Layer-dependent spin properties of charge carriers in vertically coupled telecom quantum dots

We investigate the spin properties of charge carriers in vertically coupled InAs/InAlGaAs quantum dots grown by molecular beam epitaxy, emitting at telecom C-band wavelengths, with a silicon <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>δ</a:mi> </a:math> -doped layer. Using time-resolved pump-probe Faraday ellipticity measurements, we systematically study single-, two-, and four-layer quantum dot (QD) configurations to quantify how vertical coupling affects key spin-coherence parameters. Our measurements reveal distinct layer-dependent effects: (i) Adding a second QD layer flips the resident charge from electrons to holes, consistent with optically induced electron tunneling into lower-energy dots and resultant hole charging. (ii) Starting from the four-layer sample, the pump-probe signal develops an additional nonoscillating, decaying component absent in single- and two-layer samples, attributed to multiple layer growth changing the strain environment, which reduces heavy-hole and light-hole mixing. (iii) With four layers or more, hole spin mode locking (SML) can be observed, enabling quantitative extraction of the hole coherence time <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:msub> <b:mi>T</b:mi> <b:mn>2</b:mn> </b:msub> <b:mo>≈</b:mo> <b:mn>13</b:mn> </b:mrow> </b:math> ns from SML amplitude saturation. We also extract longitudinal spin relaxation ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msub> <c:mi>T</c:mi> <c:mn>1</c:mn> </c:msub> </c:math> ) and transverse ( <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msubsup> <d:mi>T</d:mi> <d:mn>2</d:mn> <d:mo>*</d:mo> </d:msubsup> </d:math> ) spin dephasing times, g-factors, and inhomogeneous dephasing parameters for both electrons and holes across all layer configurations. The hole spin dephasing times <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:msubsup> <e:mi>T</e:mi> <e:mn>2</e:mn> <e:mo>*</e:mo> </e:msubsup> </e:math> remain relatively constant (2.3–2.7 ns) across layer counts, while longitudinal relaxation times <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:msub> <f:mi>T</f:mi> <f:mn>1</f:mn> </f:msub> </f:math> decrease with increasing layers (from <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:mn>0.9</g:mn> <g:mspace width="0.28em"/> <g:mi>µ</g:mi> <g:mi mathvariant="normal">s</g:mi> </g:mrow> </g:math> for single-layer to <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:mrow> <j:mn>0.32</j:mn> <j:mspace width="0.28em"/> <j:mi>µ</j:mi> <j:mi mathvariant="normal">s</j:mi> </j:mrow> </j:math> for four-layer samples). These findings provide potential design guidelines for engineering spin coherence in telecom-band QDs for quantum information applications.