Lars Persson, Avdelningen för fotonik

Diskussionsledare Prof. Uli Schwarz

Sammanfattning:

For wavelength-sensitive applications, such as gas sensing and atomic clocks, it is paramount to use lasers with a stable emission wavelength. Vertical-cavity surface-emitting lasers (VCSELs) are often preferred for these applications since the lasing wavelength is set by the cavity resonance rather than by the gain peak, and the lasing wavelength therefore shifts less with temperature. In VCSELs, the lasing wavelength typically redshifts with increasing temperature. This is attributed to the increasing refractive index, n, with temperature, T, of the semiconductor materials used in the cavity (dn/dT>0). Solutions to mitigate this redshift often rely on the use of temperature-stabilizing feedback loops and power-hungry thermo-electric coolers. These solutions can be both expensive and bulky.

We here present a way of compensating for the inherent redshift in the VCSEL by utilizing the dielectric material HfO2 that has a negative dn/dT in the distributed Bragg reflectors (DBRs). This concept is implemented in AlGaN-based VCSELs with a short (2.5λ) cavity to achieve a temperature stable emission wavelength at around 310 nm.

For high-performing VCSELs it is beneficial with a longer cavity length to reduce thermal resistance. However, then the effect caused by the optical field’s penetration into the DBRs is insufficient to compensate for a longer semiconductor cavity. Instead, we show numerically that incorporating a HfO2 spacer layer within the cavity can compensate for the redshift caused by the longer cavity. The two approaches proposed and demonstrated here to achieve a temperature-stable lasing wavelength are generic and could be applied to VCSELs in all material systems and emission wavelengths if they allow for the use of materials with negative dn/dT.