplier tubes.

Here, we will discuss our work on the digital alloy growth of AlInAsSb alloys and its impact on key APD performance criteria (noise, gain, dark current, breakdown, etc.), enabling single photon detection up to room temperature and out to record long wavelengths for III-Vs with conventional APDs, as well as near-ideal noise and gain scaling with staircase APDs.In the spirit of a workshop, we will also discuss our ongoing work (1) covering broader swaths of the IR with low-noise APDs by taking advantage of the large digital alloy design space, (2) translating AlInAsSb to InP and silicon substrates, and (3) combining with MBE selective-area regrowth for dense focal plane arrays. This work is in close collaboration with Prof. Joe Campbell’s group at UVA and we acknowledge support from ARO, DARPA, NASA, AFRL, Northrop Grumman, and Lockheed Martin.

Semiconductor quantum dots (QDs), also referred to as artificial atoms, offer unique properties such as discrete and size-controlled energy levels. They have recently emerged as a pivotal platform for various optoelectronic applications including emitters and detectors. Specifically, epitaxial III-V QDs serve as the active component of choice in solid-state infrared emitters such as lasers and single/entangled photon sources. QD-based emitters in the near IR regime have demonstrated exceptional performance with state-of-the-art device parameters. A critical step towards realizing these QD-based IR emitters is high-quality epitaxial growth, with separate optimization strategies required for different device classes. This presentation will focus on our recent work in developing epitaxial strategies for realizing QD-based IR emitters, specifically lasers and single/entangled photon sources (SPS).

Different methods for realizing III-V epitaxial QDs - Stranski-Krastanov, droplet epitaxy, in-situ etching and patterned growth - will be reviewed. The QDs are grown using molecular beam epitaxy (MBE) on various III-V substrates including GaAs, GaSb and InP. Structural optimization studies to tune areal density, shape, size and position will be presented and are based on atomic force microscopy and transmission electron microscopy results. It is to be noted that the objectives for structural optimization are slightly different between laser- and SPS-based applications. Preliminary work on deterministic placement of QDs will be discussed. From the optical perspective, all emitter applications demand wavelength control and higher photon counts (on different scales) and these findings will be presented based on photoluminescence (PL) measurements. Optimized QD recipes are used to grow both laser and SPS structures and device fabrication is carried out. Device-specific characterization results will be shared: for lasers - LIV (light-current-voltage) characteristics, light traces and spectrum measurements; for SPS - low-temperature imaging/spectrum and g2 measurements.