diff --git a/FDTD/engine.cpp b/FDTD/engine.cpp
index df1f63b..30e3671 100644
--- a/FDTD/engine.cpp
+++ b/FDTD/engine.cpp
@@ -16,8 +16,8 @@
 */
 
 #include "engine.h"
-#include "engine_extension.h"
-#include "operator_extension.h"
+#include "extensions/engine_extension.h"
+#include "extensions/operator_extension.h"
 #include "tools/array_ops.h"
 
 //! \brief construct an Engine instance
diff --git a/FDTD/engine_cylindermultigrid.cpp b/FDTD/engine_cylindermultigrid.cpp
index a32785e..956472c 100644
--- a/FDTD/engine_cylindermultigrid.cpp
+++ b/FDTD/engine_cylindermultigrid.cpp
@@ -17,7 +17,7 @@
 
 #include "engine_cylindermultigrid.h"
 #include "operator_cylindermultigrid.h"
-#include "engine_ext_cylindermultigrid.h"
+#include "extensions/engine_ext_cylindermultigrid.h"
 
 Engine_CylinderMultiGrid* Engine_CylinderMultiGrid::New(const Operator_CylinderMultiGrid* op, unsigned int numThreads)
 {
diff --git a/FDTD/engine_multithread.cpp b/FDTD/engine_multithread.cpp
index fbd0c5a..05ee0ce 100644
--- a/FDTD/engine_multithread.cpp
+++ b/FDTD/engine_multithread.cpp
@@ -26,7 +26,7 @@
 
 
 #include "engine_multithread.h"
-#include "engine_extension.h"
+#include "extensions/engine_extension.h"
 #include "tools/array_ops.h"
 
 #include "boost/date_time/posix_time/posix_time.hpp"
diff --git a/FDTD/engine_ext_cylinder.cpp b/FDTD/extensions/engine_ext_cylinder.cpp
similarity index 99%
rename from FDTD/engine_ext_cylinder.cpp
rename to FDTD/extensions/engine_ext_cylinder.cpp
index 476b9f2..e7720c3 100644
--- a/FDTD/engine_ext_cylinder.cpp
+++ b/FDTD/extensions/engine_ext_cylinder.cpp
@@ -15,7 +15,7 @@
 *	along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#include "engine.h"
+#include "FDTD/engine.h"
 #include "engine_ext_cylinder.h"
 #include "operator_ext_cylinder.h"
 
diff --git a/FDTD/engine_ext_cylinder.h b/FDTD/extensions/engine_ext_cylinder.h
similarity index 94%
rename from FDTD/engine_ext_cylinder.h
rename to FDTD/extensions/engine_ext_cylinder.h
index a3ec90f..d468bc6 100644
--- a/FDTD/engine_ext_cylinder.h
+++ b/FDTD/extensions/engine_ext_cylinder.h
@@ -18,9 +18,9 @@
 #ifndef ENGINE_CYLINDER_H
 #define ENGINE_CYLINDER_H
 
-#include "engine.h"
+#include "FDTD/engine.h"
 #include "engine_extension.h"
-#include "operator_cylinder.h"
+#include "FDTD/operator_cylinder.h"
 
 class Operator_Ext_Cylinder;
 
diff --git a/FDTD/engine_ext_cylindermultigrid.cpp b/FDTD/extensions/engine_ext_cylindermultigrid.cpp
similarity index 98%
rename from FDTD/engine_ext_cylindermultigrid.cpp
rename to FDTD/extensions/engine_ext_cylindermultigrid.cpp
index 39577fc..f271f5c 100644
--- a/FDTD/engine_ext_cylindermultigrid.cpp
+++ b/FDTD/extensions/engine_ext_cylindermultigrid.cpp
@@ -15,9 +15,9 @@
 *	along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#include "engine.h"
+#include "FDTD/engine.h"
 #include "engine_ext_cylindermultigrid.h"
-#include "engine_cylindermultigrid.h"
+#include "FDTD/engine_cylindermultigrid.h"
 
 Engine_Ext_CylinderMultiGrid::Engine_Ext_CylinderMultiGrid(Operator_Extension* op_ext, bool isBase) : Engine_Extension(op_ext)
 {
diff --git a/FDTD/engine_ext_cylindermultigrid.h b/FDTD/extensions/engine_ext_cylindermultigrid.h
similarity index 94%
rename from FDTD/engine_ext_cylindermultigrid.h
rename to FDTD/extensions/engine_ext_cylindermultigrid.h
index b995a1c..e9226f5 100644
--- a/FDTD/engine_ext_cylindermultigrid.h
+++ b/FDTD/extensions/engine_ext_cylindermultigrid.h
@@ -18,9 +18,9 @@
 #ifndef ENGINE_EXT_CYLINDERMULTIGRID_H
 #define ENGINE_EXT_CYLINDERMULTIGRID_H
 
-#include "engine_cylindermultigrid.h"
+#include "FDTD/engine_cylindermultigrid.h"
 #include "engine_extension.h"
-#include "operator_cylindermultigrid.h"
+#include "FDTD/operator_cylindermultigrid.h"
 
 class Operator_Ext_CylinderMultiGrid;
 
diff --git a/FDTD/engine_ext_dispersive.cpp b/FDTD/extensions/engine_ext_dispersive.cpp
similarity index 99%
rename from FDTD/engine_ext_dispersive.cpp
rename to FDTD/extensions/engine_ext_dispersive.cpp
index c2d4eb3..fb766a8 100644
--- a/FDTD/engine_ext_dispersive.cpp
+++ b/FDTD/extensions/engine_ext_dispersive.cpp
@@ -17,8 +17,7 @@
 
 #include "engine_ext_dispersive.h"
 #include "operator_ext_dispersive.h"
-#include "engine.h"
-#include "engine_sse.h"
+#include "FDTD/engine_sse.h"
 
 Engine_Ext_Dispersive::Engine_Ext_Dispersive(Operator_Ext_Dispersive* op_ext_disp) : Engine_Extension(op_ext_disp)
 {
diff --git a/FDTD/engine_ext_dispersive.h b/FDTD/extensions/engine_ext_dispersive.h
similarity index 95%
rename from FDTD/engine_ext_dispersive.h
rename to FDTD/extensions/engine_ext_dispersive.h
index b69b98c..536a5bf 100644
--- a/FDTD/engine_ext_dispersive.h
+++ b/FDTD/extensions/engine_ext_dispersive.h
@@ -19,8 +19,8 @@
 #define ENGINE_EXT_DISPERSIVE_H
 
 #include "engine_extension.h"
-#include "engine.h"
-#include "operator.h"
+#include "FDTD/engine.h"
+#include "FDTD/operator.h"
 
 class Operator_Ext_Dispersive;
 
diff --git a/FDTD/engine_ext_lorentzmaterial.cpp b/FDTD/extensions/engine_ext_lorentzmaterial.cpp
similarity index 99%
rename from FDTD/engine_ext_lorentzmaterial.cpp
rename to FDTD/extensions/engine_ext_lorentzmaterial.cpp
index 3ba30f6..6d21aba 100644
--- a/FDTD/engine_ext_lorentzmaterial.cpp
+++ b/FDTD/extensions/engine_ext_lorentzmaterial.cpp
@@ -17,8 +17,7 @@
 
 #include "engine_ext_lorentzmaterial.h"
 #include "operator_ext_lorentzmaterial.h"
-#include "engine.h"
-#include "engine_sse.h"
+#include "FDTD/engine_sse.h"
 
 Engine_Ext_LorentzMaterial::Engine_Ext_LorentzMaterial(Operator_Ext_LorentzMaterial* op_ext_lorentz) : Engine_Ext_Dispersive(op_ext_lorentz)
 {
diff --git a/FDTD/engine_ext_lorentzmaterial.h b/FDTD/extensions/engine_ext_lorentzmaterial.h
similarity index 100%
rename from FDTD/engine_ext_lorentzmaterial.h
rename to FDTD/extensions/engine_ext_lorentzmaterial.h
diff --git a/FDTD/engine_ext_mur_abc.cpp b/FDTD/extensions/engine_ext_mur_abc.cpp
similarity index 99%
rename from FDTD/engine_ext_mur_abc.cpp
rename to FDTD/extensions/engine_ext_mur_abc.cpp
index c160ef4..bca8b0d 100644
--- a/FDTD/engine_ext_mur_abc.cpp
+++ b/FDTD/extensions/engine_ext_mur_abc.cpp
@@ -17,8 +17,8 @@
 
 #include "engine_ext_mur_abc.h"
 #include "operator_ext_mur_abc.h"
-#include "engine.h"
-#include "engine_sse.h"
+#include "FDTD/engine.h"
+#include "FDTD/engine_sse.h"
 #include "tools/array_ops.h"
 #include "tools/useful.h"
 
diff --git a/FDTD/engine_ext_mur_abc.h b/FDTD/extensions/engine_ext_mur_abc.h
similarity index 97%
rename from FDTD/engine_ext_mur_abc.h
rename to FDTD/extensions/engine_ext_mur_abc.h
index c358596..aadb2fd 100644
--- a/FDTD/engine_ext_mur_abc.h
+++ b/FDTD/extensions/engine_ext_mur_abc.h
@@ -19,8 +19,8 @@
 #define ENGINE_EXT_MUR_ABC_H
 
 #include "engine_extension.h"
-#include "engine.h"
-#include "operator.h"
+#include "FDTD/engine.h"
+#include "FDTD/operator.h"
 
 class Operator_Ext_Mur_ABC;
 
diff --git a/FDTD/engine_ext_pml_sf.cpp b/FDTD/extensions/engine_ext_pml_sf.cpp
similarity index 99%
rename from FDTD/engine_ext_pml_sf.cpp
rename to FDTD/extensions/engine_ext_pml_sf.cpp
index d0e0271..47bbdec 100644
--- a/FDTD/engine_ext_pml_sf.cpp
+++ b/FDTD/extensions/engine_ext_pml_sf.cpp
@@ -17,8 +17,7 @@
 
 #include "engine_ext_pml_sf.h"
 #include "operator_ext_pml_sf.h"
-#include "engine.h"
-#include "engine_sse.h"
+#include "FDTD/engine_sse.h"
 #include "tools/array_ops.h"
 
 /************************************************  Engine_Ext_PML_SF  **************************************************************************/
diff --git a/FDTD/engine_ext_pml_sf.h b/FDTD/extensions/engine_ext_pml_sf.h
similarity index 97%
rename from FDTD/engine_ext_pml_sf.h
rename to FDTD/extensions/engine_ext_pml_sf.h
index 615beab..8e910d5 100644
--- a/FDTD/engine_ext_pml_sf.h
+++ b/FDTD/extensions/engine_ext_pml_sf.h
@@ -19,8 +19,8 @@
 #define ENGINE_EXT_PML_SF_H
 
 #include "engine_extension.h"
-#include "engine.h"
-#include "operator.h"
+#include "FDTD/engine.h"
+#include "FDTD/operator.h"
 
 class Operator_Ext_PML_SF;
 class Operator_Ext_PML_SF_Plane;
diff --git a/FDTD/engine_ext_upml.cpp b/FDTD/extensions/engine_ext_upml.cpp
similarity index 99%
rename from FDTD/engine_ext_upml.cpp
rename to FDTD/extensions/engine_ext_upml.cpp
index 37ac614..62d51bb 100644
--- a/FDTD/engine_ext_upml.cpp
+++ b/FDTD/extensions/engine_ext_upml.cpp
@@ -17,8 +17,8 @@
 
 #include "engine_ext_upml.h"
 #include "operator_ext_upml.h"
-#include "engine.h"
-#include "engine_sse.h"
+#include "FDTD/engine.h"
+#include "FDTD/engine_sse.h"
 #include "tools/array_ops.h"
 #include "tools/useful.h"
 
diff --git a/FDTD/engine_ext_upml.h b/FDTD/extensions/engine_ext_upml.h
similarity index 97%
rename from FDTD/engine_ext_upml.h
rename to FDTD/extensions/engine_ext_upml.h
index 5fe98e8..283c886 100644
--- a/FDTD/engine_ext_upml.h
+++ b/FDTD/extensions/engine_ext_upml.h
@@ -19,8 +19,8 @@
 #define ENGINE_EXT_UPML_H
 
 #include "engine_extension.h"
-#include "engine.h"
-#include "operator.h"
+#include "FDTD/engine.h"
+#include "FDTD/operator.h"
 
 class Operator_Ext_UPML;
 
diff --git a/FDTD/engine_extension.cpp b/FDTD/extensions/engine_extension.cpp
similarity index 98%
rename from FDTD/engine_extension.cpp
rename to FDTD/extensions/engine_extension.cpp
index 242d28c..6f7993c 100644
--- a/FDTD/engine_extension.cpp
+++ b/FDTD/extensions/engine_extension.cpp
@@ -17,7 +17,7 @@
 
 #include "engine_extension.h"
 
-#include "engine.h"
+#include "FDTD/engine.h"
 
 Engine_Extension::Engine_Extension(Operator_Extension* op_ext)
 {
diff --git a/FDTD/engine_extension.h b/FDTD/extensions/engine_extension.h
similarity index 100%
rename from FDTD/engine_extension.h
rename to FDTD/extensions/engine_extension.h
diff --git a/FDTD/operator_ext_cylinder.cpp b/FDTD/extensions/operator_ext_cylinder.cpp
similarity index 98%
rename from FDTD/operator_ext_cylinder.cpp
rename to FDTD/extensions/operator_ext_cylinder.cpp
index a5e9458..0dff575 100644
--- a/FDTD/operator_ext_cylinder.cpp
+++ b/FDTD/extensions/operator_ext_cylinder.cpp
@@ -16,7 +16,7 @@
 */
 
 #include "operator_ext_cylinder.h"
-#include "operator_cylinder.h"
+#include "FDTD/operator_cylinder.h"
 #include "engine_ext_cylinder.h"
 
 Operator_Ext_Cylinder::Operator_Ext_Cylinder(Operator_Cylinder* op) : Operator_Extension(op)
diff --git a/FDTD/operator_ext_cylinder.h b/FDTD/extensions/operator_ext_cylinder.h
similarity index 98%
rename from FDTD/operator_ext_cylinder.h
rename to FDTD/extensions/operator_ext_cylinder.h
index 9da72ae..d65fb89 100644
--- a/FDTD/operator_ext_cylinder.h
+++ b/FDTD/extensions/operator_ext_cylinder.h
@@ -19,7 +19,7 @@
 #define OPERATOR_EXT_CYLINDER_H
 
 #include "operator_extension.h"
-#include "operator.h"
+#include "FDTD/operator.h"
 
 class Operator_Cylinder;
 
diff --git a/FDTD/operator_ext_dispersive.cpp b/FDTD/extensions/operator_ext_dispersive.cpp
similarity index 100%
rename from FDTD/operator_ext_dispersive.cpp
rename to FDTD/extensions/operator_ext_dispersive.cpp
diff --git a/FDTD/operator_ext_dispersive.h b/FDTD/extensions/operator_ext_dispersive.h
similarity index 98%
rename from FDTD/operator_ext_dispersive.h
rename to FDTD/extensions/operator_ext_dispersive.h
index 21e2329..e647632 100644
--- a/FDTD/operator_ext_dispersive.h
+++ b/FDTD/extensions/operator_ext_dispersive.h
@@ -18,7 +18,7 @@
 #ifndef OPERATOR_EXT_DISPERSIVE_H
 #define OPERATOR_EXT_DISPERSIVE_H
 
-#include "operator.h"
+//#include "operator.h"
 #include "operator_extension.h"
 
 //! Abstract base class for all dispersive material models, based on an ADE (additional differential equation)
diff --git a/FDTD/operator_ext_lorentzmaterial.cpp b/FDTD/extensions/operator_ext_lorentzmaterial.cpp
similarity index 100%
rename from FDTD/operator_ext_lorentzmaterial.cpp
rename to FDTD/extensions/operator_ext_lorentzmaterial.cpp
diff --git a/FDTD/operator_ext_lorentzmaterial.h b/FDTD/extensions/operator_ext_lorentzmaterial.h
similarity index 98%
rename from FDTD/operator_ext_lorentzmaterial.h
rename to FDTD/extensions/operator_ext_lorentzmaterial.h
index 9da4aaa..ec7b2bc 100644
--- a/FDTD/operator_ext_lorentzmaterial.h
+++ b/FDTD/extensions/operator_ext_lorentzmaterial.h
@@ -18,7 +18,7 @@
 #ifndef OPERATOR_EXT_LORENTZMATERIAL_H
 #define OPERATOR_EXT_LORENTZMATERIAL_H
 
-#include "operator.h"
+#include "FDTD/operator.h"
 #include "operator_ext_dispersive.h"
 
 class Operator_Ext_LorentzMaterial : public Operator_Ext_Dispersive
diff --git a/FDTD/operator_ext_mur_abc.cpp b/FDTD/extensions/operator_ext_mur_abc.cpp
similarity index 100%
rename from FDTD/operator_ext_mur_abc.cpp
rename to FDTD/extensions/operator_ext_mur_abc.cpp
diff --git a/FDTD/operator_ext_mur_abc.h b/FDTD/extensions/operator_ext_mur_abc.h
similarity index 98%
rename from FDTD/operator_ext_mur_abc.h
rename to FDTD/extensions/operator_ext_mur_abc.h
index e05a7d0..765355d 100644
--- a/FDTD/operator_ext_mur_abc.h
+++ b/FDTD/extensions/operator_ext_mur_abc.h
@@ -18,7 +18,7 @@
 #ifndef OPERATOR_EXT_MUR_ABC_H
 #define OPERATOR_EXT_MUR_ABC_H
 
-#include "operator.h"
+#include "FDTD/operator.h"
 #include "operator_extension.h"
 
 class Operator_Ext_Mur_ABC : public Operator_Extension
diff --git a/FDTD/operator_ext_pml_sf.cpp b/FDTD/extensions/operator_ext_pml_sf.cpp
similarity index 99%
rename from FDTD/operator_ext_pml_sf.cpp
rename to FDTD/extensions/operator_ext_pml_sf.cpp
index 1b7f35b..46de18d 100644
--- a/FDTD/operator_ext_pml_sf.cpp
+++ b/FDTD/extensions/operator_ext_pml_sf.cpp
@@ -17,7 +17,7 @@
 
 #include "operator_ext_pml_sf.h"
 #include "engine_ext_pml_sf.h"
-#include "operator_cylinder.h"
+#include "FDTD/operator_cylinder.h"
 #include "tools/array_ops.h"
 #include "fparser.hh"
 
diff --git a/FDTD/operator_ext_pml_sf.h b/FDTD/extensions/operator_ext_pml_sf.h
similarity index 99%
rename from FDTD/operator_ext_pml_sf.h
rename to FDTD/extensions/operator_ext_pml_sf.h
index eb15f26..c4379ef 100644
--- a/FDTD/operator_ext_pml_sf.h
+++ b/FDTD/extensions/operator_ext_pml_sf.h
@@ -18,8 +18,8 @@
 #ifndef OPERATOR_EXT_PML_SF_H
 #define OPERATOR_EXT_PML_SF_H
 
-#include "operator.h"
 #include "operator_extension.h"
+#include "FDTD/operator.h"
 
 class FunctionParser;
 
diff --git a/FDTD/operator_ext_upml.cpp b/FDTD/extensions/operator_ext_upml.cpp
similarity index 99%
rename from FDTD/operator_ext_upml.cpp
rename to FDTD/extensions/operator_ext_upml.cpp
index f85f693..4a8ad5d 100644
--- a/FDTD/operator_ext_upml.cpp
+++ b/FDTD/extensions/operator_ext_upml.cpp
@@ -16,7 +16,7 @@
 */
 
 #include "operator_ext_upml.h"
-#include "operator_cylindermultigrid.h"
+#include "FDTD/operator_cylindermultigrid.h"
 #include "engine_ext_upml.h"
 #include "tools/array_ops.h"
 #include "fparser.hh"
diff --git a/FDTD/operator_ext_upml.h b/FDTD/extensions/operator_ext_upml.h
similarity index 99%
rename from FDTD/operator_ext_upml.h
rename to FDTD/extensions/operator_ext_upml.h
index fdfeeb2..1f54364 100644
--- a/FDTD/operator_ext_upml.h
+++ b/FDTD/extensions/operator_ext_upml.h
@@ -18,7 +18,7 @@
 #ifndef OPERATOR_EXT_UPML_H
 #define OPERATOR_EXT_UPML_H
 
-#include "operator.h"
+#include "FDTD/operator.h"
 #include "operator_extension.h"
 
 class FunctionParser;
diff --git a/FDTD/operator_extension.cpp b/FDTD/extensions/operator_extension.cpp
similarity index 97%
rename from FDTD/operator_extension.cpp
rename to FDTD/extensions/operator_extension.cpp
index 4a52a93..20ed422 100644
--- a/FDTD/operator_extension.cpp
+++ b/FDTD/extensions/operator_extension.cpp
@@ -16,8 +16,7 @@
 */
 
 #include "operator_extension.h"
-
-#include "operator.h"
+#include "FDTD/operator.h"
 
 Operator_Extension::Operator_Extension(Operator* op)
 {
diff --git a/FDTD/operator_extension.h b/FDTD/extensions/operator_extension.h
similarity index 100%
rename from FDTD/operator_extension.h
rename to FDTD/extensions/operator_extension.h
diff --git a/FDTD/operator.cpp b/FDTD/operator.cpp
index 4b298fd..d0e08cc 100644
--- a/FDTD/operator.cpp
+++ b/FDTD/operator.cpp
@@ -18,7 +18,7 @@
 #include <fstream>
 #include "operator.h"
 #include "engine.h"
-#include "operator_extension.h"
+#include "extensions/operator_extension.h"
 #include "Common/processfields.h"
 #include "tools/array_ops.h"
 #include "fparser.hh"
diff --git a/FDTD/operator_cylinder.cpp b/FDTD/operator_cylinder.cpp
index fb41a11..c1e790d 100644
--- a/FDTD/operator_cylinder.cpp
+++ b/FDTD/operator_cylinder.cpp
@@ -18,8 +18,8 @@
 #include "engine.h"
 #include "Common/processfields.h"
 #include "operator_cylinder.h"
-#include "operator_extension.h"
-#include "operator_ext_cylinder.h"
+#include "extensions/operator_extension.h"
+#include "extensions/operator_ext_cylinder.h"
 
 Operator_Cylinder* Operator_Cylinder::New(unsigned int numThreads)
 {
diff --git a/FDTD/operator_cylindermultigrid.cpp b/FDTD/operator_cylindermultigrid.cpp
index eadf0e7..2d9d866 100644
--- a/FDTD/operator_cylindermultigrid.cpp
+++ b/FDTD/operator_cylindermultigrid.cpp
@@ -17,7 +17,7 @@
 
 #include "operator_cylindermultigrid.h"
 #include "engine_cylindermultigrid.h"
-#include "operator_ext_cylinder.h"
+#include "extensions/operator_ext_cylinder.h"
 #include "tools/useful.h"
 
 Operator_CylinderMultiGrid::Operator_CylinderMultiGrid(vector<double> Split_Radii) : Operator_Cylinder()
diff --git a/openEMS.pro b/openEMS.pro
index 26be764..a85eaa6 100644
--- a/openEMS.pro
+++ b/openEMS.pro
@@ -44,6 +44,7 @@ win32 {
 QMAKE_LFLAGS += \'-Wl,-rpath,\$$ORIGIN/../CSXCAD\'
 QMAKE_LFLAGS += \'-Wl,-rpath,\$$ORIGIN/../fparser\'
 QMAKE_LFLAGS += \'-Wl,-rpath,\$$ORIGIN/../tinyxml\'
+
 SOURCES += main.cpp \
     tools/ErrorMsg.cpp \
     tools/AdrOp.cpp \
@@ -55,30 +56,33 @@ SOURCES += main.cpp \
     FDTD/operator_cylinder.cpp \
     FDTD/engine_sse.cpp \
     FDTD/operator_sse.cpp \
-    FDTD/operator_extension.cpp \
-    FDTD/engine_extension.cpp \
-    FDTD/engine_ext_mur_abc.cpp \
-    FDTD/operator_ext_mur_abc.cpp \
-    FDTD/excitation.cpp \
-    FDTD/operator_ext_cylinder.cpp \
-    FDTD/engine_ext_cylinder.cpp \
     FDTD/operator_sse_compressed.cpp \
     FDTD/engine_sse_compressed.cpp \
     FDTD/operator_multithread.cpp \
     tools/global.cpp \
     tools/useful.cpp \
-    FDTD/operator_ext_dispersive.cpp \
-    FDTD/operator_ext_lorentzmaterial.cpp \
-    FDTD/engine_ext_dispersive.cpp \
-    FDTD/engine_ext_lorentzmaterial.cpp \
-    FDTD/operator_ext_pml_sf.cpp \
-    FDTD/engine_ext_pml_sf.cpp \
+    FDTD/excitation.cpp \
     FDTD/operator_cylindermultigrid.cpp \
     FDTD/engine_cylindermultigrid.cpp \
-    FDTD/engine_ext_cylindermultigrid.cpp \
-    FDTD/operator_ext_upml.cpp \
-    FDTD/engine_ext_upml.cpp \
     FDTD/engine_interface_fdtd.cpp
+
+# FDTD/extensions source files
+SOURCES += FDTD/extensions/engine_extension.cpp \
+    FDTD/extensions/operator_ext_dispersive.cpp \
+    FDTD/extensions/operator_ext_lorentzmaterial.cpp \
+    FDTD/extensions/engine_ext_dispersive.cpp \
+    FDTD/extensions/engine_ext_lorentzmaterial.cpp \
+    FDTD/extensions/operator_ext_pml_sf.cpp \
+    FDTD/extensions/engine_ext_pml_sf.cpp \
+    FDTD/extensions/engine_ext_cylindermultigrid.cpp \
+    FDTD/extensions/operator_ext_upml.cpp \
+    FDTD/extensions/engine_ext_upml.cpp \
+    FDTD/extensions/operator_extension.cpp \
+    FDTD/extensions/engine_ext_mur_abc.cpp \
+    FDTD/extensions/operator_ext_mur_abc.cpp \
+    FDTD/extensions/operator_ext_cylinder.cpp \
+    FDTD/extensions/engine_ext_cylinder.cpp
+
 # Common source files
 SOURCES += Common/operator_base.cpp \
     Common/engine_interface_base.cpp \
@@ -103,31 +107,34 @@ HEADERS += tools/ErrorMsg.h \
     FDTD/operator_cylinder.h \
     FDTD/engine_sse.h \
     FDTD/operator_sse.h \
-    FDTD/operator_extension.h \
-    FDTD/engine_extension.h \
-    FDTD/engine_ext_mur_abc.h \
-    FDTD/operator_ext_mur_abc.h \
     FDTD/excitation.h \
-    FDTD/operator_ext_cylinder.h \
-    FDTD/engine_ext_cylinder.h \
     FDTD/operator_sse_compressed.h \
     FDTD/engine_sse_compressed.h \
     FDTD/operator_multithread.h \
     tools/global.h \
     tools/useful.h \
-    FDTD/operator_ext_dispersive.h \
-    FDTD/operator_ext_lorentzmaterial.h \
-    FDTD/engine_ext_dispersive.h \
-    FDTD/engine_ext_lorentzmaterial.h \
-    FDTD/operator_ext_pml_sf.h \
-    FDTD/engine_ext_pml_sf.h \
     FDTD/operator_cylindermultigrid.h \
     FDTD/engine_cylindermultigrid.h \
-    FDTD/engine_ext_cylindermultigrid.h \
     tools/aligned_allocator.h \
-    FDTD/operator_ext_upml.h \
-    FDTD/engine_ext_upml.h \
-    FDTD/engine_interface_fdtd.h     
+    FDTD/engine_interface_fdtd.h  
+
+# FDTD/extensions header files   
+HEADERS += FDTD/operator_extension.h \
+    FDTD/extensions/engine_extension.h \
+    FDTD/extensions/engine_ext_mur_abc.h \
+    FDTD/extensions/operator_ext_mur_abc.h \
+    FDTD/extensions/operator_ext_cylinder.h \
+    FDTD/extensions/engine_ext_cylinder.h \
+    FDTD/extensions/operator_ext_dispersive.h \
+    FDTD/extensions/operator_ext_lorentzmaterial.h \
+    FDTD/extensions/engine_ext_dispersive.h \
+    FDTD/extensions/engine_ext_lorentzmaterial.h \
+    FDTD/extensions/operator_ext_pml_sf.h \
+    FDTD/extensions/engine_ext_pml_sf.h \
+    FDTD/extensions/engine_ext_cylindermultigrid.h \
+    FDTD/extensions/operator_ext_upml.h \
+    FDTD/extensions/engine_ext_upml.h
+
 # Common header files
 HEADERS += Common/operator_base.h \
     Common/engine_interface_base.h \
diff --git a/openems.cpp b/openems.cpp
index 7ab4d8f..4a5d4bb 100644
--- a/openems.cpp
+++ b/openems.cpp
@@ -22,9 +22,10 @@
 #include "FDTD/operator_cylindermultigrid.h"
 #include "FDTD/engine_multithread.h"
 #include "FDTD/operator_multithread.h"
-#include "FDTD/operator_ext_mur_abc.h"
-#include "FDTD/operator_ext_pml_sf.h"
-#include "FDTD/operator_ext_upml.h"
+#include "FDTD/extensions/operator_ext_mur_abc.h"
+#include "FDTD/extensions/operator_ext_pml_sf.h"
+#include "FDTD/extensions/operator_ext_upml.h"
+#include "FDTD/extensions/operator_ext_lorentzmaterial.h"
 #include "FDTD/engine_interface_fdtd.h"
 #include "Common/processvoltage.h"
 #include "Common/processcurrent.h"
@@ -37,8 +38,6 @@
 #include <H5Cpp.h> // only for H5get_libversion()
 #include <boost/version.hpp> // only for BOOST_LIB_VERSION
 
-#include "FDTD/operator_ext_lorentzmaterial.h"
-
 //external libs
 #include "tinyxml.h"
 #include "ContinuousStructure.h"