Longitudinal Computer Generated Holograms for Digital Frequency Control in Electronically Tunable Terahertz Lasers

TL;DR

This paper introduces a novel longitudinal computer-generated hologram (LCGH) that enables digital frequency control in terahertz lasers, allowing electronic tuning of multiple discrete frequencies in a single device.

Contribution

The work presents the design and demonstration of an LCGH that provides high-resolution spectral control and electronic tunability in THz quantum cascade lasers, a new approach in holographic and laser technology.

Findings

Achieved electronic switching between six discrete THz frequencies.

Demonstrated full spectral control by defining reflectivity at each k point.

Simplified integration of tunable THz lasers for spectroscopic applications.

Abstract

A transverse computer-generated hologram (CGH) diffracts and provides flexible control of incident light by steering it to any point in the projected image plane - i.e. CGHs are able to direct the light to where it is needed and away from where it is not. In addition, the number of resolvable points in the image projection plane is a function of the CGH's pixel count. Here we report a longitudinal CGH (LCGH), a photonic structure, which swaps the ability to steer light toward fixed spatial points for digital control in the frequency domain. This is of particular interest in the context of tunable lasers. In this regard, an LCGH offers two important degrees-of-freedom (DOFs): 1) provides high-resolution wavevector or k space resolution within the Brillouin zone; 2) enables full control to define or modify the reflectivity at each resolvable k point, so attaining a target spectral response. We demonstrate the flexibility of our LCGH approach by achieving purely electronic tuning between six digitally-selected operating frequencies in a single section terahertz (THz) quantum cascade laser (QCL). These switchable single-frequency devices will simplify combining the power and flexibility of THz QCLs with spectroscopic applications, such as remote sensing, spectral analysis, and both security and medical imaging.