Optical phase change in bismuth through structural distortions induced by laser irradiation

ABSTRACT

Semimetal bismuth (Bi) is known to possess a wide range of peculiar properties, owing to its unique electronic band structure. Its electronic band can easily be distorted by structural changes, and thereby undergo transitions between semimetal to either semiconductor or metal states. Utilising a focused laser beam, one can easily introduce structural defects, along with phase changes, oxidation, and morphological modifications. Confocal Raman microscopy indicated that the as-fabricated Bi droplets inhibit the Raman signal from the underlying silicon (Si) substrate. After a laser flash heating step, the intensity of Si optical phonons was strongly enhanced at the positions of Bi droplets, and exceeding the intensity from the bare Si substrate. Thus, such laser irradiating step on the Bi droplets induces an optical phase change. The optical phase change was detected as going from inhibition to strong enhancement of the underlying Si substrate Raman signal. From the observed Bi optical phonon modes (E_g and A_1g), alterations in the Raman peaks due to laser exposure indicated that the ordered crystallinity in pristine Bi droplets became deteriorated. The effects of atomic displacements and loss of structural order in Bi droplets impacts its dielectric response. The observed Si Raman signal enhancement is similar to the surface-enhanced Raman scattering effect typically known for noble metals.

KEYWORDS:

• Bismuth
• phase change
• laser irradiation
• structural defects
• Raman spectroscopy

Acknowledgements

The authors are thankful to S. Ishii, T. D. Dao, and H. D. Ngo for scientific discussions.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Ørjan S. Handegård http://orcid.org/0000-0002-4618-2204

Tadaaki Nagao http://orcid.org/0000-0002-6746-2686

Additional information

Funding

This work is partially supported by JSPS KAKENHI (16F16315, 16H06364, 16H03820), and CREST ‘Phase Interface Science for Highly Efficient Energy Utilization’ (JPMJCR13C3) from Japan Science and Technology Agency.