diff --git a/lasy/utils/grid.py b/lasy/utils/grid.py
index 1168cec7..dc091f87 100644
--- a/lasy/utils/grid.py
+++ b/lasy/utils/grid.py
@@ -1,4 +1,5 @@
-from lasy.backend import xp
+import numpy as np
+from lasy.backend import xp, use_cupy
 
 time_axis_indx = -1
 
@@ -77,7 +78,9 @@ def set_temporal_field(self, field):
         """
         assert field.shape == self.temporal_field.shape
         assert field.dtype == "complex128"
-        self.temporal_field[:, :, :] = field
+        if use_cupy and type(field) == np.ndarray:
+            field = xp.asarray(field) # Copy to GPU
+        self.temporal_field[:,:,:] = field
         self.temporal_field_valid = True
         self.spectral_field_valid = False  # Invalidates the spectral field
 
@@ -92,11 +95,13 @@ def set_spectral_field(self, field):
         """
         assert field.shape == self.spectral_field.shape
         assert field.dtype == "complex128"
-        self.spectral_field[:, :, :] = field
+        if use_cupy and type(field) == np.ndarray:
+            field = xp.asarray(field) # Copy to GPU
+        self.spectral_field[:,:,:] = field
         self.spectral_field_valid = True
         self.temporal_field_valid = False  # Invalidates the temporal field
 
-    def get_temporal_field(self):
+    def get_temporal_field(self, to_cpu=False):
         """
         Return a copy of the temporal field.
 
@@ -108,37 +113,41 @@ def get_temporal_field(self):
         field : ndarray of complexs
             The temporal field.
         """
-        # We return a copy, so that the user cannot modify
-        # the original field, unless get_temporal_field is called
-        if self.temporal_field_valid:
-            return self.temporal_field.copy()
-        elif self.spectral_field_valid:
+        if not self.temporal_field_valid:
             self.spectral2temporal_fft()
-            return self.temporal_field.copy()
+        # Return a copy of the field, either on CPU or GPU, so that the user
+        # cannot modify the original field, unless set_spectral_field is called
+        if to_cpu and use_cupy:
+            return xp.asnumpy(self.temporal_field)
         else:
-            raise ValueError("Both temporal and spectral fields are invalid")
+            return self.temporal_field.copy()
 
-    def get_spectral_field(self):
+    def get_spectral_field(self, to_cpu=False):
         """
         Return a copy of the spectral field.
 
         (Modifying the returned object will not modify the original field stored
         in the Grid object ; one must use set_spectral_field to do so.)
 
+        Parameters
+        ----------
+        to_cpu : bool
+            If True, the returned field is always returned as a numpy array on CPU
+            (even when the lasy backend is cupy)
+
         Returns
         -------
         field : ndarray of complexs
             The spectral field.
         """
-        # We return a copy, so that the user cannot modify
-        # the original field, unless set_spectral_field is called
-        if self.spectral_field_valid:
-            return self.spectral_field.copy()
-        elif self.temporal_field_valid:
+        if not self.spectral_field_valid:
             self.temporal2spectral_fft()
-            return self.spectral_field.copy()
+        # Return a copy of the field, either on CPU or GPU, so that the user
+        # cannot modify the original field, unless set_spectral_field is called
+        if to_cpu and use_cupy:
+            return xp.asnumpy(self.spectral_field)
         else:
-            raise ValueError("Both temporal and spectral fields are invalid")
+            return self.spectral_field.copy()
 
     def temporal2spectral_fft(self):
         """