Mode-locked THz QC-VECSELs

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This research addresses the challenge of making terahertz semiconductor laser sources that emit electromagnetic waves with frequencies between 2 and 5 THz (i.e. wavelengths between 60 and 150 microns). The terahertz frequency range is relatively underdeveloped part of the electromagnetic spectrum, which resides between the infrared and microwave. In past research, this team demonstrated a new laser architecture that addressed the challenge of how to generate high power THz light simultaneously with high beam quality, known as the THz quantum-cascade vertical-external-cavity surface-emitting-laser. The goal of this research is how to adapt this laser architecture to create a so-called “mode-locked” terahertz laser. In such a laser, instead of emitting steady continuous light, the laser would emit a series of extremely short high-intensity light pulses at regular intervals. While mode-locked lasers are ubiquitous in the visible and infrared wavelengths, they have proven very difficult to implement for terahertz lasers. Work will focus on several approaches to adapt the vertical-external-cavity surface-emitting-laser approach, including coaxing the laser to emit many wavelengths at once, controlling the speed of propagation of all of these various wavelengths within the laser cavity, and introducing a fast “saturable absorber” switch into the laser cavity to encourage these various wavelengths to travel in lock-step so that they all constructively interfere to create a short pulse. If successful, this research would result in a new terahertz source for applications in the fields of astrophysics, atmospheric science, biological and medical sciences, security screening, illicit material detection, combustion science, antiquities, waste-sorting, next-generation wireless communications, and non-destructive evaluation. As a part of the project, the research will train graduate and undergraduate students, and will support recruitment and retention of underrepresented minorities to engineering through participation in a targeted research project course.Technical descriptionThe research goal of this proposal is the development of active and passive mode-locked terahertz quantum-cascade lasers that emit picosecond pulses based upon the metasurface vertical-external-cavity surface-emitting-laser (VECSEL) concept. Two primary schemes will be investigated: active mode-locking in which the gain metasurface is modulated via radiofrequency (RF) electrical injection at the round trip frequency, and passive/hybrid mode-locking where a fast saturable absorber is used with optional RF loss modulation. The enabling component of the VECSEL is a reflectarray metasurface made up of sub-wavelength antenna-coupled microcavities loaded with laser gain material; this creates an active amplifying mirror which serves as one mirror in an open cavity. The intellectual merit in the proposed work lies in the use of the metasurface VECSEL architecture to investigate mode-locked terahertz lasers. Such a configuration gives the possible of engineering the metasurface to achieve broadband gain and dispersion compensation; furthermore the external cavity allows power combining for high-power output along with control of the cavity repetition rate. Additionally, the external cavity will allow integration of a fast saturable absorber, which can be separately optimized to have a fast recovery time while the quantum-cascade active material is optimized to have a long gain recovery time. The broader impacts are addressed at several levels including undergraduate and graduate research experiences, dissemination of results, technology advancement, and outreach to underrepresented minorities. Outreach will specifically occur through development of research projects for a course designed for the recruitment and retention of underrepresented minority first-year engineering students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.