Design of a THz-TDS system optimised for high signal-to-noise ratio

Hirsch, Lennart (2025) Design of a THz-TDS system optimised for high signal-to-noise ratio. PhD thesis, University of Glasgow.

Abstract

This thesis represents a summary of the activities I performed throughout my PhD, focusing mainly on terahertz time-domain spectroscopy (THz-TDS) and developing techniques by which the THz-TDS method may be optimised for use with Ytterbium-doped Pottasium Gaddolinium Tungstate (Yb:KGW) laser systems, culminating in the application of quantum metrological methods to improve the sensitivity of the system further.

The THz region of the electromagnetic spectrum holds great promise for sensing, given the strong resonances exhibited by many biological molecules in the region, as well as the ability of THz to penetrate many dielectric materials whilst being non-ionising. This has lead to interest and application in a wide range of fields and industries such as, but certainly not limited to, defense, security, and medicine. Despite the inherent difficulty of generating and detecting coherent THz radiation, the field of THz research, and the filling of the THz gap, has attracted widespread curiosity within the academic optics and electronics communities.

As Yb:KGW laser systems become more commonplace in nonlinear optical laboratories, their use in the generation and detection of THz radiation through THz-TDS has increased. However, in order to make the best use of these laser sources, the inherent challenges that come with using high peak power laser sources such as these must be addressed. Excessive heating of the THz generation medium can result in reabsorption of THz and suboptimal optical conversion efficiency, and challenges in pump light rejection can results in either scattered/stray pump light or loss of THz radiation. These challenges, if unaddressed, result in excess noise, defeating the advantage of the higher THz signals generated by such sources. The temporal pulse duration achieved by Yb:KGWlasers is also longer than those achieved by Titanium Sapphire (Ti:Sapph) laser sources, putting these sources at a disadvantage in terms of temporal resolution, and generation and detection bandwidth. Some groups address some of these concerns through the use of highly customised pulse compressors and cryogenic cooling of nonlinear optical materials. This thesis describes the development of methods by which these Yb:KGW-driven THz-TDS systems may be improved by simpler means, without the need for cryogenic systems or complex pulse compression systems, using instead tools that are readily available in many labs working in nonlinear optics. This results in a more accessible and less complex system, opening the subject of THz-TDS sensing to a wider audience. The experimental design is optimised using simulation tools, cooling of the THz generation medium is achieved through water cooling using pre-existing water-cooling systems used by the laser source itself, and pulse compression is achieved using an optical parametric amplifier, present in many nonlinear optical laboratories already. The additional components required to implement these optimisations are commercially available and do not require custom fabrication. These improvements aim to enhance the performance of THz-TDS, broaden the adoption of the technology in both academic and industrial lab settings, and improve its applicability to industries such as defense, security, and medicine by increasing the achievable sensitivity of the system. Furthermore, these systems could feasibly be applied as remote sensing platforms, reducing or entirely removing the requirement to come into direct contact with the analyte to be measured. This is of special interest in applications that deal with potentially hazardous or unknown substances such as those in defense and security.

The use of quantum metrological techniques to reduce the noise of a system below the classical shot noise limit is also of great interest in the domain of THz-TDS. A further improvement of the measurement sensitivity potentially enables detection of smaller quantities of hazardous substances in security defense applications, allowing for earlier detection of such materials and reduced operator risk. This thesis extends the efforts to improve the performance of THz-TDS systems by implementing amplitude-squeezed twin beams (TWBs) as a probe, successfully pushing the noise performance of the system below the shot noise limit in a modified TDS scheme. While this scheme cannot be applied to the conventional TDS scheme, it represents, to the author’s best knowledge, the first demonstration of a quantum-enhanced THz-TDS measurement that reduces the system noise below the shot noise limit when compared to an analogous scheme using a coherent/classical probe.

Finally, this thesis covers works investigating sensing of turbulent environments. The use of THz-TDS systems within the industries mentioned above will inevitably require application outside a laboratory environment. As such, an experimentwas designed and constructed to assess the applicability of light with orbital angular momentum (OAM) to such problems. These phasestructured states of light show promise in environmental sensing due to their phase-sensitive nature. Therefore there is potential to apply such a scheme to correct for turbulent channels when using THz-TDS systems within such environments.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Supported by funding from the Defence Science and Technology Laboratory (DSTL).
• Subjects: T Technology > T Technology (General)
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Funder's Name: Defence, Science and Technology Laboratory (DSTL)
• Supervisor's Name: Clerici, Professor Matteo and Lavery, Professor Martin
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85214
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 18 Jun 2025 12:38
• Last Modified: 18 Jun 2025 12:42
• Thesis DOI: 10.5525/gla.thesis.85214
• URI: https://theses.gla.ac.uk/id/eprint/85214

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