Micro-optical studies of optical properties and electronic states of ridge quantum wire lasers Presented at Department of Physics, Graduate School of Science, University of Tokyo Shinichi Watanabe Room 201a

Table of contents 3. Lasing properties 4. Top-View imaging measurement 5. Temperature dependence of carrier migration 6. Polarization measurement from the cleaved edge 7. Recomposition of the hole wavefunctions to numerically investigate the linear polarization dependence 8. Correspondance between model calculation and experimental polarization measurement 9. Application to T-shaped quantum wire lasers 2. Sample structure 1. Introduction

Chapter 1 Chapter 2 Introduction Sample structure

One dimensional (1D) quantum wire (QWR) lasers ; Peaked structure of the 1D Density of states ; Valence band mixing at the band edge ; Chapter 1 Figure (1.1) Page 14 Reduced temperature dependence Narrow gain spectra High differential gain Low threshold pump power for lasing Polarization control

Probability of electrons E QW -E QWR ～ 15 meV E QW -E QWR ～ meV

Chapter 2 Figure (2.2) Page 21 TEM image

Formation of ridge QWR by MBE [facet growth] GaAs patterned substrate S.Koshiba et.al,J. Appl. Phys. 76, 4138 (1994). MBE growth Chapter 2 Figure (2.4) (2.5) Page 22,23 2  m (111)B (001) Ga atoms flow (111)B → (001)

Chapter 2 Figure (2.1) Page 20 Sample structure

Chapter 2 Figure (2.2) Page 21 TEM image

Lasing properties of ridge quantum wire laser I. PL Imaging to investigate the origin of the emission II. Lasing properties III. Origin of Lasing Chapter 3

Figure (3.1) Page 29 Experimental Setup

Chapter 3 Figure (3.2) Page 30 PL spectrum and corresponding images z y Ti:Sapphire Laser 3 x 10 2 W/cm 2 z y

Chapter 3 Figure (3.4) Page 32 Excitation power dependence of PL spectra

Chapter 3 Figure (3.5) Page 32 Excitation power dependence of Emission intensity (QWR)

Temperature dependence of Lasing spectra and Threshold Power Chapter 3 Figure (3.7) Page 34 Th. Power

Excitation power dependence of PL spectra II Chapter 3 Figure (3.4) Page 32

Lasing image and PL images from QWR and Side-QW Chapter 3 Figure (3.9) Page 35

--- Stimulated emission characteristics --- Summary of chapter 3 The origin of lasing is ; (PL) spectrum and image support this conclusion Stimulated emission from the 1D ridge QWR laser structures from 4 K to R. T. The transition between higher order subbands in 1D QWRs S. Watanabe et al., Appl. Phys. Lett. 73, 511 (1998) Reference :

Top-View imaging measurement of ridge quantum wire laser I. Various Top-View images II. Energy-resolved Top-View PL images III. Excitation power dependence of Top-View PL images Chapter 4

Figure (4.2) Page 40 Mono Layer fluctuation of the QWR (nm-scale along z-direction)

Chapter 4 Figure (4.3) Page 41 3  m QWR ; 2-3  m Side-QW ; 1  m (I) Along the wire (// x) (II) Lateral direction (// y) Carrier migration Side-QW QWR

Lasing origin = QWR No scattered lasing emission inside the cavity sub-  m uniformity Chapter 4 Figure (4.5) Page 43

--- Top-View measurement of ridge QWR laser --- Summary of chapter 4 (I) Carriers can move along QWR in 2-3-  m scale S. Watanabe et al., Appl. Phys. Lett. 75, 2190 (1999) Reference : (II) Carrier migration from Side-QW to QWR (III) Origin of lasing = QWR We further confirm by the Top-View image. Position dependence of the migration

Temperature dependence of carrier migration in ridge quantum wire laser I. Temperature dependence of PL intensity (II. Temperature dependence of Top-View images) (III. Origin of the low-energy states) Chapter 5

Figure (5.1) Page 47 Sharp spectral line-width (QWR)

Side-QW QWR Sum Temperature dependence of PL Integrated Intensity T < 35 K Side-QW QWR Loss ; small Move along the QWR T > 35 K Loss ; large Chapter 5 Figure (5.1) Page 47

--- Temperature dependence of PL --- Summary of chapter 5 S. Watanabe et al., Proc. APF-6, pp. 376(2001) Reference : T < 35 K Side-QW QWR Loss ; small Move along the QWR T > 35 K Loss ; large

Polarization measurement from the cleaved edge of ridge quantum wire laser I. Uniform excitation II. Point excitation Chapter 6

Polarizer PL Polarization Measurement (Uniform excitation) Chapter 6 Figure (6.6) Page 59 YLF Laser

Polarizer YLF Laser PL Polarization Measurement (Point excitation) Chapter 6 Figure (6.10) Page deg.+30 deg.

--- Polarization measurement of ridge QWR laser --- Summary of chapter 6 (I) Uniform excitation (Structure fluctuation) Lasing polarization (II) Point excitation +45 degree → +20 degree → +30 degree

Recomposition of the hole wavefunctions to numerically investigate the linear polarization dependence I. Electron envelope wavefunctions II.Recomposition of the hole wavefunctions III. Transition matrix element IV. The case of Asymmetric potential profile Chapter 7

Calculation of the transition matrix element e1 e2 x y z e1-h1 x y z e2-h1 h1 even odd Chapter 7 Figure (7.3) (7.4) Page 74, 76

Oscillator strength of the e1-h1 transition Chapter 7 Table (7.2) Page 79 e1-h1 : y – polarization [inside y-z plane]

Oscillator strength of the e2-h1 transition Chapter 7 Table (7.2) Page 79 e2-h1 : z – polarization [inside y-z plane]

Oscillator strength for the asymmetric potential Chapter 7 Figure (7.8) Page 83

e1 e2 h1 e1-h1 e2-h1 Chapter 7 Figure (7.7-9) Page 82-84

--- Recomposition of the hole wavefunctions --- Summary of chapter 7 S. Watanabe et al., submitted to Phys. Rev. B Reference : Electron envelopHole recomposed envelop e1-h1 : nearly y-polarization e2-h1 : nearly z-polarization

The correspondance between model calculation and experimental polarization measurement(chap. 6) (I. Model Calculation) II.Calculation from the TEM potential Chapter 8

Figure (8.13) Page 99 Wave functions of a real ridge QWR laser Hole wave functions largely spread over Side-QW. e1-h1, e2-h2,... transition ; small oscillator strength e2-h1, e3-h2,... transition ; large oscillator strength (nearly y-polarization) (nearly z-polarization)

Chapter 8 Figure (8.14) Page 100 e(n+1)-h(n) (-10 ～ 35 deg.) e1-h1 (55 deg.) e(n)-h(n) (-65 ～ -90 deg.)

--- Correspondance between Model calc. and Measurement --- Summary of chapter 8 (I) Model calculation (II) Calculation for a real ridge QWR laser (W dependence) (Asymmetry) (d dependence) e(n+1)-h(n) transition has large oscillator strength, which causes the nearly z-polarized transition.

Micro-imaging measurement and waveguide calculation for T-shaped quantum wire lasers I. Microscopic imaging measurement to investigate the origin of lasing II. Numerical calculation to optimize the waveguide structure Chapter 9

PL and Lasing Image Chapter 9 Figure (9.4) (9.7-9) Page 107,110,111 x= 0.5 x= 0.35 x=0.1 T-wire

Design of a single T-wire laser Chapter 9 Figure (9.14) (9.15) Page 117,118  = 5.13 x (optimized)

--- Microscopy and calculation of T-wire laser --- Summary of chapter 9 (I) Microscopic imaging measurement Y. Takahashi et al., submitted to QELS conference (2002). Reference : (II) Optimization of the T-shaped waveguide Stimulated emission from a single T-wire laser Lasing origin of L1(T-wire), L2(arm-QW), and L3(stem-QW) Y. Hayamizu et al., submitted to CLEO conference (2002).

Summary Lasing properties and its origin (Chapter 3, 4) Nano-scale electronic structure (Chapter 4, 6) Carrier migration (Chapter 4, 5) Calculation for electronic states (Chapter 7, 8) Application to T-shaped quantum wire laser (Chapter 9) Ridge quantum wire laser ; Understanding of physical phenomena inside 1D quantum wire laser